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LetterMay 8, 2020

Oxidative Coupling of Aldehydes with Alcohol for the Synthesis of Esters Promoted by Polystyrene-Supported N-Heterocyclic Carbene: Unraveling the Solvent Effect on the Catalyst Behavior Using NMR Relaxation
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Graziano Di Carmine
Graziano Di Carmine
Department of Chemical Engineering and Analytical Science, The University of Manchester, The Mill, Sackville Street, Manchester, M13 9PL, U.K.
More by Graziano Di Carmine
http://orcid.org/0000-0002-2591-9633
Daniele Ragno
Daniele Ragno
Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara, Via Luigi Borsari, 46, I-44121 Ferrara, Italy
More by Daniele Ragno
http://orcid.org/0000-0003-0016-290X
Alessandro Massi
Alessandro Massi
Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara, Via Luigi Borsari, 46, I-44121 Ferrara, Italy
More by Alessandro Massi
http://orcid.org/0000-0001-8303-5441
Carmine D’Agostino*
Carmine D’Agostino
Department of Chemical Engineering and Analytical Science, The University of Manchester, The Mill, Sackville Street, Manchester, M13 9PL, U.K.
*E-mail: [email protected]
More by Carmine D’Agostino
http://orcid.org/0000-0003-3391-8320

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Organic Letters

Cite this: Org. Lett. 2020, 22, 13, 4927–4931

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https://doi.org/10.1021/acs.orglett.0c01188

Published May 8, 2020

Abstract

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Heterogeneous organocatalysts hold great potential as they offer practical advantages in terms of purification and reusability compared with the homogeneous counterpart. A puzzling aspect is the solvent effect on their catalytic performance. Here we propose a new approach whereby T₁/T₂ NMR relaxation measurements are used to evaluate the strength of solvent–surface interactions in the polystyrene-supported N-heterocyclic carbene-promoted oxidation of aldehydes. The results reveal that solvents with high surface affinity lead to a decrease in catalyst activity.

Subjects

Preparation of esters plays a crucial role in the synthesis of chemicals of interest for both academia and industry due to their ubiquitous presence in natural products and polymers and as APIs (active pharmaceutical ingredients). (1) Traditional routes involve toxic coupling reagents, and their use leads to the production of hazardous waste. Nevertheless, in the past decade, the demand of green protocols in response to the more stringent environmental regulations has encouraged chemists to develop new sustainable methods for the preparation of these compounds. (2) In the toolkit of organic chemists, oxidative coupling of aldehydes is among the most desirable routes due to the high stability of the aldehyde group compared to the acyl halide and the better atom economy of the overall process. (3) A mild and metal-free approach to convert aldehydes into esters in the presence of alcohols employs N-heterocyclic carbene (NHC) in oxidative conditions. (4) External oxidants in stoichiometric amounts (TEMPO, MnO₂ and Kharasch reagent) can be used. (5) However, cooperative systems that employ atmospheric oxygen as terminal oxidant would be more desirable. Surprisingly, after the seminal works of Shinkai and Diederich in 1980 and 1992, respectively, this approach remained uninvestigated until the first decade of the millennium. (6) In fact, in 2015 Sundén and co-workers (7) disclosed an efficient protocol to convert α,β-unsaturated aldehydes into the corresponding esters employing a triazolium salt (NHC) along with a mediator couple composed of an FePc (iron(II) phthalocyanine) and 2,6-di-tert-butylphenol. This mediator couple is able to generate the required oxidant species directly in situ working in homogeneous catalytic conditions and using atmospheric oxygen as terminal oxidant. (7) Unfortunately, in organocatalysis a higher catalytic loading is required to efficiently promote the reaction, compared to that for metal catalysis, making this protocol less attractive for industrial purposes.

The most promising approach to overcome this drawback encountered in the homogeneous catalytic process is that of anchoring the catalyst onto an insoluble support and reusing the catalyst several times after a simple filtration. (8) In addition, this approach has the potential to increase the sustainability of the process by facilitating catalyst separation, for example, by avoiding energy-intensive distillation procedures, which can in some cases make catalyst recovery unfeasible. In this context, Massi and co-workers reported the esterification of glycerol and solketal employing the strategy adopted by Sundén, using a simple triazolium salt supported on polystyrene for this purpose. (9) Subsequently, the same authors presented the oxidation of 5-hydroxymethylfurfural (5-HMF). (10)

Unlike homogeneous organocatalytic processes, the behavior of organocatalysts immobilized over a solid support, that is, heterogeneous organocatalysts, still remains puzzling, and this has limited their use and development both in academia and in industry, relative to their homogeneous counterparts. (11) New experimental protocols, in addition to the existing ones, are needed to gain new insights into the behavior of such materials, if they are to be developed as alternative, greener catalysts. In particular, unlike reactions in the presence of homogeneous organocatalysts, reactions over solid heterogeneous organocatalysts can be significantly affected by diffusion and adsorption behavior of reaction species over the surface. Yet, the influence of these aspects in heterogeneous organocatalysis remains unexplored. In recent years, NMR relaxation time measurements have emerged as a promising method to investigate and evaluate the adsorption behavior of molecules over porous materials due to the noninvasive nature and the fast acquisition time of the methodology. (12) It was demonstrated that the T₁/T₂ ratio can be used as a robust indicator of an activation energy for surface diffusion, which is intrinsically linked to the strength of molecule–surface interactions. In particular, a higher T₁/T₂ ratio indicates a stronger interaction with the surface. T₁/T₂ measurements have also been applied by Habina et al. to different systems, such as studies of surface interactions in oil and gas shales. (13) In a recent work carried out by D’Agostino and Gladden, NMR T₁–T₂ experiments were successfully used to explain the size effect of gold (Au) nanoparticles on glycerol adsorption over Au/TiO₂ catalysts. (14) Subsequently, these authors applied the same method to rationalize the inhibition effect of additives on catalyst activity in the oxidation of glycerol promoted by Au/TiO₂ catalyst (15) and to explain the solvent effect in aerobic oxidations of polyols over Au/TiO₂ catalysts. (16)

Prompted by these studies conducted over purely heterogeneous inorganic catalysts, we have envisaged that NMR relaxation techniques could play a significant role in starting to unravel the behavior of organocatalysts immobilized over solid surfaces, with solvent effects being among key aspects to understand in these materials. The role of solvent on the reaction rate is well-known, and it can be manifested in several ways: polar solvents may stabilize TSs (transition states) if they are characterized by charge separation; in addition, the polarity of the medium strongly influences the performance of acid–base reactions, and in metal catalysis, solvents can compete with the substrate in the complexation step, affecting the reaction rate dramatically. (17)

Even though several studies have been performed to investigate the solvent effect in heterogeneous catalysis, only inorganic supports have been investigated. Marchi and co-workers disclosed the solvent effect in the hydrogenation of acetophenone promoted by Ni/SiO₂, and they found a correlation between the solvent adsorption energy and the reaction rate concluding that the adsorption of the solvent molecules on the surface may partially block the active sites. (18)

While it is clear that efforts have been made to understand solvent effects in traditional metal-supported catalysts, insights into solvent effects in organocatalysts supported on solids have remained elusive. Herein, we present new insights into how the type of solvent can affect reactivity in heterogeneous organocatalysis. In particular, NMR relaxation time measurements have been used to probe the strength of surface interactions of several solvents over a polystyrene-supported triazolium salt (NHC). In addition, NMR diffusion measurements have also been performed to assess the presence of any mass transport limitations affecting the process.

As we were interested in studying the oxidation of aldehydes into the corresponding esters through heterogeneous NHC catalysis, we chose the reaction between 2-chlorobenzaldehyde 1 and methanol 2 as a benchmark reaction because of its simple monitoring by NMR spectroscopy. The quinone oxidant 4 has been selected as the oxidant for the process and DBU (1,5-diazabiciclo(5.4.0)undec-7-ene) as the base necessary to activate the triazolium salts (as homogeneous catalyst, C, and the same salt immobilized over polystyrene, PS1) into the corresponding NHC in situ. The polystyrene-supported triazolium salt PS1 was selected as the catalyst due to its well-known broad compatibility in oxidative NHC catalysis, and it was prepared following a previously reported protocol. (9) The average pore size of the porous matrix of this material is approximately 5.4 nm (see Supporting Information, page S2, for details). A homogeneous version of the reaction was performed under the same conditions using the analogous triazolium salt in solution, C. Several solvents were used, with different features (results summarized in Table 1). THF (tetrahydrofuran), DMF (dimethylformamide), and DCM (dichloromethane) were chosen as polar solvents, whereas toluene and cyclohexane were chosen as slightly polar and nonpolar solvents, respectively.

Table 1. Solvent Screening for the Oxidative Coupling of 2-Chlorobenzaldehyde with Methanol^a

catalyst	solvent	TOF^b
C	THF	(1.56 ± 0.04) × 10^–3
PS1	THF	(1.51 ± 0.04) × 10^–3
C	DMF	(4.00 ± 0.10) × 10^–4
PS1	DMF	(3.33 ± 0.08) × 10^–4
C	DCM	(2.13 ± 0.05) × 10^–3
PS1	DCM	(1.91 ± 0.05) × 10^–3
C	toluene	(5.28 ± 0.13) × 10^–4
PS1	toluene	(3.11 ± 0.08) × 10^–4
C	cyclohexane	(2.78 ± 0.07) × 10^–4
PS1	cyclohexane	(1.39 ± 0.03) × 10^–4

^{^a}

Reaction conditions: Aldehyde 1 (0.5 mmol), alcohol 2 (1.0 mmol), 4 (0.5 mmol), C or PS1 (12.5 μmol), DBU (0.05 mmol), diphenyl ether (0.5 mmol) as internal standard for the NMR yield quantification using a 300 MHz Gemini Varian spectrometer, solvent (4.0 mL).

^{^b}

Turnover frequency (TOF) in s^–1.

The ratios between TOFs (heterogeneous over homogeneous) were calculated and are summarized in Table 2. Since we are primarily interested in the effect of the solvent inside the pores of the heterogeneous NHC over the porous solid support, the use of this ratio is crucial because it rules out the influence of solvent effects in the homogeneous reaction, that is, removing any effect not caused by molecule–surface interaction or diffusion within the catalyst pores (such as proticity, polarizability, or basicity that could be involved in the stabilization or destabilization of the transition states in the mechanism).

Table 2. TOF Ratio between Heterogeneous and Homogeneous Oxidative Coupling by NHC

solvent	TOF_hetero/TOF_homo
THF	0.97 ± 0.03
DMF	0.83 ± 0.02
DCM	0.90 ± 0.03
toluene	0.59 ± 0.02
cyclohexane	0.50 ± 0.02

From the data in Table 2, it is possible to observe that a similar reactivity is observed for polar solvents (THF, DMF, DCM) when comparing the heterogeneous reaction with the homogeneous one. This finding is of high relevance as it is often the case that the reactivity of heterogeneous organocatalysts is much lower than that of the homogeneous counterpart, whereas in this case, for those solvents, a comparable reactivity is observed. The drop in TOF ratio appears more remarkable for the slightly polar solvent toluene and for the nonpolar solvent cyclohexane. Scheme 1 shows the general accepted mechanism for the oxidation of aldehydes promoted by NHC and an external oxidant. The Breslow intermediate is the key species involved in the redox step, and it is generated through the nucleophilic addition of the in situ formed catalyst act-PS1. The catalytic species involved in the process is act-PS1, whereas PS1 is the protonated form in which usually the catalyst is stored for stability purposes. To switch from PS1 to act-PS1in situ, a base such as DBU is required. For the NMR experiments, as we were interested in investigating the physical behavior of act-PS1, because it is the active form, we treated the precatalyst with a strong base in excess (NaH) before the preparation of the NMR samples (see Supporting Information for details).

Scheme 1. Generally Accepted Mechanism for the Oxidative NHC Catalyzed Reaction (5c)

The activated polystyrene-supported catalyst was soaked in anhydrous solvent for 24 h to allow full saturation of the pores. After this time, the catalyst particles were put onto a presoaked filter paper, the external surface of the catalyst particles was dried to remove any excess bulk solvent around the particles, and the solid was then transferred into a 5 mm NMR tube. The spin–lattice relaxation time, T₁, was measured using an inversion recovery pulse sequence, and the transverse relaxation time, T₂, was measured with the CMPG pulse sequence (19) (see Supporting Information for details on acquired NMR relaxation data and fitting of the data). T₁/T₂ values are summarized in Table 3.

Table 3. Values of T₁/T₂ Ratio of Solvents in the Polystyrene-Supported NHC Catalyst Used in This Work

solvent	T₁/T₂
THF	2.39 ± 0.07
DCM	2.94 ± 0.09
DMF	3.11 ± 0.09
toluene	3.97 ± 0.12
cyclohexane	4.61 ± 0.14

For fast tumbling bulk liquids, such as the solvents used in this work, single values of T₁ and T₂ are higher than for liquids confined in porous materials, and it is well-known that T₁/T₂ ≈ 1, as also suggested by the theory. (20) Aksnes and Gjerdåker have reported a T₁ significantly different from T₂ for bulk cyclohexane, although this was measured at 400 MHz. (21) We note here that while the T₁ of bulk liquids at room temperature does not depend on frequency, (22) the behavior of T₂ is more complex as it is affected by frequency as well as scalar coupling. (23) Previous experimental results at frequencies below 400 MHz have shown that for cyclohexane T₁ = T₂ = 2.9 s, (16a) which is in agreement with the value of 2.8 s reported here (see Supporting Information, Table SI4). We also note that in our case, NMR relaxation times of the bulk liquid are not necessary for our analysis, which is based on the T₁/T₂ of solvents confined in the porous matrix of the catalyst. It is important to highlight here that while single values of relaxation times can be affected by several factors other than the surface influence, such as amount of bulk liquid, degree of pore saturation, and specific surface area of contact, the T₁/T₂ ratio is to a large extent not affected by these factors and is a more direct and robust indication of the influence of surface interactions. (13a) In particular, as mentioned above, the T₁/T₂ ratio value is related to the surface–molecule energy interaction. Higher T₁/T₂ values indicate a stronger affinity of the solvent for the surface, and in our case this occurs for the less polar solvents; this is expected due to the hydrophobic nature of the matrix, which leads to preferential interactions with the less polar solvents. The results reported here give experimental evidence of such effects and provide quantitative metrics to assess them. Diffusion measurements were also performed in order to assess the influence of the pore diffusion on reaction rate, in addition to solvent adsorption effects. The Weisz–Prater criterion was applied to estimate the presence of diffusion limitation. In all cases, the calculated values were below 1; hence pore diffusion limitation can be excluded (see Supporting Information for more details). (24)

A plot of the TOF ratio versus T₁/T₂ of the solvent is shown in Figure 1, and it clearly depicts a remarkable trend showing that solvents with higher affinity for the surface result in lower catalytic activity. This strongly suggests that the decrease in catalytic activity is closely related to a stronger surface affinity of the solvent, which inhibits reactivity by preventing access of reactant molecules to the catalytic sites over the surface. A similar effect has been reported for metal/support catalysts used in oxidation of diols using NMR spectroscopic analysis, (25) and here, for the first time, we observe a similar effect on immobilized organocatalysts. This suggests that solvent selection is a key parameter to consider when optimizing and developing such materials. It is important to highlight that other effects may affect reactivity, most notably diffusion and transition state stabilization; however, as previously discussed, based on our calculations and considerations, such effects have already been ruled out. Hence, the results shown here demonstrate that the solvent interaction with the catalyst surface plays a key role in determining catalyst activity.

Figure 1. T₁/T₂ of solvents used in the oxidative coupling of 2-chlorobenzaldehyde with methanol versus the TOF_hetero/TOF_homo. The solid line is a linear fit, which represents a guide for the eyes.

In conclusion, we have used NMR relaxation and diffusion studies coupled with reaction screening to rationalize for the first time how solvent–surface interactions can affect the reaction rate of immobilized organocatalysts, using the oxidative coupling of 2-chlorobenzaldehyde with methanol promoted by NHC as a model reaction. Other systems will be studied in the near future by our group in order to expand this approach. The initial findings reported here are of high importance as they suggest that the design of a solid matrix with suitable adsorption properties is a key parameter to take into account when designing these catalysts, which could lead to new and exciting developments in this area of catalysis so far underexploited.

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.orglett.0c01188.

Experimental procedures, characterization data, and T₁, T₂, and diffusion measurements (PDF)

ol0c01188_si_001.pdf (1.38 MB)

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Author Information

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Corresponding Author
- Carmine D’Agostino - Department of Chemical Engineering and Analytical Science, The University of Manchester, The Mill, Sackville Street, Manchester, M13 9PL, U.K.; http://orcid.org/0000-0003-3391-8320; Email: [email protected]
Authors
- Graziano Di Carmine - Department of Chemical Engineering and Analytical Science, The University of Manchester, The Mill, Sackville Street, Manchester, M13 9PL, U.K.; http://orcid.org/0000-0002-2591-9633
- Daniele Ragno - Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara, Via Luigi Borsari, 46, I-44121 Ferrara, Italy; http://orcid.org/0000-0003-0016-290X
- Alessandro Massi - Dipartimento di Scienze Chimiche e Farmaceutiche, Università di Ferrara, Via Luigi Borsari, 46, I-44121 Ferrara, Italy; http://orcid.org/0000-0001-8303-5441
Author Contributions
All authors have given approval to the final version of the manuscript.
Notes
The authors declare no competing financial interest.

Acknowledgments

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We acknowledge the EPSRC, grant no. EP/S019138/1, for funding the research activities carried out for this work. We also acknowledge Dr. Andy York from Johnson Matthey for the fruitful discussions and Gianluca Mazzolani for his help on the graphical work.

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Oxidn. of aldehydes and α-keto acids to carboxylic acids occurs readily in the presence of N-hexadecylthiazolium bromide (I) and 3-methyltetra-O-acetylriboflavin (II). Thus, 4-ClC6H4CHO (III) in the presence of 0.2 equiv I, 1 equiv II and a cationic micelle gave 63% 4-ClC6H4CO2H. In the absence of II, III gave 37% benzoin IV (R = H) and 47% benzil IV (R2 = bond). Kinetic studies are reported, and a mechanism involving heterocycle V (R1 = aryl) as intermediate is proposed.
(b) Tam, S.; Jimenez, L.; Diederich, F. One pot synthesis of aromatic methyl esters by electrochemical oxidation of aldehydes mediated by biscoenzyme catalysis. J. Am. Chem. Soc. 1992, 114, 1503, DOI: 10.1021/ja00030a069
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One pot synthesis of aromatic methyl esters by electrochemical oxidation of aldehydes mediated by biscoenzyme catalysis
Tam, Suk Wah; Jimenez, Leslie; Diederich, Francois
Journal of the American Chemical Society (1992), 114 (4), 1503-5CODEN: JACSAT; ISSN:0002-7863.
Electrochem. oxidn. of aldehydes over flavin and thiazolium as catalysts in MeOH as solvent gave Me carboxylates selectively. The electrochem. flavin/N-benzyl-4-methylthiazolium bromide-catalyzed oxidn. of 4-chlorobenzaldehyde gave 87% Me 4-chlorobenzoate. The electrochem. oxidn. of aliph. aldehydes was less efficient due to formation of hemiacetals.
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Axelsson, A.; Hammarvid, E.; Ta, L.; Sundén, H. Asymmetric aerobic oxidative NHC-catalysed synthesis of dihydropyranones utilising a system of electron transfer mediators. Chem. Commun. 2016, 52, 11571, DOI: 10.1039/C6CC06060A
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Asymmetric aerobic oxidative NHC-catalysed synthesis of dihydropyranones utilising a system of electron transfer mediators
Axelsson, A.; Hammarvid, E.; Ta, L.; Sunden, H.
Chemical Communications (Cambridge, United Kingdom) (2016), 52 (77), 11571-11574CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
The asym. aerobic oxidative synthesis of dihydropyranones I (R = CH2CH2CH3, 2-O2NC6H4, thiophen-2-yl, etc.; R1 = Me, Ph, OMe, OEt, Ot-Bu, OBn) was isolated in high to excellent yields, with high ee (up to 95%). The oxidn. is aided by a system of electron transfer mediators and is selective toward the homoenolate.
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Gruttadauria, M.; Giacalone, F.; Noto, R. Supported proline and proline-derivatives as recyclable organocatalysts. Chem. Soc. Rev. 2008, 37, 1666, DOI: 10.1039/b800704g
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Supported proline and proline-derivatives as recyclable organocatalysts
Gruttadauria, Michelangelo; Giacalone, Francesco; Noto, Renato
Chemical Society Reviews (2008), 37 (8), 1666-1688CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
A review. In the last eight years, l-proline and l-proline derivs., such as substituted prolinamides or pyrrolidines, were successfully used as organocatalysts in several reactions. In this crit. review the authors summarized the immobilization procedures of such organocatalysts highlighting their application, recoverability and reusability (86 refs.).
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Ragno, D.; Brandolese, A.; Urbani, D.; Di Carmine, G.; De Risi, C.; Bortolini, O.; Giovannini, P. P.; Massi, A. Esterification of glycerol and solketal by oxidative NHC-catalysis under heterogeneous batch and flow conditions. React. Chem. Eng. 2018, 3, 816, DOI: 10.1039/C8RE00143J
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Esterification of glycerol and solketal by oxidative NHC-catalysis under heterogeneous batch and flow conditions
Ragno, Daniele; Brandolese, Arianna; Urbani, Daniele; Di Carmine, Graziano; De Risi, Carmela; Bortolini, Olga; Giovannini, Pier Paolo; Massi, Alessandro
Reaction Chemistry & Engineering (2018), 3 (5), 816-825CODEN: RCEEBW; ISSN:2058-9883. (Royal Society of Chemistry)
The design and synthesis of a set of supported azolium salt pre-catalysts is presented along with their utilization in the prodn. of monoesters of glycerol and solketal by oxidative N-heterocyclic carbene (NHC)-catalysis through batch and continuous-flow approaches. After a propaedeutic study with sol. NHCs, the heterogeneous analogs (silica and polystyrene supports) were tested in a model monoesterification of glycerol using either the Kharasch oxidant or air (in the presence of electron transfer mediators) as the terminal oxidants. The best performing polystyrene-supported triazolium salt pre-catalyst afforded monoacylglycerols (MAGs) in high yields (up to 95%) and almost complete selectivity (monoester/diester >95:5) using air and the green solvent Me-THF. The synthesis of fully bio-based MAGs from furfural, 5-hydroxymethylfurfural (HMF), citronellal, and vanillin is also reported. Continuous-flow expts. have been finally performed by fabricating the corresponding packed-bed microreactor, which could be operated for ca. 120 h with maintenance of conversion efficiency and selectivity.
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Brandolese, A.; Ragno, D.; Di Carmine, G.; Bernardi, T.; Bortolini, O.; Giovannini, P. P.; Pandoli, O. G.; Altomare, A.; Massi, A. Aerobic oxidation of 5-hydroxymethylfurfural to 5-hydroxymethyl-2-furancarboxylic acid and its derivatives by heterogeneous NHC-catalysis. Org. Biomol. Chem. 2018, 16, 8955, DOI: 10.1039/C8OB02425A
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Aerobic oxidation of 5-hydroxymethylfurfural to 5-hydroxymethyl-2-furancarboxylic acid and its derivatives by heterogeneous NHC-catalysis
Brandolese, Arianna; Ragno, Daniele; Di Carmine, Graziano; Bernardi, Tatiana; Bortolini, Olga; Giovannini, Pier Paolo; Pandoli, Omar Ginoble; Altomare, Alessandra; Massi, Alessandro
Organic & Biomolecular Chemistry (2018), 16 (46), 8955-8964CODEN: OBCRAK; ISSN:1477-0520. (Royal Society of Chemistry)
The application of the oxidative system composed of a heterogeneous triazolium pre-catalyst, iron(II) phthalocyanine and air is described for the selective conversion of 5-hydroxymethylfurfural (HMF) into the added-value 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). The disclosed one-pot two-step procedure involved sequential oxidative esterifications of HMF to afford a polyester oligomer having hydroxyl and carboxyl terminal groups (Mw = 389-1258), which in turn was hydrolyzed by a supported base (Ambersep 900 OH) to yield HMFCA in 87% overall yield. The same strategy was adopted for the effective synthesis of ester and amide derivs. of HMFCA by nucleophilic depolymn. of the oligomeric intermediate with methanol and butylamine, resp. The utilization of the disclosed oxidative system for the direct conversion of HMF and furfural into their corresponding ester, amide, and thioester derivs. is also reported.
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Selected reviews on heterogeneous organocatalysis:
(a) Atodiresei, I.; Vila, C.; Rueping, M. Asymmetric Organocatalysis in Continuous Flow: Opportunities for Impacting Industrial Catalysis. ACS Catal. 2015, 5, 1972, DOI: 10.1021/acscatal.5b00002
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Asymmetric Organocatalysis in Continuous Flow: Opportunities for Impacting Industrial Catalysis
Atodiresei, Iuliana; Vila, Carlos; Rueping, Magnus
ACS Catalysis (2015), 5 (3), 1972-1985CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
A review. The review highlights the different advantages assocd. with organocatalytic transformations performed in continuous-flow systems and presents the reactions which have been successfully achieved to date. Particular focus is placed on the comparison between batch and flow applications in order to show the advantages and disadvantages and to demonstrate the great potential for applying organocatalysis as well as combined organo and photoredox catalyzed reactions in continuous flow.
(b) Savateev, A.; Antonietti, M. Heterogeneous Organocatalysis for Photoredox Chemistry. ACS Catal. 2018, 8, 9790, DOI: 10.1021/acscatal.8b02595
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Heterogeneous Organocatalysis for Photoredox Chemistry
Savateev, Aleksandr; Antonietti, Markus
ACS Catalysis (2018), 8 (10), 9790-9808CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Photoredox catalysis is a tool enabling a wide variety of chem. reactions with high selectivity under mild conditions of visible light. In this Review, we summarize recent expts. which use heterogeneous, covalent, metal-free semiconductors with adjustable reactivity to drive such reactions. This class started with mesoporous graphitic carbon nitride, then continued with poly(heptazine imides), but is meanwhile extended to other polymers and solid-state orgs. with conjugated electron system. Because of the high thermal and chem. stability, as well as adjustable conduction and valence band positions, the reaction space could be expanded to many org. reactions, such as photocatalytic synthesis of organosulfur compds., or unconventional halogenation and cyanation reactions. Performance of carbon nitrides and homogeneous systems in certain reactions was compared in the present Review.
(c) Corma, A.; Garcia, H. Silica-Bound Homogenous Catalysts as Recoverable and Reusable Catalysts in Organic Synthesis. Adv. Synth. Catal. 2006, 348, 1391, DOI: 10.1002/adsc.200606192
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Silica-bound homogeneous catalysts as recoverable and reusable catalysts in organic synthesis
Corma, Avelino; Garcia, Hermenegildo
Advanced Synthesis & Catalysis (2006), 348 (12+13), 1391-1412CODEN: ASCAF7; ISSN:1615-4150. (Wiley-VCH Verlag GmbH & Co. KGaA)
A review. Very frequently the most costly components in a chem. reaction are not the starting materials or the reaction products, but the catalyst. In addn. to the advantages from the economic point of view, recovery and reuse of the catalyst is equally important in order to avoid wastes, so improving the greenness of the process. There is a current tendency to transform homogeneous into heterogeneous catalysis, that will even be accelerated in the near future. Starting from a successful homogeneous catalyst, one general methodol. allowing its recovery and reuse is to immobilize a suitable deriv. of the active catalyst on an insol. solid support. When the catalyst does not deactivate and is sufficiently stable under the reaction conditions, the ultimate immobilization methodol. is to attach covalently the catalytically active species to the support. In this contribution, after introducing some general principles describing the fundamentals of the covalent anchoring, the emphasis is placed more on giving an overview of the most important types of covalently anchored catalysts, including Bronsted and Lewis acids, covalently anchored bases and hydrogenation complexes. Hot topics such as silica-bound organocatalysts and the application of periodic mesoporous organosilicas as heterogeneous catalysts is also covered.
(d) Bartók, M. Advances in Immobilized Organocatalysts for the Heterogeneous Asymmetric Direct Aldol Reactions. Catal. Rev.: Sci. Eng. 2015, 57, 192, DOI: 10.1080/01614940.2015.1039432
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Advances in Immobilized Organocatalysts for the Heterogeneous Asymmetric Direct Aldol Reactions
Bartok, Mihaly
Catalysis Reviews: Science and Engineering (2015), 57 (2), 192-255CODEN: CRSEC9; ISSN:0161-4940. (Taylor & Francis, Inc.)
A review. The results published from the beginning (2000) on asym. direct aldol reactions taking place in the presence of immobilized chiral organocatalysts was summarized. The seven groups of organocatalysts discussed were: asym. direct aldol reactions catalyzed by: (1) covalently bonded immobilized hydroxyproline organocatalysts; (2) covalently bonded immobilized prolinamide organocatalysts; (3) covalently bonded immobilized peptide organocatalysts; (4) other covalently bonded immobilized chiral organocatalysts; (5) chiral organocatalysts bonded by ionic groups; (6) chiral organocatalysts with adsorptive bonding; and (7) other types of immobilized chiral organocatalysts. The main objective of this article based on results obtained by using about 360 immobilized organocatalysts, was to focus on catalyst systems in which the aldol reactions yielded beta-hydroxyketones of maximal optical purity, while keeping catalyst concns. and reaction times as low and as short as possible. Trends recognized in data obtained in this field of investigation may mark out further tasks for the purpose of practical application.
Rostamnia, S.; Doustkhah, E. Nanoporous silica-supported organocatalyst: a heterogeneous and green hybrid catalyst for organic transformations. RSC Adv. 2014, 4, 28238, DOI: 10.1039/C4RA03773A
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Nanoporous silica-supported organocatalyst: a heterogeneous and green hybrid catalyst for organic transformations
Rostamnia, Sadegh; Doustkhah, Esmail
RSC Advances (2014), 4 (54), 28238-28248CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
A review. Organically modified hybrid silica mesostructures are becoming increasingly promising candidates as catalysts for various kinds of org. reactions and green chem. Org. moieties with an amine base and sulfonic acid have been extensively incorporated into these mesostructures. In this review, the authors discuss recent advances of the most common types of organically modified hybrid materials that act as organocatalysts in org. transformations and processes.
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(a) Weber, D.; Mitchell, J.; McGregor, J.; Gladden, L. F. Comparing Strengths of Surface Interactions for Reactants and Solvents in Porous Catalysts Using Two-Dimensional NMR Relaxation Correlations. J. Phys. Chem. C 2009, 113, 6610, DOI: 10.1021/jp811246j
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Comparing Strengths of Surface Interactions for Reactants and Solvents in Porous Catalysts Using Two-Dimensional NMR Relaxation Correlations
Weber, Daniel; Mitchell, Jonathan; McGregor, James; Gladden, Lynn F.
Journal of Physical Chemistry C (2009), 113 (16), 6610-6615CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
Two-dimensional NMR relaxation time correlation measurements have been used to observe the behavior of liqs. inside porous catalyst pellets; in particular, liqs. of relevance to the hydrogenation of 2-butanone over a silica-supported ruthenium catalyst (Ru/SiO2). The behavior of 2-butanone is studied and compared to that of water and 2-propanol, which are used as solvents in this hydrogenation reaction. From the ratio of NMR relaxation times, T1/T2, for the liqs. confined in the pores, it is possible to infer the relative strengths of the surface interaction for each liq. Water is seen to have the strongest surface interaction, and 2-butanone has the weakest surface interaction. These results are supported by displacement expts., in which one liq. replaces the other over time within the pore space of the catalyst. For comparison, the behavior of the same liqs. in an alumina-supported palladium catalyst (Pd/Al2O3) was also studied. The variation in the strengths of surface interactions was more pronounced in the Pd/Al2O3 catalyst than in the Ru/SiO2 catalyst. This work demonstrates the applicability of NMR relaxation time correlation expts. to real catalytic systems contg. metallic components. From these measurements, information on the access of reactants to surface adsorption sites can be inferred.
(b) Krzyżak, A. T.; Habina, I. Low field ¹H NMR characterization of mesoporous silica MCM-41 and SBA-15 filled with different amount of water. Microporous Mesoporous Mater. 2016, 231, 230, DOI: 10.1016/j.micromeso.2016.05.032
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Low field 1H NMR characterization of mesoporous silica MCM-41 and SBA-15 filled with different amount of water
Krzyzak, A. T.; Habina, I.
Microporous and Mesoporous Materials (2016), 231 (), 230-239CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
The low field 1H NMR characterization of pure mesoporous SiO2 materials - MCM-41 and SBA-15 as a function of H2O content is presented. The values of surface relaxivities for both materials were estd. to compare pore size distributions obtained from relaxation spectra and N2 adsorption measurements. This comparison led to differentiation between contributions from inner-bulk H2O mols. in pores and these from the surface. The T1/T2 ratio reflecting the strength of interaction between H2O mols. and the mesoporous SiO2 surface is higher for SBA-15. Measurements for a mesopore surface partially covered by H2O provided quant. information on the hydroxylation rate.
(c) Aksnes, D. W.; Førland, K.; Kimtys, L. ¹H and ²H NMR studies of cyclohexane nanocrystals in controlled pore glasses. J. Mol. Struct. 2004, 708, 23, DOI: 10.1016/j.molstruc.2004.02.055
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1H and 2H NMR studies of cyclohexane nanocrystals in controlled pore glasses
Aksnes, D. W.; Forland, K.; Kimtys, L.
Journal of Molecular Structure (2004), 708 (1-3), 23-31CODEN: JMOSB4; ISSN:0022-2860. (Elsevier B.V.)
The formation and behavior of cyclohexane and cyclohexane-d12 nanocrystals in mesoporous solids of well-defined dimensional constraints are studied by 1H and 2H NMR. The NMR line widths, spin-spin relaxation times (T2), spin-lattice relaxation times (T1) and diffusivities (D) were measured as a function of temp., and the results are discussed with ref. to the values obtained for the bulk materials. The confined solids exhibit substantial changes in the phase behavior and mol. dynamics. Thus, the line-shape measurements reveal a two-phase system consisting of a highly mobile component at the surface of the pore and a plastically cryst. phase in the center of the pore. The liq.-like surface layer in the mesopores is observable well below the reduced transition temp. of the confined cyclohexane. However, the T2 and diffusion measurements show that the mobile phase also embraces a minor component attributed to non-frozen liq. in pockets or offshoots.
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(a) D’Agostino, C.; Mitchell, J.; Mantle, M. D.; Gladden, L. F. Interpretation of NMR Relaxation as a Tool for Characterising the Adsorption Strength of Liquids inside Porous Materials. Chem. - Eur. J. 2014, 20, 13009, DOI: 10.1002/chem.201403139
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13a
Interpretation of NMR Relaxation as a Tool for Characterising the Adsorption Strength of Liquids inside Porous Materials
D'Agostino, Carmine; Mitchell, Jonathan; Mantle, Michael D.; Gladden, Lynn F.
Chemistry - A European Journal (2014), 20 (40), 13009-13015CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
NMR relaxation times are shown to provide a unique probe of adsorbate-adsorbent interactions in liq.-satd. porous materials. A short theor. anal. is presented, which shows that the ratio of the longitudinal to transverse relaxation times (T1/T2) is related to an adsorbate-adsorbent interaction energy, and the authors introduce a quant. metric esurf (based on the relaxation time ratio) characterizing the strength of this surface interaction. The authors then consider the interaction of water with a range of oxide surfaces (TiO2 anatase, TiO2 rutile, γ-Al2O3, SiO2, θ-Al2O3, and ZrO2) and show that esurf correlates with the strongest adsorption sites present, as detd. by temp. programmed desorption (TPD). Thus, NMR relaxation measurements have a direct phys. interpretation in terms of the characterization of activation energy of desorption from the surface. Further, for a series of chem. similar solid materials, in this case a range of oxide materials, for which at least two calibration values are obtainable by TPD, the esurf parameter yields a direct est. of the max. activation energy of desorption from the surface. The results suggest that T1/T2 measurements may become a useful addn. to the methods available to characterize liq.-phase adsorption in porous materials. The particular motivation for this work was to characterize adsorbate-surface interactions in liq.-phase catalysis.
(b) Habina, I.; Radzik, N.; Topór, T.; Krzyžak, A. T. Insight into oil and gas-shales compounds signatures in low field ¹H NMR and its application in porosity evaluation. Microporous Mesoporous Mater. 2017, 252, 37, DOI: 10.1016/j.micromeso.2017.05.054
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Insight into oil and gas-shales compounds signatures in low field 1H NMR and its application in porosity evaluation
Habina, I.; Radzik, N.; Topor, T.; Krzyzak, A. T.
Microporous and Mesoporous Materials (2017), 252 (), 37-49CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
The low field 1H NMR characterization of rock components that are specific for oil- and gas- shales is presented. 1D T2 distributions together with T1-T2 maps and T1/T2 ratios for the samples of smectite, illite, illite-smectite, kaolinite, chlorite and kerogen served to obtain characteristic features. T1-T2 maps measured in three satn. states (native, satd. and dried) allowed the observation of different fractions of water such as water confined in interlayer spaces, filling pore spaces and hydroxyl groups from crystallog. lattice. Differential images are proposed for the evaluation of water distribution in the pore spaces and values of porosity, directly related to the fluid removed after the drying of the sample. Porosity values for exemplary shale samples, calcd. for differential results are consistent with the porosity obtained using T2 cut-offs. The relations between cation exchange capacity and T2 log-mean values for the samples at different satn. states are also shown.
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D’Agostino, C.; Brett, G.; Divitini, G.; Ducati, C.; Hutchings, G. J.; Mantle, M. D.; Gladden, L. F. Increased Affinity of Small Gold Particles for Glycerol Oxidation over Au/TiO₂ Probed by NMR Relaxation Methods. ACS Catal. 2017, 7, 4235, DOI: 10.1021/acscatal.7b01255
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Increased Affinity of Small Gold Particles for Glycerol Oxidation over Au/TiO2 Probed by NMR Relaxation Methods
D'Agostino, Carmine; Brett, Gemma; Divitini, Giorgio; Ducati, Caterina; Hutchings, Graham J.; Mantle, Michael D.; Gladden, Lynn F.
ACS Catalysis (2017), 7 (7), 4235-4241CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
The aerobic oxidn. of glycerol in aq. soln. over Au/TiO2 catalysts has been studied, and the effect of Au loading by wet impregnation, in the range 0.5-5% Au, has been assessed. Low metal loading favors the deposition of smaller particles, whereas higher loadings lead to the formation of much larger gold particles, as revealed by scanning transmission electron microscopy (STEM) anal. Reaction studies show that a higher metal loading has a detrimental effect on the catalyst activity, which decreases significantly as the Au load increases. In addn. to reaction studies, 1H NMR T1/T2 relaxation time measurements have been used to assess the effect of metal loading and particle size on the adsorption properties of glycerol (reactant) and water (solvent) within the catalyst. The NMR results show that the adsorption properties of glycerol relative to water as a function of the Au loading have a similar trend to that obsd. for the reactivity, with glycerol exhibiting a higher surface affinity relative to water for the catalyst with low Au loading. The overall results indicate that metal loading significantly affects the typical Au particle size, which, in turn, affects both the reaction and adsorption properties of glycerol over the catalyst surface. In particular, the trend in T1/T2 ratio clearly indicates that glycerol has a much stronger affinity with smaller gold particles, which is an important factor in promoting glycerol oxidn. This result is of great significance in understanding the reactivity of polyols over supported gold catalysts and gives the first exptl. evidence that smaller gold particles tend to be stronger adsorption sites for glycerol, in agreement with computational and theor. studies.
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D’Agostino, C.; Armstrong, R. D.; Hutchings, G. J.; Gladden, L. F. Product Inhibition in Glycerol Oxidation over Au/TiO₂ Catalysts Quantified by NMR Relaxation. ACS Catal. 2018, 8, 7334, DOI: 10.1021/acscatal.8b01516
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Product Inhibition in Glycerol Oxidation over Au/TiO2 Catalysts Quantified by NMR Relaxation
D'Agostino, Carmine; Armstrong, Robert D.; Hutchings, Graham J.; Gladden, Lynn F.
ACS Catalysis (2018), 8 (8), 7334-7339CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
Liq.-phase catalytic oxidn. of glycerol in aq. solns. using porous solid catalysts represents a viable strategy for the sustainable prodn. of fine chems. from renewable resources. Various aspects of this novel type of reactions are still under investigation. Catalyst deactivation is one of those issues that need to be understood and addressed in order to make these processes com. viable. In a previous study it has been reported that the catalytic activity of Au/TiO2 catalysts for the oxidn. of glycerol with O2 under basic conditions can be severely inhibited by some reaction intermediates or products. It was suggested that the presence of certain species blocks the active sites of the catalyst, preventing the adsorption of glycerol, which in turn results in a decrease of reaction rate. In this work, we used NMR relaxation time measurements in order to assess surface interactions of glycerol in Au/TiO2 catalyst pre-treated with aq. solns. of various oxygenates, including intermediates and products of glycerol oxidn., under basic conditions, in particular evaluating changes in glycerol adsorption properties. The NMR T1/T2 ratio of glycerol, which is indicative of the strength of interaction of glycerol with the catalyst surface, traces out well the trend in catalytic activity in the presence of different additives, suggesting that adsorption of glycerol onto the catalyst surface play a crucial role in the reaction, which supports the hypothesis previously made in the literature. This exptl. approach and the related results represent a significant advance in the understanding of liq.-phase catalytic reactions occurring over solid surfaces, which can be used to understand and optimize catalytic processes and the effect of intermediate and product inhibition.
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(a) D’Agostino, C.; Kotionova, T.; Mitchell, J.; Miedziak, P. J.; Knight, D. W.; Taylor, S. H.; Hutchings, G. J.; Gladden, L. F.; Mantle, M. D. Solvent Effect and Reactivity Trend in the Aerobic Oxidation of 1,3-Propanediols over Gold Supported on Titania: NMR Diffusion and Relaxation Studies. Chem. - Eur. J. 2013, 19, 11725, DOI: 10.1002/chem.201300502
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Solvent Effect and Reactivity Trend in the Aerobic Oxidation of 1,3-Propanediols over Gold Supported on Titania: NMR Diffusion and Relaxation Studies
D'Agostino, Carmine; Kotionova, Tatyana; Mitchell, Jonathan; Miedziak, Peter J.; Knight, David W.; Taylor, Stuart H.; Hutchings, Graham J.; Gladden, Lynn F.; Mantle, Mick D.
Chemistry - A European Journal (2013), 19 (35), 11725-11732CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
In recent work, it was reported that changes in solvent compn., precisely the addn. of water, significantly inhibits the catalytic activity of Au/TiO2 catalyst in the aerobic oxidn. of 1,4-butanediol in methanol due to changes in diffusion and adsorption properties of the reactant. In order to understand whether the inhibition mechanism of water on diol oxidn. in methanol is generally valid, the solvent effect on the aerobic catalytic oxidn. of 1,3-propanediol and its two methyl-substituted homologues, 2-methyl-1,3-propanediol and 2,2-dimethyl-1,3-propanediol, over a Au/TiO2 catalyst has been studied here using conventional catalytic reaction monitoring in combination with pulsed-field gradient NMR (PFG-NMR) diffusion and NMR relaxation time measurements. Diol conversion is significantly lower when water is present in the initial diol/methanol mixt. A reactivity trend within the group of diols was also obsd. Combined NMR diffusion and relaxation time measurements suggest that mol. diffusion and, in particular, the relative strength of diol adsorption, are important factors in detg. the conversion. These results highlight NMR diffusion and relaxation techniques as novel, non-invasive characterization tools for catalytic materials, which complement conventional reaction data.
(b) D’Agostino, C.; Brett, G. L.; Miedziak, P. J.; Knight, D. W.; Hutchings, G. J.; Gladden, L. F.; Mantle, M. D. Understanding the Solvent Effect on the Catalytic Oxidation of 1,4-Butanediol in Methanol over Au/TiO₂ Catalyst: NMR Diffusion and Relaxation Studies. Chem. - Eur. J. 2012, 18, 14426, DOI: 10.1002/chem.201201922
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Understanding the Solvent Effect on the Catalytic Oxidation of 1,4-Butanediol in Methanol over Au/TiO2 Catalyst: NMR Diffusion and Relaxation Studies
D'Agostino, Carmine; Brett, Gemma L.; Miedziak, Peter J.; Knight, David W.; Hutchings, Graham J.; Gladden, Lynn F.; Mantle, Mick D.
Chemistry - A European Journal (2012), 18 (45), 14426-14433, S14426/1-S14426/3CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
The effect of water on the catalytic oxidn. of 1,4-butanediol in methanol over Au/TiO2 was studied by catalytic reaction studies and NMR diffusion and relaxation studies. The addn. of water to the dry catalytic system decreased both conversion and selectivity towards di-Me succinate. Pulsed-field gradient (PFG)-NMR spectroscopy was used to assess the effect of water addn. on the effective self-diffusivity of the reactant within the catalyst. NMR relaxation studies were also carried out to probe the strength of surface interaction of the reactant in the absence and presence of water. PFG-NMR studies revealed that the addn. of water to the initial system, although increasing the diln. of the system, leads to a significant decrease of effective diffusion rate of the reactant within the catalyst. From T1 and T2 relaxation measurements it was possible to infer the strength of surface interaction of the reactant with the catalyst surface. The addn. of water was found to inhibit the adsorption of the reactant over the catalyst surface, with the T1/T2 ratio of 1,4-butanediol decreasing significantly when water was added. The results overall suggest that both the decrease of diffusion rate and adsorption strength of the reactant within the catalyst, due to water addn., limits the access of reactant mols. to the catalytic sites, which results in a decrease of reaction rate and conversion.
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(a) Scalambra, F.; Holzmann, N.; Bernasconi, L.; Imberti, S.; Romerosa, A. Water Participation in Catalysis: An Atomistic Approach to Solvent Effects in the Catalytic Isomerization of Allylic Alcohols. ACS Catal. 2018, 8, 3812, DOI: 10.1021/acscatal.8b00199
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Water Participation in Catalysis: An Atomistic Approach to Solvent Effects in the Catalytic Isomerization of Allylic Alcohols
Scalambra, Franco; Holzmann, Nicole; Bernasconi, Leonardo; Imberti, Silvia; Romerosa, Antonio
ACS Catalysis (2018), 8 (5), 3812-3819CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
We study the intermediate of the reaction between a ruthenium complex and 1-propen-3-ol in water using an atomistic approach for gaining information about the conformation and dynamics of complex mols. in aq. soln., which combines d.-functional-theory-based ab initio mol. dynamics and neutron scattering data based on empirical potential structure refinement simulations. We apply our method to the study of the water-sol. η2-allylic complex [RuCp(exo-η2-CH2=CH-CH2-OH)(PTA)2]+ (2) (PTA = 1,3,5-triaza-7-phosphaadamantane), an important intermediate in the isomerization of 1-propen-3-ol into propanal catalyzed by {RuCp(H2O-κO)(PTA)2}+. We identify the factors responsible for the stabilization of a specific conformer of 2 in water soln. and we examine the involvement of water mols. in the formation of this species. In particular, we show that long-lived (ca. 10 ps) bonded chains of water mols. play a crucial role in influencing the conformation and, potentially, the chem. reactivity of 2.
(b) Satpathy, L.; Sahu, P. K.; Behera, P. K.; Mishra, B. K. Solvent Effect on the Potential Energy Surfaces of the F^– + CH₃CH₂Br Reaction. J. Phys. Chem. A 2018, 122, 5861, DOI: 10.1021/acs.jpca.8b02687
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Solvent Effect on the Potential Energy Surfaces of the F- + CH3CH2Br Reaction
Satpathy, Lopamudra; Sahu, Prabhat K.; Behera, Pradipta K.; Mishra, Bijay K.
Journal of Physical Chemistry A (2018), 122 (27), 5861-5869CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
Although substantial work has been undertaken on reaction pathways involved in base-promoted elimination reactions and bimol. nucleophilic substitution reaction of F- on CH3CH2X (X = Cl, Br, I), the effect of solvents with varying dielec. consts. on the stereochem. of each of the reaction species involved across the reaction profile have not yet been clearly understood. The present investigation reports the effect of solvents on the potential energy surfaces (PES) and structures of the species appearing in the reaction pathway of F- with bromoethane. The PESs in the gas phase have been computed at MP2 level and CCSD(T) level. The performance of several hybrid d. functional, such as B3LYP, M06, M06L, BHandH, X3LYP, M05, M05-2X, and M06-2X have also been investigated toward describing the elimination and nucleophilic substitution reactions. With respect to MAE values and to make the computation cost-effective, we have explored the implicit continuum solvent model, CPCM in solvents like cyclohexane, methanol, acetonitrile, DMSO and water. The reactant complexes proceed through the subsequent steps to produce fluoroethane as the substitution product and ethylene as one of the elimination products. For elimination reaction both syn and anti elimination have been explored. The calcd. relatives energies values, which are neg. in the gas phase, are found to be pos. in polar solvents since the point charge in the sepd. reactants are more stabilized than the dispersed charge in the transient complex, which has also been analyzed through NBO anal.
(c) Dyson, P. J.; Jessop, P. G. Solvent effects in catalysis: rational improvements of catalysts via manipulation of solvent interactions. Catal. Sci. Technol. 2016, 6, 3302, DOI: 10.1039/C5CY02197A
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17c
Solvent effects in catalysis: rational improvements of catalysts via manipulation of solvent interactions
Dyson, Paul J.; Jessop, Philip G.
Catalysis Science & Technology (2016), 6 (10), 3302-3316CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
A review; in homogeneous catalysis, the main emphasis on improving catalyst performance (rate, yield and selectivity) is directed towards manipulation of the ligands. The steric and electronic effects of ligands are extremely well understood and the rational design of new ligands that leads to improved catalysts is feasible. Similarly, in heterogeneous catalysis structural changes to the catalyst are used to improve their properties. In contrast, the role that solvents play in catalytic processes is often given cursory attention. The environmental impact of solvents is often considered, as it should be, and the use of org.-immiscible solvents, e.g. water, ionic liqs., etc., is frequently considered with respect to catalyst recovery, product isolation, and recycling. Nevertheless, the direct role of solvents in reactions is often overlooked, although solvents interact directly with the catalyst, the substrates and products, and all these interactions can increase or decrease reaction rate and/or selectivity. Herein we consider the role of solvents in catalysis and illustrate the crit. role of solvents viewed from a mechanistic approach. In particular, we focus largely, but not exclusively, on hydrogenation reactions and cross-coupling reactions as the main types of solvents used for these two classes of reactions tend to be very different and illustrate different functions of solvents in catalysis.
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(a) Bertero, N. M.; Trasarti, a. F.; Apesteguía, C. R.; Marchi, A. J. Solvent effect in the liquid-phase hydrogenation of acetophenone over Ni/SiO₂: A comprehensive study of the phenomenon. Appl. Catal., A 2011, 394, 228, DOI: 10.1016/j.apcata.2011.01.003
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18a
Solvent effect in the liquid-phase hydrogenation of acetophenone over Ni/SiO2: A comprehensive study of the phenomenon
Bertero, Nicolas M.; Trasarti, Andres F.; Apesteguia, Carlos R.; Marchi, Alberto J.
Applied Catalysis, A: General (2011), 394 (1-2), 228-238CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
The solvent effect on catalyst activity and selectivity for the liq.-phase hydrogenation of acetophenone (AP) to 1-phenylethanol was thoroughly investigated over Ni/SiO2. Solvents of different nature were used: protic (C1-C3 primary and secondary alcs.), aprotic polar (THF, γ-butyrolactone, and acetonitrile) and apolar solvents (cyclohexane, toluene, and benzene). The solvent had a strong influence on the AP hydrogenation rate but did not modify significantly the selectivity to 1-phenylethanol that was always higher than 92%. The AP hydrogenation activity followed the order: C2-C3 alcs. > cyclohexane > toluene > THF > γ-butyrolactone > methanol » benzene ≃ acetonitrile. In order to explain this activity pattern, the solvent-AP, solvent-H2 and solvent-catalyst interactions were analyzed. For the anal. of the solvent-AP interactions in liq. phase, both classical measures of polarity and others based on different solvatochromic scales were considered. The H2 availability in the liq. phase was estd. from the H2 soly. at reaction conditions. Solvent-catalyst interactions were characterized by means of the adsorption enthalpies measured calorimetrically. A reasonable correlation between the catalyst activity and some solvatochromic parameters was found only when solvents of similar nature were compared. For protic solvents, the AP hydrogenation rate decreased with the solvent polarity and its ability for H-bond formation with AP. Instead, the solvent-AP interactions were weak when using apolar solvents and thereby the activity pattern was essentially detd. by the strength of solvent-catalyst interactions. In the case of aprotic polar solvents, both the solvent-AP interactions in the liq. phase and the solvent adsorption strength on the catalyst surface influenced the hydrogenation activity. The highest catalytic activities were obtained when using C2-C3 alc. solvents. These protic solvents adsorbed dissociatively on metal nickel surface increasing the no. of active H available for the hydrogenation reaction; this effect was much more important in the case of 2-propanol.
(b) Wang, S.; Ge, B.; Yin, Y.; Wu, X.; Zhu, H.; Yue, Y.; Bai, Z.; Bao, X.; Yuan, P. Solvent effect in heterogeneous catalytic selective hydrogenation of nitrile butadiene rubber: relationship between reaction activity and solvents with density functional theory analysis. ChemCatChem 2020, 12, 663, DOI: 10.1002/cctc.201901555
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18b
Solvent Effect in Heterogeneous Catalytic Selective Hydrogenation of Nitrile Butadiene Rubber: Relationship between Reaction Activity and Solvents with Density Functional Theory Analysis
Wang, Shuhan; Ge, Bingqing; Yin, Yixuan; Wu, Xinru; Zhu, Haibo; Yue, Yuanyuan; Bai, Zhengshuai; Bao, Xiaojun; Yuan, Pei
ChemCatChem (2020), 12 (2), 663-672CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
Herein, a systematical investigation is devoted to explore nitrile butadiene rubber (NBR) hydrogenation activity and selectivity in different org. solvents, and the dependence between reactivity and solvent is revealed with the assistance of the d. functional theory (DFT). The results show that the electron-pair donor solvents have better performance in NBR hydrogenation activity than the weak electron-pair acceptor solvents, but they do not modify the selectivity to HNBR that always keeps up to 100%. A direct correlation between reactivity and hydrogen bond acceptance ability (β) is discovered in both solvent families and the higher β, the better hydrogenation performance. The preferred solvent is cyclohexanone with hydrogenation degree of 91.4% in 10 min and the highest conversion of 94.2%. Our contribution is to provide useful information about solvent and solvent effect on the hydrogenation of NBR and gain new insight into NBR hydrogenation reaction process.
(c) Li, Y.; Cheng, H.; Lin, W.; Zhang, C.; Wu, Q.; Zhao, F.; Arai, M. Solvent effects on heterogeneous catalysis in the selective hydrogenation of cinnamaldehyde over a conventional Pd/C catalyst. Catal. Sci. Technol. 2018, 8, 3580, DOI: 10.1039/C8CY00943K
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18c
Solvent effects on heterogeneous catalysis in the selective hydrogenation of cinnamaldehyde over a conventional Pd/C catalyst
Li, Yan; Cheng, Haiyang; Lin, Weiwei; Zhang, Chao; Wu, Qifan; Zhao, Fengyu; Arai, Masahiko
Catalysis Science & Technology (2018), 8 (14), 3580-3589CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
Solvent effects in the selective hydrogenation of an α,β-unsatd. aldehyde of cinnamaldehyde (CAL) were investigated under gas-liq.-solid reaction conditions. A conventional 5 wt% Pd/C catalyst and twelve org. solvents were used, including apolar solvents (three) and protic (two) and aprotic (seven) polar ones. The total rate of CAL hydrogenation strongly depended on the solvents used. The relationship between the rate of CAL hydrogenation obsd. and a few different solvent parameters was examd. The main product was hydrocinnamaldehyde (HCAL) with a selectivity of 80% or higher in most of the solvents examd. except for pyridine and 4-methylpyridine. In these two solvents, the CAL hydrogenation was slow and the main product was changed to cinnamyl alc. (COL) with a selectivity of about 60%. The addn. of a small vol. of pyridine to other solvents such as 2-propanol and THF could change the rate of hydrogenation and switch the main product from HCAL to COL. Possible interactions among the solvents, CAL substrate, and Pd/C catalyst were examd. by FTIR (attenuated total reflection (ATR-) and diffuse reflectance modes (DR-)) and TPD to discuss the solvent effects obsd. in the Pd-catalyzed heterogeneous CAL hydrogenation.
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Aksnes, D. W.; Gjerdåker, L. NMR line width, relaxation and diffusion studies of cyclohexane confined in porous silica. J. Mol. Struct. 1999, 475, 27, DOI: 10.1016/S0022-2860(98)00493-1
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NMR line width, relaxation and diffusion studies of cyclohexane confined in porous silica
Aksnes, D. W.; Gjerdaker, L.
Journal of Molecular Structure (1999), 475 (1), 27-34CODEN: JMOSB4; ISSN:0022-2860. (Elsevier Science B.V.)
The rotational and translational dynamics of cyclohexane confined within silica pores of 4-50 nm in diam., have been studied as a function of temp. by measuring 1H NMR line widths, self-diffusion coeffs., spin-spin relaxation times, and spin-lattice relaxation times, and the results are compared with values obtained for the bulk material. The confinement in the pores gives rise to substantial changes in the mol. dynamics and phase behavior. The line shape and T2 measurements revealed a two-phase system, consisting of a liq.-like component at the surface and a plastic phase in the center of the pore. The highly mobile surface layer is observable far below the reduced transition temp. of the confined cyclohexane. A high diffusion rate of the adsorbed cyclohexane is obsd. over a wide temp. range - even well below the region of the depressed f.p. The diffusion coeff. of the mols. in the surface layer is three orders of magnitude larger than in the plastic phase of bulk cyclohexane.
22
Stapf, S.; Kimmich, R.; Zavada, T. Dynamics of plastic and liquid cyclohexane in bulk and in porous glasses studied by NMR methods. Appl. Magn. Reson. 1997, 12, 199, DOI: 10.1007/BF03162187
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Dynamics of plastic and liquid cyclohexane in bulk and in porous glasses studied by NMR methods
Stapf, Siegfried; Kimmich, R.; Zavada, T.
Applied Magnetic Resonance (1997), 12 (2-3), 199-212CODEN: APMREI; ISSN:0937-9347. (Springer)
Cyclohexane was investigated in bulk and in porous glasses with pore diams. of 4-208 nm at 136 ≤ T ≤ 300 K with field-cycling NMR relaxometry, field-gradient NMR diffusometry, transverse-relaxation spectroscopy, and DSC. The field-cycling data for the bulk material are described assuming translational modulation of intermol. dipole-dipole coupling. This is confirmed by expts. with different degrees of deuteration, and is in accordance with diffusion coeffs. detd. with the aid of field-gradient diffusometry. The confinement in pores produces substantial changes in the phase behavior and in mol. dynamics. For pore diams. of 30 nm and above, a non-frozen 2 monolayer thick film on the surface retains a diffusivity about 1 order of magnitude lower than in bulk liq., but 2 orders of magnitude larger than in the bulk plastic phase. Expts. indicate an exchange mechanism between this layer and the crystallite inside the pore. In glass with a pore diam. of 4 nm, all applied methods corroborate DSC results of the virtual absence of a phase transition and reveal a continuously decreasing translational mobility down to 100 K below the bulk liq./cubic phase transition temp.
23
(a) Deelchand, D. K.; Van de Moortele, P.-F.; Adriany, G.; Iltis, I.; Andersen, P.; Strupp, J. P.; Vaughan, J. T.; Uğurbil, K.; Henry, P.-G. In vivo ¹H NMR spectroscopy of the human brain at 9.4 T: Initial results. J. Magn. Reson. 2010, 206, 74, DOI: 10.1016/j.jmr.2010.06.006
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23a
In vivo 1H NMR spectroscopy of the human brain at 9.4 T: Initial results
Deelchand, Dinesh Kumar; Van de Moortele, Pierre-Francois; Adriany, Gregor; Iltis, Isabelle; Andersen, Peter; Strupp, John P.; Thomas Vaughan, J.; Ugurbil, Kamil; Henry, Pierre-Gilles
Journal of Magnetic Resonance (2010), 206 (1), 74-80CODEN: JMARF3; ISSN:1090-7807. (Elsevier B.V.)
In vivo proton NMR spectroscopy allows non-invasive detection and quantification of a wide range of biochem. compds. in the brain. Higher field strength is generally considered advantageous for spectroscopy due to increased signal-to-noise and increased spectral dispersion. So far 1H NMR spectra have been reported in the human brain up to 7 T. In this study we show that excellent quality short echo time STEAM and LASER 1H NMR spectra can be measured in the human brain at 9.4 T. The information content of the human brain spectra appears very similar to that measured in the past decade in rodent brains at the same field strength, in spite of broader linewidth in human brain. Compared to lower fields, the T1 relaxation times of metabolites were slightly longer while T2 relaxation values of metabolites were shorter (<100 ms) at 9.4 T. The linewidth of the total creatine (tCr) resonance at 3.03 ppm increased linearly with magnetic field (1.35 Hz/T from 1.5 T to 9.4 T), with a min. achievable tCr linewidth of around 12.5 Hz at 9.4 T. At very high field, B 0 microsusceptibility effects are the main contributor to the min. achievable linewidth.
(b) Deelchand, D. K.; Henry, P.-G.; Uğurbil, K.; Marjanska, M. Measurement of Transverse Relaxation Times of J-Coupled Metabolites in the Human Visual Cortex at 4 T. Magn. Reson. Med. 2012, 67, 891, DOI: 10.1002/mrm.23080
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23b
Measurement of transverse relaxation times of J-coupled metabolites in the human visual cortex at 4 T
Deelchand, Dinesh Kumar; Henry, Pierre-Gilles; Ugurbil, Kamil; Marjanska, Malgorzata
Magnetic Resonance in Medicine (2012), 67 (4), 891-897CODEN: MRMEEN; ISSN:0740-3194. (Wiley-Blackwell)
Accurate quantification of 1H NMR spectra often requires knowledge of the relaxation times to correct for signal losses due to relaxation and satn. In human brain, T2 values for singlets such as N-acetylaspartate, creatine, and choline have been reported, but few T2 values are available for J-coupled spin systems. The purpose of this study was to measure the T2 relaxation times of J-coupled metabolites in the human occipital lobe using the LASER sequence. Spectra were acquired at multiple echo times and were analyzed with an LCModel using basis sets simulated at each echo time. Sep. basis spectra were used for resonances of protons belonging to the same mol. but having very different T2 values (e.g., two sep. basis spectra were used for the singlet and multiplet signal in N-acetylaspartate). The T2 values for the N-acetylaspartate multiplet (149 ± 12 ms), glutamate (125 ± 10 ms), myo-inositol (139 ± 20 ms), and taurine (196 ± 28 ms) were successfully measured in the human visual cortex at 4 T. These measured T2 relaxation times have enabled the accurate and abs. quantification of cerebral metabolites at longer echo times.
24
Fogler, H. S. Elements of Chemical Reaction Engineering; Prentice Hall: New Jersey, USA, 2005.
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D’Agostino, C.; Feaviour, M. R.; Brett, G. L.; Mitchell, J.; York, A. P. E.; Hutchings, G. J.; Mantle, M. D.; Gladden, L. D. Solvent inhibition in the liquid-phase catalytic oxidation of 1,4-butanediol: understanding the catalyst behaviour from NMR relaxation time measurements. Catal. Sci. Technol. 2016, 6, 7896, DOI: 10.1039/C6CY01458E
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25
Solvent inhibition in the liquid-phase catalytic oxidation of 1,4-butanediol: understanding the catalyst behaviour from NMR relaxation time measurements
D'Agostino, Carmine; Feaviour, Mark R.; Brett, Gemma L.; Mitchell, Jonathan; York, Andrew P. E.; Hutchings, Graham J.; Mantle, Mick D.; Gladden, Lynn F.
Catalysis Science & Technology (2016), 6 (21), 7896-7901CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
Catalytic reaction studies and NMR (NMR) relaxation time measurements have been compared to study the influence of competitive adsorption of reactant and solvent on catalytic conversion. The reaction chosen is the aerobic catalytic oxidn. of 1,4-butanediol in methanol over different supported-metal catalysts. From the NMR T1/T2 ratio, where T1 is the longitudinal and T2 the transverse spin relaxation time, the relative affinity of reactant and solvent for different catalytic surfaces is detd. The catalysts with the lowest activity show a preferential surface affinity for the solvent compared to the reactant. Conversely, the catalyst with the highest activity shows a preferential surface affinity for the reactant compared to the solvent. Significantly, Ru/SiO2, which is totally inactive for the oxidn. of 1,4-butanediol, exhibited a lower T1/T2 ratio (surface affinity) for 1,4-butanediol (reactant) than for a "weakly-interacting" alkane, indicating a very poor surface affinity for the diol functionality. The results provide direct evidence of the importance of the adsorbate-adsorbent interactions on catalyst activity in liq.-phase oxidns. and indicate that the competitive adsorption of the solvent plays an important role in these reactions. This work demonstrates that NMR relaxation time anal. is a powerful method for comparing adsorption of liqs. in porous catalysts, providing valuable information on the affinity of different chem. species for a catalyst surface. Moreover, the results demonstrate that NMR relaxation time measurements can be used not only to guide selection of solvent for use with a specific catalyst, but also selection of the catalyst itself. The results suggest that this method may be used to predict catalyst behavior, enabling improved design and optimization of heterogeneous catalytic processes.

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Organic Letters

Cite this: Org. Lett. 2020, 22, 13, 4927–4931

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https://doi.org/10.1021/acs.orglett.0c01188

Published May 8, 2020

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Abstract
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Scheme 1
Scheme 1. Generally Accepted Mechanism for the Oxidative NHC Catalyzed Reaction (5c)
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Figure 1
Figure 1. T₁/T₂ of solvents used in the oxidative coupling of 2-chlorobenzaldehyde with methanol versus the TOF_hetero/TOF_homo. The solid line is a linear fit, which represents a guide for the eyes.
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References
This article references 25 other publications.
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  Recent developments in oxidative esterification and amidation of aldehydes
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  Tetrahedron Letters (2016), 57 (31), 3433-3440CODEN: TELEAY; ISSN:0040-4039. (Elsevier Ltd.)
  A review. Esters and amides, which are functional groups of fundamental importance in org. chem. are traditionally prepd. by reacting carboxylic acid derivs. (carboxylic halides, anhydrides and activated esters) with alcs. and amines, often implying multistep processes. Recently the oxidative esterification and amidation of aldehydes have been raised as sustainable and efficient alternatives to classical synthesis. This digest reports more recent advances in this emergent field.
2. 2
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  Metrics of Green Chemistry and Sustainability: Past, Present, and Future
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  A review concerning historic, current, and future green chem. and sustainability metrics for fine org. chem. and pharmaceutical prodn. is given. Topics discussed include: green chem. origins; catalysis soln. to pollution; mass-based green metrics (atom economy and the E [environmental] factor, other mass-based metrics, system boundaries and intrinsic E factors); sustainability metrics and the environmental impact of wastes (energy efficiency metrics, environmental impact of wastes, life cycle assessment); from environmental impact to sustainability (circular economy); the bio-based economy; and summary and future outlook.
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  Davis, S. E.; Ide, M. S.; Davis, R. J. Selective oxidation of alcohols and aldehydes over supported metal nanoparticles. Green Chem. 2013, 15, 17, DOI: 10.1039/C2GC36441G
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  Selective oxidation of alcohols and aldehydes over supported metal nanoparticles
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  A review. Oxidn. is a key reaction in org. synthesis and will likely play a significant role in the development of value-added chems. from biomass. The application of heterogeneous catalysis and mol. oxygen to oxidn. reactions offers a green alternative to traditional, toxic chem. oxidants. However, making comparisons of catalyst performance (reaction rate, product selectivity) between reports in the literature is difficult because of inconsistencies in the ways results are reported. Herein, we examine the literature on supported metal catalysts for the oxidn. of mols. of interest in biomass conversion (primary alcs., polyols, 5-hydroxymethylfurfural, and various sugars). Reaction rates are calcd. and compared in a consistent manner and recommendations for avoiding common pitfalls in kinetic investigations are made.
4. 4
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  Catalysis with N-Heterocyclic Carbenes under Oxidative Conditions
  De Sarkar, Suman; Biswas, Anup; Samanta, Ramesh C.; Studer, Armido
  Chemistry - A European Journal (2013), 19 (15), 4664-4678CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. This article discusses the potential of oxidative carbene catalysis in synthesis and comprehensively covers pioneering studies as well as recent developments. Oxidative carbene catalysis can be conducted by using inorg. and org. oxidants.
5. 5
  (a) Guin, J.; De Sarkar, S.; Grimme, S.; Studer, A. Biomimetic carbene-catalyzed oxidations of aldehydes using TEMPO. Angew. Chem., Int. Ed. 2008, 47, 8727, DOI: 10.1002/anie.200802735
  5a
  Biomimetic carbene-catalyzed oxidations of aldehydes using TEMPO
  Guin, Joyram; De Sarkar, Suman; Grimme, Stefan; Studer, Armido
  Angewandte Chemie, International Edition (2008), 47 (45), 8727-8730CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Transition-metal-free organocatalytic oxidns. of various aldehydes proceed with the TEMPO radical as a mild org. oxidant; the resulting TEMPO esters are formed in moderate to excellent yields. N-Heterocyclic carbenes (NHCs) are efficient catalysts and activate aldehydes for electron-transfer reactions. The TEMPO esters are readily hydrolyzed and the nitroxide can be regenerated by aerobic oxidn.
  (b) Maki, B. E.; Chan, A.; Phillips, E. M.; Scheidt, K. A. Tandem Oxidation of Allylic and Benzylic Alcohols to Esters Catalyzed by N-Heterocyclic Carbenes. Org. Lett. 2007, 9, 371, DOI: 10.1021/ol062940f
  5b
  Tandem Oxidation of Allylic and Benzylic Alcohols to Esters Catalyzed by N-Heterocyclic Carbenes
  Maki, Brooks E.; Chan, Audrey; Phillips, Eric M.; Scheidt, Karl A.
  Organic Letters (2007), 9 (2), 371-374CODEN: ORLEF7; ISSN:1523-7060. (American Chemical Society)
  N-Heterocyclic carbenes catalyze the oxidn. of allylic, propargylic, and benzylic alcs. to esters with manganese(IV) oxide in excellent yields. A variety of ester derivs. can be synthesized, including protected carboxylates. This one-pot tandem oxidn. represents the first organocatalytic oxidn. of alcs. to esters. Satd. esters can also be accessed from aldehydes using this method. Through the utilization of a chiral catalyst, the acyl-heteroazolium intermediate becomes a chiral acylating agent, which can desymmetrize meso-1,2-diols.
  (c) De Sarkar, S.; Grimme, A.; Studer, A. NHC Catalyzed Oxidations of Aldehydes to Esters: Chemoselective Acylation of Alcohols in Presence of Amines. J. Am. Chem. Soc. 2010, 132, 1190– 1191, DOI: 10.1021/ja910540j
  5c
  NHC Catalyzed Oxidations of Aldehydes to Esters: Chemoselective Acylation of Alcohols in Presence of Amines
  De Sarkar, Suman; Grimme, Stefan; Studer, Armido
  Journal of the American Chemical Society (2010), 132 (4), 1190-1191CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
  Not just one but two carbenes of the same structure act cooperatively in oxidative acylations of alcs. with aldehydes by using a readily available cheap org. oxidant. Alcs. are selectively acylated in the presence of amines by cooperative carbene catalysis. E.g., treatment of (E)-cinnamaldehyde with 1,3-dimethyltriazolium iodide, DBU, and 3,3',5,5'-tetra-tert-butyldiphenoquinone in THF/MeOH at rt for 2 h afforded quant. (E)-Me cinnamate. An important aspect herein is the rare chemoselective acylation of alcs. in the presence of amines. E.g., a 1:1 mixt. of benzyl alc. and benzyl amine was acylated with cinnamaldehyde to give benzyl cinnamate (96%). Acylated benzylamine was not identified as checked by GC anal. Quantum chem. calcns. support the suggested mechanism.
  (d) Di Carmine, G.; Ragno, D.; Brandolese, A.; Bortolini, O.; Pecorari, D.; Sabuzi, F.; Mazzanti, A.; Massi, A. Enantioselective Desymmetrization of 1,4-Dihydropyridines by Oxidative NHC Catalysis. Chem. - Eur. J. 2019, 25, 7469, DOI: 10.1002/chem.201901243
  5d
  Enantioselective Desymmetrization of 1,4-Dihydropyridines by Oxidative NHC Catalysis
  Di Carmine, Graziano; Ragno, Daniele; Brandolese, Arianna; Bortolini, Olga; Pecorari, Daniel; Sabuzi, Federica; Mazzanti, Andrea; Massi, Alessandro
  Chemistry - A European Journal (2019), 25 (31), 7469-7474CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The unprecedented desymmetrization of prochiral dialdehydes catalyzed by N-heterocyclic carbenes under oxidative conditions was applied to the highly enantioselective synthesis of 1,4-dihydropyridines (DHPs) starting from 3,5-dicarbaldehyde substrates. Synthetic elaboration of the resulting 5-formyl-1,4-DHP-3-carboxylates allowed for access to the class of pharmaceutically relevant 1,4-DHP-3,5-dicarboxylates (Hantzsch esters). DFT calcns. suggested that the enantioselectivity of the process is detd. by the transition state involving the oxidn. of the Breslow intermediate by the external quinone oxidant.
  (e) Ragno, D.; Di Carmine, G.; Brandolese, A.; Bortolini, O.; Giovannini, P. P.; Fantin, G.; Bertoldo, M.; Massi, A. Oxidative NHC-Catalysis as Organocatalytic Platform for the Synthesis of Polyester Oligomers by Step-Growth Polymerization. Chem. - Eur. J. 2019, 25, 14701, DOI: 10.1002/chem.201903557
  5e
  Oxidative NHC-Catalysis as Organocatalytic Platform for the Synthesis of Polyester Oligomers by Step-Growth Polymerization
  Ragno, Daniele; Di Carmine, Graziano; Brandolese, Arianna; Bortolini, Olga; Giovannini, Pier Paolo; Fantin, Giancarlo; Bertoldo, Monica; Massi, Alessandro
  Chemistry - A European Journal (2019), 25 (64), 14701-14710CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The application of N-heterocyclic carbene (NHC) catalysis to the polycondensation of diols and dialdehydes under oxidative conditions is herein presented for the synthesis of polyesters using fossil-based (ethylene glycol, phthalaldehydes) and bio-based (furan derivs., glycerol, isosorbide) monomers. The catalytic di-Me triazolium/1,8-diazabicyclo[5.4.0]undec-7-ene couple and stoichiometric quinone oxidant afforded polyester oligomers with a no.-av. mol. wt. (Mn) in the range of 1.5-7.8 kg mol-1 as detd. by NMR anal. The synthesis of a higher mol. wt. polyester (polyethylene terephthalate, PET) by an NHC-promoted two-step procedure via oligoester intermediates is also illustrated together with the catalyst-controlled prepn. of crosslinked or linear polyesters derived from the trifunctional glycerol. The thermal properties (TGA and DSC analyses) of the synthesized oligoesters are also reported.
6. 6
  (a) Shinkai, S.; Yamashita, T.; Kusano, Y.; Manabe, O. Facile oxidation of aldehydes and α-keto acids as catalyzed by flavin and thiazolium ion. Tetrahedron Lett. 1980, 21, 2543, DOI: 10.1016/0040-4039(80)80123-7
  6a
  Facile oxidation of aldehydes and α-keto acids as catalyzed by flavin and thiazolium ion
  Shinkai, Seiji; Yamashita, Takaharu; Kusano, Yumiko; Manabe, Osamu
  Tetrahedron Letters (1980), 21 (26), 2543-6CODEN: TELEAY; ISSN:0040-4039.
  Oxidn. of aldehydes and α-keto acids to carboxylic acids occurs readily in the presence of N-hexadecylthiazolium bromide (I) and 3-methyltetra-O-acetylriboflavin (II). Thus, 4-ClC6H4CHO (III) in the presence of 0.2 equiv I, 1 equiv II and a cationic micelle gave 63% 4-ClC6H4CO2H. In the absence of II, III gave 37% benzoin IV (R = H) and 47% benzil IV (R2 = bond). Kinetic studies are reported, and a mechanism involving heterocycle V (R1 = aryl) as intermediate is proposed.
  (b) Tam, S.; Jimenez, L.; Diederich, F. One pot synthesis of aromatic methyl esters by electrochemical oxidation of aldehydes mediated by biscoenzyme catalysis. J. Am. Chem. Soc. 1992, 114, 1503, DOI: 10.1021/ja00030a069
  6b
  One pot synthesis of aromatic methyl esters by electrochemical oxidation of aldehydes mediated by biscoenzyme catalysis
  Tam, Suk Wah; Jimenez, Leslie; Diederich, Francois
  Journal of the American Chemical Society (1992), 114 (4), 1503-5CODEN: JACSAT; ISSN:0002-7863.
  Electrochem. oxidn. of aldehydes over flavin and thiazolium as catalysts in MeOH as solvent gave Me carboxylates selectively. The electrochem. flavin/N-benzyl-4-methylthiazolium bromide-catalyzed oxidn. of 4-chlorobenzaldehyde gave 87% Me 4-chlorobenzoate. The electrochem. oxidn. of aliph. aldehydes was less efficient due to formation of hemiacetals.
7. 7
  Axelsson, A.; Hammarvid, E.; Ta, L.; Sundén, H. Asymmetric aerobic oxidative NHC-catalysed synthesis of dihydropyranones utilising a system of electron transfer mediators. Chem. Commun. 2016, 52, 11571, DOI: 10.1039/C6CC06060A
  7
  Asymmetric aerobic oxidative NHC-catalysed synthesis of dihydropyranones utilising a system of electron transfer mediators
  Axelsson, A.; Hammarvid, E.; Ta, L.; Sunden, H.
  Chemical Communications (Cambridge, United Kingdom) (2016), 52 (77), 11571-11574CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
  The asym. aerobic oxidative synthesis of dihydropyranones I (R = CH2CH2CH3, 2-O2NC6H4, thiophen-2-yl, etc.; R1 = Me, Ph, OMe, OEt, Ot-Bu, OBn) was isolated in high to excellent yields, with high ee (up to 95%). The oxidn. is aided by a system of electron transfer mediators and is selective toward the homoenolate.
8. 8
  Gruttadauria, M.; Giacalone, F.; Noto, R. Supported proline and proline-derivatives as recyclable organocatalysts. Chem. Soc. Rev. 2008, 37, 1666, DOI: 10.1039/b800704g
  8
  Supported proline and proline-derivatives as recyclable organocatalysts
  Gruttadauria, Michelangelo; Giacalone, Francesco; Noto, Renato
  Chemical Society Reviews (2008), 37 (8), 1666-1688CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
  A review. In the last eight years, l-proline and l-proline derivs., such as substituted prolinamides or pyrrolidines, were successfully used as organocatalysts in several reactions. In this crit. review the authors summarized the immobilization procedures of such organocatalysts highlighting their application, recoverability and reusability (86 refs.).
9. 9
  Ragno, D.; Brandolese, A.; Urbani, D.; Di Carmine, G.; De Risi, C.; Bortolini, O.; Giovannini, P. P.; Massi, A. Esterification of glycerol and solketal by oxidative NHC-catalysis under heterogeneous batch and flow conditions. React. Chem. Eng. 2018, 3, 816, DOI: 10.1039/C8RE00143J
  9
  Esterification of glycerol and solketal by oxidative NHC-catalysis under heterogeneous batch and flow conditions
  Ragno, Daniele; Brandolese, Arianna; Urbani, Daniele; Di Carmine, Graziano; De Risi, Carmela; Bortolini, Olga; Giovannini, Pier Paolo; Massi, Alessandro
  Reaction Chemistry & Engineering (2018), 3 (5), 816-825CODEN: RCEEBW; ISSN:2058-9883. (Royal Society of Chemistry)
  The design and synthesis of a set of supported azolium salt pre-catalysts is presented along with their utilization in the prodn. of monoesters of glycerol and solketal by oxidative N-heterocyclic carbene (NHC)-catalysis through batch and continuous-flow approaches. After a propaedeutic study with sol. NHCs, the heterogeneous analogs (silica and polystyrene supports) were tested in a model monoesterification of glycerol using either the Kharasch oxidant or air (in the presence of electron transfer mediators) as the terminal oxidants. The best performing polystyrene-supported triazolium salt pre-catalyst afforded monoacylglycerols (MAGs) in high yields (up to 95%) and almost complete selectivity (monoester/diester >95:5) using air and the green solvent Me-THF. The synthesis of fully bio-based MAGs from furfural, 5-hydroxymethylfurfural (HMF), citronellal, and vanillin is also reported. Continuous-flow expts. have been finally performed by fabricating the corresponding packed-bed microreactor, which could be operated for ca. 120 h with maintenance of conversion efficiency and selectivity.
10. 10
  Brandolese, A.; Ragno, D.; Di Carmine, G.; Bernardi, T.; Bortolini, O.; Giovannini, P. P.; Pandoli, O. G.; Altomare, A.; Massi, A. Aerobic oxidation of 5-hydroxymethylfurfural to 5-hydroxymethyl-2-furancarboxylic acid and its derivatives by heterogeneous NHC-catalysis. Org. Biomol. Chem. 2018, 16, 8955, DOI: 10.1039/C8OB02425A
  10
  Aerobic oxidation of 5-hydroxymethylfurfural to 5-hydroxymethyl-2-furancarboxylic acid and its derivatives by heterogeneous NHC-catalysis
  Brandolese, Arianna; Ragno, Daniele; Di Carmine, Graziano; Bernardi, Tatiana; Bortolini, Olga; Giovannini, Pier Paolo; Pandoli, Omar Ginoble; Altomare, Alessandra; Massi, Alessandro
  Organic & Biomolecular Chemistry (2018), 16 (46), 8955-8964CODEN: OBCRAK; ISSN:1477-0520. (Royal Society of Chemistry)
  The application of the oxidative system composed of a heterogeneous triazolium pre-catalyst, iron(II) phthalocyanine and air is described for the selective conversion of 5-hydroxymethylfurfural (HMF) into the added-value 5-hydroxymethyl-2-furancarboxylic acid (HMFCA). The disclosed one-pot two-step procedure involved sequential oxidative esterifications of HMF to afford a polyester oligomer having hydroxyl and carboxyl terminal groups (Mw = 389-1258), which in turn was hydrolyzed by a supported base (Ambersep 900 OH) to yield HMFCA in 87% overall yield. The same strategy was adopted for the effective synthesis of ester and amide derivs. of HMFCA by nucleophilic depolymn. of the oligomeric intermediate with methanol and butylamine, resp. The utilization of the disclosed oxidative system for the direct conversion of HMF and furfural into their corresponding ester, amide, and thioester derivs. is also reported.
11. 11
  Selected reviews on heterogeneous organocatalysis:
  (a) Atodiresei, I.; Vila, C.; Rueping, M. Asymmetric Organocatalysis in Continuous Flow: Opportunities for Impacting Industrial Catalysis. ACS Catal. 2015, 5, 1972, DOI: 10.1021/acscatal.5b00002
  11a
  Asymmetric Organocatalysis in Continuous Flow: Opportunities for Impacting Industrial Catalysis
  Atodiresei, Iuliana; Vila, Carlos; Rueping, Magnus
  ACS Catalysis (2015), 5 (3), 1972-1985CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  A review. The review highlights the different advantages assocd. with organocatalytic transformations performed in continuous-flow systems and presents the reactions which have been successfully achieved to date. Particular focus is placed on the comparison between batch and flow applications in order to show the advantages and disadvantages and to demonstrate the great potential for applying organocatalysis as well as combined organo and photoredox catalyzed reactions in continuous flow.
  (b) Savateev, A.; Antonietti, M. Heterogeneous Organocatalysis for Photoredox Chemistry. ACS Catal. 2018, 8, 9790, DOI: 10.1021/acscatal.8b02595
  11b
  Heterogeneous Organocatalysis for Photoredox Chemistry
  Savateev, Aleksandr; Antonietti, Markus
  ACS Catalysis (2018), 8 (10), 9790-9808CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Photoredox catalysis is a tool enabling a wide variety of chem. reactions with high selectivity under mild conditions of visible light. In this Review, we summarize recent expts. which use heterogeneous, covalent, metal-free semiconductors with adjustable reactivity to drive such reactions. This class started with mesoporous graphitic carbon nitride, then continued with poly(heptazine imides), but is meanwhile extended to other polymers and solid-state orgs. with conjugated electron system. Because of the high thermal and chem. stability, as well as adjustable conduction and valence band positions, the reaction space could be expanded to many org. reactions, such as photocatalytic synthesis of organosulfur compds., or unconventional halogenation and cyanation reactions. Performance of carbon nitrides and homogeneous systems in certain reactions was compared in the present Review.
  (c) Corma, A.; Garcia, H. Silica-Bound Homogenous Catalysts as Recoverable and Reusable Catalysts in Organic Synthesis. Adv. Synth. Catal. 2006, 348, 1391, DOI: 10.1002/adsc.200606192
  11c
  Silica-bound homogeneous catalysts as recoverable and reusable catalysts in organic synthesis
  Corma, Avelino; Garcia, Hermenegildo
  Advanced Synthesis & Catalysis (2006), 348 (12+13), 1391-1412CODEN: ASCAF7; ISSN:1615-4150. (Wiley-VCH Verlag GmbH & Co. KGaA)
  A review. Very frequently the most costly components in a chem. reaction are not the starting materials or the reaction products, but the catalyst. In addn. to the advantages from the economic point of view, recovery and reuse of the catalyst is equally important in order to avoid wastes, so improving the greenness of the process. There is a current tendency to transform homogeneous into heterogeneous catalysis, that will even be accelerated in the near future. Starting from a successful homogeneous catalyst, one general methodol. allowing its recovery and reuse is to immobilize a suitable deriv. of the active catalyst on an insol. solid support. When the catalyst does not deactivate and is sufficiently stable under the reaction conditions, the ultimate immobilization methodol. is to attach covalently the catalytically active species to the support. In this contribution, after introducing some general principles describing the fundamentals of the covalent anchoring, the emphasis is placed more on giving an overview of the most important types of covalently anchored catalysts, including Bronsted and Lewis acids, covalently anchored bases and hydrogenation complexes. Hot topics such as silica-bound organocatalysts and the application of periodic mesoporous organosilicas as heterogeneous catalysts is also covered.
  (d) Bartók, M. Advances in Immobilized Organocatalysts for the Heterogeneous Asymmetric Direct Aldol Reactions. Catal. Rev.: Sci. Eng. 2015, 57, 192, DOI: 10.1080/01614940.2015.1039432
  11d
  Advances in Immobilized Organocatalysts for the Heterogeneous Asymmetric Direct Aldol Reactions
  Bartok, Mihaly
  Catalysis Reviews: Science and Engineering (2015), 57 (2), 192-255CODEN: CRSEC9; ISSN:0161-4940. (Taylor & Francis, Inc.)
  A review. The results published from the beginning (2000) on asym. direct aldol reactions taking place in the presence of immobilized chiral organocatalysts was summarized. The seven groups of organocatalysts discussed were: asym. direct aldol reactions catalyzed by: (1) covalently bonded immobilized hydroxyproline organocatalysts; (2) covalently bonded immobilized prolinamide organocatalysts; (3) covalently bonded immobilized peptide organocatalysts; (4) other covalently bonded immobilized chiral organocatalysts; (5) chiral organocatalysts bonded by ionic groups; (6) chiral organocatalysts with adsorptive bonding; and (7) other types of immobilized chiral organocatalysts. The main objective of this article based on results obtained by using about 360 immobilized organocatalysts, was to focus on catalyst systems in which the aldol reactions yielded beta-hydroxyketones of maximal optical purity, while keeping catalyst concns. and reaction times as low and as short as possible. Trends recognized in data obtained in this field of investigation may mark out further tasks for the purpose of practical application.
  Rostamnia, S.; Doustkhah, E. Nanoporous silica-supported organocatalyst: a heterogeneous and green hybrid catalyst for organic transformations. RSC Adv. 2014, 4, 28238, DOI: 10.1039/C4RA03773A
  11
  Nanoporous silica-supported organocatalyst: a heterogeneous and green hybrid catalyst for organic transformations
  Rostamnia, Sadegh; Doustkhah, Esmail
  RSC Advances (2014), 4 (54), 28238-28248CODEN: RSCACL; ISSN:2046-2069. (Royal Society of Chemistry)
  A review. Organically modified hybrid silica mesostructures are becoming increasingly promising candidates as catalysts for various kinds of org. reactions and green chem. Org. moieties with an amine base and sulfonic acid have been extensively incorporated into these mesostructures. In this review, the authors discuss recent advances of the most common types of organically modified hybrid materials that act as organocatalysts in org. transformations and processes.
12. 12
  (a) Weber, D.; Mitchell, J.; McGregor, J.; Gladden, L. F. Comparing Strengths of Surface Interactions for Reactants and Solvents in Porous Catalysts Using Two-Dimensional NMR Relaxation Correlations. J. Phys. Chem. C 2009, 113, 6610, DOI: 10.1021/jp811246j
  12a
  Comparing Strengths of Surface Interactions for Reactants and Solvents in Porous Catalysts Using Two-Dimensional NMR Relaxation Correlations
  Weber, Daniel; Mitchell, Jonathan; McGregor, James; Gladden, Lynn F.
  Journal of Physical Chemistry C (2009), 113 (16), 6610-6615CODEN: JPCCCK; ISSN:1932-7447. (American Chemical Society)
  Two-dimensional NMR relaxation time correlation measurements have been used to observe the behavior of liqs. inside porous catalyst pellets; in particular, liqs. of relevance to the hydrogenation of 2-butanone over a silica-supported ruthenium catalyst (Ru/SiO2). The behavior of 2-butanone is studied and compared to that of water and 2-propanol, which are used as solvents in this hydrogenation reaction. From the ratio of NMR relaxation times, T1/T2, for the liqs. confined in the pores, it is possible to infer the relative strengths of the surface interaction for each liq. Water is seen to have the strongest surface interaction, and 2-butanone has the weakest surface interaction. These results are supported by displacement expts., in which one liq. replaces the other over time within the pore space of the catalyst. For comparison, the behavior of the same liqs. in an alumina-supported palladium catalyst (Pd/Al2O3) was also studied. The variation in the strengths of surface interactions was more pronounced in the Pd/Al2O3 catalyst than in the Ru/SiO2 catalyst. This work demonstrates the applicability of NMR relaxation time correlation expts. to real catalytic systems contg. metallic components. From these measurements, information on the access of reactants to surface adsorption sites can be inferred.
  (b) Krzyżak, A. T.; Habina, I. Low field ¹H NMR characterization of mesoporous silica MCM-41 and SBA-15 filled with different amount of water. Microporous Mesoporous Mater. 2016, 231, 230, DOI: 10.1016/j.micromeso.2016.05.032
  12b
  Low field 1H NMR characterization of mesoporous silica MCM-41 and SBA-15 filled with different amount of water
  Krzyzak, A. T.; Habina, I.
  Microporous and Mesoporous Materials (2016), 231 (), 230-239CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
  The low field 1H NMR characterization of pure mesoporous SiO2 materials - MCM-41 and SBA-15 as a function of H2O content is presented. The values of surface relaxivities for both materials were estd. to compare pore size distributions obtained from relaxation spectra and N2 adsorption measurements. This comparison led to differentiation between contributions from inner-bulk H2O mols. in pores and these from the surface. The T1/T2 ratio reflecting the strength of interaction between H2O mols. and the mesoporous SiO2 surface is higher for SBA-15. Measurements for a mesopore surface partially covered by H2O provided quant. information on the hydroxylation rate.
  (c) Aksnes, D. W.; Førland, K.; Kimtys, L. ¹H and ²H NMR studies of cyclohexane nanocrystals in controlled pore glasses. J. Mol. Struct. 2004, 708, 23, DOI: 10.1016/j.molstruc.2004.02.055
  12c
  1H and 2H NMR studies of cyclohexane nanocrystals in controlled pore glasses
  Aksnes, D. W.; Forland, K.; Kimtys, L.
  Journal of Molecular Structure (2004), 708 (1-3), 23-31CODEN: JMOSB4; ISSN:0022-2860. (Elsevier B.V.)
  The formation and behavior of cyclohexane and cyclohexane-d12 nanocrystals in mesoporous solids of well-defined dimensional constraints are studied by 1H and 2H NMR. The NMR line widths, spin-spin relaxation times (T2), spin-lattice relaxation times (T1) and diffusivities (D) were measured as a function of temp., and the results are discussed with ref. to the values obtained for the bulk materials. The confined solids exhibit substantial changes in the phase behavior and mol. dynamics. Thus, the line-shape measurements reveal a two-phase system consisting of a highly mobile component at the surface of the pore and a plastically cryst. phase in the center of the pore. The liq.-like surface layer in the mesopores is observable well below the reduced transition temp. of the confined cyclohexane. However, the T2 and diffusion measurements show that the mobile phase also embraces a minor component attributed to non-frozen liq. in pockets or offshoots.
13. 13
  (a) D’Agostino, C.; Mitchell, J.; Mantle, M. D.; Gladden, L. F. Interpretation of NMR Relaxation as a Tool for Characterising the Adsorption Strength of Liquids inside Porous Materials. Chem. - Eur. J. 2014, 20, 13009, DOI: 10.1002/chem.201403139
  13a
  Interpretation of NMR Relaxation as a Tool for Characterising the Adsorption Strength of Liquids inside Porous Materials
  D'Agostino, Carmine; Mitchell, Jonathan; Mantle, Michael D.; Gladden, Lynn F.
  Chemistry - A European Journal (2014), 20 (40), 13009-13015CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  NMR relaxation times are shown to provide a unique probe of adsorbate-adsorbent interactions in liq.-satd. porous materials. A short theor. anal. is presented, which shows that the ratio of the longitudinal to transverse relaxation times (T1/T2) is related to an adsorbate-adsorbent interaction energy, and the authors introduce a quant. metric esurf (based on the relaxation time ratio) characterizing the strength of this surface interaction. The authors then consider the interaction of water with a range of oxide surfaces (TiO2 anatase, TiO2 rutile, γ-Al2O3, SiO2, θ-Al2O3, and ZrO2) and show that esurf correlates with the strongest adsorption sites present, as detd. by temp. programmed desorption (TPD). Thus, NMR relaxation measurements have a direct phys. interpretation in terms of the characterization of activation energy of desorption from the surface. Further, for a series of chem. similar solid materials, in this case a range of oxide materials, for which at least two calibration values are obtainable by TPD, the esurf parameter yields a direct est. of the max. activation energy of desorption from the surface. The results suggest that T1/T2 measurements may become a useful addn. to the methods available to characterize liq.-phase adsorption in porous materials. The particular motivation for this work was to characterize adsorbate-surface interactions in liq.-phase catalysis.
  (b) Habina, I.; Radzik, N.; Topór, T.; Krzyžak, A. T. Insight into oil and gas-shales compounds signatures in low field ¹H NMR and its application in porosity evaluation. Microporous Mesoporous Mater. 2017, 252, 37, DOI: 10.1016/j.micromeso.2017.05.054
  13b
  Insight into oil and gas-shales compounds signatures in low field 1H NMR and its application in porosity evaluation
  Habina, I.; Radzik, N.; Topor, T.; Krzyzak, A. T.
  Microporous and Mesoporous Materials (2017), 252 (), 37-49CODEN: MIMMFJ; ISSN:1387-1811. (Elsevier B.V.)
  The low field 1H NMR characterization of rock components that are specific for oil- and gas- shales is presented. 1D T2 distributions together with T1-T2 maps and T1/T2 ratios for the samples of smectite, illite, illite-smectite, kaolinite, chlorite and kerogen served to obtain characteristic features. T1-T2 maps measured in three satn. states (native, satd. and dried) allowed the observation of different fractions of water such as water confined in interlayer spaces, filling pore spaces and hydroxyl groups from crystallog. lattice. Differential images are proposed for the evaluation of water distribution in the pore spaces and values of porosity, directly related to the fluid removed after the drying of the sample. Porosity values for exemplary shale samples, calcd. for differential results are consistent with the porosity obtained using T2 cut-offs. The relations between cation exchange capacity and T2 log-mean values for the samples at different satn. states are also shown.
14. 14
  D’Agostino, C.; Brett, G.; Divitini, G.; Ducati, C.; Hutchings, G. J.; Mantle, M. D.; Gladden, L. F. Increased Affinity of Small Gold Particles for Glycerol Oxidation over Au/TiO₂ Probed by NMR Relaxation Methods. ACS Catal. 2017, 7, 4235, DOI: 10.1021/acscatal.7b01255
  14
  Increased Affinity of Small Gold Particles for Glycerol Oxidation over Au/TiO2 Probed by NMR Relaxation Methods
  D'Agostino, Carmine; Brett, Gemma; Divitini, Giorgio; Ducati, Caterina; Hutchings, Graham J.; Mantle, Michael D.; Gladden, Lynn F.
  ACS Catalysis (2017), 7 (7), 4235-4241CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  The aerobic oxidn. of glycerol in aq. soln. over Au/TiO2 catalysts has been studied, and the effect of Au loading by wet impregnation, in the range 0.5-5% Au, has been assessed. Low metal loading favors the deposition of smaller particles, whereas higher loadings lead to the formation of much larger gold particles, as revealed by scanning transmission electron microscopy (STEM) anal. Reaction studies show that a higher metal loading has a detrimental effect on the catalyst activity, which decreases significantly as the Au load increases. In addn. to reaction studies, 1H NMR T1/T2 relaxation time measurements have been used to assess the effect of metal loading and particle size on the adsorption properties of glycerol (reactant) and water (solvent) within the catalyst. The NMR results show that the adsorption properties of glycerol relative to water as a function of the Au loading have a similar trend to that obsd. for the reactivity, with glycerol exhibiting a higher surface affinity relative to water for the catalyst with low Au loading. The overall results indicate that metal loading significantly affects the typical Au particle size, which, in turn, affects both the reaction and adsorption properties of glycerol over the catalyst surface. In particular, the trend in T1/T2 ratio clearly indicates that glycerol has a much stronger affinity with smaller gold particles, which is an important factor in promoting glycerol oxidn. This result is of great significance in understanding the reactivity of polyols over supported gold catalysts and gives the first exptl. evidence that smaller gold particles tend to be stronger adsorption sites for glycerol, in agreement with computational and theor. studies.
15. 15
  D’Agostino, C.; Armstrong, R. D.; Hutchings, G. J.; Gladden, L. F. Product Inhibition in Glycerol Oxidation over Au/TiO₂ Catalysts Quantified by NMR Relaxation. ACS Catal. 2018, 8, 7334, DOI: 10.1021/acscatal.8b01516
  15
  Product Inhibition in Glycerol Oxidation over Au/TiO2 Catalysts Quantified by NMR Relaxation
  D'Agostino, Carmine; Armstrong, Robert D.; Hutchings, Graham J.; Gladden, Lynn F.
  ACS Catalysis (2018), 8 (8), 7334-7339CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  Liq.-phase catalytic oxidn. of glycerol in aq. solns. using porous solid catalysts represents a viable strategy for the sustainable prodn. of fine chems. from renewable resources. Various aspects of this novel type of reactions are still under investigation. Catalyst deactivation is one of those issues that need to be understood and addressed in order to make these processes com. viable. In a previous study it has been reported that the catalytic activity of Au/TiO2 catalysts for the oxidn. of glycerol with O2 under basic conditions can be severely inhibited by some reaction intermediates or products. It was suggested that the presence of certain species blocks the active sites of the catalyst, preventing the adsorption of glycerol, which in turn results in a decrease of reaction rate. In this work, we used NMR relaxation time measurements in order to assess surface interactions of glycerol in Au/TiO2 catalyst pre-treated with aq. solns. of various oxygenates, including intermediates and products of glycerol oxidn., under basic conditions, in particular evaluating changes in glycerol adsorption properties. The NMR T1/T2 ratio of glycerol, which is indicative of the strength of interaction of glycerol with the catalyst surface, traces out well the trend in catalytic activity in the presence of different additives, suggesting that adsorption of glycerol onto the catalyst surface play a crucial role in the reaction, which supports the hypothesis previously made in the literature. This exptl. approach and the related results represent a significant advance in the understanding of liq.-phase catalytic reactions occurring over solid surfaces, which can be used to understand and optimize catalytic processes and the effect of intermediate and product inhibition.
16. 16
  (a) D’Agostino, C.; Kotionova, T.; Mitchell, J.; Miedziak, P. J.; Knight, D. W.; Taylor, S. H.; Hutchings, G. J.; Gladden, L. F.; Mantle, M. D. Solvent Effect and Reactivity Trend in the Aerobic Oxidation of 1,3-Propanediols over Gold Supported on Titania: NMR Diffusion and Relaxation Studies. Chem. - Eur. J. 2013, 19, 11725, DOI: 10.1002/chem.201300502
  16a
  Solvent Effect and Reactivity Trend in the Aerobic Oxidation of 1,3-Propanediols over Gold Supported on Titania: NMR Diffusion and Relaxation Studies
  D'Agostino, Carmine; Kotionova, Tatyana; Mitchell, Jonathan; Miedziak, Peter J.; Knight, David W.; Taylor, Stuart H.; Hutchings, Graham J.; Gladden, Lynn F.; Mantle, Mick D.
  Chemistry - A European Journal (2013), 19 (35), 11725-11732CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  In recent work, it was reported that changes in solvent compn., precisely the addn. of water, significantly inhibits the catalytic activity of Au/TiO2 catalyst in the aerobic oxidn. of 1,4-butanediol in methanol due to changes in diffusion and adsorption properties of the reactant. In order to understand whether the inhibition mechanism of water on diol oxidn. in methanol is generally valid, the solvent effect on the aerobic catalytic oxidn. of 1,3-propanediol and its two methyl-substituted homologues, 2-methyl-1,3-propanediol and 2,2-dimethyl-1,3-propanediol, over a Au/TiO2 catalyst has been studied here using conventional catalytic reaction monitoring in combination with pulsed-field gradient NMR (PFG-NMR) diffusion and NMR relaxation time measurements. Diol conversion is significantly lower when water is present in the initial diol/methanol mixt. A reactivity trend within the group of diols was also obsd. Combined NMR diffusion and relaxation time measurements suggest that mol. diffusion and, in particular, the relative strength of diol adsorption, are important factors in detg. the conversion. These results highlight NMR diffusion and relaxation techniques as novel, non-invasive characterization tools for catalytic materials, which complement conventional reaction data.
  (b) D’Agostino, C.; Brett, G. L.; Miedziak, P. J.; Knight, D. W.; Hutchings, G. J.; Gladden, L. F.; Mantle, M. D. Understanding the Solvent Effect on the Catalytic Oxidation of 1,4-Butanediol in Methanol over Au/TiO₂ Catalyst: NMR Diffusion and Relaxation Studies. Chem. - Eur. J. 2012, 18, 14426, DOI: 10.1002/chem.201201922
  16b
  Understanding the Solvent Effect on the Catalytic Oxidation of 1,4-Butanediol in Methanol over Au/TiO2 Catalyst: NMR Diffusion and Relaxation Studies
  D'Agostino, Carmine; Brett, Gemma L.; Miedziak, Peter J.; Knight, David W.; Hutchings, Graham J.; Gladden, Lynn F.; Mantle, Mick D.
  Chemistry - A European Journal (2012), 18 (45), 14426-14433, S14426/1-S14426/3CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
  The effect of water on the catalytic oxidn. of 1,4-butanediol in methanol over Au/TiO2 was studied by catalytic reaction studies and NMR diffusion and relaxation studies. The addn. of water to the dry catalytic system decreased both conversion and selectivity towards di-Me succinate. Pulsed-field gradient (PFG)-NMR spectroscopy was used to assess the effect of water addn. on the effective self-diffusivity of the reactant within the catalyst. NMR relaxation studies were also carried out to probe the strength of surface interaction of the reactant in the absence and presence of water. PFG-NMR studies revealed that the addn. of water to the initial system, although increasing the diln. of the system, leads to a significant decrease of effective diffusion rate of the reactant within the catalyst. From T1 and T2 relaxation measurements it was possible to infer the strength of surface interaction of the reactant with the catalyst surface. The addn. of water was found to inhibit the adsorption of the reactant over the catalyst surface, with the T1/T2 ratio of 1,4-butanediol decreasing significantly when water was added. The results overall suggest that both the decrease of diffusion rate and adsorption strength of the reactant within the catalyst, due to water addn., limits the access of reactant mols. to the catalytic sites, which results in a decrease of reaction rate and conversion.
17. 17
  (a) Scalambra, F.; Holzmann, N.; Bernasconi, L.; Imberti, S.; Romerosa, A. Water Participation in Catalysis: An Atomistic Approach to Solvent Effects in the Catalytic Isomerization of Allylic Alcohols. ACS Catal. 2018, 8, 3812, DOI: 10.1021/acscatal.8b00199
  17a
  Water Participation in Catalysis: An Atomistic Approach to Solvent Effects in the Catalytic Isomerization of Allylic Alcohols
  Scalambra, Franco; Holzmann, Nicole; Bernasconi, Leonardo; Imberti, Silvia; Romerosa, Antonio
  ACS Catalysis (2018), 8 (5), 3812-3819CODEN: ACCACS; ISSN:2155-5435. (American Chemical Society)
  We study the intermediate of the reaction between a ruthenium complex and 1-propen-3-ol in water using an atomistic approach for gaining information about the conformation and dynamics of complex mols. in aq. soln., which combines d.-functional-theory-based ab initio mol. dynamics and neutron scattering data based on empirical potential structure refinement simulations. We apply our method to the study of the water-sol. η2-allylic complex [RuCp(exo-η2-CH2=CH-CH2-OH)(PTA)2]+ (2) (PTA = 1,3,5-triaza-7-phosphaadamantane), an important intermediate in the isomerization of 1-propen-3-ol into propanal catalyzed by {RuCp(H2O-κO)(PTA)2}+. We identify the factors responsible for the stabilization of a specific conformer of 2 in water soln. and we examine the involvement of water mols. in the formation of this species. In particular, we show that long-lived (ca. 10 ps) bonded chains of water mols. play a crucial role in influencing the conformation and, potentially, the chem. reactivity of 2.
  (b) Satpathy, L.; Sahu, P. K.; Behera, P. K.; Mishra, B. K. Solvent Effect on the Potential Energy Surfaces of the F^– + CH₃CH₂Br Reaction. J. Phys. Chem. A 2018, 122, 5861, DOI: 10.1021/acs.jpca.8b02687
  17b
  Solvent Effect on the Potential Energy Surfaces of the F- + CH3CH2Br Reaction
  Satpathy, Lopamudra; Sahu, Prabhat K.; Behera, Pradipta K.; Mishra, Bijay K.
  Journal of Physical Chemistry A (2018), 122 (27), 5861-5869CODEN: JPCAFH; ISSN:1089-5639. (American Chemical Society)
  Although substantial work has been undertaken on reaction pathways involved in base-promoted elimination reactions and bimol. nucleophilic substitution reaction of F- on CH3CH2X (X = Cl, Br, I), the effect of solvents with varying dielec. consts. on the stereochem. of each of the reaction species involved across the reaction profile have not yet been clearly understood. The present investigation reports the effect of solvents on the potential energy surfaces (PES) and structures of the species appearing in the reaction pathway of F- with bromoethane. The PESs in the gas phase have been computed at MP2 level and CCSD(T) level. The performance of several hybrid d. functional, such as B3LYP, M06, M06L, BHandH, X3LYP, M05, M05-2X, and M06-2X have also been investigated toward describing the elimination and nucleophilic substitution reactions. With respect to MAE values and to make the computation cost-effective, we have explored the implicit continuum solvent model, CPCM in solvents like cyclohexane, methanol, acetonitrile, DMSO and water. The reactant complexes proceed through the subsequent steps to produce fluoroethane as the substitution product and ethylene as one of the elimination products. For elimination reaction both syn and anti elimination have been explored. The calcd. relatives energies values, which are neg. in the gas phase, are found to be pos. in polar solvents since the point charge in the sepd. reactants are more stabilized than the dispersed charge in the transient complex, which has also been analyzed through NBO anal.
  (c) Dyson, P. J.; Jessop, P. G. Solvent effects in catalysis: rational improvements of catalysts via manipulation of solvent interactions. Catal. Sci. Technol. 2016, 6, 3302, DOI: 10.1039/C5CY02197A
  17c
  Solvent effects in catalysis: rational improvements of catalysts via manipulation of solvent interactions
  Dyson, Paul J.; Jessop, Philip G.
  Catalysis Science & Technology (2016), 6 (10), 3302-3316CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
  A review; in homogeneous catalysis, the main emphasis on improving catalyst performance (rate, yield and selectivity) is directed towards manipulation of the ligands. The steric and electronic effects of ligands are extremely well understood and the rational design of new ligands that leads to improved catalysts is feasible. Similarly, in heterogeneous catalysis structural changes to the catalyst are used to improve their properties. In contrast, the role that solvents play in catalytic processes is often given cursory attention. The environmental impact of solvents is often considered, as it should be, and the use of org.-immiscible solvents, e.g. water, ionic liqs., etc., is frequently considered with respect to catalyst recovery, product isolation, and recycling. Nevertheless, the direct role of solvents in reactions is often overlooked, although solvents interact directly with the catalyst, the substrates and products, and all these interactions can increase or decrease reaction rate and/or selectivity. Herein we consider the role of solvents in catalysis and illustrate the crit. role of solvents viewed from a mechanistic approach. In particular, we focus largely, but not exclusively, on hydrogenation reactions and cross-coupling reactions as the main types of solvents used for these two classes of reactions tend to be very different and illustrate different functions of solvents in catalysis.
18. 18
  (a) Bertero, N. M.; Trasarti, a. F.; Apesteguía, C. R.; Marchi, A. J. Solvent effect in the liquid-phase hydrogenation of acetophenone over Ni/SiO₂: A comprehensive study of the phenomenon. Appl. Catal., A 2011, 394, 228, DOI: 10.1016/j.apcata.2011.01.003
  18a
  Solvent effect in the liquid-phase hydrogenation of acetophenone over Ni/SiO2: A comprehensive study of the phenomenon
  Bertero, Nicolas M.; Trasarti, Andres F.; Apesteguia, Carlos R.; Marchi, Alberto J.
  Applied Catalysis, A: General (2011), 394 (1-2), 228-238CODEN: ACAGE4; ISSN:0926-860X. (Elsevier B.V.)
  The solvent effect on catalyst activity and selectivity for the liq.-phase hydrogenation of acetophenone (AP) to 1-phenylethanol was thoroughly investigated over Ni/SiO2. Solvents of different nature were used: protic (C1-C3 primary and secondary alcs.), aprotic polar (THF, γ-butyrolactone, and acetonitrile) and apolar solvents (cyclohexane, toluene, and benzene). The solvent had a strong influence on the AP hydrogenation rate but did not modify significantly the selectivity to 1-phenylethanol that was always higher than 92%. The AP hydrogenation activity followed the order: C2-C3 alcs. > cyclohexane > toluene > THF > γ-butyrolactone > methanol » benzene ≃ acetonitrile. In order to explain this activity pattern, the solvent-AP, solvent-H2 and solvent-catalyst interactions were analyzed. For the anal. of the solvent-AP interactions in liq. phase, both classical measures of polarity and others based on different solvatochromic scales were considered. The H2 availability in the liq. phase was estd. from the H2 soly. at reaction conditions. Solvent-catalyst interactions were characterized by means of the adsorption enthalpies measured calorimetrically. A reasonable correlation between the catalyst activity and some solvatochromic parameters was found only when solvents of similar nature were compared. For protic solvents, the AP hydrogenation rate decreased with the solvent polarity and its ability for H-bond formation with AP. Instead, the solvent-AP interactions were weak when using apolar solvents and thereby the activity pattern was essentially detd. by the strength of solvent-catalyst interactions. In the case of aprotic polar solvents, both the solvent-AP interactions in the liq. phase and the solvent adsorption strength on the catalyst surface influenced the hydrogenation activity. The highest catalytic activities were obtained when using C2-C3 alc. solvents. These protic solvents adsorbed dissociatively on metal nickel surface increasing the no. of active H available for the hydrogenation reaction; this effect was much more important in the case of 2-propanol.
  (b) Wang, S.; Ge, B.; Yin, Y.; Wu, X.; Zhu, H.; Yue, Y.; Bai, Z.; Bao, X.; Yuan, P. Solvent effect in heterogeneous catalytic selective hydrogenation of nitrile butadiene rubber: relationship between reaction activity and solvents with density functional theory analysis. ChemCatChem 2020, 12, 663, DOI: 10.1002/cctc.201901555
  18b
  Solvent Effect in Heterogeneous Catalytic Selective Hydrogenation of Nitrile Butadiene Rubber: Relationship between Reaction Activity and Solvents with Density Functional Theory Analysis
  Wang, Shuhan; Ge, Bingqing; Yin, Yixuan; Wu, Xinru; Zhu, Haibo; Yue, Yuanyuan; Bai, Zhengshuai; Bao, Xiaojun; Yuan, Pei
  ChemCatChem (2020), 12 (2), 663-672CODEN: CHEMK3; ISSN:1867-3880. (Wiley-VCH Verlag GmbH & Co. KGaA)
  Herein, a systematical investigation is devoted to explore nitrile butadiene rubber (NBR) hydrogenation activity and selectivity in different org. solvents, and the dependence between reactivity and solvent is revealed with the assistance of the d. functional theory (DFT). The results show that the electron-pair donor solvents have better performance in NBR hydrogenation activity than the weak electron-pair acceptor solvents, but they do not modify the selectivity to HNBR that always keeps up to 100%. A direct correlation between reactivity and hydrogen bond acceptance ability (β) is discovered in both solvent families and the higher β, the better hydrogenation performance. The preferred solvent is cyclohexanone with hydrogenation degree of 91.4% in 10 min and the highest conversion of 94.2%. Our contribution is to provide useful information about solvent and solvent effect on the hydrogenation of NBR and gain new insight into NBR hydrogenation reaction process.
  (c) Li, Y.; Cheng, H.; Lin, W.; Zhang, C.; Wu, Q.; Zhao, F.; Arai, M. Solvent effects on heterogeneous catalysis in the selective hydrogenation of cinnamaldehyde over a conventional Pd/C catalyst. Catal. Sci. Technol. 2018, 8, 3580, DOI: 10.1039/C8CY00943K
  18c
  Solvent effects on heterogeneous catalysis in the selective hydrogenation of cinnamaldehyde over a conventional Pd/C catalyst
  Li, Yan; Cheng, Haiyang; Lin, Weiwei; Zhang, Chao; Wu, Qifan; Zhao, Fengyu; Arai, Masahiko
  Catalysis Science & Technology (2018), 8 (14), 3580-3589CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
  Solvent effects in the selective hydrogenation of an α,β-unsatd. aldehyde of cinnamaldehyde (CAL) were investigated under gas-liq.-solid reaction conditions. A conventional 5 wt% Pd/C catalyst and twelve org. solvents were used, including apolar solvents (three) and protic (two) and aprotic (seven) polar ones. The total rate of CAL hydrogenation strongly depended on the solvents used. The relationship between the rate of CAL hydrogenation obsd. and a few different solvent parameters was examd. The main product was hydrocinnamaldehyde (HCAL) with a selectivity of 80% or higher in most of the solvents examd. except for pyridine and 4-methylpyridine. In these two solvents, the CAL hydrogenation was slow and the main product was changed to cinnamyl alc. (COL) with a selectivity of about 60%. The addn. of a small vol. of pyridine to other solvents such as 2-propanol and THF could change the rate of hydrogenation and switch the main product from HCAL to COL. Possible interactions among the solvents, CAL substrate, and Pd/C catalyst were examd. by FTIR (attenuated total reflection (ATR-) and diffuse reflectance modes (DR-)) and TPD to discuss the solvent effects obsd. in the Pd-catalyzed heterogeneous CAL hydrogenation.
19. 19
  Fukushima, E.; Roeder, S. B. W. Experimental Pulse NMR: A nuts and bolts approach; Addison-Wesley Publishing Company: Boston, MA, 1981; p 539.
  There is no corresponding record for this reference.
20. 20
  Abragam, A. Principles of Nuclear Magnetism; Oxford Press: London, 1961.
  There is no corresponding record for this reference.
21. 21
  Aksnes, D. W.; Gjerdåker, L. NMR line width, relaxation and diffusion studies of cyclohexane confined in porous silica. J. Mol. Struct. 1999, 475, 27, DOI: 10.1016/S0022-2860(98)00493-1
  21
  NMR line width, relaxation and diffusion studies of cyclohexane confined in porous silica
  Aksnes, D. W.; Gjerdaker, L.
  Journal of Molecular Structure (1999), 475 (1), 27-34CODEN: JMOSB4; ISSN:0022-2860. (Elsevier Science B.V.)
  The rotational and translational dynamics of cyclohexane confined within silica pores of 4-50 nm in diam., have been studied as a function of temp. by measuring 1H NMR line widths, self-diffusion coeffs., spin-spin relaxation times, and spin-lattice relaxation times, and the results are compared with values obtained for the bulk material. The confinement in the pores gives rise to substantial changes in the mol. dynamics and phase behavior. The line shape and T2 measurements revealed a two-phase system, consisting of a liq.-like component at the surface and a plastic phase in the center of the pore. The highly mobile surface layer is observable far below the reduced transition temp. of the confined cyclohexane. A high diffusion rate of the adsorbed cyclohexane is obsd. over a wide temp. range - even well below the region of the depressed f.p. The diffusion coeff. of the mols. in the surface layer is three orders of magnitude larger than in the plastic phase of bulk cyclohexane.
22. 22
  Stapf, S.; Kimmich, R.; Zavada, T. Dynamics of plastic and liquid cyclohexane in bulk and in porous glasses studied by NMR methods. Appl. Magn. Reson. 1997, 12, 199, DOI: 10.1007/BF03162187
  22
  Dynamics of plastic and liquid cyclohexane in bulk and in porous glasses studied by NMR methods
  Stapf, Siegfried; Kimmich, R.; Zavada, T.
  Applied Magnetic Resonance (1997), 12 (2-3), 199-212CODEN: APMREI; ISSN:0937-9347. (Springer)
  Cyclohexane was investigated in bulk and in porous glasses with pore diams. of 4-208 nm at 136 ≤ T ≤ 300 K with field-cycling NMR relaxometry, field-gradient NMR diffusometry, transverse-relaxation spectroscopy, and DSC. The field-cycling data for the bulk material are described assuming translational modulation of intermol. dipole-dipole coupling. This is confirmed by expts. with different degrees of deuteration, and is in accordance with diffusion coeffs. detd. with the aid of field-gradient diffusometry. The confinement in pores produces substantial changes in the phase behavior and in mol. dynamics. For pore diams. of 30 nm and above, a non-frozen 2 monolayer thick film on the surface retains a diffusivity about 1 order of magnitude lower than in bulk liq., but 2 orders of magnitude larger than in the bulk plastic phase. Expts. indicate an exchange mechanism between this layer and the crystallite inside the pore. In glass with a pore diam. of 4 nm, all applied methods corroborate DSC results of the virtual absence of a phase transition and reveal a continuously decreasing translational mobility down to 100 K below the bulk liq./cubic phase transition temp.
23. 23
  (a) Deelchand, D. K.; Van de Moortele, P.-F.; Adriany, G.; Iltis, I.; Andersen, P.; Strupp, J. P.; Vaughan, J. T.; Uğurbil, K.; Henry, P.-G. In vivo ¹H NMR spectroscopy of the human brain at 9.4 T: Initial results. J. Magn. Reson. 2010, 206, 74, DOI: 10.1016/j.jmr.2010.06.006
  23a
  In vivo 1H NMR spectroscopy of the human brain at 9.4 T: Initial results
  Deelchand, Dinesh Kumar; Van de Moortele, Pierre-Francois; Adriany, Gregor; Iltis, Isabelle; Andersen, Peter; Strupp, John P.; Thomas Vaughan, J.; Ugurbil, Kamil; Henry, Pierre-Gilles
  Journal of Magnetic Resonance (2010), 206 (1), 74-80CODEN: JMARF3; ISSN:1090-7807. (Elsevier B.V.)
  In vivo proton NMR spectroscopy allows non-invasive detection and quantification of a wide range of biochem. compds. in the brain. Higher field strength is generally considered advantageous for spectroscopy due to increased signal-to-noise and increased spectral dispersion. So far 1H NMR spectra have been reported in the human brain up to 7 T. In this study we show that excellent quality short echo time STEAM and LASER 1H NMR spectra can be measured in the human brain at 9.4 T. The information content of the human brain spectra appears very similar to that measured in the past decade in rodent brains at the same field strength, in spite of broader linewidth in human brain. Compared to lower fields, the T1 relaxation times of metabolites were slightly longer while T2 relaxation values of metabolites were shorter (<100 ms) at 9.4 T. The linewidth of the total creatine (tCr) resonance at 3.03 ppm increased linearly with magnetic field (1.35 Hz/T from 1.5 T to 9.4 T), with a min. achievable tCr linewidth of around 12.5 Hz at 9.4 T. At very high field, B 0 microsusceptibility effects are the main contributor to the min. achievable linewidth.
  (b) Deelchand, D. K.; Henry, P.-G.; Uğurbil, K.; Marjanska, M. Measurement of Transverse Relaxation Times of J-Coupled Metabolites in the Human Visual Cortex at 4 T. Magn. Reson. Med. 2012, 67, 891, DOI: 10.1002/mrm.23080
  23b
  Measurement of transverse relaxation times of J-coupled metabolites in the human visual cortex at 4 T
  Deelchand, Dinesh Kumar; Henry, Pierre-Gilles; Ugurbil, Kamil; Marjanska, Malgorzata
  Magnetic Resonance in Medicine (2012), 67 (4), 891-897CODEN: MRMEEN; ISSN:0740-3194. (Wiley-Blackwell)
  Accurate quantification of 1H NMR spectra often requires knowledge of the relaxation times to correct for signal losses due to relaxation and satn. In human brain, T2 values for singlets such as N-acetylaspartate, creatine, and choline have been reported, but few T2 values are available for J-coupled spin systems. The purpose of this study was to measure the T2 relaxation times of J-coupled metabolites in the human occipital lobe using the LASER sequence. Spectra were acquired at multiple echo times and were analyzed with an LCModel using basis sets simulated at each echo time. Sep. basis spectra were used for resonances of protons belonging to the same mol. but having very different T2 values (e.g., two sep. basis spectra were used for the singlet and multiplet signal in N-acetylaspartate). The T2 values for the N-acetylaspartate multiplet (149 ± 12 ms), glutamate (125 ± 10 ms), myo-inositol (139 ± 20 ms), and taurine (196 ± 28 ms) were successfully measured in the human visual cortex at 4 T. These measured T2 relaxation times have enabled the accurate and abs. quantification of cerebral metabolites at longer echo times.
24. 24
  Fogler, H. S. Elements of Chemical Reaction Engineering; Prentice Hall: New Jersey, USA, 2005.
  There is no corresponding record for this reference.
25. 25
  D’Agostino, C.; Feaviour, M. R.; Brett, G. L.; Mitchell, J.; York, A. P. E.; Hutchings, G. J.; Mantle, M. D.; Gladden, L. D. Solvent inhibition in the liquid-phase catalytic oxidation of 1,4-butanediol: understanding the catalyst behaviour from NMR relaxation time measurements. Catal. Sci. Technol. 2016, 6, 7896, DOI: 10.1039/C6CY01458E
  25
  Solvent inhibition in the liquid-phase catalytic oxidation of 1,4-butanediol: understanding the catalyst behaviour from NMR relaxation time measurements
  D'Agostino, Carmine; Feaviour, Mark R.; Brett, Gemma L.; Mitchell, Jonathan; York, Andrew P. E.; Hutchings, Graham J.; Mantle, Mick D.; Gladden, Lynn F.
  Catalysis Science & Technology (2016), 6 (21), 7896-7901CODEN: CSTAGD; ISSN:2044-4753. (Royal Society of Chemistry)
  Catalytic reaction studies and NMR (NMR) relaxation time measurements have been compared to study the influence of competitive adsorption of reactant and solvent on catalytic conversion. The reaction chosen is the aerobic catalytic oxidn. of 1,4-butanediol in methanol over different supported-metal catalysts. From the NMR T1/T2 ratio, where T1 is the longitudinal and T2 the transverse spin relaxation time, the relative affinity of reactant and solvent for different catalytic surfaces is detd. The catalysts with the lowest activity show a preferential surface affinity for the solvent compared to the reactant. Conversely, the catalyst with the highest activity shows a preferential surface affinity for the reactant compared to the solvent. Significantly, Ru/SiO2, which is totally inactive for the oxidn. of 1,4-butanediol, exhibited a lower T1/T2 ratio (surface affinity) for 1,4-butanediol (reactant) than for a "weakly-interacting" alkane, indicating a very poor surface affinity for the diol functionality. The results provide direct evidence of the importance of the adsorbate-adsorbent interactions on catalyst activity in liq.-phase oxidns. and indicate that the competitive adsorption of the solvent plays an important role in these reactions. This work demonstrates that NMR relaxation time anal. is a powerful method for comparing adsorption of liqs. in porous catalysts, providing valuable information on the affinity of different chem. species for a catalyst surface. Moreover, the results demonstrate that NMR relaxation time measurements can be used not only to guide selection of solvent for use with a specific catalyst, but also selection of the catalyst itself. The results suggest that this method may be used to predict catalyst behavior, enabling improved design and optimization of heterogeneous catalytic processes.
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- Experimental procedures, characterization data, and T₁, T₂, and diffusion measurements (PDF)
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Oxidative Coupling of Aldehydes with Alcohol for the Synthesis of Esters Promoted by Polystyrene-Supported N-Heterocyclic Carbene: Unraveling the Solvent Effect on the Catalyst Behavior Using NMR Relaxation
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Oxidative Coupling of Aldehydes with Alcohol for the Synthesis of Esters Promoted by Polystyrene-Supported N-Heterocyclic Carbene: Unraveling the Solvent Effect on the Catalyst Behavior Using NMR RelaxationClick to copy article linkArticle link copied!

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Oxidative Coupling of Aldehydes with Alcohol for the Synthesis of Esters Promoted by Polystyrene-Supported N-Heterocyclic Carbene: Unraveling the Solvent Effect on the Catalyst Behavior Using NMR Relaxation
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