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Double-Carrousel Mechanism for Mn-Catalyzed Dehydrogenative Amide Synthesis from Alcohols and Amines

  • Jesús A. Luque-Urrutia
    Jesús A. Luque-Urrutia
    Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, C/ Maria Aurèlia Capmany 69, 17003 Girona, Catalonia, Spain
    More by Jesús A. Luque-Urrutia
  • , 
  • Tània Pèlachs
    Tània Pèlachs
    Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, C/ Maria Aurèlia Capmany 69, 17003 Girona, Catalonia, Spain
    More by Tània Pèlachs
  • , 
  • Miquel Solà*
    Miquel Solà
    Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, C/ Maria Aurèlia Capmany 69, 17003 Girona, Catalonia, Spain
    *Email: [email protected]
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    Orcidhttp://orcid.org/0000-0002-1917-7450
  • , and 
  • Albert Poater*
    Albert Poater
    Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, C/ Maria Aurèlia Capmany 69, 17003 Girona, Catalonia, Spain
    *Email: [email protected]
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    Orcidhttp://orcid.org/0000-0002-8997-2599
Cite this: ACS Catal. 2021, 11, 10, 6155–6161
Publication Date (Web):May 6, 2021
https://doi.org/10.1021/acscatal.1c00693

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Abstract

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We study with density functional theory calculations the mechanism of the original example of a base-metal-catalyzed synthesis of amides from alcohols and amines. A preliminary proposal of the mechanism of this reaction was experimentally reported by Milstein and co-workers. Instead of the proposed reaction mechanism with a hemilabile pincer amine arm, our DFT calculations describe a facile protocol, where the catalyst only produces aldehydes from alcohols. Once formaldehyde is formed from methanol, it reacts with the amine to form a second alcohol. This new alcohol undergoes the same procedure as methanol and creates the desired amide through a double-carrousel mechanism. The rate-determining step in the catalytic aldehyde synthesis corresponds to the H2 formation. However, in the nonmetal-catalyzed part of the mechanism, the interaction of formaldehyde with the amine is also quite kinetically demanding.

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Introduction

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In the third decade of the third millennium, there is a fever to develop efficient electric cars (1) with the aim to reduce pollution in large cities, and, in general, to alleviate the greenhouse effect, (2) apart from the ozone depletion. (3) Among the main culprits are carbon dioxide (CO2) and nitrous oxide (N2O) generated by the burning of fossil fuels. (4−6) We can use experiments (7) and theory (8) to generate ways to fix both CO2 and N2O, (9) but also pull toward hydrogen (H2) generation, to avoid their undesired production.
It has been believed that the ideal source of hydrogen production could be the environmentally friendly oxidation of water, but such an approach is still far from efficient; thus, alternatives must be sought. The process called acceptorless dehydrogenative coupling (ADC) of alcohols, (10) with organic substrates, such as alkenes, (11) amines, (12,13) or nitriles, (14,15) is one of such alternatives. In these reactions, the involved hydrogen atoms do not become part of any subsequent organic molecule but are simply free as byproducts. These processes might become a potential way to convert alcohol via molecular hydrogen as the main fuel in the future, (10b) and these alcohols are obtained from a sustainable source such as biomass. (16) Surely, thinking of hydrogen as an energy source from this reaction is very pretentious, in quantitative terms, but this ADC process can be applied toward the development of liquid organic hydrogen carriers. (17) Returning to the origins of the ADC developed by Milstein and co-workers, ADC was first demonstrated by coupling of primary alcohols to form esters, (18) whereas the first example of amide formation by ADC of alcohols and amines was reported 2 years later. (19) Even though obtaining hydrogen in ADC reactions might be considered as a positive accident since the primary goal was to obtain an imine, or if condensation is avoided, an amide, ADC reactions represent a good example of a successful design of green chemical processes. In the synthesis of these chemicals, however, a metal complex is needed to facilitate them. This role was first played by ruthenium complexes, (18,20) followed by other precious metals, (21) and then during the last decade, it was possible to use more abundant first-row transition metals, (22) such as manganese (11,23) coordinating pincer ligands. (24)
Due to the important appearance of amides in medical treatments (they are present in about a quarter of current drugs, (25) plastics, diverse materials, and even in the DNA as a link between amino acids), (26) it is necessary to optimize the amide bond formation to reduce chemical waste and increase the production yield. (27) This line of research was followed by Milstein and co-workers who reported the first base-metal-catalyzed synthesis of amides employing primary amines with alcohols, using a pincer-based MnPtBuNNH catalyst (1), as shown in Figure 1. This synthetic methodology for amide synthesis avoids dealing with either carboxylic acids or their amine-activated derivatives in the presence of promoters, (28) with the corresponding undesired generation of stoichiometric amounts of residues. (29) This MnPtBuNNH-type of a catalyst was shown before to catalyze the dehydrogenative coupling of amines and diols to form cyclic imides in the presence of a base. (30)

Figure 1

Figure 1. Mn-based catalyst involved in the dehydrogenative amide synthesis from alcohols and amines.

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Catalyst 1 can convert up to 94% of alcohol into amide without the need of a promoter molecule and forming hydrogen gas as a byproduct. In the search for a plausible mechanism for the catalytic conversion schematized in Figure 1, we envisaged density functional theory (DFT) calculations. We were particularly interested in analyzing whether a plausible Mn–N bond cleavage during the process could reduce the potential of this catalyst due to possible decomposition. This cleavage was suggested in the originally proposed mechanism by Milstein et al. (13a) for the amide synthesis reaction catalyzed by 1 (see step 4→II in the preliminary mechanistic proposal of Figure 2).

Figure 2

Figure 2. Initial proposed mechanism for the amide synthesis reaction catalyzed by 1.

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Computational Details

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All DFT calculations were performed with the Gaussian 16 set of programs. (31) The electronic configuration of the molecular systems was described with the BP86 functional of Becke and Perdew, (32) using the Ahlrichs basis set def2SVP. (33) Since corrections due to dispersion are essential to study the reactivity, we have included them through Grimme’s method with Becke–Johnson damping (GD3BJ keyword in Gaussian). (34,35) The geometry optimizations were performed without symmetry constraints and the characterization of the local stationary points was carried out by analytical frequency calculations. These frequencies were used to calculate unscaled zero-point energies (ZPEs) as well as thermal corrections and entropy effects at 383.15 K and 1 atm. Solvent effects were included with the polarizable continuous solvation model (PCM) (36) using toluene as a solvent in single-point energy calculations on the optimized geometries with the M06 functional (37) and the cc-pVTZ basis set. (38) The reported Gibbs energies in this work include M06/cc-pVTZ//BP86-D3BJ/Def2SVP electronic energies with solvent effects obtained at the same level of theory, (13b,15,39) corrected with zero-point energies, thermal corrections, and entropy effects evaluated at 383.15 K with the BP86-D3BJ/Def2SVP method. The resulting solvation Gibbs energies were added to the final Gibbs energies in the gas phase to obtain Gibbs energies in solution. (40) Standard Gibbs energies in a solution refer to a 1 M standard-state concentration for all species. The change of the conventional 1 atm standard state for gas-phase calculations to a standard state of 1 M concentration in a solution requires the introduction of a correction in the Gibbs energy term of 2.62 kcal/mol. (41) Finally, according to Shaik and Kozuch, (42) in most catalytic cycles, only one transition state and one intermediate determine the turnover frequency (TOF). They are called the TOF-determining transition state (TDTS) and the TOF-determining intermediate (TDI).

Results and Discussion

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The mechanism of the reaction of alcohols with amines to form amides in the presence of catalyst 1 is shown in Figure 3. The first step includes the formation of the adduct A between the metal complex with an alcohol molecule, with a destabilizing Gibbs energy of 6.1 kcal/mol. The next step is a concerted double hydrogen transfer from methanol to the catalyst. In detail, one hydrogen atom goes directly to the metal and the other to the NtBu group bonded to the metal itself, leading to intermediate B. This last step, with methanol as alcohol, allows at the same time the release of a molecule of formaldehyde. This process is barrierless in the Gibbs energy surface and has a thermodynamic cost 13.6 kcal/mol with respect to the initial catalyst. Then, reductive elimination in B leads to the formation of H2 coordinated to manganese. This reductive elimination involves the upper energy point of the catalytic cycle, with a value of 29.6 kcal/mol from the TDI that is the initial catalyst to the TDTS, (42) 16.0 kcal/mol from B, forming a relatively unstable intermediate C. In the next step, C overcomes an insignificant energy barrier of 0.3 kcal/mol to release H2 and regenerate the catalyst. Thermodynamically, the whole catalytic cycle releases 10.3 kcal/mol, whereas kinetically, the formation of intermediate C defines the rate-determining step (RDS) with the overall cost of 29.6 kcal/mol (see Figure 4a). This is in agreement with the experimental temperature of 383.15 K. In addition, even though the energy difference is not that significant, the substitution in para of the pyridyl group by NH2 and CN groups modified this energy barrier to 29.4 and 29.7 kcal/mol, respectively, showing that an electron-donating group in this position of the ring slightly facilitates overcoming the energy barrier of the RDS.

Figure 3

Figure 3. Full mechanism of catalyst I. The top cycle shows the aldehyde formation from methanol (black) and hemiaminal [2] (magenta). The bottom pathway shows the formation of hemiaminal [2] from aldehyde (relative Gibbs energies in kcal/mol).

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Figure 4

Figure 4. Transition states (a) B → C, (b) [1] → [2] (assisted by water), and (c) [1] → [2] (assisted by methanol); selected distances given in Å.

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Once the first catalytic cycle finishes and formaldehyde appears in the media, there is a combination of formaldehyde, obtained in the first catalytic cycle, with amine, and thus the nonmetal-catalyzed pathway starts. It is a simple reaction of formaldehyde with the amine, which develops into hemiaminal [2]. The corresponding energy barrier is high (35.6 kcal/mol), but when assisted by an explicit molecule of methanol, the energy barrier is lowered by 11.6–24.0 kcal/mol (see Figure 4b,c). Even though this barrier is relatively high, it does not dispute the RDS through TS B-C of the initial catalytic cycle. (42) Once [2] is formed, it enters the second catalytic cycle when combined with catalyst I. The steps are mimetic of those found in the first catalytic cycle. Despite the analogies, there are certain differences. Specifically, TS A-B is 1.4 kcal/mol lower for [2] than for methanol, and the following steps are considerably less kinetically demanding. Actually, the RDS previously described by TS B-C requires only 16.0 kcal/mol in this second catalytic pathway, taking now B as the TDI, instead of the initial catalyst for the first catalytic pathway.
Overall, the above mechanism is comprised by (1) synthesis of formaldehyde from methanol, (2) formation of hemiaminal [2] from formaldehyde, and (3) synthesis of the product from compound [2]. We call this mechanism a “double carrousel” because it first synthesizes an aldehyde from alcohol, and then, it forms compound [2], a second alcohol that restarts the initial catalytic cycle. Consequently, it repeats the catalytic mechanism twice. Completing the double carrousel gives two hydrogen molecules, which complies with the experimental finding of 1.7 equiv compared to either the amine or the alcohol.
To have absolute certainty of having recognized the correct mechanism, we still need to compare it to the initial guess (Figure 2) to find out why this mechanism was not plausible even though the cleavage of a Mn–N bond does not require the presence of a nonmetal-catalyzed step. DFT calculations of the hypothesized mechanism in Figure 5 rule out the existence of this alternative mechanism.

Figure 5

Figure 5. Alternative mechanism of catalyst 1 incorporating the Mn–N bond cleavage (relative Gibbs energies in kcal/mol).

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Starting from catalyst I, the methanol molecule attacks the metal center via the oxygen atom, leading to intermediate 4, once the hydroxyl transfers its hydrogen to the NtBu moiety. This step is easy to achieve since the energy barrier is only 6.9 kcal/mol. Next, there are two concerted transformations before reaching compound II. First, we must open the Mn–NtBu bond, with a calculated energy barrier of 21.1 kcal/mol. Then, there is a hydrogen transfer from the former alcohol leading to the formation of a hydride. In complex II, there is an agostic interaction of the closest methylenic hydrogen with the metal that provides some additional stabilization of this complex. This last hydrogen transfer facilitates the attack of the amine on the newly dehydrated carbon (TS II-E), which involves the formation of H2 on the metal. It is defined as the RDS of this alternative reaction pathway, with a kinetic cost of 52.6 kcal/mol when assisted by a water molecule. The next release of a hydrogen molecule again has a low cost as in the mechanism of Figure 3, specifically 2.5 kcal/mol, and has a very favorable thermodynamics of 17.8 kcal/mol, partly justified by the coordination of the labile arm previously dissociated from the metal. But this step precedes an expensive transfer of hydrogen to the metal as a hydride (42.8 kcal/mol from initial catalyst I) to release the amide product. Final reductive elimination in 5 releases the second molecule of hydrogen and regenerates initial catalyst I. Either way, the energy barrier for the RDS is insurmountable under the reaction conditions. Thus, this alternative pathway is not the optimal pathway for the catalyst, not for the initial or final steps, but for the RDS that occurs once the Mn–N bond is broken. (43) Thus, in consistency with the past ruthenium-based catalyst, Mn catalysis involving a hemilabile pincer amine arm is found to be not feasible. (44)

Conclusions

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The mechanism for the dehydrogenative amide synthesis from alcohols and amines has been studied with DFT calculations. The most plausible reaction mechanism found involves a double-carrousel catalytic cycle to get aldehydes twice, first, the reagent aldehyde in the metal-catalyzed cycle, and second, the product aldehyde (the amide) from the hemiaminal generated by the combination of formaldehyde and amine in a nonmetal-catalyzed step. Thus, the reaction mechanism is divided into aldehyde synthesis and hemiaminal formation, metal- and nonmetal-catalyzed, respectively. The overall RDS appears at the hydrogen formation (29.6 kcal/mol), thus in the metal-catalyzed cycle, whereas the most energetically demanding nonmetal-catalyzed step unveils a rather high kinetic cost (24.0 kcal/mol) as well. An alternative mechanism, in which all steps are catalyzed by the metal and which involves the breaking of a Mn–N bond, was found unrealistic under the experimental conditions.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acscatal.1c00693.

  • Computational details and all XYZ coordinates, absolute energies, and 3D structures of all species (XYZ)

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Most electronic Supporting Information files are available without a subscription to ACS Web Editions. Such files may be downloaded by article for research use (if there is a public use license linked to the relevant article, that license may permit other uses). Permission may be obtained from ACS for other uses through requests via the RightsLink permission system: http://pubs.acs.org/page/copyright/permissions.html.

Author Information

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  • Corresponding Authors
  • Authors
    • Jesús A. Luque-Urrutia - Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, C/ Maria Aurèlia Capmany 69, 17003 Girona, Catalonia, Spain
    • Tània Pèlachs - Institut de Química Computacional i Catàlisi and Departament de Química, Universitat de Girona, C/ Maria Aurèlia Capmany 69, 17003 Girona, Catalonia, Spain
  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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J.A.L.-U. thanks Universitat de Girona for an IFUdG2017 Ph.D. fellowship. A.P. is a Serra Húnter Fellow. A.P. and M.S. thank the Ministerio de Economía y Competitividad (MINECO) of Spain for projects PGC2018-097722-B-I00 and CTQ2017-85341-P and the Generalitat de Catalunya for project 2017SGR39 and ICREA Academia prize 2019 to A.P. The authors thank Prof. David Milstein for helpful discussion, especially centered on the hemilability of the pincer ligand.

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    A review. An overview is given on extensive studies which are involve N2O in org. synthesis as an oxidant. Due to its unique oxidative properties, nitrous oxide provides remarkable prospects in catalytic and non-catalytic oxidn. chem. in both gas and liq. phases. The interest in this new oxidant resulted in development of a new technol. for the prodn. of N2O.
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    .
    (d) Tolman, W. B. Binding and Activation of N2O at Transition Metal Centers: Recent Mechanistic Insight. Angew. Chem., Int. Ed. 2010, 49, 1018– 1024,  DOI: 10.1002/anie.200905364
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    5d
    Binding and Activation of N2O at Transition-Metal Centers: Recent Mechanistic Insights
    Tolman, William B.
    Angewandte Chemie, International Edition (2010), 49 (6), 1018-1024CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review of mechanistic insights concerning the binding and activation of N2O at transition metal centers in relation to it decompn. by those transition metals, is given. Recent advances in the understanding of metal ion gaseous-phase N2O redn. reaction pathways and in heterogeneous, homogeneous, and bio-catalytic systems provide provocative ideas concerning the structure and properties of metal-N2O adducts and derived intermediates. These ideas are likely to inform efforts to design more effective N2O remediation catalysts. Topics discussed include: metal/N2O interactions in the gas phase or inert matrixes; N2O activation during heterogeneous catalysis; N2O activation in biol.: nitrous oxide reductase; N2O binding and activation by metal complexes in soln.; and conclusions and perspective.
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  6. 6
    (a) Hsu, J.; Prather, M. J. Global Long-Lived Chemical Modes Excited in a 3-D Chemistry Transport Model: Stratospheric N2O, NOy, O3 and CH4 Chemistry. Geophys. Res. Lett. 2010, 37, L07805  DOI: 10.1029/2009GL042243
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    6a
    Global long-lived chemical modes excited in a 3-D chemistry transport model: stratospheric N2O, NOy, O3 and CH4 chemistry
    Hsu, Juno; Prather, Michael J.
    Geophysical Research Letters (2010), 37 (7), L07805/1-L07805/5CODEN: GPRLAJ; ISSN:0094-8276. (American Geophysical Union)
    The two longest-lived, major chem. response patterns (eigenmodes) of the atm., coupling N2O and CH4, are identified with the UCI chem.-transport model using a linearized (N2O, NOy, O3, CH4, H2O)-system for stratospheric chem. and specified tropospheric losses. As in previous 1D and 2D studies, these century-long 3D simulations show that the e-folding decay time of a N2O perturbation (mode-1: 108.4 y) caused by a pulse emission of N2O is 10-years shorter than the N2O atm. lifetime (118.2 y). This mode-1 can also be excited by CH4 emissions due to CH4-O3 stratospheric chem.: a pulse emission of 100 Tg CH4 creates a +0.1 Tg N2O perturbation in mode-1 with a 108-yr e-folding decay time, thus increasing the CH4 global warming potential by 1.2%. Almost all of the 100 Tg CH4 appears in mode-2 (10.1 y).
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    (b) Montzka, S. A.; Dlugokencky, E. J.; Butler, J. H. Non-CO2 Greenhouse Gases and Climate Change. Nature 2011, 476, 43– 50,  DOI: 10.1038/nature10322
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    6b
    Non-CO2 greenhouse gases and climate change
    Montzka, S. A.; Dlugokencky, E. J.; Butler, J. H.
    Nature (London, United Kingdom) (2011), 476 (7358), 43-50CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    A review. Earth's climate is warming as a result of anthropogenic emissions of greenhouse gases, particularly CO2 from fossil fuel combustion. Anthropogenic emissions of non-CO2 greenhouse gases, such as CH4, N2O, and ozone-depleting substances (largely from sources other than fossil fuels), also contribute significantly to warming. Some non-CO2 greenhouse gases have much shorter lifetimes than CO2, so reducing their emissions offers an addnl. opportunity to lessen future climate change. Although it is clear that sustainably reducing the warming influence of greenhouse gases will be possible only with substantial cuts in emissions of CO2, reducing non-CO2 greenhouse gas emissions would be a relatively quick way of contributing to this goal.
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    .
    (c) Prather, M. J.; Hsu, J.; DeLuca, N. M.; Jackman, C. H.; Oman, L. D.; Douglass, A. R.; Fleming, E. L.; Strahan, S. S.; Steenrod, S. D.; Søvde, O. A.; Isaksen, I. S. A.; Froidevaux, L.; Funke, B. Measuring and Modeling the Lifetime of Nitrous Oxide Including its Variability. J. Geophys. Res. 2015, 120, 5693– 5705,  DOI: 10.1002/2015JD023267
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    6c
    Measuring and modeling the lifetime of nitrous oxide including its variability
    Prather, Michael J.; Hsu, Juno; DeLuca, Nicole M.; Jackman, Charles H.; Oman, Luke D.; Douglass, Anne R.; Fleming, Eric L.; Strahan, Susan E.; Steenrod, Stephen D.; Sovde, O. Amund; Isaksen, Ivar S. A.; Froidevaux, Lucien; Funke, Bernd
    Journal of Geophysical Research: Atmospheres (2015), 120 (11), 5693-5705CODEN: JGRDE3; ISSN:2169-8996. (Wiley-Blackwell)
    The lifetime of nitrous oxide, the third-most-important human-emitted greenhouse gas, is based to date primarily on model studies or scaling to other gases. This work calcs. a semiempirical lifetime based on Microwave Limb Sounder satellite measurements of stratospheric profiles of nitrous oxide, ozone, and temp.; lab. cross-section data for ozone and mol. oxygen plus kinetics for O(1D); the obsd. solar spectrum; and a simple radiative transfer model. The result is 116 ± 9 years. The obsd. monthly-to-biennial variations in lifetime and tropical abundance are well matched by four independent chem.-transport models driven by reanal. meteorol. fields for the period of observation (2005-2010), but all these models overestimate the lifetime due to lower abundances in the crit. loss region near 32 km in the tropics. These models plus a chem.-climate model agree on the nitrous oxide feedback factor on its own lifetime of 0.94 ± 0.01, giving N2O perturbations an effective residence time of 109 years. Combining this new empirical lifetime with model ests. of residence time and preindustrial lifetime (123 years) adjusts our best ests. of the human-natural balance of emissions today and improves the accuracy of projected nitrous oxide increases over this century.
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  7. 7
    (a) Zeng, R.; Feller, M.; Ben-David, Y.; Milstein, D. Hydrogenation and Hydrosilylation of Nitrous Oxide Homogeneously Catalyzed by a Metal Complex. J. Am. Chem. Soc. 2017, 139, 5720– 5723,  DOI: 10.1021/jacs.7b02124
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    7a
    Hydrogenation and Hydrosilylation of Nitrous Oxide Homogeneously Catalyzed by a Metal Complex
    Zeng, Rong; Feller, Moran; Ben-David, Yehoshoa; Milstein, David
    Journal of the American Chemical Society (2017), 139 (16), 5720-5723CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Due to its significant contribution to stratospheric ozone depletion and its potent greenhouse effect, nitrous oxide has stimulated much research interest regarding its reactivity modes and its transformations, which can lead to its abatement. We report the homogeneously catalyzed reaction of nitrous oxide (N2O) with H2. The reaction is catalyzed by a PNP pincer ruthenium complex, generating efficiently only dinitrogen and water, under mild conditions, thus providing a green, mild methodol. for removal of nitrous oxide. The reaction proceeds through a sequence of dihydrogen activation, "O"-atom transfer, and dehydration, in which metal-ligand cooperation plays a central role. This approach was further developed to catalytic O-transfer from N2O to Si-H bonds.
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    .
    (b) Pal, T. K.; De, D.; Bharadwaj, P. K. Metal-organic Frameworks for the Chemical Fixation of CO2 into Cyclic Carbonates. Coord. Chem. Rev. 2020, 408, 213173  DOI: 10.1016/j.ccr.2019.213173
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    7b
    Metal-organic frameworks for the chemical fixation of CO2 into cyclic carbonates
    Pal, Tapan K.; De, Dinesh; Bharadwaj, Parimal K.
    Coordination Chemistry Reviews (2020), 408 (), 213173CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. Presently, we find a rapid pace of CO2 emission into the atm. causing major problems facing our planet. If no action is taken, it would have harmful consequences to humanity and the biosphere. More CO2 in the atm. will cause global warming. This will lead to climate upheavals disturbing the ecosystems, modification of the conditions and cycles of plant reprodn. and numerous assocd. problems. Therefore, present CO2 content in the atm. should be drastically reduced to a much lower level on an urgent basis. Alternatively, CO2 represents an abundant C1 feedstock and its chem. utilization has caught the imagination of chemists in recent years. Thus, fixation of CO2 with epoxide to form cyclic carbonate (hereafter, CC) via cycloaddn. reaction is significantly important and vigorously pursued in different labs. around the world. This is because removal of CO2 takes place from the atm. and simultaneously it can be converted into value-added products. Metal org. frameworks (MOFs) have attracted enormous attention in recent years as potential systems for gas storage, sepn., heterogeneous catalysis and so on, owing to their unique features such as designable architecture, controllable pore size, high surface area, permanent porosity, etc. In the present review, we discuss the recent progress made on catalytic conversion of CO2 to CCs by specially designed MOFs. It should be emphasized here that in the present review the literature survey is not exhaustive and we apologize for missing any important result in this review.
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    .
    (c) Zeng, R.; Feller, M.; Diskin-Posner, Y.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. CO Oxidation by N2O Homogeneously Catalyzed by Ruthenium Hydride Pincer Complexes Indicating a New Mechanism. J. Am. Chem. Soc. 2018, 140, 7061– 7064,  DOI: 10.1021/jacs.8b03927
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    7c
    CO Oxidation by N2O Homogeneously Catalyzed by Ruthenium Hydride Pincer Complexes Indicating a New Mechanism
    Zeng, Rong; Feller, Moran; Diskin-Posner, Yael; Shimon, Linda J. W.; Ben-David, Yehoshoa; Milstein, David
    Journal of the American Chemical Society (2018), 140 (23), 7061-7064CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Both CO and N2O are important, environmentally harmful industrial gases. The reaction of CO and N2O to produce CO2 and N2 has stimulated much research interest aimed at degrdn. of these two gases in a single step. Herein, we report an efficient CO oxidn. by N2O catalyzed by a (PNN)Ru-H pincer complex under mild conditions, even with no added base. The reaction is proposed to proceed through a sequence of O-atom transfer (OAT) from N2O to the Ru-H bond to form a Ru-OH intermediate, followed by intramol. OH attack on an adjacent CO ligand, forming CO2 and N2. Thus, the Ru-H bond of the catalyst plays a central role in facilitating the OAT from N2O to CO, providing an efficient and novel protocol for CO oxidn.
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  8. 8
    (a) Luque-Urrutia, J. A.; Poater, A. The Fundamental non Innocent Role of Water for the Hydrogenation of Nitrous Oxide by PNP Pincer Ru-based Catalysts. Inorg. Chem. 2017, 56, 14383– 14387,  DOI: 10.1021/acs.inorgchem.7b02630
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    8a
    The Fundamental Noninnocent Role of Water for the Hydrogenation of Nitrous Oxide by PNP Pincer Ru-based Catalysts
    Luque-Urrutia, Jesus A.; Poater, Albert
    Inorganic Chemistry (2017), 56 (23), 14383-14387CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The hydrogenation of nitrous oxide by PNP pincer ruthenium complexes supposes a promising way to functionalize a hazardous gas and reduce the greenhouse effect, generating dinitrogen and water. Here, by DFT calcns. we describe not only the whole mechanism for such a green transformation but we unravel the fundamental role of water, without which the reaction could not go forward. Water assists mandatorily in the H transfer to generate the hydroxyl group together with the release of dinitrogen.
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    .
    (b) Escayola, S.; Solà, M.; Poater, A. Mechanism of the Facile Nitrous Oxide Fixation by Homogeneous Ruthenium Hydride Pincer Catalysts. Inorg. Chem. 2020, 59, 9374– 9383,  DOI: 10.1021/acs.inorgchem.0c01252
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    8b
    Mechanism of the Facile Nitrous Oxide Fixation by Homogeneous Ruthenium Hydride Pincer Catalysts
    Escayola, Silvia; Sola, Miquel; Poater, Albert
    Inorganic Chemistry (2020), 59 (13), 9374-9383CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    Solving ozone depletion and climate change problems requires the development of effective methods for sustainably curbing them. With this aim, Milstein and co-workers developed a PNP pincer ruthenium catalyst for the homogeneous hydrogenation of nitrous oxide (N2O), an ozone-depleting substance and the third most important greenhouse gas, to generate dinitrogen and water as resultant products. The mechanism of this promising transformation was unveiled by means of expts. together with d. functional theory (DFT) calcns., which inspired Milstein and co-workers to use similar (PNN)Ru-H pincer catalysts for the redn. of N2O by CO to produce N2 and CO2. The use of the latter type of catalysts resulted in the proposition of a new reaction protocol and allowed to work under milder conditions. Here we describe the detailed mechanism of the last transformation catalyzed by a (PNN)Ru-H catalyst by means of DFT calcns., and not only this, but we also discover the way to block undesired parasitic reactions. Apart from that, we have explored a new evolution of this family of catalysts to go beyond previous exptl. outcomes. The mechanism consists of a cascade of easy steps, starting from an insertion of the N2O oxygen into the Ru-H bond generating a hydroxo intermediate and releasing N2 and ending with a β-hydride elimination to form CO2 and regenerate the catalyst. The whole process occurs in a facile way with the exception of two steps: the formation of the hydroxyl ligand and the final β-hydride elimination to form CO2. However, the energy barriers of these two steps are not the bottleneck in the catalysis but rather the ease of the pyridyl group bonded to Ru to isomerize by C-H activation. We propose to solve this drawback by tuning the PNN ligand to block the pyridyl free rotation. Better catalytic activity of a (PNN)Ru-based catalyst for the redn. of N2O by CO from DFT calcns.
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  9. 9
    (a) Maity, B.; Koley, D. Computational Investigation on the Role of Disilene Substituents Toward N2O Activation. J. Phys. Chem. A 2017, 121, 401– 417,  DOI: 10.1021/acs.jpca.6b11988
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    9a
    Computational Investigation on the Role of Disilene Substituents Toward N2O Activation
    Maity, Bholanath; Koley, Debasis
    Journal of Physical Chemistry A (2017), 121 (1), 401-417CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The effect of substituents in disilene mediated N2O activation was studied at the M06-2X/QZVP//ωB97xD/TZVP level of theory. The relationship between structural diversity and the corresponding reactivity of six disilenes (IA-Ft) in the presence of four different substituents (-NMe2, -Cl, -Me, -SiMe3) is addressed in this investigation. We primarily propose two plausible mechanistic routes: Pathway I featuring disilene → silylene decompn. followed by N2O coordination and Pathway II constituting the N2O attack without Si-Si bond cleavage. Depending on the fashion of N2O approach the latter route was further differentiated into Pathway IIa and Pathway IIb detailing the "end-on" and "side-on" attack to the disilene scaffold. Interestingly, the lone pair contg. substituents (-NMe2, -Cl,) facilitates disilene → silylene dissocn.; on the contrary it reduces the electrophilicity at Si center in silylene, a feature manifested with higher activation barrier during N2O attack. In the absence of any lone-pair influence from substituents (-Me, -SiMe3), the decompn. of disilenes is considerably endothermic. Therefore, Pathway I appears to be the less preferred route for both types of substituents. In Pathway IIa, the N2O moiety uniformly approaches via O-end to both the silicon centers in disilenes. However, the calcns. reveal that Pathway IIa, although not operational for all disilenes, is unlikely to be a viable route due to the predominantly higher transition barrier (ca. 36 kcal/mol). The most feasible route in this current study accompanying moderately low activation barriers (∼19-26 kcal/mol) is Pathway IIb, which involves successive addn. of two N2O units proceeding via terminal N, O toward the Si centers and is applicable for all disilenes. The reactivity of substituted disilenes can be estd. in terms of the first activation barrier of N2O attack. Surprisingly, in Pathway IIb, the initial activation barrier and hence the reactivity shows negligible correlation with Si-Si bond strength, indicating toward the versatility of the reaction route.
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    .
    (b) Wendel, D.; Szilvási, T.; Henschel, D.; Altmann, P. J.; Jandl, C.; Inoue, S.; Rieger, B. Precise Activation of Ammonia and Carbon Dioxide by an Iminodisilene. Angew. Chem., Int. Ed. 2018, 57, 14575– 14579,  DOI: 10.1002/anie.201804472
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    9b
    Precise activation of ammonia and carbon dioxide by an iminodisilene
    Wendel, Daniel; Szilvasi, Tibor; Henschel, Daniel; Altmann, Philipp J.; Jandl, Christian; Inoue, Shigeyoshi; Rieger, Bernhard
    Angewandte Chemie, International Edition (2018), 57 (44), 14575-14579CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The activation of NH3 and CO2 was achieved in reaction with 1,2-bis(supersilyl)-1,2-bis(imidazolimino)disilene; amination, carboxylation and insertion reactions produced silanamines and carboxylates. The reaction is still an ambitious target for multiply bonded sub-valent silicon compds. Now, the precise splitting of the N-H bond of ammonia by (Z)-imino(silyl)disilene 1 to give trans-1,2-adduct 2 a at low temps. (-78°) is presented. According to DFT calcns., the stereospecific hydroamination follows a similar mechanism as the recently reported anti-addn. of H2 to the Si:Si bond of 1. The aminosilane 2b could also be obtained as the formal silylene addn. product under thermodn. reaction control. By applying low temps., the activation of CO2 with 1 selectively afforded the cis-oxadisilacyclobutanone 7-c as [2+2] cycloadduct. By performing the reaction directly at ambient temps., a mixt. of three different-sized silacycles (4-6) was obsd. Their formation was investigated theor. and their structures were revealed with sep. expts. using 1 and the oxygenation agents N2O and O2.
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    .
    (c) Sodpiban, O.; Del Gobbo, S.; Barman, S.; Aomchad, V.; Kidkhunthod, P.; Ould-Chikh, S.; Poater, A.; D’Elia, V.; Basset, J.-M. Synthesis of Well-defined Yttrium-based Lewis Acids by Capture of a Reaction Intermediate and Catalytic Application for Cycloaddition of CO2 to Epoxides Under Atmospheric Pressure. Catal. Sci. Technol. 2019, 9, 6152– 6165,  DOI: 10.1039/C9CY01642B
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    9c
    Synthesis of well-defined yttrium-based Lewis acids by capturing a reaction intermediate and catalytic application for cycloaddition of CO2 to epoxides under atmospheric pressure
    Sodpiban, Ounjit; Del Gobbo, Silvano; Barman, Samir; Aomchad, Vatcharaporn; Kidkhunthod, Pinit; Ould-Chikh, Samy; Poater, Albert; D'Elia, Valerio; Basset, Jean-Marie
    Catalysis Science & Technology (2019), 9 (21), 6152-6165CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    Despite widespread use of yttrium halide complexes as powerful Lewis acids in catalysis, no strategies have yet been developed to prep. well-defined heterogeneous systems. Herein, we show that by applying the methodol. of surface organometallic chem. (SOMC), a readily available intermediate of the mechanism of the cycloaddn. of CO2 to epoxides catalyzed by YCl3/TBAB (TBAB: tetrabutylammonium bromide) can be grafted on silica resulting in a well-defined complex [(SiO-)YCl(-OCH(CH3)CH2Cl)]. The complex was thoroughly characterized by means of elemental anal., FT-IR, solid state (SS) NMR, XPS and XANES techniques. The thus-prepd. surface complex serves as heterogeneous Lewis acid for the cycloaddn. of CO2 to several epoxides under atm. pressure performing as a simple but efficient and recyclable material. Remarkably, the isolated complex prepd. on highly dehydroxylated silica performed as the most efficient compd. Addnl. catalytic studies show that the yttrium complexes prepd. in this study have the potential to be employed also as versatile Lewis acid catalyst for 5-hydroxymethyl furfural (HMF) reductive etherification. DFT calcns. were carried out to investigate the possible grafting pathways and the mechanistic pathways of CO2-epoxide cycloaddn. catalyzed by different surface model complexes.
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    (d) Coufourier, S.; Gaignard-Gaillard, Q.; Lohier, J.-F.; Poater, A.; Gaillard, S.; Renaud, J.-L. Hydrogenation of CO2, Hydrogenocarbonate, and Carbonate to Formate in Water using Phosphine Free Bifunctional Iron Complexes. ACS Catal. 2020, 10, 2108– 2116,  DOI: 10.1021/acscatal.9b04340
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    9d
    Hydrogenation of CO2, Hydrogenocarbonate, and Carbonate to Formate in Water using Phosphine Free Bifunctional Iron Complexes
    Coufourier, Sebastien; Gaignard Gaillard, Quentin; Lohier, Jean-Francois; Poater, Albert; Gaillard, Sylvain; Renaud, Jean-Luc
    ACS Catalysis (2020), 10 (3), 2108-2116CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The development of efficient and low-cost catalytic systems is important for the replacement of the robust noble metal complexes. A highly efficient, stable, phosphine-free, and easy-to-synthesize Fe catalyst system for the redn. of CO2, hydrogenocarbonate, and carbonate in pure H2O is reported. In the presence of the bifunctional cyclopentadienone Fe tricarbonyl Fe4a-d, the hydrogenation of carbonic derivs. proceeds in good yields with good catalyst productivity. Turnover nos. (TON) of up to 3343, 4234, and 40 for the hydrogenation of CO2, hydrogenocarbonate, and carbonate, resp., to formate in pure H2O were achieved. For the CO2 hydrogenation, a base was required, and triethanolamine emerged as the best one. DFT calcns. rationalized the mechanism as well as the better performance of triethanolamine as a base.
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    (e) Sarkar, D.; Weetman, C.; Dutta, S.; Schubert, E.; Jandl, C.; Koley, D.; Inoue, S. N-Heterocyclic Carbene-Stabilized Germa-acylium Ion: Reactivity and Utility in Catalytic CO2 Functionalizations. J. Am. Chem. Soc. 2020, 142, 15403– 15411,  DOI: 10.1021/jacs.0c06287
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    9e
    N-Heterocyclic Carbene-Stabilized Germa-acylium Ion: Reactivity and Utility in Catalytic CO2 Functionalizations
    Sarkar, Debotra; Weetman, Catherine; Dutta, Sayan; Schubert, Emeric; Jandl, Christian; Koley, Debasis; Inoue, Shigeyoshi
    Journal of the American Chemical Society (2020), 142 (36), 15403-15411CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The first acceptor-free heavier germanium analog of an acylium ion, [RGe(O)(NHC)2]X (R = MesTer = 2,6-(2,4,6-Me3C6H2)2C6H3; NHC = IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene; X = Cl, BArF4, where BArF4 = [3,5-(CF3)2C6H5]4B), was isolated by reacting [RGe(NHC)2]X with N2O. Conversion of the germa-acylium ion to the first solely donor-stabilized germanium ester [(NHC)RGe(O)(OSiPh3)] and corresponding heavier analogs ([RGe(S)(NHC)2]X and [RGe(Se)(NHC)2]X) demonstrated its classical acylium-like behavior. The polarized terminal GeO bond in the germa-acylium ion was utilized to activate CO2 and silane, with the former found to be an example of reversible activation of CO2, thus mimicking the behavior of transition metal oxides. Furthermore, its transition-metal-like nature is demonstrated as it was found to be an active catalyst in both CO2 hydrosilylation and reductive N-functionalization of amines using CO2 as the C1 source. Mechanistic studies were undertaken both exptl. and computationally, which revealed that the reaction proceeds via an N-heterocyclic carbene (NHC) siloxygermylene [(NHC)RGe(OSiHPh2)].
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  10. 10
    (a) Crabtree, R. H. Homogeneous Transition Metal Catalysis of Acceptorless Dehydrogenative Alcohol Oxidation: Applications in Hydrogen Storage and to Heterocycle Synthesis. Chem. Rev. 2017, 117, 9228– 9246,  DOI: 10.1021/acs.chemrev.6b00556
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    10a
    Homogeneous Transition Metal Catalysis of Acceptorless Dehydrogenative Alcohol Oxidation: Applications in Hydrogen Storage and to Heterocycle Synthesis
    Crabtree, Robert H.
    Chemical Reviews (Washington, DC, United States) (2017), 117 (13), 9228-9246CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    The different types of acceptorless alc. dehydrogenation (AAD) reactions are discussed, followed by the catalysts and mechanisms involved. Special emphasis is put on the common appearance in AAD of pincer ligands, of noninnocent ligands, and of outer sphere mechanisms. Early work emphasized precious metals, mainly Ru and Ir, but interest in nonprecious metal AAD catalysis is growing. Alc.-amine combinations are discussed to the extent that net oxidn. occurs by loss of H2. These reactions are of potential synthetic interest because they can lead to N heterocycles such as pyrroles and pyridines. AAD also has green chem. credentials in that an oxidn. occurs without the need for an oxidizing agent and hence without the waste formation that would result from its use.
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    .
    (b) Gunanathan, C.; Milstein, D. Applications of Acceptorless Dehydrogenation and Related Transformations in Chemical Synthesis. Science 2013, 341, 1229712  DOI: 10.1126/science.1229712
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    10b
    Applications of acceptorless dehydrogenation and related transformations in chemical synthesis
    Gunanathan Chidambaram; Milstein David
    Science (New York, N.Y.) (2013), 341 (6143), 1229712 ISSN:.
    Conventional oxidations of organic compounds formally transfer hydrogen atoms from the substrate to an acceptor molecule such as oxygen, a metal oxide, or a sacrificial olefin. In acceptorless dehydrogenation (AD) reactions, catalytic scission of C-H, N-H, and/or O-H bonds liberates hydrogen gas with no need for a stoichiometric oxidant, thereby providing efficient, nonpolluting activation of substrates. In addition, the hydrogen gas is valuable in itself as a high-energy, clean fuel. Here, we review AD reactions selectively catalyzed by transition metal complexes, as well as related transformations that rely on intermediates derived from reversible dehydrogenation. We delineate the methodologies evolving from this recent concept and highlight the effect of these reactions on chemical synthesis.
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  11. 11
    (a) Das, U. K.; Chakraborty, S.; Diskin-Posner, Y.; Milstein, D. Direct Conversion of Alcohols into Alkenes by Dehydrogenative Coupling with Hydrazine/Hydrazone Catalyzed by Manganese. Angew. Chem., Int. Ed. 2018, 57, 13444– 13448,  DOI: 10.1002/anie.201807881
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    11a
    Direct Conversion of Alcohols into Alkenes by Dehydrogenative Coupling with Hydrazine/Hydrazone Catalyzed by Manganese
    Das, Uttam Kumar; Chakraborty, Subrata; Diskin-Posner, Yael; Milstein, David
    Angewandte Chemie, International Edition (2018), 57 (41), 13444-13448CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    We have developed unprecedented methods for the direct transformation of primary alcs. to alkenes in the presence of hydrazine, and for the synthesis of mixed alkenes by the reaction of alcs. with hydrazones. The reactions are catalyzed by a manganese pincer complex and proceed in the absence of added base or hydrogen acceptors, liberating dihydrogen, dinitrogen, and water as the only byproducts. The proposed mechanism, based on prepn. of proposed intermediates and control expts., suggests that the transformation occurs through metal-ligand cooperative N-H activation of a hydrazone intermediate.
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    .
    (b) Azofra, L. M.; Poater, A. Diastereoselective diazenyl formation: the key for manganese-catalysed alcohol conversion into (E)-alkenes. Dalton Trans. 2019, 48, 14122– 14127,  DOI: 10.1039/C9DT03379C
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    11b
    Diastereoselective diazenyl formation: the key for manganese-catalysed alcohol conversion into (E)-alkenes
    Azofra, Luis Miguel; Poater, Albert
    Dalton Transactions (2019), 48 (37), 14122-14127CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    The proposed reaction mechanism for the unprecedented direct transformation of primary alcs. into alkenes catalyzed by Mn(I)-PNP complexes consists of two cycles. First, the acceptorless dehydrogenation of the alc. into aldehyde is produced via a concerted mechanism. Secondly, in an excess of hydrazine, hydrazone is formed and reacts with the aldehyde to produce olefins. This process, taking place in base-free conditions, is characterised by the diastereoselective formation of diazenyl intermediates. Based on DFT data, the generation of the (SN,S,S) diastereoisomer is favored over the rest, leading in its decompn. to the preferential formation of an (E)-alkene and liberating N2 and H2O as the only byproducts.
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  12. 12
    Mukherjee, A.; Nerush, A.; Leitus, G.; Shimon, L. J. W.; David, Y. B.; Jalapa, N. A. E.; Milstein, D. Manganese-Catalyzed Environmentally Benign Dehydrogenative Coupling of Alcohols and Amines to Form Aldimines and H2: A Catalytic and Mechanistic Study. J. Am. Chem. Soc. 2016, 138, 4298– 4301,  DOI: 10.1021/jacs.5b13519
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    12
    Manganese-Catalyzed Environmentally Benign Dehydrogenative Coupling of Alcohols and Amines to Form Aldimines and H2: A Catalytic and Mechanistic Study
    Mukherjee, Arup; Nerush, Alexander; Leitus, Gregory; Shimon, Linda J. W.; Ben David, Yehoshoa; Espinosa Jalapa, Noel Angel; Milstein, David
    Journal of the American Chemical Society (2016), 138 (13), 4298-4301CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The catalytic dehydrogenative coupling of alcs. and amines to form aldimines represents an environmentally benign methodol. in org. chem. This has been accomplished in recent years mainly with precious-metal-based catalysts. We present the dehydrogenative coupling of alcs. and amines to form imines and H2 that is catalyzed, for the first time, by a complex of the earth-abundant Mn. Detailed mechanistic study was carried out with the aid of NMR spectroscopy, intermediate isolation, and X-ray anal.
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  13. 13
    (a) Kumar, A.; Espinosa-Jalapa, N. A.; Leitus, G.; Diskin-Posner, Y.; Avram, L.; Milstein, D. Direct Synthesis of Amides by Dehydrogenative Coupling of Amines with either Alcohols or Esters: Manganese Pincer Complex as Catalyst. Angew. Chem., Int. Ed. 2017, 56, 14992– 14996,  DOI: 10.1002/anie.201709180
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    13a
    Direct Synthesis of Amides by Dehydrogenative Coupling of Amines with either Alcohols or Esters: Manganese Pincer Complex as Catalyst
    Kumar, Amit; Espinosa-Jalapa, Noel Angel; Leitus, Gregory; Diskin-Posner, Yael; Avram, Liat; Milstein, David
    Angewandte Chemie, International Edition (2017), 56 (47), 14992-14996CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The first example of base-metal-catalyzed synthesis of amides from the coupling of primary amines with either alcs. or esters is reported. The reactions are catalyzed by a new manganese pincer complex and generate hydrogen gas as the sole byproduct, thus making the overall process atom-economical and sustainable.
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    .
    (b) Masdemont, J.; Luque-Urrutia, J. A.; Gimferrer, M.; Milstein, D.; Poater, A. Mechanism of Coupling of Alcohols and Amines to Generate Aldimines and H2 by a Pincer Manganese Catalyst. ACS Catal. 2019, 9, 1662– 1669,  DOI: 10.1021/acscatal.8b04175
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    13b
    Mechanism of Coupling of Alcohols and Amines To Generate Aldimines and H2 by a Pincer Manganese Catalyst
    Masdemont, Judit; Luque-Urrutia, Jesus A.; Gimferrer, Marti; Milstein, David; Poater, Albert
    ACS Catalysis (2019), 9 (3), 1662-1669CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Acceptorless dehydrogenative coupling (ADC) of alcs. and amines using a manganese-based catalyst is able to produce aldimines, which have a wide reactivity, without underestimating the generation of a clean fuel, since as a result of this coupling, mol. hydrogen is also obtained. Therefore, the aldimine synthesis represents an interesting reaction from chem. and clean energy points of view. In this work, the computational study, via d. functional theory (DFT) calcns., of a manganese-based catalyst for the acceptorless dehydrogenative coupling of alcs. and amines, together with the elucidation of its catalytic cycle using benzene as a solvent, is carried out. Calcns. provided insight not only into the catalytic pathway but also into the noncatalyzed org. transformation to reach the desired aldimine, on the basis of the aldehyde generated in the catalytic cycle. The importance of the alternative isomers of the Mn-pincer complex, previously characterized exptl., and the assisting role of alc. or water are also described.
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  14. 14
    (a) Chakraborty, S.; Das, U. K.; Ben-David, Y.; Milstein, D. Manganese Catalyzed α-Olefination of Nitriles by Primary Alcohols. J. Am. Chem. Soc. 2017, 139, 11710– 11713,  DOI: 10.1021/jacs.7b06993
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    14a
    Manganese Catalyzed α-Olefination of Nitriles by Primary Alcohols
    Chakraborty, Subrata; Das, Uttam Kumar; Ben-David, Yehoshoa; Milstein, David
    Journal of the American Chemical Society (2017), 139 (34), 11710-11713CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    In the presence of a manganese (phosphinoethyl)(phosphinobenzyl)amine pincer complex, primary alcs. such as benzyl alc. underwent diastereoselective acceptorless dehydrogenative α-olefination reactions with acetonitriles such as phenylacetonitrile in toluene in a sealed tube at 135° to yield (Z)-α,β-unsatd. nitriles such as (Z)-PhCH:CPhCN in 20-91% yields. (Z)-PhCH:CPhCN was prepd. under open air in 66% yield and in 87% yield in a glovebox. A salt of the manganese pincer complex with the carbanion of 4-fluorophenylacetonitrile was obsd. by NMR.
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    .
    (b) Chakraborty, S.; Gellrich, U.; Diskin-Posner, Y.; Leitus, G.; Avram, L.; Milstein, D. Manganese-Catalyzed N-Formylation of Amines by Methanol Liberating H2: A Catalytic and Mechanistic Study. Angew. Chem., Int. Ed. 2017, 56, 4229– 4233,  DOI: 10.1002/anie.201700681
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    14b
    Manganese-Catalyzed N-Formylation of Amines by Methanol Liberating H2: A Catalytic and Mechanistic Study
    Chakraborty, Subrata; Gellrich, Urs; Diskin-Posner, Yael; Leitus, Gregory; Avram, Liat; Milstein, David
    Angewandte Chemie, International Edition (2017), 56 (15), 4229-4233CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    The first example of a base metal (manganese) catalyzed acceptorless dehydrogenative coupling of methanol and amines to form formamides is reported herein. The novel pincer complex (iPr-PNHP)Mn(H)(CO)2 catalyzes the reaction under mild conditions in the absence of any additives, bases, or hydrogen acceptors. Mechanistic insight based on the observation of an intermediate and DFT calcns. is also provided.
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  15. 15
    Luque-Urrutia, J. A.; Solà, M.; Milstein, D.; Poater, A. Mechanism of the Manganese-Pincer Catalyzed Acceptorless Dehydrogenative Coupling of Nitriles and Alcohols. J. Am. Chem. Soc. 2019, 141, 2398– 2403,  DOI: 10.1021/jacs.8b11308
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    15
    Mechanism of the Manganese-Pincer-Catalyzed Acceptorless Dehydrogenative Coupling of Nitriles and Alcohols
    Luque-Urrutia, Jesus A.; Sola, Miquel; Milstein, David; Poater, Albert
    Journal of the American Chemical Society (2019), 141 (6), 2398-2403CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    A recent study showed that a Mn-pincer could catalyze the acceptorless dehydrogenative coupling of nitriles and alco-hols to yield acrylonitriles. The reaction mechanism pro-posed in that work contained some intermediates that, in most of the cases, were not characterized. Moreover, one of the intermediates involved a charged sepn., which is unlikely in apolar solvents. To clarify the reaction mecha-nism of this crit. reaction, we decided to perform a DFT study. Our results prove the existence of a cooperative effect of the metal and the ligand in several steps of the catalytic cycle. We also find the presence of several equilib-ria between isomeric intermediates where water, or the same alc. reagent, take part in assisting the proton transfer. Furthermore, we have analyzed the charge sepd. structure proposed exptl. and have found a nearly pure covalent bond between the two expected charged moieties. Finally, the Knoevenagel condensation step that generates the acrylonitrile is found to be the rate-detg. step.
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  16. 16
    (a) Barta, K.; Ford, P. C. Catalytic Conversion of Nonfood Woody Biomass Solids to Organic Liquids. Acc. Chem. Res. 2014, 47, 1503– 1512,  DOI: 10.1021/ar4002894
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    16a
    Catalytic Conversion of Nonfood Woody Biomass Solids to Organic Liquids
    Barta, Katalin; Ford, Peter C.
    Accounts of Chemical Research (2014), 47 (5), 1503-1512CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. This Account outlines recent efforts in the labs. addressing a fundamental challenge of sustainability chem., the effective use of biomass for prodn. of chems. and fuels. Efficient methods for converting renewable biomass solids to chems. and liq. fuels would reduce society's dependence on nonrenewable petroleum resources while easing the atm. CO2 burden. The major nonfood component of biomass is lignocellulose, a matrix of the biopolymers cellulose, hemicellulose, and lignin. New approaches are needed to effect facile conversion of lignocellulose solids to liq. fuels and to other chem. precursors without the formation of intractable side products and with sufficient specificity to give economically sustainable product streams. The authors have devised a novel catalytic system whereby the renewable feedstocks cellulose, organosolv lignin, and even lignocellulose composites such as sawdust are transformed into org. liqs. The reaction medium is supercrit. MeOH (s.c.-MeOH), while the catalyst is a Cu-doped porous metal oxide (PMO) prepd. from inexpensive, Earth-abundant starting materials. This transformation occurs in a single stage reactor operating at 300-320° and 160-220 bar. The reducing equiv. for these transformations are derived by the reforming of MeOH (to H2 and CO), which thereby serves as a liq. syngas in the present case. H2O generated by deoxygenation processes is quickly removed by the water-gas shift reaction. The Cu-doped PMO serves multiple purposes, catalyzing substrate hydrogenolysis and hydrogenation as well as the MeOH reforming and shift reactions. This 1-pot UCSB process is quant., giving little or no biochar residual. Provided is an overview of these catalysis studies beginning with reactions of the model compd. dihydrobenzofuran that help define the key processes occurring. The initial step is phenyl-ether bond hydrogenolysis, and this is followed by arom. ring hydrogenation. The complete catalytic disassembly of the more complex organosolv lignin to monomeric units, largely propyl-cyclohexanol derivs. is then described. Operational indexes based on 1H NMR anal. are also presented that facilitate holistic evaluation of these product streams that within several hours consist largely of propyl-cyclohexanol derivs. Lastly, the authors describe the application of this methodol. with several types of wood (pine sawdust, etc.) and with cellulose fibers. The product distribution, albeit still complex, displays unprecedented selectivity toward the prodn. of aliph. alcs. and methylated derivs. thereof. These observations clearly indicate that the Cu-doped solid metal oxide catalyst combined with s.c.-MeOH is capable of breaking down the complex biomass derived substrates to markedly deoxygenated monomeric units with increased H content. Possible implementations of this promising system on a larger scale are discussed.
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    .
    (b) Vispute, T. P.; Zhang, H.; Sanna, A.; Xiao, R.; Huber, G. W. Renewable Chemical Commodity Feedstocks From Integrated Catalytic Processing of Pyrolysis Oils. Science 2010, 330, 1222– 1227,  DOI: 10.1126/science.1194218
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    16b
    Renewable chemical commodity feedstocks from integrated catalytic processing of pyrolysis oils
    Vispute, Tushar P.; Zhang, Huiyan; Sanna, Aimaro; Xiao, Rui; Huber, George W.
    Science (Washington, DC, United States) (2010), 330 (6008), 1222-1227CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Fast pyrolysis of lignocellulosic biomass produces a renewable liq. fuel called pyrolysis oil that is the cheapest liq. fuel produced from biomass today. Here we show that pyrolysis oils can be converted into industrial commodity chem. feedstocks using an integrated catalytic approach that combines hydroprocessing with zeolite catalysis. The hydroprocessing increases the intrinsic hydrogen content of the pyrolysis oil, producing polyols and alcs. The zeolite catalyst then converts these hydrogenated products into light olefins and arom. hydrocarbons in a yield as much as three times higher than that produced with the pure pyrolysis oil. The yield of arom. hydrocarbons and light olefins from the biomass conversion over zeolite is proportional to the intrinsic amt. of hydrogen added to the biomass feedstock during hydroprocessing. The total product yield can be adjusted depending on market values of the chem. feedstocks and the relative prices of the hydrogen and biomass.
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  17. 17
    Kumar, A.; Janes, T.; Espinosa-Jalapa, N. A.; Milstein, D. Selective Hydrogenation of Cyclic Imides to Diols and Amines and Its Application in the Development of a Liquid Organic Hydrogen Carrier. J. Am. Chem. Soc. 2018, 140, 7453– 7457,  DOI: 10.1021/jacs.8b04581
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    17
    Selective Hydrogenation of Cyclic Imides to Diols and Amines and Its Application in the Development of a Liquid Organic Hydrogen Carrier
    Kumar, Amit; Janes, Trevor; Espinosa-Jalapa, Noel Angel; Milstein, David
    Journal of the American Chemical Society (2018), 140 (24), 7453-7457CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Direct hydrogenation of a broad variety of cyclic imides to diols and amines using a ruthenium catalyst is reported here. We have applied this strategy toward the development of a new liq. org. hydrogen carrier system based on the hydrogenation of bis-cyclic imide that is formed by the dehydrogenative coupling of 1,4-butanediol and ethylenediamine using a new ruthenium catalyst. The rechargeable system has a max. gravimetric hydrogen storage capacity of 6.66 wt%.
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    Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Facile Conversion of Alcohols into Esters and Dihydrogen Catalyzed by New Ruthenium Complexes. J. Am. Chem. Soc. 2005, 127, 10840– 10841,  DOI: 10.1021/ja052862b
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    18
    Facile Conversion of Alcohols into Esters and Dihydrogen Catalyzed by New Ruthenium Complexes
    Zhang, Jing; Leitus, Gregory; Ben-David, Yehoshoa; Milstein, David
    Journal of the American Chemical Society (2005), 127 (31), 10840-10841CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    An efficient, environmentally benign method for the prepn. of esters from alcs. under mild, neutral conditions without the need for carboxylic acid derivs. and condensing agents was developed. Catalyst design, based on new Ru(II) hydrido carbonyl complexes incorporating electron-rich bis(phosphinomethyl)pyridine (PNP) and aminomethylphosphinomethylpyridine (PNN) ligands resulted in Ru(PNP/PNN)(CO)Cl(H) and Ru(PNN)(CO)H the latter of which (PNN = diethylaminomethylbis(tert-butyl)phosphinomethylpyridine) is an outstanding catalyst for the dehydrogenation of primary alcs. to esters and H2 under neutral conditions.
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    Gunanathan, C.; Ben-David, Y.; Milstein, D. Direct Synthesis of Amides from Alcohols and Amines with Liberation of H2. Science 2007, 317, 790– 792,  DOI: 10.1126/science.1145295
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    19
    Direct synthesis of amides from alcohols and amines with liberation of H2
    Gunanathan, Chidambaram; Ben-David, Yehoshoa; Milstein, David
    Science (Washington, DC, United States) (2007), 317 (5839), 790-792CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Given the widespread importance of amides in biochem. and chem. systems, an efficient synthesis that avoids wasteful use of stoichiometric coupling reagents or corrosive acidic and basic media is highly desirable. We report a reaction in which primary amines are directly acylated by equimolar amts. of alcs. to produce amides and mol. hydrogen (the only products) in high yields and high turnover nos. This reaction is catalyzed by a ruthenium complex based on a dearomatized PNN-type ligand [where PNN is 2-(di-tert-butylphosphinomethyl)-6-(diethylaminomethyl)pyridine], and no base or acid promoters are required. Use of primary diamines in the reaction leads to bis-amides, whereas with a mixed primary-secondary amine substrate, chemoselective acylation of the primary amine group takes place. The proposed mechanism involves dehydrogenation of hemiaminal intermediates formed by the reaction of an aldehyde intermediate with the amine.
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  20. 20
    (a) Gunanathan, C.; Milstein, D. Metal-Ligand Cooperation by Aromatization-Dearomatization: A New Paradigm in Bond Activation and “Green” Catalysis. Acc. Chem. Res. 2011, 44, 588– 602,  DOI: 10.1021/ar2000265
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    20a
    Metal-Ligand Cooperation by Aromatization-Dearomatization: A New Paradigm in Bond Activation and "Green" Catalysis
    Gunanathan, Chidambaram; Milstein, David
    Accounts of Chemical Research (2011), 44 (8), 588-602CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review. In view of global concerns regarding the environment and sustainable energy resources, there is a strong need for the discovery of new, green catalytic reactions. For this purpose, fresh approaches to catalytic design are desirable. In recent years, complexes based on "cooperating" ligands have exhibited remarkable catalytic activity. These ligands cooperate with the metal center by undergoing reversible structural changes in the processes of substrate activation and product formation. We have discovered a new mode of metal-ligand cooperation, involving aromatization-dearomatization of ligands. Pincer-type ligands based on pyridine or acridine exhibit such cooperation, leading to unusual bond activation processes and to novel, environmentally benign catalysis. Bond activation takes place with no formal change in the metal oxidn. state, and so far the activation of H-H, C-H (sp2 and sp3), O-H, and N-H bonds has been demonstrated. Using this approach, we have demonstrated a unique water splitting process, which involves consecutive thermal liberation of H2 and light-induced liberation of O2, using no sacrificial reagents, promoted by a pyridine-based pincer ruthenium complex. An acridine pincer complex displays unique "long-range" metal-ligand cooperation in the activation of H2 and in reaction with ammonia. In this Account, we begin by providing an overview of the metal-ligand cooperation based on aromatization-dearomatization processes. We then describe a range of novel catalytic reactions that we developed guided by these new modes of metal-ligand cooperation. These reactions include the following: (1) acceptorless dehydrogenation of secondary alcs. to ketones, (2) acceptorless dehydrogenative coupling of alcs. to esters, (3) acylation of secondary alcs. by esters with dihydrogen liberation, (4) direct coupling of alcs. and amines to form amides and polyamides with liberation of dihydrogen, (5) coupling of esters and amines to form amides with H2 liberation, (6) selective synthesis of imines from alcs. and amines, (6) facile catalytic hydrogenolysis of esters to alcs., (7) hydrogenolysis of amides to alcs. and amines, (8) hydrogenation of ketones to secondary alcs. under mild hydrogen pressures, (9) direct conversion of alcs. to acetals and dihydrogen, and (10) selective synthesis of primary amines directly from alcs. and ammonia. These reactions are efficient, proceed under neutral conditions, and produce no waste, the only byproduct being mol. hydrogen and/or water, providing a foundation for new, highly atom economical, green synthetic processes.
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    .
    (b) Zhang, J.; Balaraman, E.; Leitus, G.; Milstein, D. Electron-Rich PNP- and PNN-Type Ruthenium(II) Hydrido Borohydride Pincer Complexes. Synthesis, Structure, and Catalytic Dehydrogenation of Alcohols and Hydrogenation of Esters. Organometallics 2011, 30, 5716– 5724,  DOI: 10.1021/om200595m
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    20b
    Electron-rich PNP- and PNN-type ruthenium(II) hydrido borohydride pincer complexes. Synthesis, structure, and catalytic dehydrogenation of alcohols and hydrogenation of esters
    Zhang, Jing; Balaraman, Ekambaram; Leitus, Gregory; Milstein, David
    Organometallics (2011), 30 (21), 5716-5724CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    Electron-rich PNP- and PNN-type ruthenium(II) hydrido borohydride pincer complexes I (5, R = tBu, X = tBu2P; 6, R = tBu, X = NEt2), were prepd. from their corresponding N2-bridged dinuclear Ru(II) complexes [(tBu-PNP)RuCl2]2(μ-N2) (3) and [(tBu-PNN)RuCl2]2(μ-N2) (4), resp. The x-ray structure of 5 reveals a BH4- anion η2-coordinated to ruthenium through two bridging hydrides. A variable-temp. 1H NMR study of 6 exhibits interesting fluxional behavior of the BH4- ligand. Similarly, the Ru(II) hydrido borohydride complex [(iPr-PNP)RuH(PPh3)(HBH3)] (9), in which the BH4- moiety is coordinated in a η1 bonding mode, was obtained by reaction of [RuCl2(PPh3)(iPr-PNP)] (8, iPr-PNP = 2,6-bis(diisopropylphosphinomethyl)pyridine) with two equiv of NaBH4 at room temp. The hydrido borohydride pincer complexes 5, 6, and 9 catalyze the acceptorless dehydrogenative coupling of primary alcs. to esters and the dehydrogenation of secondary alcs. to the corresponding ketones, accompanied by evolution of hydrogen gas. The reactivity follows the order 6 > 9 > 5. With the hydrido borohydride complex 6 as catalyst, high yields (up to 98%) and high turnover nos. (TON ∼1000) were obtained in the dehydrogenation of primary alcs. under mild and neutral conditions. In addn., 6 effectively catalyzes the hydrogenation of nonactivated arom. and aliph. esters to the corresponding alcs. with TON ∼200 under a relatively mild pressure of dihydrogen and neutral and homogeneous conditions. Thus, an efficient homogeneous catalytic system for the dehydrogenation-hydrogenation reactions of alcs. is developed, which is relevant to the current interest in hydrogen storage.
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    (c) Gnanaprakasam, B.; Milstein, D. Synthesis of Amides from Esters and Amines with Liberation of H2 under Neutral Conditions. J. Am. Chem. Soc. 2011, 133, 1682– 1685,  DOI: 10.1021/ja109944n
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    20c
    Synthesis of amides from esters and amines with liberation of H2 under neutral conditions
    Gnanaprakasam, Boopathy; Milstein, David
    Journal of the American Chemical Society (2011), 133 (6), 1682-1685CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Efficient synthesis of amides directly from esters and amines is achieved under mild, neutral conditions with the liberation of mol. hydrogen. Both primary and secondary amines can be utilized. This unprecedented, general, environmentally benign reaction is homogeneously catalyzed under neutral conditions by a dearomatized ruthenium-pincer PNN complex and proceeds in toluene under an inert atm. with a high turnover no. (up to 1000). PNP analogs do not catalyze this transformation, underlining the crucial importance of the amine arm of the pincer ligand. A mechanism is proposed involving metal-ligand cooperation via aromatization-dearomatization of the pyridine moiety and hemilability of the amine arm.
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  21. 21
    Jin, H.; Xie, J.; Pan, C.; Zhu, Z.; Cheng, Y.; Zhu, C. Rhenium-Catalyzed Acceptorless Dehydrogenative Coupling via Dual Activation of Alcohols and Carbonyl Compounds. ACS Catal. 2013, 3, 2195– 2198,  DOI: 10.1021/cs400572q
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    21
    Rhenium-Catalyzed Acceptorless Dehydrogenative Coupling via Dual Activation of Alcohols and Carbonyl Compounds
    Jin, Hongming; Xie, Jin; Pan, Changduo; Zhu, Zhengbo; Cheng, Yixiang; Zhu, Chengjian
    ACS Catalysis (2013), 3 (10), 2195-2198CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    A rhenium hetaphydride complex was found to be a versatile, homogeneous catalyst for dehydrogenative functionalization of alc. The dehydrogenative C-C coupling of alcs. and carbonyl compds. was carried out in the absence of base and hydrogen acceptors to afford a series of α,β-unsatd. carbonyl compds. [e.g., N-phenyloxindole + PhCH2OH in presence of ReH7(PCy3)2 and LiCl afforded 3-benzylidene-N-phenyloxindole (E/Z 25:3, 75% yield)]. A possible dual activation pathway was proposed by mechanistic investigations.
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  22. 22
    (a) Mastalir, M.; Tomsu, G.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Co(II) PCP Pincer Complexes as Catalysts for the Alkylation of Aromatic Amines with Primary Alcohols. Org. Lett. 2016, 18, 3462– 3465,  DOI: 10.1021/acs.orglett.6b01647
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    22a
    Co(II) PCP Pincer complexes as catalysts for the alkylation of aromatic amines with primary alcohols
    Mastalir, Matthias; Tomsu, Gerald; Pittenauer, Ernst; Allmaier, Guenter; Kirchner, Karl
    Organic Letters (2016), 18 (14), 3462-3465CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
    Efficient alkylations of amines by alcs. catalyzed by well-defined Co(II) complexes are described that are stabilized by a PCP ligand (N,N'-bis(diisopropylphosphino)-N,N'-dimethyl-1,3-diaminobenzene) based on the 1,3-diaminobenzene scaffold. This reaction is an environmentally benign process implementing inexpensive, earth-abundant nonprecious metal catalysts and is based on the acceptorless alc. dehydrogenation concept. 16A range of primary alcs. and arom. amines were efficiently converted into mono-N-alkylated amines in good to excellent isolated yields.
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    (b) Mastalir, M.; Glatz, M.; Gorgas, N.; Stöger, B.; Pittenauer, E.; Allmaier, G.; Veiros, L. F.; Kirchner, K. Divergent Coupling of Alcohols and Amines Catalyzed by Isoelectronic Hydride MnI and FeII PNP Pincer Complexes. Chem. – Eur. J. 2016, 22, 12316– 12320,  DOI: 10.1002/chem.201603148
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    22b
    Divergent Coupling of Alcohols and Amines Catalyzed by Isoelectronic Hydride MnI and FeII PNP Pincer Complexes
    Mastalir, Matthias; Glatz, Mathias; Gorgas, Nikolaus; Stoeger, Berthold; Pittenauer, Ernst; Allmaier, Guenter; Veiros, Luis F.; Kirchner, Karl
    Chemistry - A European Journal (2016), 22 (35), 12316-12320CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Herein, we describe an efficient coupling of alcs. and amines catalyzed by well-defined isoelectronic hydride MnI and FeII complexes, which are stabilized by a PNP ligand based on the 2,6-diaminopyridine scaffold. This reaction is an environmentally benign process implementing inexpensive, earth-abundant non-precious metal catalysts, and is based on the acceptorless alc. dehydrogenation concept. A range of alcs. and amines including both arom. and aliph. substrates were efficiently converted in good to excellent isolated yields. In the case of Mn, imines were selectively obtained; however, with Fe, monoalkylated amines were exclusively formed. These reactions proceed under base-free conditions and required the addn. of mol. sieves.
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    (c) Lane, E. M.; Hazari, N.; Bernskoetter, W. H. Iron-Catalyzed Urea Synthesis: Dehydrogenative Coupling of Methanol and Amines. Chem. Sci. 2018, 9, 4003– 4008,  DOI: 10.1039/C8SC00775F
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    22c
    Iron-catalyzed urea synthesis: dehydrogenative coupling of methanol and amines
    Lane, Elizabeth M.; Hazari, Nilay; Bernskoetter, Wesley H.
    Chemical Science (2018), 9 (16), 4003-4008CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
    Herein, an efficient synthesis of sym. ureas RHNC(O)NHR [R = i-Pr, c-hexyl, Bn, etc.] was reported via dehydrogenative coupling of methanol with primary amines using a pincer supported iron catalyst with isolated yields of up to 80% (corresponding to a catalytic turnover of up to 160) and with H2 as the sole byproduct. Further, mechanistic insights enabled the development of an iron-catalyzed method for the synthesis of unsym. ureas R1HNC(O)NHR [R1 = i-Pr, Ph, 4-MeOC6H4, etc.] from formamides and amines. These studies indicated a stepwise pathway beginning with methanol dehydrogenation to give formaldehyde, which was trapped by amine to afford a formamide. The formamide was then dehydrogenated to produce a transient isocyanate, which reacted with another equiv. of amine to form a urea.
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    (d) Mukherjee, A.; Milstein, D. Homogeneous Catalysis by Cobalt and Manganese Pincer Complexes. ACS Catal. 2018, 8, 11435– 11469,  DOI: 10.1021/acscatal.8b02869
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    22d
    Homogeneous Catalysis by Cobalt and Manganese Pincer Complexes
    Mukherjee, Arup; Milstein, David
    ACS Catalysis (2018), 8 (12), 11435-11469CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    A review. Homogeneous catalysis of org. transformations by metal complexes has been mostly based on complexes of noble metals. In recent years, tremendous progress has been made in the field of base-metal catalysis, mostly with pincer-type complexes, such as iron, cobalt, nickel and manganese pincer systems. Particularly impressive is the explosive growth in the catalysis by Mn-based pincer complexes, the first such complexes being reported as recent as 2016. This review covers recent progress in the field of homogenously catalyzed reactions using pincer-type complexes of cobalt and manganese. Various reactions are described, including acceptorless dehydrogenation, hydrogenation, dehydrogenative coupling, hydrogen borrowing, hydrogen transfer, H-X addns., C-C coupling, alkene polymn. and N2 fixation are described, including scope and brief mechanistic comments.
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    (e) Alig, L.; Fritz, M.; Schneider, S. First-Row Transition Metal (De)Hydrogenation Catalysis Based on Functional Pincer Ligands. Chem. Rev. 2019, 119, 2681– 2751,  DOI: 10.1021/acs.chemrev.8b00555
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    22e
    First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands
    Alig, Lukas; Fritz, Maximilian; Schneider, Sven
    Chemical Reviews (Washington, DC, United States) (2019), 119 (4), 2681-2751CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodol. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.
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    (f) Jayarathne, U.; Zhang, Y.; Hazari, N.; Bernskoetter, W. H. Selective Iron-Catalyzed Deaminative Hydrogenation of Amides. Organometallics 2017, 36, 409– 416,  DOI: 10.1021/acs.organomet.6b00816
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    Selective Iron-Catalyzed Deaminative Hydrogenation of Amides
    Jayarathne, Upul; Zhang, Yuanyuan; Hazari, Nilay; Bernskoetter, Wesley H.
    Organometallics (2017), 36 (2), 409-416CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
    The five-coordinate Fe(II) hydride complex (iPrPNP)Fe(H)CO (iPrPNP = N[CH2CH2(PiPr2)]2) was found to selectively catalyze deaminative hydrogenation of amides to the corresponding amines and primary alcs. It is one of the most active amide hydrogenation catalysts reported to date, with turnover nos. (TONs) >1000 obsd. for multiple substrates and TONs >4000 obtained for activated formanilides. The amide C-N cleavage reactions occur with a preference for electron-withdrawing substituents and with greater activity for formamides compared with acetamides and benzamides. Stoichiometric reactions between (iPrPNP)Fe(H)CO and formanilide afforded the new Fe(II) complex (iPrPNHP)Fe(H)CO(N(Ph)HCO) resulting from N-H addn. across the Fe-N bond. Complexes of this type were identified as the resting state during catalytic hydrogenation reactions contg. secondary amides. Addn. of a Lewis acid cocatalyst provided further enhancement of the productivity of catalytic amide hydrogenations.
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    (a) Waiba, S.; Maji, B. Manganese Catalyzed Acceptorless Dehydrogenative Coupling Reactions. ChemCatChem 2020, 12, 1891– 1902,  DOI: 10.1002/cctc.201902180
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    Manganese Catalyzed Acceptorless Dehydrogenative Coupling Reactions
    Waiba, Satyadeep; Maji, Biplab
    ChemCatChem (2020), 12 (7), 1891-1902CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A review. In this mini-review, the recent progress that has been made in manganese catalyzed acceptorless dehydrogenative coupling reactions was highlighted.
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    (b) Chandra, P.; Ghosh, T.; Choudhary, N.; Mohammad, A.; Mobin, S. M. Recent Advancement in Oxidation or Acceptorless Dehydrogenation of Alcohols to Valorised Products Using Manganese Based Catalysts. Coord. Chem. Rev. 2020, 411, 213241  DOI: 10.1016/j.ccr.2020.213241
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    Recent advancement in oxidation or acceptorless dehydrogenation of alcohols to valorised products using manganese based catalysts
    Chandra, Prakash; Ghosh, Topi; Choudhary, Neha; Mohammad, Akbar; Mobin, Shaikh M.
    Coordination Chemistry Reviews (2020), 411 (), 213241CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
    A review. Heterogeneous manganese oxides (MnOx) or manganese oxide-based octahedral mol. sieves were used as catalyts for the alc. oxidn. under aerobic conditions. The advantages assocd. with these manganese-based heterogeneous catalysts are due its recyclability, additives free process and employing aerobic oxidants (such as air or mol. oxygen) instead of peroxide-based oxidant. Recently, acceptor-less dehydrogenation (AD) has become a vital alternative technique in the toolbox of the synthetic chemist for alc. valorisation without generating a huge amt. of waste. Several manganese-based pincer, salen and pyrazolyl - imidazolyl complexes were used for the AD reactions of alcs.
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    Manganese-Catalyzed Multicomponent Synthesis of Pyrroles through Acceptorless Dehydrogenation Hydrogen Autotransfer Catalysis: Experiment and Computation
    Borghs, Jannik C.; Azofra, Luis Miguel; Biberger, Tobias; Linnenberg, Oliver; Cavallo, Luigi; Rueping, Magnus; El-Sepelgy, Osama
    ChemSusChem (2019), 12 (13), 3083-3088CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A new base metal catalyzed sustainable multicomponent synthesis of pyrroles from readily available substrates is reported. The developed protocol utilizes an air- and moisture-stable catalyst system and enables the replacement of the mutagenic α-haloketones with readily abundant 1,2-diols. Moreover, the presented method was catalytic in base and the sole byproducts of this transformation were water and hydrogen gas. Exptl. and computational mechanistic studies indicated that the reaction takes place through a combined acceptorless dehydrogenation hydrogen autotransfer methodol.
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    Key factors in pincer ligand design
    Peris, Eduardo; Crabtree, Robert H.
    Chemical Society Reviews (2018), 47 (6), 1959-1968CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    Pincers, tridentate ligands that prefer a meridional geometry, are a rising class because of their distinctive combination of properties. They permit a good level of control on the nature of the coordination sphere by holding the donor groups in a predictable arrangement. Some groups, such as an aryl or a pyridine, that would normally be easily lost as monodentate ligands, become reliably coordinated, esp. if they form the central donor unit of the three. Many pincer complexes show exceptional thermal stability, a property that is particularly prized in homogeneous catalysis where they can permit high temp. operation. The connectors between the three donor groups are often rigid, enforcing a strict mer geometry but flexible linkers permit fac binding and even fluxionality between the two forms. Rigid pincers can make good ligands for asym. catalysis-if the wingtip groups cannot easily rotate they may instead maintain a geometry in which suitable substituents project into the active site area of the catalyst where they help enantio-differentiation of the relevant transition states. Examples have been selected to illustrate these and other properties of this promising ligand class.
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    (a) Bray, B. L. Large-Scale Manufacture of Peptide Therapeutics by Chemical Synthesis. Nat. Rev. Drug Discovery 2003, 2, 587– 593,  DOI: 10.1038/nrd1133
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    Innovation: Large-scale manufacture of peptide therapeutics by chemical synthesis
    Bray, Brian L.
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    A review. The large-scale com. prodn. of a 36-amino-acid peptide by chem. synthesis has been demonstrated in the development of enfuvirtide (T-20 or Fuzeon), a first-in-class membrane fusion inhibitor for the treatment of HIV. The rationale behind route selection and the scale-up of the process used to manuf. enfuvirtide are discussed.
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    Analysis of the reactions used for the preparation of drug candidate molecules
    Carey, John S.; Laffan, David; Thomson, Colin; Williams, Mike T.
    Organic & Biomolecular Chemistry (2006), 4 (12), 2337-2347CODEN: OBCRAK; ISSN:1477-0520. (Royal Society of Chemistry)
    A review with 24 refs. discusses the general types of reactions used in processes for the prepn. of drug candidates from the process chem. R&D departments of GlaxoSmithKline, AstraZeneca and Pfizer in order to evaluate potential gaps in chem. technologies for the prepn. of pharmaceuticals. The presence and incorporation of asymmetry in drug candidates, the substitution patterns of arom. and heteroarom. starting materials, and the use of protecting groups and of acylation, alkylation, arylation, oxidn., redn., carbon-carbon bond-forming, and substitution reactions in processes for the prepn. of drug candidates are discussed.
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    Dunetz, Joshua R.; Magano, Javier; Weisenburger, Gerald A.
    Organic Process Research & Development (2016), 20 (2), 140-177CODEN: OPRDFK; ISSN:1083-6160. (American Chemical Society)
    A review. This review showcases various coupling reagents which have been implemented specifically for large-scale amide synthesis via the condensation of an acid and amine, while highlighting the benefits and drawbacks of each reagent on an industrial scale.
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    Cupido, T.; Tulla-Puche, J.; Spengler, J.; Albericio, F. The Synthesis of Naturally Occurring Peptides and their Analogs. Curr. Opin. Drug Discovery Dev. 2007, 10, 768– 783
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    The synthesis of naturally occurring peptides and their analogs
    Cupido, Tommaso; Tulla-Puche, Judit; Spengler, Jan; Albericio, Fernando
    Current Opinion in Drug Discovery & Development (2007), 10 (6), 768-783CODEN: CODDFF; ISSN:1367-6733. (Thomson Scientific)
    A review. The no. of peptide-based pharmaceuticals has increased enormously over recent years; some of these peptides are derived from natural sources, and many are large and complex. This work provides an overview of the current state of peptide synthesis methodol., focusing mainly on the synthesis of natural peptides and their analogs.
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  27. 27
    Constable, D. J. C.; Dunn, P. J.; Hayler, J. D.; Humphrey, G. R.; Leazer, J. L., Jr.; Linderman, R. J.; Lorenz, K.; Manley, J.; Pearlman, B. A.; Wells, A.; Zaks, A.; Zhang, T. Y. Key Green Chemistry Research Areas─a Perspective from Pharmaceutical Manufacturers. Green Chem. 2007, 9, 411– 420,  DOI: 10.1039/B703488C
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    Key green chemistry research areas-a perspective from pharmaceutical manufacturers
    Constable, David J. C.; Dunn, Peter J.; Hayler, John D.; Humphrey, Guy R.; Leazer, Johnnie L., Jr.; Linderman, Russell J.; Lorenz, Kurt; Manley, Julie; Pearlman, Bruce A.; Wells, Andrew; Zaks, Aleksey; Zhang, Tony Y.
    Green Chemistry (2007), 9 (5), 411-420CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
    A review. In 2005, the ACS Green Chem. Institute (GCI) and the global pharmaceutical corporations developed the ACS GCI Pharmaceutical Roundtable to encourage the integration of green chem. and green engineering into the pharmaceutical industry. The Roundtable has developed a list of key research areas. The purpose of this perspective is to summarize how that list was agreed, provide an assessment of the current state of the art in those areas, and to highlight areas for future improvement.
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    (a) Basha, A.; Lipton, M.; Weinreb, S. M. A Mild, General Method for Conversion of Esters to Amides. Tetrahedron Lett. 1977, 18, 4171– 4174,  DOI: 10.1016/S0040-4039(01)83457-2
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    Wang, Wei Bo; Roskamp, Eric J.
    Journal of Organic Chemistry (1992), 57 (23), 6101-3CODEN: JOCEAH; ISSN:0022-3263.
    Mixed tin(II) amides are generated, in situ, via addn. of aliph. amines to Sn[N(SiMe3)2]2 (I). These mixed tin(II) amides transfer the aliph. amines to esters yielding amides. For example, tin(II) amides derived from primary amines react with Me phenylacetate to give the corresponding amides in 63-67% yield. Similarly, tin(II) amides generated with secondary amines react with this ester in 74-87% yield. The condensation fails, however, with tin(II) amides derived from sterically encumbered amines and arom. amines. Another series of expts. evaluated esters that had reactive functional groups, such as α- and β-hydroxy esters and β-keto esters. Thus, Me lactate could be smoothly converted to a piperidinyl amide. In the case of β-hydroxy esters as substrates, one of three different products can be selectively prepd. in high yield under judiciously chosen reaction conditions. Reaction of a mixed tin(II) amide with Me 3-hydroxybutanoate at -40° yielded β-hydroxy amide HOCHMeCH2CON(CH2)5 [N(CH2)5 = piperidino], whereas reaction at room temp. gave α,β-unsatd. amide MeCH:CHCON(CH2)5. Finally, reaction of the β-hydroxy ester with 2 equiv of I and 3 equiv of piperidine for an extended period of time at room temp. produced β-amino amide (CH2)5NCHMeCH2CON(CH2)5.
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    Tetrahedron Letters (1995), 36 (31), 5461-4CODEN: TELEAY; ISSN:0040-4039. (Elsevier)
    The reaction of an ester with N,O-dimethylhydroxylamine and suitable organomagnesium reagent or lithium amide base provides a general method for the prepn. of N-methoxy-N-methylamides. Application in the direct conversion of a highly hindered ester to a ketone, the azasteroid 5α-reductase inhibitor MK-0434, is described.
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    (a) Han, S.-Y.; Kim, Y.-A. Recent Development of Peptide Coupling Reagents in Organic Synthesis. Tetrahedron 2004, 60, 2447– 2467,  DOI: 10.1016/j.tet.2004.01.020
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    Recent development of peptide coupling reagents in organic synthesis
    Han, So-Yeop; Kim, Young-Ah
    Tetrahedron (2004), 60 (11), 2447-2467CODEN: TETRAB; ISSN:0040-4020. (Elsevier Science B.V.)
    A review. Various types of peptide coupling reagents (i.e., phosphonium, uronium, immonium, carbodiimide, imidazolium, organophosphorus, acid halogenating compds., chloroformate, pyridinium, etc.) are discussed for the synthesis of bioactive mols. contg. peptide linkages. Methods used to suppress racemization during the peptide coupling step are presented.
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    Amide bond formation: beyond the myth of coupling reagents
    Valeur, Eric; Bradley, Mark
    Chemical Society Reviews (2009), 38 (2), 606-631CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    This crit. review is focussed on the most recently developed coupling reagents with particular attention paid to the pros and cons of the plethora of "acronym" based reagents. Amide bond formation is a fundamentally important reaction in org. synthesis, and is typically mediated by one of a myriad of so-called coupling reagents. It aims to demystify the process allowing the chemist to make a sensible and educated choice when carrying out an amide coupling reaction (179 refs.).
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    (c) de Figueiredo, R. M.; Suppo, J.-S.; Campagne, J.-M. Nonclassical Routes for Amide Bond Formation. Chem. Rev. 2016, 116, 12029– 12122,  DOI: 10.1021/acs.chemrev.6b00237
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    Nonclassical Routes for Amide Bond Formation
    de Figueiredo Renata Marcia; Suppo Jean-Simon; Campagne Jean-Marc
    Chemical reviews (2016), 116 (19), 12029-12122 ISSN:.
    The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .
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    Espinosa-Jalapa, N. A.; Kumar, A.; Leitus, G.; Diskin-Posner, Y.; Milstein, D. Synthesis of Cyclic Imides by Acceptorless Dehydrogenative Coupling of Diols and Amines Catalyzed by a Manganese Pincer Complex. J. Am. Chem. Soc. 2017, 139, 11722– 11725,  DOI: 10.1021/jacs.7b08341
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    Synthesis of Cyclic Imides by Acceptorless Dehydrogenative Coupling of Diols and Amines Catalyzed by a Manganese Pincer Complex
    Espinosa-Jalapa, Noel Angel; Kumar, Amit; Leitus, Gregory; Diskin-Posner, Yael; Milstein, David
    Journal of the American Chemical Society (2017), 139 (34), 11722-11725CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The first example of base-metal-catalyzed dehydrogenative coupling of diols and amines to form cyclic imides, i.e. I, is reported. The reaction is catalyzed by a pincer complex of the earth abundant manganese and forms hydrogen gas as the sole byproduct, making the overall process atom economical and environmentally benign.
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    (a) Becke, A. Density-Functional Exchange-Energy Approximation with Correct Asymptotic Behaviour. Phys. Rev. A 1988, 38, 3098  DOI: 10.1103/PhysRevA.38.3098
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    Physical Review A: Atomic, Molecular, and Optical Physics (1988), 38 (6), 3098-100CODEN: PLRAAN; ISSN:0556-2791.
    Current gradient-cor. d.-functional approxns. for the exchange energies of at. and mol. systems fail to reproduce the correct 1/r asymptotic behavior of the exchange-energy d. A gradient-cor. exchange-energy functional is given with the proper asymptotic limit. This functional, contg. only one parameter, fits the exact Hartree-Fock exchange energies of a wide variety of at. systems with remarkable accuracy, surpassing the performance of previous functionals contg. two parameters or more.
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    (a) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297– 3305,  DOI: 10.1039/b508541a
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    Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy
    Weigend, Florian; Ahlrichs, Reinhart
    Physical Chemistry Chemical Physics (2005), 7 (18), 3297-3305CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    Gaussian basis sets of quadruple zeta valence quality for Rb-Rn are presented, as well as bases of split valence and triple zeta valence quality for H-Rn. The latter were obtained by (partly) modifying bases developed previously. A large set of more than 300 mols. representing (nearly) all elements-except lanthanides-in their common oxidn. states was used to assess the quality of the bases all across the periodic table. Quantities investigated were atomization energies, dipole moments and structure parameters for Hartree-Fock, d. functional theory and correlated methods, for which we had chosen Moller-Plesset perturbation theory as an example. Finally recommendations are given which type of basis set is used best for a certain level of theory and a desired quality of results.
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    Accurate Coulomb-fitting basis sets for H to Rn
    Weigend, Florian
    Physical Chemistry Chemical Physics (2006), 8 (9), 1057-1065CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
    A series of auxiliary basis sets to fit Coulomb potentials for the elements H to Rn (except lanthanides) is presented. For each element only one auxiliary basis set is needed to approx. Coulomb energies in conjunction with orbital basis sets of split valence, triple zeta valence and quadruple zeta valence quality with errors of typically below ca. 0.15 kJ mol-1 per atom; this was demonstrated in conjunction with the recently developed orbital basis sets of types def2-SV(P), def2-TZVP and def2-QZVPP for a large set of small mols. representing (nearly) each element in all of its common oxidn. states. These auxiliary bases are slightly more than three times larger than orbital bases of split valence quality. Compared to non-approximated treatments, computation times for the Coulomb part are reduced by a factor of ca. 8 for def2-SV(P) orbital bases, ca. 25 for def2-TZVP and ca. 100 for def2-QZVPP orbital bases.
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    Johnson, E. R.; Becke, A. D. A Post-Hartree-Fock Model of Intermolecular Interactions: Inclusion of Higher-Order Corrections. J. Chem. Phys. 2006, 124, 174104  DOI: 10.1063/1.2190220
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    A post-Hartree-Fock model of intermolecular interactions: Inclusion of higher-order corrections
    Johnson, Erin R.; Becke, Axel D.
    Journal of Chemical Physics (2006), 124 (17), 174104/1-174104/9CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    We have previously demonstrated that the dipole moment of the exchange hole can be used to derive intermol. C6 dispersion coeffs. [J. Chem. Phys. 122, 154104 (2005)]. This was subsequently the basis for a novel post-Hartree-Fock model of intermol. interactions [J. Chem. Phys. 123, 024101 (2005)]. In the present work, the model is extended to include higher-order dispersion coeffs. C8 and C10. The extended model performs very well for prediction of intermonomer sepns. and binding energies of 45 van der Waals complexes. In particular, it performs twice as well as basis-set extrapolated MP2 theory for dispersion-bound complexes, with minimal computational cost.
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    (a) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104  DOI: 10.1063/1.3382344
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    A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu
    Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge
    Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
    The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
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    Effect of the damping function in dispersion corrected density functional theory
    Grimme, Stefan; Ehrlich, Stephan; Goerigk, Lars
    Journal of Computational Chemistry (2011), 32 (7), 1456-1465CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
    It is shown by an extensive benchmark on mol. energy data that the math. form of the damping function in DFT-D methods has only a minor impact on the quality of the results. For 12 different functionals, a std. "zero-damping" formula and rational damping to finite values for small interat. distances according to Becke and Johnson (BJ-damping) has been tested. The same (DFT-D3) scheme for the computation of the dispersion coeffs. is used. The BJ-damping requires one fit parameter more for each functional (three instead of two) but has the advantage of avoiding repulsive interat. forces at shorter distances. With BJ-damping better results for nonbonded distances and more clear effects of intramol. dispersion in four representative mol. structures are found. For the noncovalently-bonded structures in the S22 set, both schemes lead to very similar intermol. distances. For noncovalent interaction energies BJ-damping performs slightly better but both variants can be recommended in general. The exception to this is Hartree-Fock that can be recommended only in the BJ-variant and which is then close to the accuracy of cor. GGAs for non-covalent interactions. According to the thermodn. benchmarks BJ-damping is more accurate esp. for medium-range electron correlation problems and only small and practically insignificant double-counting effects are obsd. It seems to provide a phys. correct short-range behavior of correlation/dispersion even with unmodified std. functionals. In any case, the differences between the two methods are much smaller than the overall dispersion effect and often also smaller than the influence of the underlying d. functional. © 2011 Wiley Periodicals, Inc.; J. Comput. Chem., 2011.
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    (a) Barone, V.; Cossi, M. Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model. J. Phys. Chem. A 1998, 102, 1995– 2001,  DOI: 10.1021/jp9716997
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    Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model
    Barone, Vincenzo; Cossi, Maurizio
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    A new implementation of the conductor-like screening solvation model (COSMO) in the GAUSSIAN94 package is presented. It allows Hartree-Fock (HF), d. functional (DF) and post-HF energy, and HF and DF gradient calcns.: the cavities are modeled on the mol. shape, using recently optimized parameters, and both electrostatic and nonelectrostatic contributions to energies and gradients are considered. The calcd. solvation energies for 19 neutral mols. in water are found in very good agreement with exptl. data; the solvent-induced geometry relaxation is studied for some closed and open shell mols., at HF and DF levels. The computational times are very satisfying: the self-consistent energy evaluation needs a time 15-30% longer than the corresponding procedure in vacuo, whereas the calcn. of energy gradients is only 25% longer than in vacuo for medium size mols.
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    Molecular Interactions in Solution: An Overview of Methods Based on Continuous Distributions of the Solvent
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    A review with 838 refs. on different approaches to studying noncovalent mol. interactions in soln. with regard to the basic continuum model, the classical electrostatic problem, and the quantum problem. Equil. thermodn. functions, dynamic effects in the continuum model and inhomogeneities and anisotropies in the solvent are also discussed.
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    Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215– 241,  DOI: 10.1007/s00214-007-0310-x
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    The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals
    Zhao, Yan; Truhlar, Donald G.
    Theoretical Chemistry Accounts (2008), 120 (1-3), 215-241CODEN: TCACFW; ISSN:1432-881X. (Springer GmbH)
    We present two new hybrid meta exchange-correlation functionals, called M06 and M06-2X. The M06 functional is parametrized including both transition metals and nonmetals, whereas the M06-2X functional is a high-nonlocality functional with double the amt. of nonlocal exchange (2X), and it is parametrized only for nonmetals. The functionals, along with the previously published M06-L local functional and the M06-HF full-Hartree-Fock functionals, constitute the M06 suite of complementary functionals. We assess these four functionals by comparing their performance to that of 12 other functionals and Hartree-Fock theory for 403 energetic data in 29 diverse databases, including ten databases for thermochem., four databases for kinetics, eight databases for noncovalent interactions, three databases for transition metal bonding, one database for metal atom excitation energies, and three databases for mol. excitation energies. We also illustrate the performance of these 17 methods for three databases contg. 40 bond lengths and for databases contg. 38 vibrational frequencies and 15 vibrational zero point energies. We recommend the M06-2X functional for applications involving main-group thermochem., kinetics, noncovalent interactions, and electronic excitation energies to valence and Rydberg states. We recommend the M06 functional for application in organometallic and inorganometallic chem. and for noncovalent interactions.
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    Kendall, Rick A.; Dunning, Thom H., Jr.; Harrison, Robert J.
    Journal of Chemical Physics (1992), 96 (9), 6796-806CODEN: JCPSA6; ISSN:0021-9606.
    The authors describe a reliable procedure for calcg. the electron affinity of an atom and present results for H, B, C, O, and F (H is included for completeness). This procedure involves the use of the recently proposed correlation-consistent basis sets augmented with functions to describe the more diffuse character of the at. anion coupled with a straightforward, uniform expansion of the ref. space for multireference singles and doubles configuration-interaction (MRSD-CI) calcns. A comparison is given with previous results and with corresponding full CI calcns. The most accurate EAs obtained from the MRSD-CI calcns. are (with exptl. values in parentheses): H 0.740 eV (0.754), B 0.258 (0.277), C 1.245 (1.263), O 1.384 (1.461), and F 3.337 (3.401). The EAs obtained from the MR-SDCI calcns. differ by less than 0.03 eV from those predicted by the full CI calcns.
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    (a) Leitgeb, A.; Abbas, M.; Fischer, R. C.; Poater, A.; Cavallo, L.; Slugovc, C. A latent Ruthenium Based Olefin Metathesis Catalyst with a Sterically Demanding NHC Ligand (Pre)Catalysts. Catal. Sci. Technol. 2012, 2, 1640– 1643,  DOI: 10.1039/c2cy20311a
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    A latent ruthenium based olefin metathesis catalyst with a sterically demanding NHC ligand
    Leitgeb, Anita; Abbas, Mudassar; Fischer, Roland C.; Poater, Albert; Cavallo, Luigi; Slugovc, Christian
    Catalysis Science & Technology (2012), 2 (8), 1640-1643CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
    An olefin metathesis catalyst featuring a SIPr NHC and an ester chelating carbene ligand is introduced. In contrast to its previously published SIMes analog, only the trans dichloro configurated isomer was obtained. The two counterparts are tested in various olefin metathesis reactions, revealing a striking superiority of the new complex in the cross metathesis of olefins with Me vinyl ketone allowing for full conversion with only 500 ppm catalyst loading.
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    Iron-Catalyzed Chemoselective Reduction of α,β-Unsaturated Ketones
    Lator, Alexis; Gaillard, Sylvain; Poater, Albert; Renaud, Jean-Luc
    Chemistry - A European Journal (2018), 24 (22), 5770-5774CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    An iron-catalyzed chemo- and diastereoselective redn. of α,β-unsatd. ketones into the corresponding satd. ketones in mild reaction conditions is reported herein. DFT calcns. and exptl. work underline that transfer hydride redn. is a more facile process than hydrogenation, unveiling the fundamental role of the base.
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    On the Mechanism of the Digold(I)-Hydroxide-Catalysed Hydrophenoxylation of Alkynes
    Gomez-Suarez, Adrian; Oonishi, Yoshihiro; Martin, Anthony R.; Vummaleti, Sai V. C.; Nelson, David J.; Cordes, David B.; Slawin, Alexandra M. Z.; Cavallo, Luigi; Nolan, Steven P.; Poater, Albert
    Chemistry - A European Journal (2016), 22 (3), 1125-1132CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Herein, we present a detailed investigation of the mechanistic aspects of the dual gold-catalyzed hydrophenoxylation of alkynes by both exptl. and computational methods. The dissocn. of [{Au(NHC)}2(μ-OH)][BF4] is essential to enter the catalytic cycle, and this step is favored by the presence of bulky, non-coordinating counter ions. Moreover, in silico studies confirmed that phenol does not only act as a reactant, but also as a co-catalyst, lowering the energy barriers of several transition states. A gem-diaurated species might form during the reaction, but this lies deep within a potential energy well, and is likely to be an "off-cycle" rather than an "in-cycle" intermediate.
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    (d) Poater, A.; Pump, E.; Vummaleti, S. V. C.; Cavallo, L. The Right Computational Recipe for Olefin Metathesis with Ru-based Catalysts: the Whole Mechanism of Ring-Closing Olefin Metathesis. J. Chem. Theory Comput. 2014, 10, 4442– 4448,  DOI: 10.1021/ct5003863
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    The Right Computational Recipe for Olefin Metathesis with Ru-Based Catalysts: The Whole Mechanism of Ring-Closing Olefin Metathesis
    Poater, Albert; Pump, Eva; Vummaleti, Sai Vikrama Chaitanya; Cavallo, Luigi
    Journal of Chemical Theory and Computation (2014), 10 (10), 4442-4448CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
    The initiation mechanism of ruthenium methylidene complexes was studied detailing mechanistic insights of all involved reaction steps within a classical olefin metathesis pathway. Computational studies reached a good agreement with the rarely available exptl. data and even enabled to complement them. As a result, a highly accurate computational and rather cheap recipe is presented; M06/TZVP//BP86/SVP (PCM, P = 1354 atm).
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    Besora, M.; Vidossich, P.; Lledós, A.; Ujaque, G.; Maseras, F. Calculation of Reaction Free Energies in Solution: A Comparison of Current Approaches. J. Phys. Chem. A 2018, 122, 1392– 1399,  DOI: 10.1021/acs.jpca.7b11580
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    Calculation of Reaction Free Energies in Solution: A Comparison of Current Approaches
    Besora, Maria; Vidossich, Pietro; Lledos, Agusti; Ujaque, Gregori; Maseras, Feliu
    Journal of Physical Chemistry A (2018), 122 (5), 1392-1399CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
    The result of the application of different approaches based on the ideal gas/rigid rotor/harmonic oscillator (IGRRHO) model, commonly used in popular software packages, for the calcn. of free energies in soln. is compared with that of ab initio mol. dynamics for a process involving ligand exchange in palladium complexes. The IGRRHO-based approaches considered differ in most cases in the extent to which the rotational and translational contributions are included in the correction. Our study supports the use the free energy values directly obtained from dispersion-cor. DFT functionals without any correction or with minor corrections at most.
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    (a) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. SM6: A Density Functional Theory Continuum Solvation Model for Calculating Aqueous Solvation Free Energies of Neutrals, Ions, and Solute–Water Clusters. J. Chem. Theory Comput. 2005, 1, 1133– 1152,  DOI: 10.1021/ct050164b
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    SM6: A Density Functional Theory Continuum Solvation Model for Calculating Aqueous Solvation Free Energies of Neutrals, Ions, and Solute-Water Clusters
    Kelly, Casey P.; Cramer, Christopher J.; Truhlar, Donald G.
    Journal of Chemical Theory and Computation (2005), 1 (6), 1133-1152CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
    A new charge model, called Charge Model 4 (CM4), and a new continuum solvent model, called Solvation Model 6 (SM6), are presented. Using a database of aq. solvation free energies for 273 neutrals, 112 ions, and 31 ion-water clusters, parameter sets for the mPW0 hybrid d. functional of Adamo and Barone (Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664-675) were optimized for use with the following four basis sets: MIDI!6D, 6-31G(d), 6-31+G(d), and 6-31+G(d,p). SM6 separates the observable aq. solvation free energy into two different components: one arising from long-range bulk electrostatic effects and a second from short-range interactions between the solute and solvent mols. in the first solvation shell. This partition of the observable solvation free energy allows SM6 to effectively model a wide range of solutes. For the 273 neutral solutes in the test set, SM6 achieves an av. error of ∼0.50 kcal/mol in the aq. solvation free energies. For solutes, esp. ions, that have highly concd. regions of charge d., adding an explicit water mol. to the calcn. significantly improves the performance of SM6 for predicting solvation free energies. The performance of SM6 was tested against several other continuum models, including SM5.43R and several different implementations of the Polarizable Continuum Model (PCM). For both neutral and ionic solutes, SM6 outperforms all of the models against which it was tested. Also, SM6 is the only model (except for one with an av. error 3.4 times larger) that improves when an explicit solvent mol. is added to solutes with concd. charge densities. Thus, in SM6, unlike the other continuum models tested here, adding one or more explicit solvent mol. to the calcn. is an effective strategy for improving the prediction of the aq. solvation free energies of solutes with strong local solute-solvent interactions. This is important, because local solute-solvent interactions are not specifically accounted for by bulk electrostatics, but modeling these interactions correctly is important for predicting the aq. solvation free energies of certain solutes. Finally, SM6 retains its accuracy when used in conjunction with the B3LYP and B3PW91 functionals, and in fact the solvation parameters obtained with a given basis set may be used with any good d. functional or fraction of Hartree-Fock exchange.
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    (b) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. Aqueous Solvation Free Energies of Ions and Ion–Water Clusters Based on an Accurate Value for the Absolute Aqueous Solvation Free Energy of the Proton. J. Phys. Chem. B 2006, 110, 16066– 16081,  DOI: 10.1021/jp063552y
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    Aqueous Solvation Free Energies of Ions and Ion-Water Clusters Based on an Accurate Value for the Absolute Aqueous Solvation Free Energy of the Proton
    Kelly, Casey P.; Cramer, Christopher J.; Truhlar, Donald G.
    Journal of Physical Chemistry B (2006), 110 (32), 16066-16081CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    Thermochem. cycles that involve pKa, gas-phase acidities, aq. solvation free energies of neutral species, and gas-phase clustering free energies have been used with the cluster pair approxn. to det. the abs. aq. solvation free energy of the proton. The best value obtained in this work is in good agreement with the value reported by Tissandier et al. (Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. J.; Earhart, A. D.; Coe, J. V., J. Phys. Chem. A 1998, 102, 7787), who applied the cluster pair approxn. to a less diverse and smaller data set of ions. We agree with previous workers who advocated the value of -265.9 kcal/mol for the abs. aq. solvation free energy of the proton. Considering the uncertainties assocd. with the exptl. gas-phase free energies of ions that are required to use the cluster pair approxn. as well as analyses of various subsets of data, we est. an uncertainty for the abs. aq. solvation free energy of the proton of no less than 2 kcal/mol. Using a value of -265.9 kcal/mol for the abs. aq. solvation free energy of the proton, we expand and update our previous compilation of abs. aq. solvation free energies; this new data set contains conventional and abs. aq. solvation free energies for 121 unclustered ions (not including the proton) and 147 conventional and abs. aq. solvation free energies for 51 clustered ions contg. from 1 to 6 water mols. When tested against the same set of ions that was recently used to develop the SM6 continuum solvation model, SM6 retains its previously detd. high accuracy; indeed, in most cases the mean unsigned error improves when it is tested against the more accurate ref. data.
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    (c) Bryantsev, V. S.; Diallo, M. S.; Goddard, W. A., III Calculation of Solvation Free Energies of Charged Solutes Using Mixed Cluster/Continuum Models. J. Phys. Chem. B 2008, 112, 9709– 9719,  DOI: 10.1021/jp802665d
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    Calculation of Solvation Free Energies of Charged Solutes Using Mixed Cluster/Continuum Models
    Bryantsev, Vyacheslav S.; Diallo, Mamadou S.; Goddard, William A., III
    Journal of Physical Chemistry B (2008), 112 (32), 9709-9719CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
    The authors derive a consistent approach for predicting the solvation free energies of charged solutes in the presence of implicit and explicit solvents. Some published methodologies make systematic errors in the computed free energies because of the incorrect accounting of the std. state corrections for water mols. or water clusters present in the thermodn. cycle. This problem can be avoided by using the same std. state for each species involved in the reaction under consideration. Two different thermodn. cycles are analyzed for calcg. the solvation free energies of ionic solutes: (1) the cluster cycle with an n water cluster as a reagent and (2) the monomer cycle with n distinct water mols. as reagents. The use of the cluster cycle gives solvation free energies that are in excellent agreement with the exptl. values obtained from studies of ion-water clusters. The mean abs. errors are 0.8 kcal/mol for H+ and 2.0 kcal/mol for Cu2+. Conversely, calcns. using the monomer cycle lead to mean abs. errors that are >10 kcal/mol for H+ and >30 kcal/mol for Cu2+. The presence of hydrogen-bonded clusters of similar size on the left- and right-hand sides of the reaction cycle results in the cancelation of the systematic errors in the calcd. free energies. Using the cluster cycle with 1 solvation shell leads to errors of 5 kcal/mol for H+ (6 waters) and 27 kcal/mol for Cu2+ (6 waters), whereas using 2 solvation shells leads to accuracies of 2 kcal/mol for Cu2+ (18 waters) and 1 kcal/mol for H+ (10 waters).
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    Kozuch, S.; Shaik, S. How to Conceptualize Catalytic Cycles? The Energetic Span Model. Acc. Chem. Res. 2011, 44, 101– 110,  DOI: 10.1021/ar1000956
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    How to Conceptualize Catalytic Cycles? The Energetic Span Model
    Kozuch, Sebastian; Shaik, Sason
    Accounts of Chemical Research (2011), 44 (2), 101-110CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
    A review; a computational study of a catalytic cycle generates state energies (the E-representation), whereas expts. lead to rate consts. (the k-representation). Based on transition state theory (TST), these are equiv. representations. Nevertheless, until recently, there has been no simple way to calc. the efficiency of a catalytic cycle, i.e., its turnover frequency (TOF), from a theor. obtained energy profile. In this Account, we introduce the energetic span model that enables one to evaluate TOFs in a straightforward manner and in affinity with the Curtin-Hammett principle. As shown herein, the model implies a change in our kinetic concepts. Analogous to Ohm's law, the catalytic chem. current (the TOF) can be defined by a chem. potential (independent of the mechanism) divided by a chem. resistance (dependent on the mechanism and the nature of the catalyst). This formulation is based on Eyring's TST and corresponds to a steady-state regime. In many catalytic cycles, only one transition state and one intermediate det. the TOF. We call them the TOF-detg. transition state (TDTS) and the TOF-detg. intermediate (TDI). These key states can be located, from among the many states available to a catalytic cycle, by assessing the degree of TOF control (XTOF); this last term resembles the structure-reactivity coeff. in classical phys. org. chem. The TDTS-TDI energy difference and the reaction driving force define the energetic span (δE) of the cycle. Whenever the TDTS appears after the TDI, δE is the energy difference between these two states; when the opposite is true, we must also add the driving force to this difference. Having δE, the TOF is expressed simply in the Arrhenius-Eyring fashion, wherein δE serves as the apparent activation energy of the cycle. An important lesson from this model is that neither one transition state nor one reaction step possess all the kinetic information that dets. the efficiency of a catalyst. Addnl., the TDI and TDTS are not necessarily the highest and lowest states, nor do they have to be adjoined as a single step. As such, we can conclude that a change in the conceptualization of catalytic cycles is in order: in catalysis, there are no rate-detg. steps, but rather rate-detg. states. We also include a study on the effect of reactant and product concns. In the energetic span approxn., only the reactants or products that are located between the TDI and TDTS accelerate or inhibit the reaction. In this manner, the energetic span model creates a direct link between exptl. quantities and theor. results. The versatility of the energetic span model is demonstrated with several catalytic cycles of organometallic reactions.
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    Li, H.; Hall, M. B. Computational Mechanistic Studies on Reactions of Transition Metal Complexes with Noninnocent Pincer Ligands: Aromatization–Dearomatization or Not. ACS Catal. 2015, 5, 1895– 1913,  DOI: 10.1021/cs501875z
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    Computational Mechanistic Studies on Reactions of Transition Metal Complexes with Noninnocent Pincer Ligands: Aromatization-Dearomatization or Not
    Li, Haixia; Hall, Michael B.
    ACS Catalysis (2015), 5 (3), 1895-1913CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    The development of green chem. has attracted chemists' attentions in recent years. Among them, Milstein and co-workers have discovered a new mode of metal-ligand cooperation in complexes in which an aromatization-dearomatization process of the pyridine- or acridine-based PNP and PNN "pincer" ligands appears to be a key element. These complexes were reported to lead to unusual X-H (X = H, C, O, N, and B) activation reactions and to environmentally benign catalysis involving dehydrogenative coupling reactions and hydrogenation reactions, representing an important development in green chem. This review provides a summary of theor. studies on the mechanisms of the reactions mediated by transition metal complexes with noninnocent pincer ligands synthesized by Milstein and co-workers. The aromatization-dearomatization process of the pyridine- or acridine-based PNP and PNN "pincer" ligands were found to play important roles in some reactions, while other reactions do not involve the aromatization-dearomatization process. For some reactions, several research groups proposed different mechanisms to explain the same reaction. Thus, to compare these mechanisms, we recalc. their rate-detg. steps by using the functionals that are calibrated to produce results close to those from coupled cluster calcns. Moreover, the understanding of the reaction mechanisms can help researchers to improve the current reactions and design new reactions.
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    Nguyen, D. H.; Trivelli, X.; Capet, F.; Paul, J.-F.; Dumeignil, F.; Gauvin, R. M. Manganese Pincer Complexes for the Base-Free, Acceptorless Dehydrogenative Coupling of Alcohols to Esters: Development, Scope, and Understanding. ACS Catal. 2017, 7, 2022– 2032,  DOI: 10.1021/acscatal.6b03554
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    Manganese Pincer Complexes for the Base-Free, Acceptorless Dehydrogenative Coupling of Alcohols to Esters: Development, Scope, and Understanding
    Nguyen, Duc Hanh; Trivelli, Xavier; Capet, Frederic; Paul, Jean-Francois; Dumeignil, Franck; Gauvin, Regis M.
    ACS Catalysis (2017), 7 (3), 2022-2032CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
    Aliph. PNP pincer-supported earth-abundant Mn(I) dicarbonyl complexes behave as effective catalysts for the acceptorless dehydrogenative coupling of a wide range of alcs. to esters under base-free conditions. The reaction proceeds under neat conditions, with modest catalyst loading and releasing only H2 as byproduct. Mechanistic aspects were addressed by synthesizing key species related to the catalytic cycle (characterized by x-ray structure detn., multinuclear (1H, 13C, 31P, 15N, 55Mn) NMR, IR spectroscopy, inter alia), by studying elementary steps connected to the postulated mechanism, and by resorting to DFT calcns. As in the case of related Ru and Fe PNP catalysts, the dehydrogenation results from cycling between the amido and amino-hydride forms of the PNP-Mn(CO)2 scaffold. For the dehydrogenation of alcs. into aldehydes, the authors' results suggest that the highest energy barrier corresponds to the H2 release from the amino-hydride form, although its value is close to that of the outer-sphere dehydrogenation of the alc. into aldehyde. This contrasts with the Ru and Fe catalytic systems, where dehydrogenation of the substrate into aldehyde is less energy-demanding compared to H2 release from the cooperative metal-ligand framework.
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  • Abstract

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    Figure 1

    Figure 1. Mn-based catalyst involved in the dehydrogenative amide synthesis from alcohols and amines.

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    Figure 2

    Figure 2. Initial proposed mechanism for the amide synthesis reaction catalyzed by 1.

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    Figure 3

    Figure 3. Full mechanism of catalyst I. The top cycle shows the aldehyde formation from methanol (black) and hemiaminal [2] (magenta). The bottom pathway shows the formation of hemiaminal [2] from aldehyde (relative Gibbs energies in kcal/mol).

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    Figure 4

    Figure 4. Transition states (a) B → C, (b) [1] → [2] (assisted by water), and (c) [1] → [2] (assisted by methanol); selected distances given in Å.

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    Figure 5

    Figure 5. Alternative mechanism of catalyst 1 incorporating the Mn–N bond cleavage (relative Gibbs energies in kcal/mol).

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  • References

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    This article references 44 other publications.

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      Pal, Tapan K.; De, Dinesh; Bharadwaj, Parimal K.
      Coordination Chemistry Reviews (2020), 408 (), 213173CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. Presently, we find a rapid pace of CO2 emission into the atm. causing major problems facing our planet. If no action is taken, it would have harmful consequences to humanity and the biosphere. More CO2 in the atm. will cause global warming. This will lead to climate upheavals disturbing the ecosystems, modification of the conditions and cycles of plant reprodn. and numerous assocd. problems. Therefore, present CO2 content in the atm. should be drastically reduced to a much lower level on an urgent basis. Alternatively, CO2 represents an abundant C1 feedstock and its chem. utilization has caught the imagination of chemists in recent years. Thus, fixation of CO2 with epoxide to form cyclic carbonate (hereafter, CC) via cycloaddn. reaction is significantly important and vigorously pursued in different labs. around the world. This is because removal of CO2 takes place from the atm. and simultaneously it can be converted into value-added products. Metal org. frameworks (MOFs) have attracted enormous attention in recent years as potential systems for gas storage, sepn., heterogeneous catalysis and so on, owing to their unique features such as designable architecture, controllable pore size, high surface area, permanent porosity, etc. In the present review, we discuss the recent progress made on catalytic conversion of CO2 to CCs by specially designed MOFs. It should be emphasized here that in the present review the literature survey is not exhaustive and we apologize for missing any important result in this review.
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      .
      (c) Zeng, R.; Feller, M.; Diskin-Posner, Y.; Shimon, L. J. W.; Ben-David, Y.; Milstein, D. CO Oxidation by N2O Homogeneously Catalyzed by Ruthenium Hydride Pincer Complexes Indicating a New Mechanism. J. Am. Chem. Soc. 2018, 140, 7061– 7064,  DOI: 10.1021/jacs.8b03927
      [ACS Full Text ACS Full Text], [CAS], Google Scholar
      7c
      CO Oxidation by N2O Homogeneously Catalyzed by Ruthenium Hydride Pincer Complexes Indicating a New Mechanism
      Zeng, Rong; Feller, Moran; Diskin-Posner, Yael; Shimon, Linda J. W.; Ben-David, Yehoshoa; Milstein, David
      Journal of the American Chemical Society (2018), 140 (23), 7061-7064CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Both CO and N2O are important, environmentally harmful industrial gases. The reaction of CO and N2O to produce CO2 and N2 has stimulated much research interest aimed at degrdn. of these two gases in a single step. Herein, we report an efficient CO oxidn. by N2O catalyzed by a (PNN)Ru-H pincer complex under mild conditions, even with no added base. The reaction is proposed to proceed through a sequence of O-atom transfer (OAT) from N2O to the Ru-H bond to form a Ru-OH intermediate, followed by intramol. OH attack on an adjacent CO ligand, forming CO2 and N2. Thus, the Ru-H bond of the catalyst plays a central role in facilitating the OAT from N2O to CO, providing an efficient and novel protocol for CO oxidn.
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    8. 8
      (a) Luque-Urrutia, J. A.; Poater, A. The Fundamental non Innocent Role of Water for the Hydrogenation of Nitrous Oxide by PNP Pincer Ru-based Catalysts. Inorg. Chem. 2017, 56, 14383– 14387,  DOI: 10.1021/acs.inorgchem.7b02630
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      8a
      The Fundamental Noninnocent Role of Water for the Hydrogenation of Nitrous Oxide by PNP Pincer Ru-based Catalysts
      Luque-Urrutia, Jesus A.; Poater, Albert
      Inorganic Chemistry (2017), 56 (23), 14383-14387CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The hydrogenation of nitrous oxide by PNP pincer ruthenium complexes supposes a promising way to functionalize a hazardous gas and reduce the greenhouse effect, generating dinitrogen and water. Here, by DFT calcns. we describe not only the whole mechanism for such a green transformation but we unravel the fundamental role of water, without which the reaction could not go forward. Water assists mandatorily in the H transfer to generate the hydroxyl group together with the release of dinitrogen.
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      .
      (b) Escayola, S.; Solà, M.; Poater, A. Mechanism of the Facile Nitrous Oxide Fixation by Homogeneous Ruthenium Hydride Pincer Catalysts. Inorg. Chem. 2020, 59, 9374– 9383,  DOI: 10.1021/acs.inorgchem.0c01252
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      8b
      Mechanism of the Facile Nitrous Oxide Fixation by Homogeneous Ruthenium Hydride Pincer Catalysts
      Escayola, Silvia; Sola, Miquel; Poater, Albert
      Inorganic Chemistry (2020), 59 (13), 9374-9383CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      Solving ozone depletion and climate change problems requires the development of effective methods for sustainably curbing them. With this aim, Milstein and co-workers developed a PNP pincer ruthenium catalyst for the homogeneous hydrogenation of nitrous oxide (N2O), an ozone-depleting substance and the third most important greenhouse gas, to generate dinitrogen and water as resultant products. The mechanism of this promising transformation was unveiled by means of expts. together with d. functional theory (DFT) calcns., which inspired Milstein and co-workers to use similar (PNN)Ru-H pincer catalysts for the redn. of N2O by CO to produce N2 and CO2. The use of the latter type of catalysts resulted in the proposition of a new reaction protocol and allowed to work under milder conditions. Here we describe the detailed mechanism of the last transformation catalyzed by a (PNN)Ru-H catalyst by means of DFT calcns., and not only this, but we also discover the way to block undesired parasitic reactions. Apart from that, we have explored a new evolution of this family of catalysts to go beyond previous exptl. outcomes. The mechanism consists of a cascade of easy steps, starting from an insertion of the N2O oxygen into the Ru-H bond generating a hydroxo intermediate and releasing N2 and ending with a β-hydride elimination to form CO2 and regenerate the catalyst. The whole process occurs in a facile way with the exception of two steps: the formation of the hydroxyl ligand and the final β-hydride elimination to form CO2. However, the energy barriers of these two steps are not the bottleneck in the catalysis but rather the ease of the pyridyl group bonded to Ru to isomerize by C-H activation. We propose to solve this drawback by tuning the PNN ligand to block the pyridyl free rotation. Better catalytic activity of a (PNN)Ru-based catalyst for the redn. of N2O by CO from DFT calcns.
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    9. 9
      (a) Maity, B.; Koley, D. Computational Investigation on the Role of Disilene Substituents Toward N2O Activation. J. Phys. Chem. A 2017, 121, 401– 417,  DOI: 10.1021/acs.jpca.6b11988
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      9a
      Computational Investigation on the Role of Disilene Substituents Toward N2O Activation
      Maity, Bholanath; Koley, Debasis
      Journal of Physical Chemistry A (2017), 121 (1), 401-417CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      The effect of substituents in disilene mediated N2O activation was studied at the M06-2X/QZVP//ωB97xD/TZVP level of theory. The relationship between structural diversity and the corresponding reactivity of six disilenes (IA-Ft) in the presence of four different substituents (-NMe2, -Cl, -Me, -SiMe3) is addressed in this investigation. We primarily propose two plausible mechanistic routes: Pathway I featuring disilene → silylene decompn. followed by N2O coordination and Pathway II constituting the N2O attack without Si-Si bond cleavage. Depending on the fashion of N2O approach the latter route was further differentiated into Pathway IIa and Pathway IIb detailing the "end-on" and "side-on" attack to the disilene scaffold. Interestingly, the lone pair contg. substituents (-NMe2, -Cl,) facilitates disilene → silylene dissocn.; on the contrary it reduces the electrophilicity at Si center in silylene, a feature manifested with higher activation barrier during N2O attack. In the absence of any lone-pair influence from substituents (-Me, -SiMe3), the decompn. of disilenes is considerably endothermic. Therefore, Pathway I appears to be the less preferred route for both types of substituents. In Pathway IIa, the N2O moiety uniformly approaches via O-end to both the silicon centers in disilenes. However, the calcns. reveal that Pathway IIa, although not operational for all disilenes, is unlikely to be a viable route due to the predominantly higher transition barrier (ca. 36 kcal/mol). The most feasible route in this current study accompanying moderately low activation barriers (∼19-26 kcal/mol) is Pathway IIb, which involves successive addn. of two N2O units proceeding via terminal N, O toward the Si centers and is applicable for all disilenes. The reactivity of substituted disilenes can be estd. in terms of the first activation barrier of N2O attack. Surprisingly, in Pathway IIb, the initial activation barrier and hence the reactivity shows negligible correlation with Si-Si bond strength, indicating toward the versatility of the reaction route.
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      .
      (b) Wendel, D.; Szilvási, T.; Henschel, D.; Altmann, P. J.; Jandl, C.; Inoue, S.; Rieger, B. Precise Activation of Ammonia and Carbon Dioxide by an Iminodisilene. Angew. Chem., Int. Ed. 2018, 57, 14575– 14579,  DOI: 10.1002/anie.201804472
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      9b
      Precise activation of ammonia and carbon dioxide by an iminodisilene
      Wendel, Daniel; Szilvasi, Tibor; Henschel, Daniel; Altmann, Philipp J.; Jandl, Christian; Inoue, Shigeyoshi; Rieger, Bernhard
      Angewandte Chemie, International Edition (2018), 57 (44), 14575-14579CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The activation of NH3 and CO2 was achieved in reaction with 1,2-bis(supersilyl)-1,2-bis(imidazolimino)disilene; amination, carboxylation and insertion reactions produced silanamines and carboxylates. The reaction is still an ambitious target for multiply bonded sub-valent silicon compds. Now, the precise splitting of the N-H bond of ammonia by (Z)-imino(silyl)disilene 1 to give trans-1,2-adduct 2 a at low temps. (-78°) is presented. According to DFT calcns., the stereospecific hydroamination follows a similar mechanism as the recently reported anti-addn. of H2 to the Si:Si bond of 1. The aminosilane 2b could also be obtained as the formal silylene addn. product under thermodn. reaction control. By applying low temps., the activation of CO2 with 1 selectively afforded the cis-oxadisilacyclobutanone 7-c as [2+2] cycloadduct. By performing the reaction directly at ambient temps., a mixt. of three different-sized silacycles (4-6) was obsd. Their formation was investigated theor. and their structures were revealed with sep. expts. using 1 and the oxygenation agents N2O and O2.
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      .
      (c) Sodpiban, O.; Del Gobbo, S.; Barman, S.; Aomchad, V.; Kidkhunthod, P.; Ould-Chikh, S.; Poater, A.; D’Elia, V.; Basset, J.-M. Synthesis of Well-defined Yttrium-based Lewis Acids by Capture of a Reaction Intermediate and Catalytic Application for Cycloaddition of CO2 to Epoxides Under Atmospheric Pressure. Catal. Sci. Technol. 2019, 9, 6152– 6165,  DOI: 10.1039/C9CY01642B
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      9c
      Synthesis of well-defined yttrium-based Lewis acids by capturing a reaction intermediate and catalytic application for cycloaddition of CO2 to epoxides under atmospheric pressure
      Sodpiban, Ounjit; Del Gobbo, Silvano; Barman, Samir; Aomchad, Vatcharaporn; Kidkhunthod, Pinit; Ould-Chikh, Samy; Poater, Albert; D'Elia, Valerio; Basset, Jean-Marie
      Catalysis Science & Technology (2019), 9 (21), 6152-6165CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      Despite widespread use of yttrium halide complexes as powerful Lewis acids in catalysis, no strategies have yet been developed to prep. well-defined heterogeneous systems. Herein, we show that by applying the methodol. of surface organometallic chem. (SOMC), a readily available intermediate of the mechanism of the cycloaddn. of CO2 to epoxides catalyzed by YCl3/TBAB (TBAB: tetrabutylammonium bromide) can be grafted on silica resulting in a well-defined complex [(SiO-)YCl(-OCH(CH3)CH2Cl)]. The complex was thoroughly characterized by means of elemental anal., FT-IR, solid state (SS) NMR, XPS and XANES techniques. The thus-prepd. surface complex serves as heterogeneous Lewis acid for the cycloaddn. of CO2 to several epoxides under atm. pressure performing as a simple but efficient and recyclable material. Remarkably, the isolated complex prepd. on highly dehydroxylated silica performed as the most efficient compd. Addnl. catalytic studies show that the yttrium complexes prepd. in this study have the potential to be employed also as versatile Lewis acid catalyst for 5-hydroxymethyl furfural (HMF) reductive etherification. DFT calcns. were carried out to investigate the possible grafting pathways and the mechanistic pathways of CO2-epoxide cycloaddn. catalyzed by different surface model complexes.
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      .
      (d) Coufourier, S.; Gaignard-Gaillard, Q.; Lohier, J.-F.; Poater, A.; Gaillard, S.; Renaud, J.-L. Hydrogenation of CO2, Hydrogenocarbonate, and Carbonate to Formate in Water using Phosphine Free Bifunctional Iron Complexes. ACS Catal. 2020, 10, 2108– 2116,  DOI: 10.1021/acscatal.9b04340
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      9d
      Hydrogenation of CO2, Hydrogenocarbonate, and Carbonate to Formate in Water using Phosphine Free Bifunctional Iron Complexes
      Coufourier, Sebastien; Gaignard Gaillard, Quentin; Lohier, Jean-Francois; Poater, Albert; Gaillard, Sylvain; Renaud, Jean-Luc
      ACS Catalysis (2020), 10 (3), 2108-2116CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The development of efficient and low-cost catalytic systems is important for the replacement of the robust noble metal complexes. A highly efficient, stable, phosphine-free, and easy-to-synthesize Fe catalyst system for the redn. of CO2, hydrogenocarbonate, and carbonate in pure H2O is reported. In the presence of the bifunctional cyclopentadienone Fe tricarbonyl Fe4a-d, the hydrogenation of carbonic derivs. proceeds in good yields with good catalyst productivity. Turnover nos. (TON) of up to 3343, 4234, and 40 for the hydrogenation of CO2, hydrogenocarbonate, and carbonate, resp., to formate in pure H2O were achieved. For the CO2 hydrogenation, a base was required, and triethanolamine emerged as the best one. DFT calcns. rationalized the mechanism as well as the better performance of triethanolamine as a base.
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      .
      (e) Sarkar, D.; Weetman, C.; Dutta, S.; Schubert, E.; Jandl, C.; Koley, D.; Inoue, S. N-Heterocyclic Carbene-Stabilized Germa-acylium Ion: Reactivity and Utility in Catalytic CO2 Functionalizations. J. Am. Chem. Soc. 2020, 142, 15403– 15411,  DOI: 10.1021/jacs.0c06287
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      9e
      N-Heterocyclic Carbene-Stabilized Germa-acylium Ion: Reactivity and Utility in Catalytic CO2 Functionalizations
      Sarkar, Debotra; Weetman, Catherine; Dutta, Sayan; Schubert, Emeric; Jandl, Christian; Koley, Debasis; Inoue, Shigeyoshi
      Journal of the American Chemical Society (2020), 142 (36), 15403-15411CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The first acceptor-free heavier germanium analog of an acylium ion, [RGe(O)(NHC)2]X (R = MesTer = 2,6-(2,4,6-Me3C6H2)2C6H3; NHC = IMe4 = 1,3,4,5-tetramethylimidazol-2-ylidene; X = Cl, BArF4, where BArF4 = [3,5-(CF3)2C6H5]4B), was isolated by reacting [RGe(NHC)2]X with N2O. Conversion of the germa-acylium ion to the first solely donor-stabilized germanium ester [(NHC)RGe(O)(OSiPh3)] and corresponding heavier analogs ([RGe(S)(NHC)2]X and [RGe(Se)(NHC)2]X) demonstrated its classical acylium-like behavior. The polarized terminal GeO bond in the germa-acylium ion was utilized to activate CO2 and silane, with the former found to be an example of reversible activation of CO2, thus mimicking the behavior of transition metal oxides. Furthermore, its transition-metal-like nature is demonstrated as it was found to be an active catalyst in both CO2 hydrosilylation and reductive N-functionalization of amines using CO2 as the C1 source. Mechanistic studies were undertaken both exptl. and computationally, which revealed that the reaction proceeds via an N-heterocyclic carbene (NHC) siloxygermylene [(NHC)RGe(OSiHPh2)].
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    10. 10
      (a) Crabtree, R. H. Homogeneous Transition Metal Catalysis of Acceptorless Dehydrogenative Alcohol Oxidation: Applications in Hydrogen Storage and to Heterocycle Synthesis. Chem. Rev. 2017, 117, 9228– 9246,  DOI: 10.1021/acs.chemrev.6b00556
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      10a
      Homogeneous Transition Metal Catalysis of Acceptorless Dehydrogenative Alcohol Oxidation: Applications in Hydrogen Storage and to Heterocycle Synthesis
      Crabtree, Robert H.
      Chemical Reviews (Washington, DC, United States) (2017), 117 (13), 9228-9246CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      The different types of acceptorless alc. dehydrogenation (AAD) reactions are discussed, followed by the catalysts and mechanisms involved. Special emphasis is put on the common appearance in AAD of pincer ligands, of noninnocent ligands, and of outer sphere mechanisms. Early work emphasized precious metals, mainly Ru and Ir, but interest in nonprecious metal AAD catalysis is growing. Alc.-amine combinations are discussed to the extent that net oxidn. occurs by loss of H2. These reactions are of potential synthetic interest because they can lead to N heterocycles such as pyrroles and pyridines. AAD also has green chem. credentials in that an oxidn. occurs without the need for an oxidizing agent and hence without the waste formation that would result from its use.
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      .
      (b) Gunanathan, C.; Milstein, D. Applications of Acceptorless Dehydrogenation and Related Transformations in Chemical Synthesis. Science 2013, 341, 1229712  DOI: 10.1126/science.1229712
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      10b
      Applications of acceptorless dehydrogenation and related transformations in chemical synthesis
      Gunanathan Chidambaram; Milstein David
      Science (New York, N.Y.) (2013), 341 (6143), 1229712 ISSN:.
      Conventional oxidations of organic compounds formally transfer hydrogen atoms from the substrate to an acceptor molecule such as oxygen, a metal oxide, or a sacrificial olefin. In acceptorless dehydrogenation (AD) reactions, catalytic scission of C-H, N-H, and/or O-H bonds liberates hydrogen gas with no need for a stoichiometric oxidant, thereby providing efficient, nonpolluting activation of substrates. In addition, the hydrogen gas is valuable in itself as a high-energy, clean fuel. Here, we review AD reactions selectively catalyzed by transition metal complexes, as well as related transformations that rely on intermediates derived from reversible dehydrogenation. We delineate the methodologies evolving from this recent concept and highlight the effect of these reactions on chemical synthesis.
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    11. 11
      (a) Das, U. K.; Chakraborty, S.; Diskin-Posner, Y.; Milstein, D. Direct Conversion of Alcohols into Alkenes by Dehydrogenative Coupling with Hydrazine/Hydrazone Catalyzed by Manganese. Angew. Chem., Int. Ed. 2018, 57, 13444– 13448,  DOI: 10.1002/anie.201807881
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      11a
      Direct Conversion of Alcohols into Alkenes by Dehydrogenative Coupling with Hydrazine/Hydrazone Catalyzed by Manganese
      Das, Uttam Kumar; Chakraborty, Subrata; Diskin-Posner, Yael; Milstein, David
      Angewandte Chemie, International Edition (2018), 57 (41), 13444-13448CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      We have developed unprecedented methods for the direct transformation of primary alcs. to alkenes in the presence of hydrazine, and for the synthesis of mixed alkenes by the reaction of alcs. with hydrazones. The reactions are catalyzed by a manganese pincer complex and proceed in the absence of added base or hydrogen acceptors, liberating dihydrogen, dinitrogen, and water as the only byproducts. The proposed mechanism, based on prepn. of proposed intermediates and control expts., suggests that the transformation occurs through metal-ligand cooperative N-H activation of a hydrazone intermediate.
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      .
      (b) Azofra, L. M.; Poater, A. Diastereoselective diazenyl formation: the key for manganese-catalysed alcohol conversion into (E)-alkenes. Dalton Trans. 2019, 48, 14122– 14127,  DOI: 10.1039/C9DT03379C
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      11b
      Diastereoselective diazenyl formation: the key for manganese-catalysed alcohol conversion into (E)-alkenes
      Azofra, Luis Miguel; Poater, Albert
      Dalton Transactions (2019), 48 (37), 14122-14127CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      The proposed reaction mechanism for the unprecedented direct transformation of primary alcs. into alkenes catalyzed by Mn(I)-PNP complexes consists of two cycles. First, the acceptorless dehydrogenation of the alc. into aldehyde is produced via a concerted mechanism. Secondly, in an excess of hydrazine, hydrazone is formed and reacts with the aldehyde to produce olefins. This process, taking place in base-free conditions, is characterised by the diastereoselective formation of diazenyl intermediates. Based on DFT data, the generation of the (SN,S,S) diastereoisomer is favored over the rest, leading in its decompn. to the preferential formation of an (E)-alkene and liberating N2 and H2O as the only byproducts.
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    12. 12
      Mukherjee, A.; Nerush, A.; Leitus, G.; Shimon, L. J. W.; David, Y. B.; Jalapa, N. A. E.; Milstein, D. Manganese-Catalyzed Environmentally Benign Dehydrogenative Coupling of Alcohols and Amines to Form Aldimines and H2: A Catalytic and Mechanistic Study. J. Am. Chem. Soc. 2016, 138, 4298– 4301,  DOI: 10.1021/jacs.5b13519
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      12
      Manganese-Catalyzed Environmentally Benign Dehydrogenative Coupling of Alcohols and Amines to Form Aldimines and H2: A Catalytic and Mechanistic Study
      Mukherjee, Arup; Nerush, Alexander; Leitus, Gregory; Shimon, Linda J. W.; Ben David, Yehoshoa; Espinosa Jalapa, Noel Angel; Milstein, David
      Journal of the American Chemical Society (2016), 138 (13), 4298-4301CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The catalytic dehydrogenative coupling of alcs. and amines to form aldimines represents an environmentally benign methodol. in org. chem. This has been accomplished in recent years mainly with precious-metal-based catalysts. We present the dehydrogenative coupling of alcs. and amines to form imines and H2 that is catalyzed, for the first time, by a complex of the earth-abundant Mn. Detailed mechanistic study was carried out with the aid of NMR spectroscopy, intermediate isolation, and X-ray anal.
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    13. 13
      (a) Kumar, A.; Espinosa-Jalapa, N. A.; Leitus, G.; Diskin-Posner, Y.; Avram, L.; Milstein, D. Direct Synthesis of Amides by Dehydrogenative Coupling of Amines with either Alcohols or Esters: Manganese Pincer Complex as Catalyst. Angew. Chem., Int. Ed. 2017, 56, 14992– 14996,  DOI: 10.1002/anie.201709180
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      13a
      Direct Synthesis of Amides by Dehydrogenative Coupling of Amines with either Alcohols or Esters: Manganese Pincer Complex as Catalyst
      Kumar, Amit; Espinosa-Jalapa, Noel Angel; Leitus, Gregory; Diskin-Posner, Yael; Avram, Liat; Milstein, David
      Angewandte Chemie, International Edition (2017), 56 (47), 14992-14996CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The first example of base-metal-catalyzed synthesis of amides from the coupling of primary amines with either alcs. or esters is reported. The reactions are catalyzed by a new manganese pincer complex and generate hydrogen gas as the sole byproduct, thus making the overall process atom-economical and sustainable.
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      .
      (b) Masdemont, J.; Luque-Urrutia, J. A.; Gimferrer, M.; Milstein, D.; Poater, A. Mechanism of Coupling of Alcohols and Amines to Generate Aldimines and H2 by a Pincer Manganese Catalyst. ACS Catal. 2019, 9, 1662– 1669,  DOI: 10.1021/acscatal.8b04175
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      13b
      Mechanism of Coupling of Alcohols and Amines To Generate Aldimines and H2 by a Pincer Manganese Catalyst
      Masdemont, Judit; Luque-Urrutia, Jesus A.; Gimferrer, Marti; Milstein, David; Poater, Albert
      ACS Catalysis (2019), 9 (3), 1662-1669CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Acceptorless dehydrogenative coupling (ADC) of alcs. and amines using a manganese-based catalyst is able to produce aldimines, which have a wide reactivity, without underestimating the generation of a clean fuel, since as a result of this coupling, mol. hydrogen is also obtained. Therefore, the aldimine synthesis represents an interesting reaction from chem. and clean energy points of view. In this work, the computational study, via d. functional theory (DFT) calcns., of a manganese-based catalyst for the acceptorless dehydrogenative coupling of alcs. and amines, together with the elucidation of its catalytic cycle using benzene as a solvent, is carried out. Calcns. provided insight not only into the catalytic pathway but also into the noncatalyzed org. transformation to reach the desired aldimine, on the basis of the aldehyde generated in the catalytic cycle. The importance of the alternative isomers of the Mn-pincer complex, previously characterized exptl., and the assisting role of alc. or water are also described.
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    14. 14
      (a) Chakraborty, S.; Das, U. K.; Ben-David, Y.; Milstein, D. Manganese Catalyzed α-Olefination of Nitriles by Primary Alcohols. J. Am. Chem. Soc. 2017, 139, 11710– 11713,  DOI: 10.1021/jacs.7b06993
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      14a
      Manganese Catalyzed α-Olefination of Nitriles by Primary Alcohols
      Chakraborty, Subrata; Das, Uttam Kumar; Ben-David, Yehoshoa; Milstein, David
      Journal of the American Chemical Society (2017), 139 (34), 11710-11713CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      In the presence of a manganese (phosphinoethyl)(phosphinobenzyl)amine pincer complex, primary alcs. such as benzyl alc. underwent diastereoselective acceptorless dehydrogenative α-olefination reactions with acetonitriles such as phenylacetonitrile in toluene in a sealed tube at 135° to yield (Z)-α,β-unsatd. nitriles such as (Z)-PhCH:CPhCN in 20-91% yields. (Z)-PhCH:CPhCN was prepd. under open air in 66% yield and in 87% yield in a glovebox. A salt of the manganese pincer complex with the carbanion of 4-fluorophenylacetonitrile was obsd. by NMR.
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      .
      (b) Chakraborty, S.; Gellrich, U.; Diskin-Posner, Y.; Leitus, G.; Avram, L.; Milstein, D. Manganese-Catalyzed N-Formylation of Amines by Methanol Liberating H2: A Catalytic and Mechanistic Study. Angew. Chem., Int. Ed. 2017, 56, 4229– 4233,  DOI: 10.1002/anie.201700681
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      14b
      Manganese-Catalyzed N-Formylation of Amines by Methanol Liberating H2: A Catalytic and Mechanistic Study
      Chakraborty, Subrata; Gellrich, Urs; Diskin-Posner, Yael; Leitus, Gregory; Avram, Liat; Milstein, David
      Angewandte Chemie, International Edition (2017), 56 (15), 4229-4233CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      The first example of a base metal (manganese) catalyzed acceptorless dehydrogenative coupling of methanol and amines to form formamides is reported herein. The novel pincer complex (iPr-PNHP)Mn(H)(CO)2 catalyzes the reaction under mild conditions in the absence of any additives, bases, or hydrogen acceptors. Mechanistic insight based on the observation of an intermediate and DFT calcns. is also provided.
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    15. 15
      Luque-Urrutia, J. A.; Solà, M.; Milstein, D.; Poater, A. Mechanism of the Manganese-Pincer Catalyzed Acceptorless Dehydrogenative Coupling of Nitriles and Alcohols. J. Am. Chem. Soc. 2019, 141, 2398– 2403,  DOI: 10.1021/jacs.8b11308
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      15
      Mechanism of the Manganese-Pincer-Catalyzed Acceptorless Dehydrogenative Coupling of Nitriles and Alcohols
      Luque-Urrutia, Jesus A.; Sola, Miquel; Milstein, David; Poater, Albert
      Journal of the American Chemical Society (2019), 141 (6), 2398-2403CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      A recent study showed that a Mn-pincer could catalyze the acceptorless dehydrogenative coupling of nitriles and alco-hols to yield acrylonitriles. The reaction mechanism pro-posed in that work contained some intermediates that, in most of the cases, were not characterized. Moreover, one of the intermediates involved a charged sepn., which is unlikely in apolar solvents. To clarify the reaction mecha-nism of this crit. reaction, we decided to perform a DFT study. Our results prove the existence of a cooperative effect of the metal and the ligand in several steps of the catalytic cycle. We also find the presence of several equilib-ria between isomeric intermediates where water, or the same alc. reagent, take part in assisting the proton transfer. Furthermore, we have analyzed the charge sepd. structure proposed exptl. and have found a nearly pure covalent bond between the two expected charged moieties. Finally, the Knoevenagel condensation step that generates the acrylonitrile is found to be the rate-detg. step.
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    16. 16
      (a) Barta, K.; Ford, P. C. Catalytic Conversion of Nonfood Woody Biomass Solids to Organic Liquids. Acc. Chem. Res. 2014, 47, 1503– 1512,  DOI: 10.1021/ar4002894
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      16a
      Catalytic Conversion of Nonfood Woody Biomass Solids to Organic Liquids
      Barta, Katalin; Ford, Peter C.
      Accounts of Chemical Research (2014), 47 (5), 1503-1512CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. This Account outlines recent efforts in the labs. addressing a fundamental challenge of sustainability chem., the effective use of biomass for prodn. of chems. and fuels. Efficient methods for converting renewable biomass solids to chems. and liq. fuels would reduce society's dependence on nonrenewable petroleum resources while easing the atm. CO2 burden. The major nonfood component of biomass is lignocellulose, a matrix of the biopolymers cellulose, hemicellulose, and lignin. New approaches are needed to effect facile conversion of lignocellulose solids to liq. fuels and to other chem. precursors without the formation of intractable side products and with sufficient specificity to give economically sustainable product streams. The authors have devised a novel catalytic system whereby the renewable feedstocks cellulose, organosolv lignin, and even lignocellulose composites such as sawdust are transformed into org. liqs. The reaction medium is supercrit. MeOH (s.c.-MeOH), while the catalyst is a Cu-doped porous metal oxide (PMO) prepd. from inexpensive, Earth-abundant starting materials. This transformation occurs in a single stage reactor operating at 300-320° and 160-220 bar. The reducing equiv. for these transformations are derived by the reforming of MeOH (to H2 and CO), which thereby serves as a liq. syngas in the present case. H2O generated by deoxygenation processes is quickly removed by the water-gas shift reaction. The Cu-doped PMO serves multiple purposes, catalyzing substrate hydrogenolysis and hydrogenation as well as the MeOH reforming and shift reactions. This 1-pot UCSB process is quant., giving little or no biochar residual. Provided is an overview of these catalysis studies beginning with reactions of the model compd. dihydrobenzofuran that help define the key processes occurring. The initial step is phenyl-ether bond hydrogenolysis, and this is followed by arom. ring hydrogenation. The complete catalytic disassembly of the more complex organosolv lignin to monomeric units, largely propyl-cyclohexanol derivs. is then described. Operational indexes based on 1H NMR anal. are also presented that facilitate holistic evaluation of these product streams that within several hours consist largely of propyl-cyclohexanol derivs. Lastly, the authors describe the application of this methodol. with several types of wood (pine sawdust, etc.) and with cellulose fibers. The product distribution, albeit still complex, displays unprecedented selectivity toward the prodn. of aliph. alcs. and methylated derivs. thereof. These observations clearly indicate that the Cu-doped solid metal oxide catalyst combined with s.c.-MeOH is capable of breaking down the complex biomass derived substrates to markedly deoxygenated monomeric units with increased H content. Possible implementations of this promising system on a larger scale are discussed.
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      .
      (b) Vispute, T. P.; Zhang, H.; Sanna, A.; Xiao, R.; Huber, G. W. Renewable Chemical Commodity Feedstocks From Integrated Catalytic Processing of Pyrolysis Oils. Science 2010, 330, 1222– 1227,  DOI: 10.1126/science.1194218
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      16b
      Renewable chemical commodity feedstocks from integrated catalytic processing of pyrolysis oils
      Vispute, Tushar P.; Zhang, Huiyan; Sanna, Aimaro; Xiao, Rui; Huber, George W.
      Science (Washington, DC, United States) (2010), 330 (6008), 1222-1227CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Fast pyrolysis of lignocellulosic biomass produces a renewable liq. fuel called pyrolysis oil that is the cheapest liq. fuel produced from biomass today. Here we show that pyrolysis oils can be converted into industrial commodity chem. feedstocks using an integrated catalytic approach that combines hydroprocessing with zeolite catalysis. The hydroprocessing increases the intrinsic hydrogen content of the pyrolysis oil, producing polyols and alcs. The zeolite catalyst then converts these hydrogenated products into light olefins and arom. hydrocarbons in a yield as much as three times higher than that produced with the pure pyrolysis oil. The yield of arom. hydrocarbons and light olefins from the biomass conversion over zeolite is proportional to the intrinsic amt. of hydrogen added to the biomass feedstock during hydroprocessing. The total product yield can be adjusted depending on market values of the chem. feedstocks and the relative prices of the hydrogen and biomass.
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    17. 17
      Kumar, A.; Janes, T.; Espinosa-Jalapa, N. A.; Milstein, D. Selective Hydrogenation of Cyclic Imides to Diols and Amines and Its Application in the Development of a Liquid Organic Hydrogen Carrier. J. Am. Chem. Soc. 2018, 140, 7453– 7457,  DOI: 10.1021/jacs.8b04581
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      17
      Selective Hydrogenation of Cyclic Imides to Diols and Amines and Its Application in the Development of a Liquid Organic Hydrogen Carrier
      Kumar, Amit; Janes, Trevor; Espinosa-Jalapa, Noel Angel; Milstein, David
      Journal of the American Chemical Society (2018), 140 (24), 7453-7457CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Direct hydrogenation of a broad variety of cyclic imides to diols and amines using a ruthenium catalyst is reported here. We have applied this strategy toward the development of a new liq. org. hydrogen carrier system based on the hydrogenation of bis-cyclic imide that is formed by the dehydrogenative coupling of 1,4-butanediol and ethylenediamine using a new ruthenium catalyst. The rechargeable system has a max. gravimetric hydrogen storage capacity of 6.66 wt%.
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    18. 18
      Zhang, J.; Leitus, G.; Ben-David, Y.; Milstein, D. Facile Conversion of Alcohols into Esters and Dihydrogen Catalyzed by New Ruthenium Complexes. J. Am. Chem. Soc. 2005, 127, 10840– 10841,  DOI: 10.1021/ja052862b
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      18
      Facile Conversion of Alcohols into Esters and Dihydrogen Catalyzed by New Ruthenium Complexes
      Zhang, Jing; Leitus, Gregory; Ben-David, Yehoshoa; Milstein, David
      Journal of the American Chemical Society (2005), 127 (31), 10840-10841CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      An efficient, environmentally benign method for the prepn. of esters from alcs. under mild, neutral conditions without the need for carboxylic acid derivs. and condensing agents was developed. Catalyst design, based on new Ru(II) hydrido carbonyl complexes incorporating electron-rich bis(phosphinomethyl)pyridine (PNP) and aminomethylphosphinomethylpyridine (PNN) ligands resulted in Ru(PNP/PNN)(CO)Cl(H) and Ru(PNN)(CO)H the latter of which (PNN = diethylaminomethylbis(tert-butyl)phosphinomethylpyridine) is an outstanding catalyst for the dehydrogenation of primary alcs. to esters and H2 under neutral conditions.
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    19. 19
      Gunanathan, C.; Ben-David, Y.; Milstein, D. Direct Synthesis of Amides from Alcohols and Amines with Liberation of H2. Science 2007, 317, 790– 792,  DOI: 10.1126/science.1145295
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      19
      Direct synthesis of amides from alcohols and amines with liberation of H2
      Gunanathan, Chidambaram; Ben-David, Yehoshoa; Milstein, David
      Science (Washington, DC, United States) (2007), 317 (5839), 790-792CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Given the widespread importance of amides in biochem. and chem. systems, an efficient synthesis that avoids wasteful use of stoichiometric coupling reagents or corrosive acidic and basic media is highly desirable. We report a reaction in which primary amines are directly acylated by equimolar amts. of alcs. to produce amides and mol. hydrogen (the only products) in high yields and high turnover nos. This reaction is catalyzed by a ruthenium complex based on a dearomatized PNN-type ligand [where PNN is 2-(di-tert-butylphosphinomethyl)-6-(diethylaminomethyl)pyridine], and no base or acid promoters are required. Use of primary diamines in the reaction leads to bis-amides, whereas with a mixed primary-secondary amine substrate, chemoselective acylation of the primary amine group takes place. The proposed mechanism involves dehydrogenation of hemiaminal intermediates formed by the reaction of an aldehyde intermediate with the amine.
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    20. 20
      (a) Gunanathan, C.; Milstein, D. Metal-Ligand Cooperation by Aromatization-Dearomatization: A New Paradigm in Bond Activation and “Green” Catalysis. Acc. Chem. Res. 2011, 44, 588– 602,  DOI: 10.1021/ar2000265
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      20a
      Metal-Ligand Cooperation by Aromatization-Dearomatization: A New Paradigm in Bond Activation and "Green" Catalysis
      Gunanathan, Chidambaram; Milstein, David
      Accounts of Chemical Research (2011), 44 (8), 588-602CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review. In view of global concerns regarding the environment and sustainable energy resources, there is a strong need for the discovery of new, green catalytic reactions. For this purpose, fresh approaches to catalytic design are desirable. In recent years, complexes based on "cooperating" ligands have exhibited remarkable catalytic activity. These ligands cooperate with the metal center by undergoing reversible structural changes in the processes of substrate activation and product formation. We have discovered a new mode of metal-ligand cooperation, involving aromatization-dearomatization of ligands. Pincer-type ligands based on pyridine or acridine exhibit such cooperation, leading to unusual bond activation processes and to novel, environmentally benign catalysis. Bond activation takes place with no formal change in the metal oxidn. state, and so far the activation of H-H, C-H (sp2 and sp3), O-H, and N-H bonds has been demonstrated. Using this approach, we have demonstrated a unique water splitting process, which involves consecutive thermal liberation of H2 and light-induced liberation of O2, using no sacrificial reagents, promoted by a pyridine-based pincer ruthenium complex. An acridine pincer complex displays unique "long-range" metal-ligand cooperation in the activation of H2 and in reaction with ammonia. In this Account, we begin by providing an overview of the metal-ligand cooperation based on aromatization-dearomatization processes. We then describe a range of novel catalytic reactions that we developed guided by these new modes of metal-ligand cooperation. These reactions include the following: (1) acceptorless dehydrogenation of secondary alcs. to ketones, (2) acceptorless dehydrogenative coupling of alcs. to esters, (3) acylation of secondary alcs. by esters with dihydrogen liberation, (4) direct coupling of alcs. and amines to form amides and polyamides with liberation of dihydrogen, (5) coupling of esters and amines to form amides with H2 liberation, (6) selective synthesis of imines from alcs. and amines, (6) facile catalytic hydrogenolysis of esters to alcs., (7) hydrogenolysis of amides to alcs. and amines, (8) hydrogenation of ketones to secondary alcs. under mild hydrogen pressures, (9) direct conversion of alcs. to acetals and dihydrogen, and (10) selective synthesis of primary amines directly from alcs. and ammonia. These reactions are efficient, proceed under neutral conditions, and produce no waste, the only byproduct being mol. hydrogen and/or water, providing a foundation for new, highly atom economical, green synthetic processes.
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      .
      (b) Zhang, J.; Balaraman, E.; Leitus, G.; Milstein, D. Electron-Rich PNP- and PNN-Type Ruthenium(II) Hydrido Borohydride Pincer Complexes. Synthesis, Structure, and Catalytic Dehydrogenation of Alcohols and Hydrogenation of Esters. Organometallics 2011, 30, 5716– 5724,  DOI: 10.1021/om200595m
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      20b
      Electron-rich PNP- and PNN-type ruthenium(II) hydrido borohydride pincer complexes. Synthesis, structure, and catalytic dehydrogenation of alcohols and hydrogenation of esters
      Zhang, Jing; Balaraman, Ekambaram; Leitus, Gregory; Milstein, David
      Organometallics (2011), 30 (21), 5716-5724CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      Electron-rich PNP- and PNN-type ruthenium(II) hydrido borohydride pincer complexes I (5, R = tBu, X = tBu2P; 6, R = tBu, X = NEt2), were prepd. from their corresponding N2-bridged dinuclear Ru(II) complexes [(tBu-PNP)RuCl2]2(μ-N2) (3) and [(tBu-PNN)RuCl2]2(μ-N2) (4), resp. The x-ray structure of 5 reveals a BH4- anion η2-coordinated to ruthenium through two bridging hydrides. A variable-temp. 1H NMR study of 6 exhibits interesting fluxional behavior of the BH4- ligand. Similarly, the Ru(II) hydrido borohydride complex [(iPr-PNP)RuH(PPh3)(HBH3)] (9), in which the BH4- moiety is coordinated in a η1 bonding mode, was obtained by reaction of [RuCl2(PPh3)(iPr-PNP)] (8, iPr-PNP = 2,6-bis(diisopropylphosphinomethyl)pyridine) with two equiv of NaBH4 at room temp. The hydrido borohydride pincer complexes 5, 6, and 9 catalyze the acceptorless dehydrogenative coupling of primary alcs. to esters and the dehydrogenation of secondary alcs. to the corresponding ketones, accompanied by evolution of hydrogen gas. The reactivity follows the order 6 > 9 > 5. With the hydrido borohydride complex 6 as catalyst, high yields (up to 98%) and high turnover nos. (TON ∼1000) were obtained in the dehydrogenation of primary alcs. under mild and neutral conditions. In addn., 6 effectively catalyzes the hydrogenation of nonactivated arom. and aliph. esters to the corresponding alcs. with TON ∼200 under a relatively mild pressure of dihydrogen and neutral and homogeneous conditions. Thus, an efficient homogeneous catalytic system for the dehydrogenation-hydrogenation reactions of alcs. is developed, which is relevant to the current interest in hydrogen storage.
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      .
      (c) Gnanaprakasam, B.; Milstein, D. Synthesis of Amides from Esters and Amines with Liberation of H2 under Neutral Conditions. J. Am. Chem. Soc. 2011, 133, 1682– 1685,  DOI: 10.1021/ja109944n
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      20c
      Synthesis of amides from esters and amines with liberation of H2 under neutral conditions
      Gnanaprakasam, Boopathy; Milstein, David
      Journal of the American Chemical Society (2011), 133 (6), 1682-1685CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Efficient synthesis of amides directly from esters and amines is achieved under mild, neutral conditions with the liberation of mol. hydrogen. Both primary and secondary amines can be utilized. This unprecedented, general, environmentally benign reaction is homogeneously catalyzed under neutral conditions by a dearomatized ruthenium-pincer PNN complex and proceeds in toluene under an inert atm. with a high turnover no. (up to 1000). PNP analogs do not catalyze this transformation, underlining the crucial importance of the amine arm of the pincer ligand. A mechanism is proposed involving metal-ligand cooperation via aromatization-dearomatization of the pyridine moiety and hemilability of the amine arm.
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    21. 21
      Jin, H.; Xie, J.; Pan, C.; Zhu, Z.; Cheng, Y.; Zhu, C. Rhenium-Catalyzed Acceptorless Dehydrogenative Coupling via Dual Activation of Alcohols and Carbonyl Compounds. ACS Catal. 2013, 3, 2195– 2198,  DOI: 10.1021/cs400572q
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      21
      Rhenium-Catalyzed Acceptorless Dehydrogenative Coupling via Dual Activation of Alcohols and Carbonyl Compounds
      Jin, Hongming; Xie, Jin; Pan, Changduo; Zhu, Zhengbo; Cheng, Yixiang; Zhu, Chengjian
      ACS Catalysis (2013), 3 (10), 2195-2198CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A rhenium hetaphydride complex was found to be a versatile, homogeneous catalyst for dehydrogenative functionalization of alc. The dehydrogenative C-C coupling of alcs. and carbonyl compds. was carried out in the absence of base and hydrogen acceptors to afford a series of α,β-unsatd. carbonyl compds. [e.g., N-phenyloxindole + PhCH2OH in presence of ReH7(PCy3)2 and LiCl afforded 3-benzylidene-N-phenyloxindole (E/Z 25:3, 75% yield)]. A possible dual activation pathway was proposed by mechanistic investigations.
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    22. 22
      (a) Mastalir, M.; Tomsu, G.; Pittenauer, E.; Allmaier, G.; Kirchner, K. Co(II) PCP Pincer Complexes as Catalysts for the Alkylation of Aromatic Amines with Primary Alcohols. Org. Lett. 2016, 18, 3462– 3465,  DOI: 10.1021/acs.orglett.6b01647
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      22a
      Co(II) PCP Pincer complexes as catalysts for the alkylation of aromatic amines with primary alcohols
      Mastalir, Matthias; Tomsu, Gerald; Pittenauer, Ernst; Allmaier, Guenter; Kirchner, Karl
      Organic Letters (2016), 18 (14), 3462-3465CODEN: ORLEF7; ISSN:1523-7052. (American Chemical Society)
      Efficient alkylations of amines by alcs. catalyzed by well-defined Co(II) complexes are described that are stabilized by a PCP ligand (N,N'-bis(diisopropylphosphino)-N,N'-dimethyl-1,3-diaminobenzene) based on the 1,3-diaminobenzene scaffold. This reaction is an environmentally benign process implementing inexpensive, earth-abundant nonprecious metal catalysts and is based on the acceptorless alc. dehydrogenation concept. 16A range of primary alcs. and arom. amines were efficiently converted into mono-N-alkylated amines in good to excellent isolated yields.
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      .
      (b) Mastalir, M.; Glatz, M.; Gorgas, N.; Stöger, B.; Pittenauer, E.; Allmaier, G.; Veiros, L. F.; Kirchner, K. Divergent Coupling of Alcohols and Amines Catalyzed by Isoelectronic Hydride MnI and FeII PNP Pincer Complexes. Chem. – Eur. J. 2016, 22, 12316– 12320,  DOI: 10.1002/chem.201603148
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      22b
      Divergent Coupling of Alcohols and Amines Catalyzed by Isoelectronic Hydride MnI and FeII PNP Pincer Complexes
      Mastalir, Matthias; Glatz, Mathias; Gorgas, Nikolaus; Stoeger, Berthold; Pittenauer, Ernst; Allmaier, Guenter; Veiros, Luis F.; Kirchner, Karl
      Chemistry - A European Journal (2016), 22 (35), 12316-12320CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Herein, we describe an efficient coupling of alcs. and amines catalyzed by well-defined isoelectronic hydride MnI and FeII complexes, which are stabilized by a PNP ligand based on the 2,6-diaminopyridine scaffold. This reaction is an environmentally benign process implementing inexpensive, earth-abundant non-precious metal catalysts, and is based on the acceptorless alc. dehydrogenation concept. A range of alcs. and amines including both arom. and aliph. substrates were efficiently converted in good to excellent isolated yields. In the case of Mn, imines were selectively obtained; however, with Fe, monoalkylated amines were exclusively formed. These reactions proceed under base-free conditions and required the addn. of mol. sieves.
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      (c) Lane, E. M.; Hazari, N.; Bernskoetter, W. H. Iron-Catalyzed Urea Synthesis: Dehydrogenative Coupling of Methanol and Amines. Chem. Sci. 2018, 9, 4003– 4008,  DOI: 10.1039/C8SC00775F
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      Iron-catalyzed urea synthesis: dehydrogenative coupling of methanol and amines
      Lane, Elizabeth M.; Hazari, Nilay; Bernskoetter, Wesley H.
      Chemical Science (2018), 9 (16), 4003-4008CODEN: CSHCCN; ISSN:2041-6520. (Royal Society of Chemistry)
      Herein, an efficient synthesis of sym. ureas RHNC(O)NHR [R = i-Pr, c-hexyl, Bn, etc.] was reported via dehydrogenative coupling of methanol with primary amines using a pincer supported iron catalyst with isolated yields of up to 80% (corresponding to a catalytic turnover of up to 160) and with H2 as the sole byproduct. Further, mechanistic insights enabled the development of an iron-catalyzed method for the synthesis of unsym. ureas R1HNC(O)NHR [R1 = i-Pr, Ph, 4-MeOC6H4, etc.] from formamides and amines. These studies indicated a stepwise pathway beginning with methanol dehydrogenation to give formaldehyde, which was trapped by amine to afford a formamide. The formamide was then dehydrogenated to produce a transient isocyanate, which reacted with another equiv. of amine to form a urea.
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      (d) Mukherjee, A.; Milstein, D. Homogeneous Catalysis by Cobalt and Manganese Pincer Complexes. ACS Catal. 2018, 8, 11435– 11469,  DOI: 10.1021/acscatal.8b02869
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      Homogeneous Catalysis by Cobalt and Manganese Pincer Complexes
      Mukherjee, Arup; Milstein, David
      ACS Catalysis (2018), 8 (12), 11435-11469CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      A review. Homogeneous catalysis of org. transformations by metal complexes has been mostly based on complexes of noble metals. In recent years, tremendous progress has been made in the field of base-metal catalysis, mostly with pincer-type complexes, such as iron, cobalt, nickel and manganese pincer systems. Particularly impressive is the explosive growth in the catalysis by Mn-based pincer complexes, the first such complexes being reported as recent as 2016. This review covers recent progress in the field of homogenously catalyzed reactions using pincer-type complexes of cobalt and manganese. Various reactions are described, including acceptorless dehydrogenation, hydrogenation, dehydrogenative coupling, hydrogen borrowing, hydrogen transfer, H-X addns., C-C coupling, alkene polymn. and N2 fixation are described, including scope and brief mechanistic comments.
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      (e) Alig, L.; Fritz, M.; Schneider, S. First-Row Transition Metal (De)Hydrogenation Catalysis Based on Functional Pincer Ligands. Chem. Rev. 2019, 119, 2681– 2751,  DOI: 10.1021/acs.chemrev.8b00555
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      First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands
      Alig, Lukas; Fritz, Maximilian; Schneider, Sven
      Chemical Reviews (Washington, DC, United States) (2019), 119 (4), 2681-2751CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodol. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.
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      (f) Jayarathne, U.; Zhang, Y.; Hazari, N.; Bernskoetter, W. H. Selective Iron-Catalyzed Deaminative Hydrogenation of Amides. Organometallics 2017, 36, 409– 416,  DOI: 10.1021/acs.organomet.6b00816
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      Selective Iron-Catalyzed Deaminative Hydrogenation of Amides
      Jayarathne, Upul; Zhang, Yuanyuan; Hazari, Nilay; Bernskoetter, Wesley H.
      Organometallics (2017), 36 (2), 409-416CODEN: ORGND7; ISSN:0276-7333. (American Chemical Society)
      The five-coordinate Fe(II) hydride complex (iPrPNP)Fe(H)CO (iPrPNP = N[CH2CH2(PiPr2)]2) was found to selectively catalyze deaminative hydrogenation of amides to the corresponding amines and primary alcs. It is one of the most active amide hydrogenation catalysts reported to date, with turnover nos. (TONs) >1000 obsd. for multiple substrates and TONs >4000 obtained for activated formanilides. The amide C-N cleavage reactions occur with a preference for electron-withdrawing substituents and with greater activity for formamides compared with acetamides and benzamides. Stoichiometric reactions between (iPrPNP)Fe(H)CO and formanilide afforded the new Fe(II) complex (iPrPNHP)Fe(H)CO(N(Ph)HCO) resulting from N-H addn. across the Fe-N bond. Complexes of this type were identified as the resting state during catalytic hydrogenation reactions contg. secondary amides. Addn. of a Lewis acid cocatalyst provided further enhancement of the productivity of catalytic amide hydrogenations.
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      (a) Waiba, S.; Maji, B. Manganese Catalyzed Acceptorless Dehydrogenative Coupling Reactions. ChemCatChem 2020, 12, 1891– 1902,  DOI: 10.1002/cctc.201902180
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      ChemCatChem (2020), 12 (7), 1891-1902CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A review. In this mini-review, the recent progress that has been made in manganese catalyzed acceptorless dehydrogenative coupling reactions was highlighted.
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      (b) Chandra, P.; Ghosh, T.; Choudhary, N.; Mohammad, A.; Mobin, S. M. Recent Advancement in Oxidation or Acceptorless Dehydrogenation of Alcohols to Valorised Products Using Manganese Based Catalysts. Coord. Chem. Rev. 2020, 411, 213241  DOI: 10.1016/j.ccr.2020.213241
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      Recent advancement in oxidation or acceptorless dehydrogenation of alcohols to valorised products using manganese based catalysts
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      Coordination Chemistry Reviews (2020), 411 (), 213241CODEN: CCHRAM; ISSN:0010-8545. (Elsevier B.V.)
      A review. Heterogeneous manganese oxides (MnOx) or manganese oxide-based octahedral mol. sieves were used as catalyts for the alc. oxidn. under aerobic conditions. The advantages assocd. with these manganese-based heterogeneous catalysts are due its recyclability, additives free process and employing aerobic oxidants (such as air or mol. oxygen) instead of peroxide-based oxidant. Recently, acceptor-less dehydrogenation (AD) has become a vital alternative technique in the toolbox of the synthetic chemist for alc. valorisation without generating a huge amt. of waste. Several manganese-based pincer, salen and pyrazolyl - imidazolyl complexes were used for the AD reactions of alcs.
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      Manganese-Catalyzed Multicomponent Synthesis of Pyrroles through Acceptorless Dehydrogenation Hydrogen Autotransfer Catalysis: Experiment and Computation
      Borghs, Jannik C.; Azofra, Luis Miguel; Biberger, Tobias; Linnenberg, Oliver; Cavallo, Luigi; Rueping, Magnus; El-Sepelgy, Osama
      ChemSusChem (2019), 12 (13), 3083-3088CODEN: CHEMIZ; ISSN:1864-5631. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A new base metal catalyzed sustainable multicomponent synthesis of pyrroles from readily available substrates is reported. The developed protocol utilizes an air- and moisture-stable catalyst system and enables the replacement of the mutagenic α-haloketones with readily abundant 1,2-diols. Moreover, the presented method was catalytic in base and the sole byproducts of this transformation were water and hydrogen gas. Exptl. and computational mechanistic studies indicated that the reaction takes place through a combined acceptorless dehydrogenation hydrogen autotransfer methodol.
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      Key factors in pincer ligand design
      Peris, Eduardo; Crabtree, Robert H.
      Chemical Society Reviews (2018), 47 (6), 1959-1968CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      Pincers, tridentate ligands that prefer a meridional geometry, are a rising class because of their distinctive combination of properties. They permit a good level of control on the nature of the coordination sphere by holding the donor groups in a predictable arrangement. Some groups, such as an aryl or a pyridine, that would normally be easily lost as monodentate ligands, become reliably coordinated, esp. if they form the central donor unit of the three. Many pincer complexes show exceptional thermal stability, a property that is particularly prized in homogeneous catalysis where they can permit high temp. operation. The connectors between the three donor groups are often rigid, enforcing a strict mer geometry but flexible linkers permit fac binding and even fluxionality between the two forms. Rigid pincers can make good ligands for asym. catalysis-if the wingtip groups cannot easily rotate they may instead maintain a geometry in which suitable substituents project into the active site area of the catalyst where they help enantio-differentiation of the relevant transition states. Examples have been selected to illustrate these and other properties of this promising ligand class.
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      (a) Bray, B. L. Large-Scale Manufacture of Peptide Therapeutics by Chemical Synthesis. Nat. Rev. Drug Discovery 2003, 2, 587– 593,  DOI: 10.1038/nrd1133
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      Bray, Brian L.
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      A review. The large-scale com. prodn. of a 36-amino-acid peptide by chem. synthesis has been demonstrated in the development of enfuvirtide (T-20 or Fuzeon), a first-in-class membrane fusion inhibitor for the treatment of HIV. The rationale behind route selection and the scale-up of the process used to manuf. enfuvirtide are discussed.
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      A review with 24 refs. discusses the general types of reactions used in processes for the prepn. of drug candidates from the process chem. R&D departments of GlaxoSmithKline, AstraZeneca and Pfizer in order to evaluate potential gaps in chem. technologies for the prepn. of pharmaceuticals. The presence and incorporation of asymmetry in drug candidates, the substitution patterns of arom. and heteroarom. starting materials, and the use of protecting groups and of acylation, alkylation, arylation, oxidn., redn., carbon-carbon bond-forming, and substitution reactions in processes for the prepn. of drug candidates are discussed.
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      Organic Process Research & Development (2016), 20 (2), 140-177CODEN: OPRDFK; ISSN:1083-6160. (American Chemical Society)
      A review. This review showcases various coupling reagents which have been implemented specifically for large-scale amide synthesis via the condensation of an acid and amine, while highlighting the benefits and drawbacks of each reagent on an industrial scale.
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      Cupido, T.; Tulla-Puche, J.; Spengler, J.; Albericio, F. The Synthesis of Naturally Occurring Peptides and their Analogs. Curr. Opin. Drug Discovery Dev. 2007, 10, 768– 783
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      A review. The no. of peptide-based pharmaceuticals has increased enormously over recent years; some of these peptides are derived from natural sources, and many are large and complex. This work provides an overview of the current state of peptide synthesis methodol., focusing mainly on the synthesis of natural peptides and their analogs.
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      Constable, D. J. C.; Dunn, P. J.; Hayler, J. D.; Humphrey, G. R.; Leazer, J. L., Jr.; Linderman, R. J.; Lorenz, K.; Manley, J.; Pearlman, B. A.; Wells, A.; Zaks, A.; Zhang, T. Y. Key Green Chemistry Research Areas─a Perspective from Pharmaceutical Manufacturers. Green Chem. 2007, 9, 411– 420,  DOI: 10.1039/B703488C
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      Green Chemistry (2007), 9 (5), 411-420CODEN: GRCHFJ; ISSN:1463-9262. (Royal Society of Chemistry)
      A review. In 2005, the ACS Green Chem. Institute (GCI) and the global pharmaceutical corporations developed the ACS GCI Pharmaceutical Roundtable to encourage the integration of green chem. and green engineering into the pharmaceutical industry. The Roundtable has developed a list of key research areas. The purpose of this perspective is to summarize how that list was agreed, provide an assessment of the current state of the art in those areas, and to highlight areas for future improvement.
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      Journal of Organic Chemistry (1992), 57 (23), 6101-3CODEN: JOCEAH; ISSN:0022-3263.
      Mixed tin(II) amides are generated, in situ, via addn. of aliph. amines to Sn[N(SiMe3)2]2 (I). These mixed tin(II) amides transfer the aliph. amines to esters yielding amides. For example, tin(II) amides derived from primary amines react with Me phenylacetate to give the corresponding amides in 63-67% yield. Similarly, tin(II) amides generated with secondary amines react with this ester in 74-87% yield. The condensation fails, however, with tin(II) amides derived from sterically encumbered amines and arom. amines. Another series of expts. evaluated esters that had reactive functional groups, such as α- and β-hydroxy esters and β-keto esters. Thus, Me lactate could be smoothly converted to a piperidinyl amide. In the case of β-hydroxy esters as substrates, one of three different products can be selectively prepd. in high yield under judiciously chosen reaction conditions. Reaction of a mixed tin(II) amide with Me 3-hydroxybutanoate at -40° yielded β-hydroxy amide HOCHMeCH2CON(CH2)5 [N(CH2)5 = piperidino], whereas reaction at room temp. gave α,β-unsatd. amide MeCH:CHCON(CH2)5. Finally, reaction of the β-hydroxy ester with 2 equiv of I and 3 equiv of piperidine for an extended period of time at room temp. produced β-amino amide (CH2)5NCHMeCH2CON(CH2)5.
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      Tetrahedron Letters (1995), 36 (31), 5461-4CODEN: TELEAY; ISSN:0040-4039. (Elsevier)
      The reaction of an ester with N,O-dimethylhydroxylamine and suitable organomagnesium reagent or lithium amide base provides a general method for the prepn. of N-methoxy-N-methylamides. Application in the direct conversion of a highly hindered ester to a ketone, the azasteroid 5α-reductase inhibitor MK-0434, is described.
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      (a) Han, S.-Y.; Kim, Y.-A. Recent Development of Peptide Coupling Reagents in Organic Synthesis. Tetrahedron 2004, 60, 2447– 2467,  DOI: 10.1016/j.tet.2004.01.020
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      A review. Various types of peptide coupling reagents (i.e., phosphonium, uronium, immonium, carbodiimide, imidazolium, organophosphorus, acid halogenating compds., chloroformate, pyridinium, etc.) are discussed for the synthesis of bioactive mols. contg. peptide linkages. Methods used to suppress racemization during the peptide coupling step are presented.
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      Amide bond formation: beyond the myth of coupling reagents
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      Chemical Society Reviews (2009), 38 (2), 606-631CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      This crit. review is focussed on the most recently developed coupling reagents with particular attention paid to the pros and cons of the plethora of "acronym" based reagents. Amide bond formation is a fundamentally important reaction in org. synthesis, and is typically mediated by one of a myriad of so-called coupling reagents. It aims to demystify the process allowing the chemist to make a sensible and educated choice when carrying out an amide coupling reaction (179 refs.).
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      Nonclassical Routes for Amide Bond Formation
      de Figueiredo Renata Marcia; Suppo Jean-Simon; Campagne Jean-Marc
      Chemical reviews (2016), 116 (19), 12029-12122 ISSN:.
      The present review offers an overview of nonclassical (e.g., with no pre- or in situ activation of a carboxylic acid partner) approaches for the construction of amide bonds. The review aims to comprehensively discuss relevant work, which was mainly done in the field in the last 20 years. Organization of the data follows a subdivision according to substrate classes: catalytic direct formation of amides from carboxylic and amines ( section 2 ); the use of carboxylic acid surrogates ( section 3 ); and the use of amine surrogates ( section 4 ). The ligation strategies (NCL, Staudinger, KAHA, KATs, etc.) that could involve both carboxylic acid and amine surrogates are treated separately in section 5 .
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      Espinosa-Jalapa, N. A.; Kumar, A.; Leitus, G.; Diskin-Posner, Y.; Milstein, D. Synthesis of Cyclic Imides by Acceptorless Dehydrogenative Coupling of Diols and Amines Catalyzed by a Manganese Pincer Complex. J. Am. Chem. Soc. 2017, 139, 11722– 11725,  DOI: 10.1021/jacs.7b08341
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      30
      Synthesis of Cyclic Imides by Acceptorless Dehydrogenative Coupling of Diols and Amines Catalyzed by a Manganese Pincer Complex
      Espinosa-Jalapa, Noel Angel; Kumar, Amit; Leitus, Gregory; Diskin-Posner, Yael; Milstein, David
      Journal of the American Chemical Society (2017), 139 (34), 11722-11725CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The first example of base-metal-catalyzed dehydrogenative coupling of diols and amines to form cyclic imides, i.e. I, is reported. The reaction is catalyzed by a pincer complex of the earth abundant manganese and forms hydrogen gas as the sole byproduct, making the overall process atom economical and environmentally benign.
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    31. 31
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      Current gradient-cor. d.-functional approxns. for the exchange energies of at. and mol. systems fail to reproduce the correct 1/r asymptotic behavior of the exchange-energy d. A gradient-cor. exchange-energy functional is given with the proper asymptotic limit. This functional, contg. only one parameter, fits the exact Hartree-Fock exchange energies of a wide variety of at. systems with remarkable accuracy, surpassing the performance of previous functionals contg. two parameters or more.
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      There is no expanded citation for this reference.
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      There is no expanded citation for this reference.
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      (a) Weigend, F.; Ahlrichs, R. Balanced Basis Sets of Split Valence, Triple Zeta Valence and Quadruple Zeta Valence Quality for H to Rn: Design and Assessment of Accuracy. Phys. Chem. Chem. Phys. 2005, 7, 3297– 3305,  DOI: 10.1039/b508541a
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      Balanced basis sets of split valence, triple zeta valence and quadruple zeta valence quality for H to Rn: Design and assessment of accuracy
      Weigend, Florian; Ahlrichs, Reinhart
      Physical Chemistry Chemical Physics (2005), 7 (18), 3297-3305CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      Gaussian basis sets of quadruple zeta valence quality for Rb-Rn are presented, as well as bases of split valence and triple zeta valence quality for H-Rn. The latter were obtained by (partly) modifying bases developed previously. A large set of more than 300 mols. representing (nearly) all elements-except lanthanides-in their common oxidn. states was used to assess the quality of the bases all across the periodic table. Quantities investigated were atomization energies, dipole moments and structure parameters for Hartree-Fock, d. functional theory and correlated methods, for which we had chosen Moller-Plesset perturbation theory as an example. Finally recommendations are given which type of basis set is used best for a certain level of theory and a desired quality of results.
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      Weigend, Florian
      Physical Chemistry Chemical Physics (2006), 8 (9), 1057-1065CODEN: PPCPFQ; ISSN:1463-9076. (Royal Society of Chemistry)
      A series of auxiliary basis sets to fit Coulomb potentials for the elements H to Rn (except lanthanides) is presented. For each element only one auxiliary basis set is needed to approx. Coulomb energies in conjunction with orbital basis sets of split valence, triple zeta valence and quadruple zeta valence quality with errors of typically below ca. 0.15 kJ mol-1 per atom; this was demonstrated in conjunction with the recently developed orbital basis sets of types def2-SV(P), def2-TZVP and def2-QZVPP for a large set of small mols. representing (nearly) each element in all of its common oxidn. states. These auxiliary bases are slightly more than three times larger than orbital bases of split valence quality. Compared to non-approximated treatments, computation times for the Coulomb part are reduced by a factor of ca. 8 for def2-SV(P) orbital bases, ca. 25 for def2-TZVP and ca. 100 for def2-QZVPP orbital bases.
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      Johnson, E. R.; Becke, A. D. A Post-Hartree-Fock Model of Intermolecular Interactions: Inclusion of Higher-Order Corrections. J. Chem. Phys. 2006, 124, 174104  DOI: 10.1063/1.2190220
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      A post-Hartree-Fock model of intermolecular interactions: Inclusion of higher-order corrections
      Johnson, Erin R.; Becke, Axel D.
      Journal of Chemical Physics (2006), 124 (17), 174104/1-174104/9CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      We have previously demonstrated that the dipole moment of the exchange hole can be used to derive intermol. C6 dispersion coeffs. [J. Chem. Phys. 122, 154104 (2005)]. This was subsequently the basis for a novel post-Hartree-Fock model of intermol. interactions [J. Chem. Phys. 123, 024101 (2005)]. In the present work, the model is extended to include higher-order dispersion coeffs. C8 and C10. The extended model performs very well for prediction of intermonomer sepns. and binding energies of 45 van der Waals complexes. In particular, it performs twice as well as basis-set extrapolated MP2 theory for dispersion-bound complexes, with minimal computational cost.
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      (a) Grimme, S.; Antony, J.; Ehrlich, S.; Krieg, H. A Consistent and Accurate Ab Initio Parametrization of Density Functional Dispersion Correction (DFT-D) for the 94 Elements H-Pu. J. Chem. Phys. 2010, 132, 154104  DOI: 10.1063/1.3382344
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      A consistent and accurate ab initio parametrization of density functional dispersion correction (DFT-D) for the 94 elements H-Pu
      Grimme, Stefan; Antony, Jens; Ehrlich, Stephan; Krieg, Helge
      Journal of Chemical Physics (2010), 132 (15), 154104/1-154104/19CODEN: JCPSA6; ISSN:0021-9606. (American Institute of Physics)
      The method of dispersion correction as an add-on to std. Kohn-Sham d. functional theory (DFT-D) has been refined regarding higher accuracy, broader range of applicability, and less empiricism. The main new ingredients are atom-pairwise specific dispersion coeffs. and cutoff radii that are both computed from first principles. The coeffs. for new eighth-order dispersion terms are computed using established recursion relations. System (geometry) dependent information is used for the first time in a DFT-D type approach by employing the new concept of fractional coordination nos. (CN). They are used to interpolate between dispersion coeffs. of atoms in different chem. environments. The method only requires adjustment of two global parameters for each d. functional, is asymptotically exact for a gas of weakly interacting neutral atoms, and easily allows the computation of at. forces. Three-body nonadditivity terms are considered. The method has been assessed on std. benchmark sets for inter- and intramol. noncovalent interactions with a particular emphasis on a consistent description of light and heavy element systems. The mean abs. deviations for the S22 benchmark set of noncovalent interactions for 11 std. d. functionals decrease by 15%-40% compared to the previous (already accurate) DFT-D version. Spectacular improvements are found for a tripeptide-folding model and all tested metallic systems. The rectification of the long-range behavior and the use of more accurate C6 coeffs. also lead to a much better description of large (infinite) systems as shown for graphene sheets and the adsorption of benzene on an Ag(111) surface. For graphene it is found that the inclusion of three-body terms substantially (by about 10%) weakens the interlayer binding. We propose the revised DFT-D method as a general tool for the computation of the dispersion energy in mols. and solids of any kind with DFT and related (low-cost) electronic structure methods for large systems. (c) 2010 American Institute of Physics.
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      (b) Grimme, S.; Ehrlich, S.; Goerigk, L. Effect of the Damping Function in Dispersion Corrected Density Functional Theory. J. Comput. Chem. 2011, 32, 1456– 1465,  DOI: 10.1002/jcc.21759
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      Effect of the damping function in dispersion corrected density functional theory
      Grimme, Stefan; Ehrlich, Stephan; Goerigk, Lars
      Journal of Computational Chemistry (2011), 32 (7), 1456-1465CODEN: JCCHDD; ISSN:0192-8651. (John Wiley & Sons, Inc.)
      It is shown by an extensive benchmark on mol. energy data that the math. form of the damping function in DFT-D methods has only a minor impact on the quality of the results. For 12 different functionals, a std. "zero-damping" formula and rational damping to finite values for small interat. distances according to Becke and Johnson (BJ-damping) has been tested. The same (DFT-D3) scheme for the computation of the dispersion coeffs. is used. The BJ-damping requires one fit parameter more for each functional (three instead of two) but has the advantage of avoiding repulsive interat. forces at shorter distances. With BJ-damping better results for nonbonded distances and more clear effects of intramol. dispersion in four representative mol. structures are found. For the noncovalently-bonded structures in the S22 set, both schemes lead to very similar intermol. distances. For noncovalent interaction energies BJ-damping performs slightly better but both variants can be recommended in general. The exception to this is Hartree-Fock that can be recommended only in the BJ-variant and which is then close to the accuracy of cor. GGAs for non-covalent interactions. According to the thermodn. benchmarks BJ-damping is more accurate esp. for medium-range electron correlation problems and only small and practically insignificant double-counting effects are obsd. It seems to provide a phys. correct short-range behavior of correlation/dispersion even with unmodified std. functionals. In any case, the differences between the two methods are much smaller than the overall dispersion effect and often also smaller than the influence of the underlying d. functional. © 2011 Wiley Periodicals, Inc.; J. Comput. Chem., 2011.
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      (a) Barone, V.; Cossi, M. Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model. J. Phys. Chem. A 1998, 102, 1995– 2001,  DOI: 10.1021/jp9716997
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      Quantum Calculation of Molecular Energies and Energy Gradients in Solution by a Conductor Solvent Model
      Barone, Vincenzo; Cossi, Maurizio
      Journal of Physical Chemistry A (1998), 102 (11), 1995-2001CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      A new implementation of the conductor-like screening solvation model (COSMO) in the GAUSSIAN94 package is presented. It allows Hartree-Fock (HF), d. functional (DF) and post-HF energy, and HF and DF gradient calcns.: the cavities are modeled on the mol. shape, using recently optimized parameters, and both electrostatic and nonelectrostatic contributions to energies and gradients are considered. The calcd. solvation energies for 19 neutral mols. in water are found in very good agreement with exptl. data; the solvent-induced geometry relaxation is studied for some closed and open shell mols., at HF and DF levels. The computational times are very satisfying: the self-consistent energy evaluation needs a time 15-30% longer than the corresponding procedure in vacuo, whereas the calcn. of energy gradients is only 25% longer than in vacuo for medium size mols.
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      Molecular Interactions in Solution: An Overview of Methods Based on Continuous Distributions of the Solvent
      Tomasi, Jacopo; Persico, Maurizio
      Chemical Reviews (Washington, DC, United States) (1994), 94 (7), 2027-94CODEN: CHREAY; ISSN:0009-2665.
      A review with 838 refs. on different approaches to studying noncovalent mol. interactions in soln. with regard to the basic continuum model, the classical electrostatic problem, and the quantum problem. Equil. thermodn. functions, dynamic effects in the continuum model and inhomogeneities and anisotropies in the solvent are also discussed.
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      Zhao, Y.; Truhlar, D. G. The M06 Suite of Density Functionals for Main Group Thermochemistry, Thermochemical Kinetics, Noncovalent Interactions, Excited States, and Transition Elements: Two New Functionals and Systematic Testing of Four M06-Class Functionals and 12 Other Functionals. Theor. Chem. Acc. 2008, 120, 215– 241,  DOI: 10.1007/s00214-007-0310-x
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      The M06 suite of density functionals for main group thermochemistry, thermochemical kinetics, noncovalent interactions, excited states, and transition elements: two new functionals and systematic testing of four M06-class functionals and 12 other functionals
      Zhao, Yan; Truhlar, Donald G.
      Theoretical Chemistry Accounts (2008), 120 (1-3), 215-241CODEN: TCACFW; ISSN:1432-881X. (Springer GmbH)
      We present two new hybrid meta exchange-correlation functionals, called M06 and M06-2X. The M06 functional is parametrized including both transition metals and nonmetals, whereas the M06-2X functional is a high-nonlocality functional with double the amt. of nonlocal exchange (2X), and it is parametrized only for nonmetals. The functionals, along with the previously published M06-L local functional and the M06-HF full-Hartree-Fock functionals, constitute the M06 suite of complementary functionals. We assess these four functionals by comparing their performance to that of 12 other functionals and Hartree-Fock theory for 403 energetic data in 29 diverse databases, including ten databases for thermochem., four databases for kinetics, eight databases for noncovalent interactions, three databases for transition metal bonding, one database for metal atom excitation energies, and three databases for mol. excitation energies. We also illustrate the performance of these 17 methods for three databases contg. 40 bond lengths and for databases contg. 38 vibrational frequencies and 15 vibrational zero point energies. We recommend the M06-2X functional for applications involving main-group thermochem., kinetics, noncovalent interactions, and electronic excitation energies to valence and Rydberg states. We recommend the M06 functional for application in organometallic and inorganometallic chem. and for noncovalent interactions.
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      Electron affinities of the first-row atoms revisited. Systematic basis sets and wave functions
      Kendall, Rick A.; Dunning, Thom H., Jr.; Harrison, Robert J.
      Journal of Chemical Physics (1992), 96 (9), 6796-806CODEN: JCPSA6; ISSN:0021-9606.
      The authors describe a reliable procedure for calcg. the electron affinity of an atom and present results for H, B, C, O, and F (H is included for completeness). This procedure involves the use of the recently proposed correlation-consistent basis sets augmented with functions to describe the more diffuse character of the at. anion coupled with a straightforward, uniform expansion of the ref. space for multireference singles and doubles configuration-interaction (MRSD-CI) calcns. A comparison is given with previous results and with corresponding full CI calcns. The most accurate EAs obtained from the MRSD-CI calcns. are (with exptl. values in parentheses): H 0.740 eV (0.754), B 0.258 (0.277), C 1.245 (1.263), O 1.384 (1.461), and F 3.337 (3.401). The EAs obtained from the MR-SDCI calcns. differ by less than 0.03 eV from those predicted by the full CI calcns.
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      (a) Leitgeb, A.; Abbas, M.; Fischer, R. C.; Poater, A.; Cavallo, L.; Slugovc, C. A latent Ruthenium Based Olefin Metathesis Catalyst with a Sterically Demanding NHC Ligand (Pre)Catalysts. Catal. Sci. Technol. 2012, 2, 1640– 1643,  DOI: 10.1039/c2cy20311a
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      A latent ruthenium based olefin metathesis catalyst with a sterically demanding NHC ligand
      Leitgeb, Anita; Abbas, Mudassar; Fischer, Roland C.; Poater, Albert; Cavallo, Luigi; Slugovc, Christian
      Catalysis Science & Technology (2012), 2 (8), 1640-1643CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
      An olefin metathesis catalyst featuring a SIPr NHC and an ester chelating carbene ligand is introduced. In contrast to its previously published SIMes analog, only the trans dichloro configurated isomer was obtained. The two counterparts are tested in various olefin metathesis reactions, revealing a striking superiority of the new complex in the cross metathesis of olefins with Me vinyl ketone allowing for full conversion with only 500 ppm catalyst loading.
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      Iron-Catalyzed Chemoselective Reduction of α,β-Unsaturated Ketones
      Lator, Alexis; Gaillard, Sylvain; Poater, Albert; Renaud, Jean-Luc
      Chemistry - A European Journal (2018), 24 (22), 5770-5774CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      An iron-catalyzed chemo- and diastereoselective redn. of α,β-unsatd. ketones into the corresponding satd. ketones in mild reaction conditions is reported herein. DFT calcns. and exptl. work underline that transfer hydride redn. is a more facile process than hydrogenation, unveiling the fundamental role of the base.
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      (c) Gómez-Suárez, A.; Oonishi, Y.; Martin, A. R.; Vummaleti, S. V. C.; Nelson, D. J.; Cordes, D. B.; Slawin, A. M. Z.; Cavallo, L.; Nolan, S. P.; Poater, A. On the Mechanism of the Digold(I)-Hydroxide-Catalysed Hydrophenoxylation of Alkynes. Chem. – Eur. J. 2016, 22, 1125– 1132,  DOI: 10.1002/chem.201503097
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      On the Mechanism of the Digold(I)-Hydroxide-Catalysed Hydrophenoxylation of Alkynes
      Gomez-Suarez, Adrian; Oonishi, Yoshihiro; Martin, Anthony R.; Vummaleti, Sai V. C.; Nelson, David J.; Cordes, David B.; Slawin, Alexandra M. Z.; Cavallo, Luigi; Nolan, Steven P.; Poater, Albert
      Chemistry - A European Journal (2016), 22 (3), 1125-1132CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Herein, we present a detailed investigation of the mechanistic aspects of the dual gold-catalyzed hydrophenoxylation of alkynes by both exptl. and computational methods. The dissocn. of [{Au(NHC)}2(μ-OH)][BF4] is essential to enter the catalytic cycle, and this step is favored by the presence of bulky, non-coordinating counter ions. Moreover, in silico studies confirmed that phenol does not only act as a reactant, but also as a co-catalyst, lowering the energy barriers of several transition states. A gem-diaurated species might form during the reaction, but this lies deep within a potential energy well, and is likely to be an "off-cycle" rather than an "in-cycle" intermediate.
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      (d) Poater, A.; Pump, E.; Vummaleti, S. V. C.; Cavallo, L. The Right Computational Recipe for Olefin Metathesis with Ru-based Catalysts: the Whole Mechanism of Ring-Closing Olefin Metathesis. J. Chem. Theory Comput. 2014, 10, 4442– 4448,  DOI: 10.1021/ct5003863
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      The Right Computational Recipe for Olefin Metathesis with Ru-Based Catalysts: The Whole Mechanism of Ring-Closing Olefin Metathesis
      Poater, Albert; Pump, Eva; Vummaleti, Sai Vikrama Chaitanya; Cavallo, Luigi
      Journal of Chemical Theory and Computation (2014), 10 (10), 4442-4448CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
      The initiation mechanism of ruthenium methylidene complexes was studied detailing mechanistic insights of all involved reaction steps within a classical olefin metathesis pathway. Computational studies reached a good agreement with the rarely available exptl. data and even enabled to complement them. As a result, a highly accurate computational and rather cheap recipe is presented; M06/TZVP//BP86/SVP (PCM, P = 1354 atm).
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      Besora, M.; Vidossich, P.; Lledós, A.; Ujaque, G.; Maseras, F. Calculation of Reaction Free Energies in Solution: A Comparison of Current Approaches. J. Phys. Chem. A 2018, 122, 1392– 1399,  DOI: 10.1021/acs.jpca.7b11580
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      40
      Calculation of Reaction Free Energies in Solution: A Comparison of Current Approaches
      Besora, Maria; Vidossich, Pietro; Lledos, Agusti; Ujaque, Gregori; Maseras, Feliu
      Journal of Physical Chemistry A (2018), 122 (5), 1392-1399CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
      The result of the application of different approaches based on the ideal gas/rigid rotor/harmonic oscillator (IGRRHO) model, commonly used in popular software packages, for the calcn. of free energies in soln. is compared with that of ab initio mol. dynamics for a process involving ligand exchange in palladium complexes. The IGRRHO-based approaches considered differ in most cases in the extent to which the rotational and translational contributions are included in the correction. Our study supports the use the free energy values directly obtained from dispersion-cor. DFT functionals without any correction or with minor corrections at most.
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    41. 41
      (a) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. SM6: A Density Functional Theory Continuum Solvation Model for Calculating Aqueous Solvation Free Energies of Neutrals, Ions, and Solute–Water Clusters. J. Chem. Theory Comput. 2005, 1, 1133– 1152,  DOI: 10.1021/ct050164b
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      41a
      SM6: A Density Functional Theory Continuum Solvation Model for Calculating Aqueous Solvation Free Energies of Neutrals, Ions, and Solute-Water Clusters
      Kelly, Casey P.; Cramer, Christopher J.; Truhlar, Donald G.
      Journal of Chemical Theory and Computation (2005), 1 (6), 1133-1152CODEN: JCTCCE; ISSN:1549-9618. (American Chemical Society)
      A new charge model, called Charge Model 4 (CM4), and a new continuum solvent model, called Solvation Model 6 (SM6), are presented. Using a database of aq. solvation free energies for 273 neutrals, 112 ions, and 31 ion-water clusters, parameter sets for the mPW0 hybrid d. functional of Adamo and Barone (Adamo, C.; Barone, V. J. Chem. Phys. 1998, 108, 664-675) were optimized for use with the following four basis sets: MIDI!6D, 6-31G(d), 6-31+G(d), and 6-31+G(d,p). SM6 separates the observable aq. solvation free energy into two different components: one arising from long-range bulk electrostatic effects and a second from short-range interactions between the solute and solvent mols. in the first solvation shell. This partition of the observable solvation free energy allows SM6 to effectively model a wide range of solutes. For the 273 neutral solutes in the test set, SM6 achieves an av. error of ∼0.50 kcal/mol in the aq. solvation free energies. For solutes, esp. ions, that have highly concd. regions of charge d., adding an explicit water mol. to the calcn. significantly improves the performance of SM6 for predicting solvation free energies. The performance of SM6 was tested against several other continuum models, including SM5.43R and several different implementations of the Polarizable Continuum Model (PCM). For both neutral and ionic solutes, SM6 outperforms all of the models against which it was tested. Also, SM6 is the only model (except for one with an av. error 3.4 times larger) that improves when an explicit solvent mol. is added to solutes with concd. charge densities. Thus, in SM6, unlike the other continuum models tested here, adding one or more explicit solvent mol. to the calcn. is an effective strategy for improving the prediction of the aq. solvation free energies of solutes with strong local solute-solvent interactions. This is important, because local solute-solvent interactions are not specifically accounted for by bulk electrostatics, but modeling these interactions correctly is important for predicting the aq. solvation free energies of certain solutes. Finally, SM6 retains its accuracy when used in conjunction with the B3LYP and B3PW91 functionals, and in fact the solvation parameters obtained with a given basis set may be used with any good d. functional or fraction of Hartree-Fock exchange.
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      .
      (b) Kelly, C. P.; Cramer, C. J.; Truhlar, D. G. Aqueous Solvation Free Energies of Ions and Ion–Water Clusters Based on an Accurate Value for the Absolute Aqueous Solvation Free Energy of the Proton. J. Phys. Chem. B 2006, 110, 16066– 16081,  DOI: 10.1021/jp063552y
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      41b
      Aqueous Solvation Free Energies of Ions and Ion-Water Clusters Based on an Accurate Value for the Absolute Aqueous Solvation Free Energy of the Proton
      Kelly, Casey P.; Cramer, Christopher J.; Truhlar, Donald G.
      Journal of Physical Chemistry B (2006), 110 (32), 16066-16081CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      Thermochem. cycles that involve pKa, gas-phase acidities, aq. solvation free energies of neutral species, and gas-phase clustering free energies have been used with the cluster pair approxn. to det. the abs. aq. solvation free energy of the proton. The best value obtained in this work is in good agreement with the value reported by Tissandier et al. (Tissandier, M. D.; Cowen, K. A.; Feng, W. Y.; Gundlach, E.; Cohen, M. J.; Earhart, A. D.; Coe, J. V., J. Phys. Chem. A 1998, 102, 7787), who applied the cluster pair approxn. to a less diverse and smaller data set of ions. We agree with previous workers who advocated the value of -265.9 kcal/mol for the abs. aq. solvation free energy of the proton. Considering the uncertainties assocd. with the exptl. gas-phase free energies of ions that are required to use the cluster pair approxn. as well as analyses of various subsets of data, we est. an uncertainty for the abs. aq. solvation free energy of the proton of no less than 2 kcal/mol. Using a value of -265.9 kcal/mol for the abs. aq. solvation free energy of the proton, we expand and update our previous compilation of abs. aq. solvation free energies; this new data set contains conventional and abs. aq. solvation free energies for 121 unclustered ions (not including the proton) and 147 conventional and abs. aq. solvation free energies for 51 clustered ions contg. from 1 to 6 water mols. When tested against the same set of ions that was recently used to develop the SM6 continuum solvation model, SM6 retains its previously detd. high accuracy; indeed, in most cases the mean unsigned error improves when it is tested against the more accurate ref. data.
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      .
      (c) Bryantsev, V. S.; Diallo, M. S.; Goddard, W. A., III Calculation of Solvation Free Energies of Charged Solutes Using Mixed Cluster/Continuum Models. J. Phys. Chem. B 2008, 112, 9709– 9719,  DOI: 10.1021/jp802665d
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      41c
      Calculation of Solvation Free Energies of Charged Solutes Using Mixed Cluster/Continuum Models
      Bryantsev, Vyacheslav S.; Diallo, Mamadou S.; Goddard, William A., III
      Journal of Physical Chemistry B (2008), 112 (32), 9709-9719CODEN: JPCBFK; ISSN:1520-6106. (American Chemical Society)
      The authors derive a consistent approach for predicting the solvation free energies of charged solutes in the presence of implicit and explicit solvents. Some published methodologies make systematic errors in the computed free energies because of the incorrect accounting of the std. state corrections for water mols. or water clusters present in the thermodn. cycle. This problem can be avoided by using the same std. state for each species involved in the reaction under consideration. Two different thermodn. cycles are analyzed for calcg. the solvation free energies of ionic solutes: (1) the cluster cycle with an n water cluster as a reagent and (2) the monomer cycle with n distinct water mols. as reagents. The use of the cluster cycle gives solvation free energies that are in excellent agreement with the exptl. values obtained from studies of ion-water clusters. The mean abs. errors are 0.8 kcal/mol for H+ and 2.0 kcal/mol for Cu2+. Conversely, calcns. using the monomer cycle lead to mean abs. errors that are >10 kcal/mol for H+ and >30 kcal/mol for Cu2+. The presence of hydrogen-bonded clusters of similar size on the left- and right-hand sides of the reaction cycle results in the cancelation of the systematic errors in the calcd. free energies. Using the cluster cycle with 1 solvation shell leads to errors of 5 kcal/mol for H+ (6 waters) and 27 kcal/mol for Cu2+ (6 waters), whereas using 2 solvation shells leads to accuracies of 2 kcal/mol for Cu2+ (18 waters) and 1 kcal/mol for H+ (10 waters).
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    42. 42
      Kozuch, S.; Shaik, S. How to Conceptualize Catalytic Cycles? The Energetic Span Model. Acc. Chem. Res. 2011, 44, 101– 110,  DOI: 10.1021/ar1000956
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      43
      How to Conceptualize Catalytic Cycles? The Energetic Span Model
      Kozuch, Sebastian; Shaik, Sason
      Accounts of Chemical Research (2011), 44 (2), 101-110CODEN: ACHRE4; ISSN:0001-4842. (American Chemical Society)
      A review; a computational study of a catalytic cycle generates state energies (the E-representation), whereas expts. lead to rate consts. (the k-representation). Based on transition state theory (TST), these are equiv. representations. Nevertheless, until recently, there has been no simple way to calc. the efficiency of a catalytic cycle, i.e., its turnover frequency (TOF), from a theor. obtained energy profile. In this Account, we introduce the energetic span model that enables one to evaluate TOFs in a straightforward manner and in affinity with the Curtin-Hammett principle. As shown herein, the model implies a change in our kinetic concepts. Analogous to Ohm's law, the catalytic chem. current (the TOF) can be defined by a chem. potential (independent of the mechanism) divided by a chem. resistance (dependent on the mechanism and the nature of the catalyst). This formulation is based on Eyring's TST and corresponds to a steady-state regime. In many catalytic cycles, only one transition state and one intermediate det. the TOF. We call them the TOF-detg. transition state (TDTS) and the TOF-detg. intermediate (TDI). These key states can be located, from among the many states available to a catalytic cycle, by assessing the degree of TOF control (XTOF); this last term resembles the structure-reactivity coeff. in classical phys. org. chem. The TDTS-TDI energy difference and the reaction driving force define the energetic span (δE) of the cycle. Whenever the TDTS appears after the TDI, δE is the energy difference between these two states; when the opposite is true, we must also add the driving force to this difference. Having δE, the TOF is expressed simply in the Arrhenius-Eyring fashion, wherein δE serves as the apparent activation energy of the cycle. An important lesson from this model is that neither one transition state nor one reaction step possess all the kinetic information that dets. the efficiency of a catalyst. Addnl., the TDI and TDTS are not necessarily the highest and lowest states, nor do they have to be adjoined as a single step. As such, we can conclude that a change in the conceptualization of catalytic cycles is in order: in catalysis, there are no rate-detg. steps, but rather rate-detg. states. We also include a study on the effect of reactant and product concns. In the energetic span approxn., only the reactants or products that are located between the TDI and TDTS accelerate or inhibit the reaction. In this manner, the energetic span model creates a direct link between exptl. quantities and theor. results. The versatility of the energetic span model is demonstrated with several catalytic cycles of organometallic reactions.
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    43. 43
      Li, H.; Hall, M. B. Computational Mechanistic Studies on Reactions of Transition Metal Complexes with Noninnocent Pincer Ligands: Aromatization–Dearomatization or Not. ACS Catal. 2015, 5, 1895– 1913,  DOI: 10.1021/cs501875z
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      42
      Computational Mechanistic Studies on Reactions of Transition Metal Complexes with Noninnocent Pincer Ligands: Aromatization-Dearomatization or Not
      Li, Haixia; Hall, Michael B.
      ACS Catalysis (2015), 5 (3), 1895-1913CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      The development of green chem. has attracted chemists' attentions in recent years. Among them, Milstein and co-workers have discovered a new mode of metal-ligand cooperation in complexes in which an aromatization-dearomatization process of the pyridine- or acridine-based PNP and PNN "pincer" ligands appears to be a key element. These complexes were reported to lead to unusual X-H (X = H, C, O, N, and B) activation reactions and to environmentally benign catalysis involving dehydrogenative coupling reactions and hydrogenation reactions, representing an important development in green chem. This review provides a summary of theor. studies on the mechanisms of the reactions mediated by transition metal complexes with noninnocent pincer ligands synthesized by Milstein and co-workers. The aromatization-dearomatization process of the pyridine- or acridine-based PNP and PNN "pincer" ligands were found to play important roles in some reactions, while other reactions do not involve the aromatization-dearomatization process. For some reactions, several research groups proposed different mechanisms to explain the same reaction. Thus, to compare these mechanisms, we recalc. their rate-detg. steps by using the functionals that are calibrated to produce results close to those from coupled cluster calcns. Moreover, the understanding of the reaction mechanisms can help researchers to improve the current reactions and design new reactions.
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    44. 44
      Nguyen, D. H.; Trivelli, X.; Capet, F.; Paul, J.-F.; Dumeignil, F.; Gauvin, R. M. Manganese Pincer Complexes for the Base-Free, Acceptorless Dehydrogenative Coupling of Alcohols to Esters: Development, Scope, and Understanding. ACS Catal. 2017, 7, 2022– 2032,  DOI: 10.1021/acscatal.6b03554
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      44
      Manganese Pincer Complexes for the Base-Free, Acceptorless Dehydrogenative Coupling of Alcohols to Esters: Development, Scope, and Understanding
      Nguyen, Duc Hanh; Trivelli, Xavier; Capet, Frederic; Paul, Jean-Francois; Dumeignil, Franck; Gauvin, Regis M.
      ACS Catalysis (2017), 7 (3), 2022-2032CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
      Aliph. PNP pincer-supported earth-abundant Mn(I) dicarbonyl complexes behave as effective catalysts for the acceptorless dehydrogenative coupling of a wide range of alcs. to esters under base-free conditions. The reaction proceeds under neat conditions, with modest catalyst loading and releasing only H2 as byproduct. Mechanistic aspects were addressed by synthesizing key species related to the catalytic cycle (characterized by x-ray structure detn., multinuclear (1H, 13C, 31P, 15N, 55Mn) NMR, IR spectroscopy, inter alia), by studying elementary steps connected to the postulated mechanism, and by resorting to DFT calcns. As in the case of related Ru and Fe PNP catalysts, the dehydrogenation results from cycling between the amido and amino-hydride forms of the PNP-Mn(CO)2 scaffold. For the dehydrogenation of alcs. into aldehydes, the authors' results suggest that the highest energy barrier corresponds to the H2 release from the amino-hydride form, although its value is close to that of the outer-sphere dehydrogenation of the alc. into aldehyde. This contrasts with the Ru and Fe catalytic systems, where dehydrogenation of the substrate into aldehyde is less energy-demanding compared to H2 release from the cooperative metal-ligand framework.
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