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Development of Nonclassical Photoprecursors for Rh2 Nitrenes
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Inorganic Chemistry

Cite this: Inorg. Chem. 2023, 62, 31, 12557–12564
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https://doi.org/10.1021/acs.inorgchem.3c01820
Published July 27, 2023

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Abstract

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Characterization of reactive intermediates in C–H functionalization is challenging due to the fleeting lifetimes of these species. Synthetic photochemistry provides a strategy to generate post-turnover-limiting-step intermediates in catalysis under cryogenic conditions that enable characterization. We have a long-standing interest in the structure and reactivity of Rh2 nitrene intermediates, which are implicated as transient intermediates in Rh2-catalyzed C–H amination. Previously, we demonstrated that Rh2 complexes bearing organic azide ligands can serve as solid-state and in crystallo photoprecursors in the synthesis of transient Rh2 nitrenoids. Complementary solution-phase experiments have not been available due to the weak binding of most organic azides to Rh2 complexes. Furthermore, the volatility of the N2 that is evolved during in crystallo nitrene synthesis from these precursors has prevented the in crystallo observation of C–H functionalization from lattice-confined nitrenes. Motivated by these challenges, here we describe the synthesis and photochemistry of nonclassical nitrene precursors based on sulfilimine ligands. Sulfilimines bind to Rh2 carboxylate complexes more tightly than the corresponding azides, which has enabled the full solid-state and solution-phase characterization of these new complexes. The higher binding affinity of sulfilimine ligands as compared with organic azides has enabled both solution-phase and solid-state nitrene photochemistry. Cryogenic photochemical studies of Rh2 sulfilimine complexes confined within polystyrene thin films demonstrate that sulfilimine photochemistry can be accomplished at low temperature but that C–H amination is rapid at temperatures compatible with N═S photoactivation. The potential of these structures to serve as platforms for multistep in crystallo cascades is discussed.

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Synopsis

The synthesis of dibenzothiophene-derived sulfilimine ligands and their coordination with Rh2esp2 is described. The coordination of the sulfilimine ligands with Rh2(II,II) complexes is characterized by UV−vis, IR, NMR spectroscopy, and X-ray crystallography. Photochemically promoted nitrogen group transfer (NGT) chemistry is observed in both solution-phase and solid-state photolyses, demonstrating the potential of sulfilimine ligands as nonclassical nitrene photoprecursors.

Introduction

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Rh2-catalyzed C–H amination reactions have emerged as among the most efficient and functional group-tolerant methods to convert readily available C–H bonds to C–N bonds and have been demonstrated in the context of complex molecule synthesis. (1−10) Motivated both by a fundamental interest in the mechanistic details of C–H amination processes and by a practical interest in extending the scope of Rh2-catalyzed amination to stronger C–H bonds, unique chemoselectivities, and intermolecular bond constructions, a series of experimental and computational investigations have been pursued to elucidate the key mechanistic details of C–H cleavage and nitrogen-group transfer (NGT). (11−17) In general, Rh2-catalyzed NGT reactions proceed through turnover-limiting nitrene transfer from iminoiodinane, organic azide, or hydroxylamine derivatives (i.e., classical nitrene precursors) to the Rh2 catalyst to generate a transient Rh2 nitrene intermediate. (11,12) Subsequent NGT to the C–H bond affords amines and regenerates the Rh2[II,II] catalyst. Because the Rh2 nitrene intermediates that engage in C–H bond cleavage and NGT chemistry is post-turnover limiting step, these structures are not observable in operando but are instead inferred based on kinetics experiments and computational studies. (11,15)
We have been interested in advancing in crystallo photochemistry as a method to prepare and characterize reactive intermediates within single crystal environments (Figure 1a). (18−22) These studies are predicated on the photolysis of single-crystal samples of molecular precursors to unveil the reactive species of interest. (23−30) The combination of lattice confinement and cryogenic temperatures extends the lifetimes of photogenerated intermediates to enable characterization by using diffraction-based methods. In the context of Rh2 nitrenes, we demonstrated that organic azide ligands represent appropriate photoprecursors to the Rh2 nitrenes (via solid-state photoextrusion of N2). (19,21) While these studies enabled observation of Rh2 nitrenes, (1) N2 mobility within the single crystal resulted in loss of crystallinity during thermal annealing experiments designed to promote in crystallo NGT at the lattice-confined Rh2 nitrene, and (2) lability of organic azide ligands in solution prevented complementary spectroscopic experiments. (18)

Figure 1

Figure 1. (a) In crystallo photogeneration and characterization of transient Rh2 nitrene from an organic azide photoprecursor. (b) Photoactivation of dibenzosulfilimines affords reactive nitrene fragments with concurrent extrusion of dibenzothiophene. (c) Here, we demonstrate the nitrene photochemistry of Rh2 sulfilimine complexes.

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We sought to overcome these challenges by designing new photolabile nitrene precursors (i.e., nonclassical nitrene precursors). (20) We envisioned that access to more strongly binding ligands could facilitate solution-phase photochemical studies and that photoprecursors based on nongaseous leaving groups may enable multistep in crystallo cascades to be accomplished without loss of crystallinity. Motivated by reports of S–N bond photoactivation from dibenzothiophene sulfilimines to unveil reactive nitrene fragments and dibenzothiophene (dbt, Figure 1b), (31−34) here, we describe the synthesis, characterization, and nitrene photochemistry of a family of Rh2 sulfilimine complexes (Figure 1c). Ligand titration experiments confirm that sulfilimine ligands bind Rh2 more tightly than do the corresponding azide ligands, which enables solution-phase characterization of these precursors. Photolysis, in either the solution phase or the solid state, results in NGT chemistry. Immobilization within polystyrene thin films enabled cryogenic photochemical experiments to be carried out, which revealed that NGT is fast relative to N–S photoactivation, even at cryogenic temperatures. The results presented demonstrate a strategy in the design of nitrene photoprecursors that enables solution-phase and solid-state experiments to be pursued.

Results and Discussion

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Synthesis and Characterization of Rh2 Sulfilimine Complexes

A small family of dibenzosulfilimine ligands (6a–6e) was prepared by treatment of dibenzo[b,d]thiophene 5-oxide (3a) or 4,6-dimethylated dibenzo[b,d]thiophene-5-oxide (3b) with the appropriate aryl sulfonamide in the presence of trifluoroacetic anhydride (TFAA, eq 1). (33) This suite of sulfilimines was selected because photochemically promoted dibenzothiophene extrusion from 9a–9e would result in differently substituted N-arylsulfonyl nitrenes and N-substitution has been shown to have a marked effect on nitrene transfer reactivity. (31−33) Photochemically promoted dibenzothiophene extrusion from 9d would result in an N-arylsulfonyl nitrene featuring pendant methine C–H bonds, which represent potential sites for intramolecular C–H amination. (35)
Complexation of these sulfilimine ligands with Rh2esp2 was pursued because (1) Rh2esp2 is commonly employed as a catalyst in nitrene transfer chemistry (1,2,5,7,8,13,14,16,17,36,37) and (2) our previous in crystallo studies employed this complex. (18,21) Treatment of Rh2esp2 with 2 equiv of sulfilimine ligands 6a–6d resulted in the formation of 2:1 complexes 9a–9d, which feature a N-coordinated sulfilimine ligand on both axial sites of the Rh2 core (Figure 2). In contrast, 6e forms an S-coordinated 2:1 complex with the Rh2(II,II) core (9e). 1H NMR analysis of a 2:1 mixture of Rh2esp2 and 6a–6e features diamagnetically shifted peaks that are integrated, consistent with the assigned structure. Complexes 9a–9e are thermally stable compounds, which based on 1H NMR studies persist in solution for hours to days in the absence of light.

Figure 2

Figure 2. Synthesis of N-coordinated bis-sulfilimine Rh2 complexes (9a–9d), and S-coordinated bis-sulfilimine Rh2 complex 9e, which were isolated by crystallization.

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UV–vis spectroscopy was utilized to evaluate the donicity of the sulfilimine ligands. (21) A CH2Cl2 solution of Rh2esp2 displays an absorbance at 425 nm, which has been ascribed to the Rh–Rh π* → Rh–O σ* transition, and a lower energy transition at 665 nm, which has been ascribed to the π* → σ* HOMO–LUMO transition. (38) In comparison, while the higher energy feature does not vary significantly as a function of the sulfilimine structure (9a–9d), the lower energy feature of these complexes is blue-shifted to 639, 641, 655, and 662 nm, respectively. The lower energy feature is sensitive to the donicity of the axial ligand as this donor perturbs the π* → σ* transition: (39) In general, the higher the donicity of the axial ligand, the larger the HOMO/LUMO gap is, because the σ* parameter would be raised by the stronger donation of the axial ligand and, consequently, the lower energy band will be more blue-shifted. (38) Based on this argument, 6a and 6b are stronger donors than 6c and 6d to the Rh2(II,II) core.
Sulfilimine complexes 9 were further characterized in the solid state by diffuse reflectance and IR spectroscopies. Diffuse reflectance UV–vis spectra were obtained for crystalline samples of compounds 1, 9b, and 9d (see Figure S1 in the Supporting Information). Similar to the solution-phase spectra of these compounds, the low-energy feature of Rh2esp2 is blue-shifted upon coordination of sulfilimines. When compared with the solution-state spectra, the solid-state spectra indicate that the Rh–Rh π* → σ* transition is sensitive to solvent polarity: In the solid state, the low energy bands of 9b and 9d are blue-shifted to 582 and 632 nm, respectively, as compared to 641 nm (9b) and 662 nm (9d) in solution. (40) The collected IR spectra indicated that the S═N stretching frequency is significantly red-shifted upon binding to Rh2esp2: For 6a, νN═S = 939 cm–1, while, for 9a, νN═S = 929 cm–1 (see Figure S2 in the Supporting Information for IR spectral comparison of sulfilimines and the corresponding Rh2 complexes).
The solid-state structures of 9a, 9b, 9c, 9d, and 9e were obtained by single-crystal X-ray diffraction analysis, and displacement ellipsoid plots are collected in Figure 3. Refinement data are collected in Table 1. Consistent with the 1H NMR analysis, X-ray structures show that 9a, 9b, 9c, 9d, and 9e are 2:1 complexes in which the sulfilimine ligands bind in the apical positions of the Rh2(II,II) core. The Rh(1)–N(1) distances in 9a–9d are 2.2992(2) Å, 2.297(4) Å, 2.3374(1) Å, and 2.363(3) Å, respectively. The significant elongation of the Rh–N bond for compounds 9c and 9d may arise due to the enhanced steric profile of sulfilimines 6c and 6d. (38) The apparent weaker coordination of the sulfilimine ligands in complexes 9c and 9d is consistent with UV vis analysis described above. (38) In contrast to 9a–9d, which feature N-bound sulfilimine ligands, 9e features an S-bound ligand. This may arise due to differences in the spatial orientations of the aryl sulfonyl substituents and the dibenzothiophene fragment of the sulfilimine ligands: For 9a–9d, the two rings are almost parallel, making the N-atom accessible to coordinate with the Rh2(II,II); in the case of 9e, the dimethylated dibenzothiophene and the aryl sulfonyl moieties are perpendicular to each other in such a way that the N-atom is unable to access the Rh2 center.

Figure 3

Figure 3. Displacement ellipsoid plots of N-coordinated Rh2 sulfilimine complexes 9a–9d and S-coordinated 9e plotted at 50% probability. H atoms and solvents are removed for clarity. The crystalline sample used in this diffraction experiment was obtained from a concentrated CH2Cl2 solution layered with pentane at −20 °C. Selected metrical parameters: for 9a, Rh(1)–N(1) = 2.2992(2) Å and Rh(1)–Rh(2) = 2.3893(3) Å; for 9b. Rh(1)–N(1) = 2.297(4) Å and Rh(1)–Rh(2) = 2.3944(8) Å; for 9c, Rh(1)–N(1) = 2.3374(1) Å and Rh(1)–Rh(2) = 2.3921(3) Å; for 9d, Rh(1)–N(1) = 2.363(3) Å and Rh(1)–Rh(2) = 2.4026(5) Å; for 9e, Rh(1)–S(1) = 2.5438(5) Å and Rh(1)–Rh(2) = 2.4048(3) Å.

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Table 1. Crystal Data and Structure Refinement
 9a·2CH2Cl29b·CH2Cl29c·4CH2Cl29d9e
formulaC70H70N2O12Rh2S4·2(CH2Cl2)C68H64N4O16Rh2S4·CH2Cl2C68H62N6O20Rh2S4·4(CH2Cl2)C86H102N2O12Rh2S4C90H110N2O12Rh2S4
temp, K110100110100110
cryst systemorthorhombicmonoclinictriclinictriclinicmonoclinic
space groupPbcaC2/cP1̅P1̅P21/C
colorgreengreengreenlight greengreen
a, Å19.1550(7)25.393(3)10.7438(6)13.044(1)15.610(1)
b, Å17.2810(5)14.204(2)13.7006(8)13.531(2)11.0136(8)
c, Å21.6940(6)22.950(3)14.487(1)14.145(1)26.606(2)
α, deg9090102.721(2)62.503(2)90
β, deg90120.729(2)93.557(2)69.314(2)95.679(2)
γ, deg9090102.459(2)72.954(2)90
V, A37181.1(4)7715.0(1)2017.7(2)2046.2(3)4551.7(5)
Z44112
R1a0.0340.06830.02730.04530.0373
wR2b0.07240.17340.05340.10980.0184
GOF (F2)c1.0621.1771.0581.0521.110
a

R1 = ∑||Fo – |Fc||/∑|Fo|.

b

wR2 = (∑(w(Fo2 – Fc2)2)/∑(w(Fo2)2))1/2.

c

GOF = (∑w(Fo2 – Fc2)2/(n – p))1/2, where n is the number of data and p is the number of parameters refined.

Synthesis and Characterization of Dibenzothiophene Complex 10

The targeted nitrene photochemistry of Rh2 sulfilimine complexes would be accompanied by the evolution of dbt, which represents a potential ligand for Rh2. To evaluate the coordination chemistry of dibenzothiophene, we treated Rh2esp2 with 2 equiv of dibenzothiophene, which resulted in the formation of adduct 10 (Figure 4). Solution-phase UV–vis spectroscopy of 10 in dichloromethane revealed absorbances centered at 636 nm and a shoulder at 434 nm. 1H NMR analysis supported the formulation as a 2:1 adduct. Refinement of single-crystal X-ray diffraction data provided the structure illustrated in Figure 4.

Figure 4

Figure 4. Reaction of dibenzothiophene with 1 affords Rh2esp2(dbt)2 (10). Displacement ellipsoid plots of 10 are plotted at 50% probability. H-atoms and and solvent are removed for the sake of clarity. Selected metrical parameters: for 10, Rh(1)–S(1) = 2.5298(4) Å and Rh(1)–Rh(2) = 2.3993(3) Å.

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Ligand Binding Thermodynamics

To assess the binding of the new family of sulfilimines to Rh2, we carried out a series of UV–vis titrations of Rh2esp2 with sulfilimines 6a–6d. Here, we discuss the UV–vis titration of sulfilimine 6a with Rh2esp2 (1), which is representative of analogous experiments with 6b–6d. The data obtained from the titration of Rh2esp2 with 6a are illustrated in Figure 5 (see Figures S3–S6 in the Supporting Information) for spectral data corresponding to titrations of Rh2esp2 with sulfilimines 6a–6d). A dichloromethane solution of 1 displays characteristic peaks centered at 665 and 425 nm (vide supra). (21,38) UV–vis spectra obtained via the addition of increasing amounts of 6a to a solution of Rh2esp2 display a blue shift of the low-energy feature from 665 nm to 639 nm. The spectral evolution ceased after the addition of 2 equiv of 6a, consistent with conversion of Rh2esp2 to 9a and well-anchored isosbestic points at 646 and 434 nm indicate the absence of steady-state intermediates in this conversion.

Figure 5

Figure 5. UV–vis spectra obtained during the titration of 1 with 6a. The well-anchored isosbestic points at 434 and 639 nm indicate the absence of steady-state intermediates in the conversion of 1 (black) to 9a (red).

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The observation that spectral evolution ceases at a 1:2 ratio of 1 and 6a indicates tight binding of the sulfilimine ligand to the Rh2 core (Keq > 10). In contrast, similar concentration-dependent UV–vis titration spectra between 1 and TsN3 (8a) continues to evolve until the addition of ∼25 equiv of 8a (Figure S7 in the Supporting Information). Similar UV–vis analysis was also performed with NsN3 (8b) (Figure S8 in the Supporting Information) and TbsN3 (8c) (Figure S9 in the Supporting Information), which also show ∼20 equiv of organic azides are required to saturate the coordination of 1.
While sulfilimine ligand 6a is more tightly binding than the corresponding azide, it can be displaced by other potentially coordinating ligands, such as THF. Titration of complex 9a with tetrahydrofuran (THF) results in conversion to Rh2esp2(THF)2; ∼2.5 equiv of THF was needed to completely displace the sulfilimine ligands from 9a (see Figure S10 in the Supporting Information for similar analysis with 9a). These data indicate that sulfilimine ligands bind more weakly than THF but more strongly than the corresponding sulfonyl azides. Furthermore, while the addition of 6b or 6c to a solution of 9a did not result in a significant spectral change, the addition of 6a or 6b to a solution of 9c did result in a significant blue shift, which suggests that the sulfilimine ligands are exchangeable and that the binding preferences are 6a > 6b > 6c (Figure S11 in the Supporting Information).

Solution-Phase Photochemistry

Because the sulfilimine ligands are tightly bound to Rh2 (vide supra), we are able to explore the photochemistry of the corresponding Rh2 complexes (i.e., 9a–d) in the absence of an exogenous sulfilimine ligand. We began our investigations of the nitrene photochemistry of Rh2 sulfilimine complexes with complex 9d, because we hypothesized that potential intramolecular amination would simplify the analysis of the reaction outcomes. Photolysis (λ > 335 nm) of a CH2Cl2 solution of 9d was monitored periodically by UV–vis spectroscopy (Figure 6). The obtained spectra display a blue shift in the lowest energy absorbance, and the spectral evolution is characterized by the presence of a well-anchored isosbestic point at 675 nm, which indicates the lack of steady-state intermediates in the photochemistry of 9d. The ultimate spectrum obtained following photolysis is superimposable with that of dibenzothiophene complex 10 (Figure S12b in the Supporting Information). The identity of the Rh-containing photoproduct (i.e., 10) was further confirmed by crystallization and X-ray diffraction analysis of the photochemical reaction mixture. Preparative-scale photolysis of 9d enabled isolation and characterization of the organic fragment generated to be heterocycle 13d, which is the product expected of intramolecular C–H amination chemistry (57% yield) (for UV–vis and NMR analysis of the photolysis of compound 9d see Figures S12 and S13 in the Supporting Information). Similarly, the photolysis of a CH2Cl2 solution of 9e results in the formation of Rh2esp2(dmdbt)2 (15) (Figure S14 in the Supporting Information). The formation of heterocycle 13d is evidenced by the 1H NMR spectra of the reaction mixture following photolysis (Figure S15 in the Supporting Information). We speculate that the nitrene photochemistry of 9e may result from the initial S- to N-photoisomerization.

Figure 6

Figure 6. Photolysis of Rh2 sulfilimine 9d results in Rh2esp2(dbt)2 (10) and heterocycle 13d, which are the products expected of intramolecular NGT photochemistry. A well-anchored isosbestic point is observed at 675 nm for the UV–vis spectra collected during photolysis of compound 9d in CH2Cl2 (λ > 335 nm), which indicates the lack of a steady-state intermediate in the conversion of 9d to 10 and 13d.

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Photolysis of CH2Cl2 solutions of sulfilimine complexes 9a–9c in the presence of appropriate nitrene traps, such as tetralin or ethylbenzene, results in the products of intermolecular nitrene transfer: Rh2esp2(dbt)2 (10) and the trap-derived benzylamine derivative (13a–13c or 14a–14c) were observed by 1H NMR spectroscopy of crude photolysis mixtures (Figure 7). In comparison, photolysis of 6a and 6b under analogous conditions resulted in no amination products; photolysis of 6c resulted in <10% yield of 13c and trace amount of 14c. In addition, no amination from 9a–9c was obtained in the absence of light or thermolytic conditions at 40 °C (for UV–vis evolution and NMR analysis of the photolysis of compounds 9a–9c with tetralin, see Figures S13–S18 in the Supporting Information).

Figure 7

Figure 7. Photolysis of compounds 9a, 9b, and 9c with tetralin and ethylbenzene in CH2Cl2 results in the respective intermolecular aminated products.

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The efficiency of intermolecular C–H amination depends intimately on the substitution of the N-sulfonyl substituents. The addition of electron-withdrawing aryl substituents, which results in progressively lower ligand-centered LUMOs, results in increasingly efficient amination of both tetralin and ethylbenzene. Similar substituent effects have been previously described in Rh2-catalyzed nitrene transfer reactions. (14)

Solid-State Photochemistry

We were interested in studying the solid-state photochemistry of compounds 9 as a platform to both examine chemistry in the absence of potentially reactive solvents and to use this as the basis for in crystallo experiments. To this end, we monitored the solid-state photolysis of 9b and 9d by using IR spectroscopy (Figure 8). The time-dependent IR spectra of 9d display the depletion of the peak at 896 cm–1, which corresponds to the N═S stretching frequency in Rh2-bound sulfilimine ligands. The concurrent disappearance of the peak at 719 cm–1 and slight red shift of the peak at 752 to 744 cm–1 indicate the formation Rh2esp2(dbt)2 (10) during the solid-state photolysis (Figures S22 and S23 in the Supporting Information). Together, these observations suggest that the solid-state photochemistry of 9d is analogous to the solution-phase photochemistry. For detailed IR analysis of the solid-state photochemistry of compound 9b, see Figures S24 and S25 in the Supporting Information. Solid-state photolysis of 9e also proceeds analogously to the solution-phase experiments (i.e., the formation of 13d and 15; see Figures S26 and S27 in the Supporting Information for relevant spectral data).

Figure 8

Figure 8. Solid-state photolysis of compound IR spectra collected during the photolysis (335 nm < λ < 610 nm) of a KBr pellet of 9d at 23 °C from 0 min (black) to 24 h (red). The disappearance of the peaks at 896 cm–1 is for the N═S cleavage. The disappearance of the peak at 719 cm–1 and slight red shift from 752 cm–1 to 744 cm–1 depicts the conversion from 9d to 10 during 24 h of photolysis.

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Photocrystallography

We have previously demonstrated in crystallo photosynthesis of transient Rh2 nitrenes using Rh2 complexes featuring Rh2-bound organic azide ligands. (18,21) While these experiments provided evidence for the formation of triplet nitrene adducts of Rh2, the synthesis of the nitrene was accompanied by the extrusion of N2. At cryogenic temperatures, N2 could be localized in electron density maps. Warming the single-crystalline samples in order to promote C–H amination resulted in a rapid loss of crystallinity, presumably due to increased mobility of intracrystalline N2. (18) Based on the fact that dbt is solid at ambient temperature, we hypothesized that dbt photoextrusion could result in a crystalline nitrene that could engage in subsequent single-crystal to single-crystal C–H amination. To this end, we examined the photolysis (λ = 365 nm) of single crystals of complexes 9a–9d by X-ray diffraction. Photolysis was carried out at 100 K and diffraction data were collected with 50 keV synchrotron radiation. Unfortunately, in no case was significant single-crystal photochemistry observed. We speculate that in the van der Waals crystals used in these experiments, photogenerated dbt was not sufficiently mobile to migrate from the reactive nitrene, thus preventing productive N═S cleavage.

Polymer Thin-Film Photochemistry

While the sulfilimine ligands utilized to prepare adducts 9a–9d are more tightly binding than the corresponding azide ligands, displacement by potentially coordinating solvents (i.e., THF) represents a challenge to cryogenic photochemical experiments aimed at the spectroscopic characterization of the transient nitrene intermediates implicated above. For example, common glassy solvents such as 2-methyltetrahydrofuran displace the sulfilimine ligands of 9. Furthermore, complexes 9 are poorly soluble in potential noncoordinating glassy solvents, such as 2,2-dimethylbutane/tert-butylbenzene mixtures. (41)
To circumvent these challenges, we pursued low-temperature photochemical experiments using polymer thin films impregnated with Rh2 sulfilimine complexes (Figure 9). (41) To this end, we prepared a 4.27 mM solution of 9d in 1,2-dichloroethane containing 2 wt % polystyrene (Mw = 350 000). We dropcast a thin film on a sapphire slide and removed residual 1,2-dichloroethane in vacuo (for comparison between the solution-phase spectrum and thin-film spectrum of compound 9d see Figure S28b in the Supporting Information). Photolysis (λ = 405 nm) of this film at 129 K for 4 h resulted in a blue-shift of the low-energy spectral feature of 9d (Figure 9). Thermal annealing (300 K) followed by recooling to 129 K did not result in further spectral evolution. Furthermore, the spectrum following photolysis and thermal annealing (300 K) overlays with that of complex 10 (recorded as a polystyrene thin film). These observations indicate that C–H amination of the proximal C–H bond takes place even under cryogenic conditions. Photolysis at lower temperatures (i.e., 77 K) did not result in appreciable spectral evolution on experimentally tractable time scales.

Figure 9

Figure 9. Solid-state cryogenic photolysis of compound 9d in a polystyrene film matrix. (a) Solid-state UV–vis spectra periodically collected during the photolysis over 4 h at 129 K where the colors black, red, and green represent UV–vis spectra at t = 0, 4.5 h, and the end of the thaw-freeze cycle, respectively. (b) Overlay of the UV–vis spectrum of independently prepared 10 (blue) and the thermally annealed spectrum (red) indicates the formation of 10 during the cryogenic photolysis of 9d.

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The observation of facile intramolecular C–H amination for a thin film of 9d but no in crystallo photoconversion suggests that dibenzothiophene photoextrusion is likely reversible below temperatures that are compatible with C–H amination. Poor solubility in polymer thin films prevented analogous experiments with sulfilimine complexes 9a–9c.

Concluding Remarks

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Synthetic nitrene photochemistry requires access to the appropriate photoprecursor molecules. Many of the commonly encountered reagents in nitrene transfer catalysts, such as organic azides and iminoiodinanes, are either weakly binding ligands to many transition metals or kinetically labile toward nitrene transfer. As a result, access to solution-stable metal nitrene photoprecursors is limited, which prevents the general application of time-resolved solution-phase methods or solid-state matrix isolation techniques to the synthesis and characterization of these species.
Here, we have introduced sulfilimine ligands as photoprecursors to reactive metal nitrene intermediates. Specifically, we reported the coordination chemistry of a small family of sulfilimine ligands with Rh2esp2. UV–vis, NMR spectroscopy, and SCXRD data indicate 2:1 coordination of these ligands with Rh2(II,II) core. Concentration-dependent UV–vis titration revealed that that the sulfilimine ligands have higher binding affinity, compared to the corresponding organic azides. Both intermolecular and intramolecular C–H amination was achieved in the solution phase via the photoextrusion of the dibenzothiophene (dbt) moiety from the complexes.
Demonstration of solution-phase nitrene photochemistry presaged examination of the solid-state photolysis of these complexes. In a KBr pellet, dbt photoextrusion and ultimate formation of Rh2 dbt adducts (i.e., 10) was observed. C–H amination products were obtained as well demonstrating the homology of solution-phase and solid-state processes. Cyogenic photolysis of these compounds in polystyrene thin film matrix revealed that C–H amination is fast, relative to dbt photoextrusion, which prevented direct observation of incipient nitrene intermediates.
Together, the results of solution-phase and solid-state photochemical experiments demonstrated the ability to obtain nitrene photochemistry from sulfilimine precursors. These results provide the foundation for the application of sulfilimine ligands, which balance ground-state binding thermodynamics with photolability, in the photosynthesis of metal nitrenes of interest to catalysis.

Supporting Information

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The Supporting Information is available free of charge at https://pubs.acs.org/doi/10.1021/acs.inorgchem.3c01820.

  • Experimental procedures and spectroscopic data (PDF)

Accession Codes

CCDC 2260853, 2261071, 2261073, 2261078, 2261080, and 2264679 contain the supplementary crystallographic data for this paper. These data can be obtained free of charge via www.ccdc.cam.ac.uk/data_request/cif, or by emailing [email protected], or by contacting The Cambridge Crystallographic Data Centre, 12 Union Road, Cambridge CB2 1EZ, UK; fax: +44 1223 336033.

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Author Information

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  • Corresponding Author
  • Authors
    • Arpan Paikar - Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
    • Gerard P. Van Trieste III - Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
    • Anuvab Das - Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States;  Orcidhttps://orcid.org/0000-0002-9344-4414
    • Chih-Wei Wang - Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States;  Orcidhttps://orcid.org/0000-0003-3056-0917
    • Tiffany E. Sill - Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States;  Orcidhttps://orcid.org/0000-0002-3620-123X
    • Nattamai Bhuvanesh - Department of Chemistry, Texas A&M University, College Station, Texas 77843, United States
  • Author Contributions

    The manuscript was written through contributions of all authors.

  • Notes
    The authors declare no competing financial interest.

Acknowledgments

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The authors acknowledge the U.S. Department of Energy (DOE), Office of Science, Office of Basic Energy Sciences, Catalysis Program (No. DE-SC0018977), and the Welch Foundation (No. A-1907) for financial support. T.E.S. acknowledges the support of the NSF under a Graduate Research Fellowship (Grant No. DGE 1746932). The authors thank Dong Hee Son for access to cryogenic photochemical equipment and Sarbajit Banerjee for the use of in situ IR spectrometer. X-ray diffraction data of compounds 9b and 9d was collected at NSF’s ChemMatCARS, Sector 15 at the Advanced Photon Source (APS), Argonne National Laboratory (ANL). This facility is supported by the Divisions of Chemistry (CHE) and Materials Research (DMR), National Science Foundation (under Grant No. NSF/CHE-1834750). This research used resources of the Advanced Photon Source; a U.S. Department of Energy (DOE) Office of Science user facility operated for the DOE Office of Science by Argonne National Laboratory (under Contract No. DE-AC02-06CH11357).

References

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This article references 41 other publications.

  1. 1
    Chiappini, N. D.; Mack, J. B. C.; Du Bois, J. Intermolecular C(sp3)–H Amination of Complex Molecules. Angew. Chem., Int. Ed. 2018, 57, 4956– 4959,  DOI: 10.1002/anie.201713225
    Google Scholar
    1
    Intermolecular C(sp3)-H Amination of Complex Molecules
    Chiappini, Nicholas D.; Mack, James B. C.; Du Bois, J.
    Angewandte Chemie, International Edition (2018), 57 (18), 4956-4959CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A general and operationally convenient method for intermol. amination of C(sp3)-H bonds is described. This technol. allows for efficient functionalization of complex mols., including numerous pharmaceutical targets. The combination of pivalonitrile as a solvent, Al2O3 as an additive, and Ph sulfamate as a nitrogen source affords differential reaction performance and substrate scope. Mechanistic data strongly implicate a pathway for catalyst decompn. that initiates with solvent oxidn., thus providing rationale for the marked influence of pivalonitrile on this reaction process.
  2. 2
    Espino, C. G.; Fiori, K. W.; Kim, M.; Du Bois, J. Expanding the Scope of C–H Amination through Catalyst Design. J. Am. Chem. Soc. 2004, 126, 15378– 15379,  DOI: 10.1021/ja0446294
    Google Scholar
    2
    Expanding the Scope of C-H Amination through Catalyst Design
    Espino, Christine G.; Fiori, Kristin Williams; Kim, Mihyong; Du Bois, J.
    Journal of the American Chemical Society (2004), 126 (47), 15378-15379CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Anal. of the mechanism for Rh-mediated C-H amination has led to the development of a remarkably effective dinuclear Rh catalyst derived from 1,3-benzenedipropionic acid. This unique complex, Rh2(esp)2, is capable of promoting both intra- and intermol. C-H oxidn. reactions, and in all cases is superior to Rh2(O2CtBu)4. For the first time, C-H insertion is described with urea and sulfamide substrates to give 1,2- and 1,3-diamine derivs., resp. In addn., intermol. amination of benzylic and secondary C-H bonds is shown to proceed efficiently even under conditions in which the starting alkane is employed as the limiting reagent.
  3. 3
    Espino, C. G.; Wehn, P. M.; Chow, J.; Du Bois, J. Synthesis of 1,3-Difunctionalized Amine Derivatives through Selective C–H Bond Oxidation. J. Am. Chem. Soc. 2001, 123, 6935– 6936,  DOI: 10.1021/ja011033x
    Google Scholar
    3
    Synthesis of 1,3-Difunctionalized Amine Derivatives through Selective C-H Bond Oxidation
    Espino, Christine G.; Wehn, Paul M.; Chow, Jessica; Du Bois, J.
    Journal of the American Chemical Society (2001), 123 (28), 6935-6936CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Cyclization of sulfamate esters via a Rh-catalyzed C-H bond oxidn./insertion reaction is described. Thus, oxathiazinanes, e.g. I, were prepd. from the stereoselective intramol. oxidative cyclization of sulfamate esters, e.g. II, using Rh2(OAc)4, PhI(OAc)2, and MgO in CH2Cl2. Nucleophilic ring opening of oxathiazinanes with water followed by oxidn. afforded β-amino acids. Thus, chiral oxathiazinane III underwent ring opening followed by oxidn. to afford (R)-(Benzyloxycarbonyl)-β-isoleucine in 81% yield.
  4. 4
    Nägeli, I.; Baud, C.; Bernardinelli, G.; Jacquier, Y.; Moraon, M.; Müllet, P. Rhodium(II)-Catalyzed CH Insertions with {[(4-Nitrophenyl)sulfonyl]imino}phenyl-λ3-iodane. Helv. Chim. Acta 1997, 80, 1087– 1105,  DOI: 10.1002/hlca.19970800407
    Google Scholar
    4
    Rhodium(II)-catalyzed CH insertions with {[(4-nitrophenyl)sulfonyl]imino}phenyl-λ3-iodane
    Nageli, Ivo; Baud, Corine; Bernardinelli, Gerald; Jacquier, Yvan; Moran, Mary; Muller, Paul
    Helvetica Chimica Acta (1997), 80 (4), 1087-1105CODEN: HCACAV; ISSN:0018-019X. (Verlag Helvetica Chimica Acta)
    The [Rh2(OAc)4]-catalyzed decompn. of {[(4-nitrophenyl)sulfonyl]imino}phenyl-λ3-iodane (NsN:IPh) resulted in formal insertions into CH bonds, activated by Ph or vinyl groups, or by O-substituents. Scope and limitations of the reaction were investigated. Yields ≤84% were achieved in the most favorable cases, and were enhanced by electron-releasing substituents and decreased by steric hindrance. Aziridination competed with allylic insertion with olefinic substrates. The insertion reaction proceeded with retention of configuration. With chiral Rh(II) catalysts, a modest asym. induction was obsd. A mechanism involving direct insertion by a Rh-complexed nitrene into the CH bond is proposed.
  5. 5
    Nguyen, Q.; Sun, K.; Driver, T. G. Rh2(II)-Catalyzed Intramolecular Aliphatic C–H Bond Amination Reactions Using Aryl Azides as the N-Atom Source. J. Am. Chem. Soc. 2012, 134, 7262– 7265,  DOI: 10.1021/ja301519q
    Google Scholar
    5
    Rh2(II)-Catalyzed intramolecular aliphatic C-H bond amination reactions using aryl azides as the N-atom source
    Nguyen, Quyen; Sun, Ke; Driver, Tom G.
    Journal of the American Chemical Society (2012), 134 (17), 7262-7265CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Rhodium(II) dicarboxylate complexes were discovered to catalyze the intramol. amination of unactivated primary, secondary, or tertiary aliph. C-H bonds using aryl azides as the N-atom precursor. While a strong electron-withdrawing group on the nitrogen atom is typically required to achieve this reaction, we found that both electron-rich and electron-poor aryl azides are efficient sources for the metal nitrene reactive intermediate.
  6. 6
    Park, Y.; Kim, Y.; Chang, S. Transition Metal-Catalyzed C–H Amination: Scope, Mechanism, and Applications. Chem. Rev. 2017, 117, 9247– 9301,  DOI: 10.1021/acs.chemrev.6b00644
    Google Scholar
    6
    Transition Metal-Catalyzed C-H Amination: Scope, Mechanism, and Applications
    Park, Yoonsu; Kim, Youyoung; Chang, Sukbok
    Chemical Reviews (Washington, DC, United States) (2017), 117 (13), 9247-9301CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. The recent advances in intra- and intermol. C-H amination reactions utilizing late transition metal-based catalysts have been reviewed.
  7. 7
    Paudyal, M. P.; Adebesin, A. M.; Burt, S. R.; Ess, D. H.; Ma, Z.; Kürti, L.; Falck, J. R. Dirhodium-catalyzed C–H arene amination using hydroxylamines. Science 2016, 353, 1144– 1147,  DOI: 10.1126/science.aaf8713
    Google Scholar
    7
    Dirhodium-catalyzed C-H arene amination using hydroxylamines
    Paudyal, Mahesh P.; Adebesin, Adeniyi Michael; Burt, Scott R.; Ess, Daniel H.; Ma, Zhiwei; Kuerti, Laszlo; Falck, John R.
    Science (Washington, DC, United States) (2016), 353 (6304), 1144-1147CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
    Primary and N-alkyl arylamine motifs are key functional groups in pharmaceuticals, agrochems., and functional materials, as well as in bioactive natural products. However, there is a dearth of generally applicable methods for the direct replacement of aryl hydrogens with NH2/NH(alkyl) moieties. Here, we present a mild dirhodium-catalyzed C-H amination for conversion of structurally diverse monocyclic and fused aroms. to the corresponding primary and N-alkyl arylamines using NH2/NH(alkyl)-O-(sulfonyl)hydroxylamines as aminating agents; the relatively weak RSO2O-N bond functions as an internal oxidant. The methodol. is operationally simple, scalable, and fast at or below ambient temp., furnishing arylamines in moderate-to-good yields and with good regioselectivity. It can be readily extended to the synthesis of fused N-heterocycles.
  8. 8
    Roizen, J. L.; Zalatan, D. N.; Du Bois, J. Selective Intermolecular Amination of C–H Bonds at Tertiary Carbon Centers. Angew. Chem., Int. Ed. 2013, 52, 11343– 11346,  DOI: 10.1002/anie.201304238
    Google Scholar
    8
    Selective Intermolecular Amination of C-H Bonds at Tertiary Carbon Centers
    Roizen, Jennifer L.; Zalatan, David N.; Du Bois, J.
    Angewandte Chemie, International Edition (2013), 52 (43), 11343-11346CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    An effective process for generating tetrasubstituted amine derivs., e.g., I, through selective, intermol. tertiary C-H bond amination have been developed. The optimized reaction is performed with limiting amts. of substrate, 1 mol % of com. available [Rh2(esp)2], and inexpensive PhI(OAc)2 as the terminal oxidant. The identification of aryloxysulfonamides, in particular DfsNH2, as functional nitrogen sources has been an instrumental find. Competition studies with substrates possessing disparate C-H bond types reveal variations in product selectivity, which derive solely from the choice of sulfamate ester.
  9. 9
    Trowbridge, A.; Walton, S. M.; Gaunt, M. J. New Strategies for the Transition-Metal Catalyzed Synthesis of Aliphatic Amines. Chem. Rev. 2020, 120, 2613– 2692,  DOI: 10.1021/acs.chemrev.9b00462
    Google Scholar
    9
    New Strategies for the Transition-Metal Catalyzed Synthesis of Aliphatic Amines
    Trowbridge, Aaron; Walton, Scarlett M.; Gaunt, Matthew J.
    Chemical Reviews (Washington, DC, United States) (2020), 120 (5), 2613-2692CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review. In light of the ever-increasing importance of aliph. amines across the range of chem. sciences, this review aims to provide a concise overview of modern transition-metal catalyzed approaches to alkylamine synthesis and their functionalization. Selected examples of amine bond forming reactions include, hydroamination and hydroaminoalkylation, transition-metal catalyzed C(sp3)-H functionalization and transition-metal catalyzed visible-light-mediated light photoredox catalysis.
  10. 10
    van den Heuvel, N.; Mason, S. M.; Mercado, B. Q.; Miller, S. J. Aspartyl β-Turn-Based Dirhodium(II) Metallopeptides for Benzylic C(sp3)–H Amination: Enantioselectivity and X-ray Structural Analysis. J. Am. Chem. Soc. 2023, 145, 12377– 12385,  DOI: 10.1021/jacs.3c03587
    Google Scholar
    10
    Aspartyl β-Turn-Based Dirhodium(II) Metallopeptides for Benzylic C(sp3)-H Amination: Enantioselectivity and X-ray Structural Analysis
    van den Heuvel, Naudin; Mason, Savannah M.; Mercado, Brandon Q.; Miller, Scott J.
    Journal of the American Chemical Society (2023), 145 (22), 12377-12385CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Amination of C(sp3)-H bonds is a powerful tool to introduce N into complex org. frameworks in a direct manner. Despite significant advances in catalyst design, full site- and enantiocontrol in complex mol. regimes remain elusive using established catalyst systems. To address these challenges, the authors herein describe a new class of peptide-based dirhodium(II) complexes derived from aspartic acid-contg. β-turn-forming tetramers. This highly modular system can serve as a platform for the rapid generation of new chiral dirhodium(II) catalyst libraries, as illustrated by the facile synthesis of 38 catalysts. Critically, the authors present the 1st crystal structure of a dirhodium(II) tetra-aspartate complex, which unveils retention of the β-turn conformation of the peptidyl ligand; a well-defined H-bonding network is evident, along with a near-C4 symmetry that renders the Rh centers inequivalent. The utility of this catalyst platform is illustrated by the enantioselective amination of benzylic C(sp3)-H bonds, in which state-of-the-art levels of enantioselectivity up to 95.5:4.5 er were obtained, even for substrates that present challenges with previously reported catalyst systems. Addnl., the authors found these complexes to be competent catalysts for the intermol. amination of N-alkylamides via insertion into the C(sp3)-H bond α to the amide N, yielding differentially protected 1,1-diamines. Of note, this type of insertion also occurs on the amide functionalities of the catalyst itself in the absence of the substrate but did not appear to be detrimental to reaction outcomes when the substrate was present.
  11. 11
    Perry, R. H.; Cahill, T. J.; Roizen, J. L.; Du Bois, J.; Zare, R. N. Capturing fleeting intermediates in a catalytic C–H amination reaction cycle. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 18295– 18299,  DOI: 10.1073/pnas.1207600109
    Google Scholar
    11
    Capturing fleeting intermediates in a catalytic C-H amination reaction cycle
    Perry, Richard H.; Cahill, Thomas J., III; Roizen, Jennifer L.; Du Bois, Justin; Zare, Richard N.
    Proceedings of the National Academy of Sciences of the United States of America (2012), 109 (45), 18295-18299, S18295/1-S18295/6CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
    The authors have applied an ambient ionization technique, desorption electrospray ionization MS, to identify transient reactive species of an archetypal C-H amination reaction catalyzed by a dirhodium tetracarboxylate complex. Using this anal. method, the authors have detected previously proposed short-lived reaction intermediates, including two nitrenoid complexes that differ in oxidn. state. Findings suggest that an Rh-nitrene oxidant can react with hydrocarbon substrates through a hydrogen atom abstraction pathway and raise the intriguing possibility that two catalytic C-H amination pathways may be operative in a typical bulk soln. reaction. As highlighted by these results, desorption electrospray ionization MS should have broad applicability for the mechanistic study of catalytic processes.
  12. 12
    Zalatan, D. N.; Du Bois, J. Understanding the Differential Performance of Rh2(esp)2 as a Catalyst for C–H Amination. J. Am. Chem. Soc. 2009, 131, 7558– 7559,  DOI: 10.1021/ja902893u
    Google Scholar
    12
    Understanding the Differential Performance of Rh2(esp)2 as a Catalyst for C-H Amination
    Zalatan, David N.; Du Bois, J.
    Journal of the American Chemical Society (2009), 131 (22), 7558-7559CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Catalytic amination of satd. C-H bonds is performed efficiently with the use of Rh2(esp)2. Efforts to identify pathways for catalyst degrdn. and/or arrest have revealed a single-electron oxidn. event that gives rise to a red-colored, mixed-valence dimer, [Rh2(esp)2]+. This species is fortuitously reduced by carboxylic acid, a byproduct generated in the reaction cycle with each turnover of the diacyloxyiodine oxidant. These findings have led to the conclusion that the high performance of Rh2(esp)2 is due in part to the superior kinetic stability of its one-electron oxidized form relative to other dimeric Rh complexes.
  13. 13
    Fiori, K. W.; Espino, C. G.; Brodsky, B. H.; Du Bois, J. A mechanistic analysis of the Rh-catalyzed intramolecular C–H amination reaction. Tetrahedron 2009, 65, 3042– 3051,  DOI: 10.1016/j.tet.2008.11.073
    Google Scholar
    13
    A mechanistic analysis of the Rh-catalyzed intramolecular C-H amination reaction
    Fiori, Kristin Williams; Espino, Christine G.; Brodsky, Benjamin H.; Du Bois, J.
    Tetrahedron (2009), 65 (16), 3042-3051CODEN: TETRAB; ISSN:0040-4020. (Elsevier Ltd.)
    A detailed mechanistic investigation of the intramol. dirhodium tetracarboxylate-catalyzed sulfamate ester C-H amination reaction is presented. These studies provide support for the formation of a sulfamate-derived iminoiodinane, which reacts rapidly with the rhodium catalyst to generate a nitrenoid-type oxidant. Reactivity patterns, Hammett anal., kinetic isotope measurement, and a cyclopropane clock expt. are indicative of a concerted, asynchronous transition structure in the product-detg. C-H insertion event.
  14. 14
    Fiori, K. W.; Du Bois, J. Catalytic Intermolecular Amination of C–H Bonds: Method Development and Mechanistic Insights. J. Am. Chem. Soc. 2007, 129, 562– 568,  DOI: 10.1021/ja0650450
    Google Scholar
    14
    Catalytic Intermolecular Amination of C-H Bonds: Method Development and Mechanistic Insights
    Fiori, Kristin Williams; Du Bois, J.
    Journal of the American Chemical Society (2007), 129 (3), 562-568CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Reaction methodol. for intermol. C-H amination of benzylic and 3° C-H bonds is described. This process uses the starting alkane as the limiting reagent, gives optically pure tetrasubstituted amines through stereospecific insertion into enantiomeric 3° centers, displays high chemoselectivity for benzylic oxidn., and enables the facile prepn. of isotopically enriched 15N-labeled compds. Access to substituted amines, amino alcs., and diamines is thereby made possible in a single transformation. Important information relevant to understanding the initial steps in the catalytic cycle, reaction chemoselectivity, the nature of the active oxidant, and pathways for catalyst inactivation has been gained through mechanistic anal.; these studies are also presented.
  15. 15
    Varela-Álvarez, A.; Yang, T.; Jennings, H.; Kornecki, K. P.; Macmillan, S. N.; Lancaster, K. M.; Mack, J. B. C.; Du Bois, J.; Berry, J. F.; Musaev, D. G. Rh2(II,III) Catalysts with Chelating Carboxylate and Carboxamidate Supports: Electronic Structure and Nitrene Transfer Reactivity. J. Am. Chem. Soc. 2016, 138, 2327– 2341,  DOI: 10.1021/jacs.5b12790
    Google Scholar
    15
    Rh2(II,III) Catalysts with Chelating Carboxylate and Carboxamidate Supports: Electronic Structure and Nitrene Transfer Reactivity
    Varela-Alvarez, Adrian; Yang, Tzuhsiung; Jennings, Heather; Kornecki, Katherine P.; Macmillan, Samantha N.; Lancaster, Kyle M.; Mack, James B. C.; Du Bois, J.; Berry, John F.; Musaev, Djamaladdin G.
    Journal of the American Chemical Society (2016), 138 (7), 2327-2341CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Dirhodium-catalyzed C-H amination is hypothesized to proceed via Rh2-nitrene intermediates in either the Rh2(II,II) or Rh2(II,III) redox state. Herein, we report joint theor. and exptl. studies of the ground electronic state (GES), redox potentials, and C-H amination of Rh2II,III(O2CCH3)4(L)n+ (1_L) (L = none, Cl-, and H2O), Rh2(esp)2+ (2), and Rh2(espn)2Cl (3) (esp = α,α,α',α'-tetramethyl-1,3-benzenedipropanoate and espn = α,α,α',α'-tetramethyl-1,3-benzenedipropanamidate). CASSCF calcns. on 1_L yield a wave function with two closely weighted configurations, (δ*)2(π1*)2(π2*)1 and (δ*)2(π1*)1(π2*)2, consistent with reported EPR g values Chem. Phys. Lett. 1986, 130, 20-23. In contrast, EPR spectra of 2 show g values consistent with the DFT-computed (π*)4(δ*)1 GES. EPR spectra and Cl K-edge XAS for 3 are consistent with a (π*)4(δ*)1 GES, as supported by DFT. Nitrene intermediates 2N_L and 3N_L are also examd. by DFT (the nitrene is an NSO3R species). DFT calcns. suggest a doublet GES for 2N_L and a quartet GES for 3N_L. CASSCF calcns. describe the GES of 2N as Rh2(II,II) with a coordinated nitrene radical cation, (π*)4(δ*)2(πnitrene,1)1(πnitrene,2)0. Conversely, the GES of 3N is Rh2(II,III) with a coordinated triplet nitrene, (π*)4(δ*)1(πnitrene,1)1(πnitrene,2)1. Quartet transition states (4TSs) are found to react via a stepwise radical mechanism, whereas 2TSs are found to react via a concerted mechanism that is lower in energy compared to 4TSs for both 2N_L and 3N_L. The exptl. (detd. by intramol. competition) and 2TS-calcd. kinetic isotopic effect (KIE) shows a KIE ∼ 3 for both 2N and 3N, which is consistent with a concerted mechanism.
  16. 16
    Bess, E. N.; DeLuca, R. J.; Tindall, D. J.; Oderinde, M. S.; Roizen, J. L.; Du Bois, J.; Sigman, M. S. Analyzing Site Selectivity in Rh2(esp)2-Catalyzed Intermolecular C–H Amination Reactions. J. Am. Chem. Soc. 2014, 136, 5783– 5789,  DOI: 10.1021/ja5015508
    Google Scholar
    16
    Analyzing Site Selectivity in Rh2(esp)2-Catalyzed Intermolecular C-H Amination Reactions
    Bess, Elizabeth N.; DeLuca, Ryan J.; Tindall, Daniel J.; Oderinde, Martins S.; Roizen, Jennifer L.; Du Bois, J.; Sigman, Matthew S.
    Journal of the American Chemical Society (2014), 136 (15), 5783-5789CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Predicting site selectivity in C-H bond oxidn. reactions involving heteroatom transfer is challenged by the small energetic differences between disparate bond types and the subtle interplay of steric and electronic effects that influence reactivity. Herein, the factors governing selective Rh2(esp)2-catalyzed C-H amination of isoamylbenzene derivs. are investigated, where modification to both the nitrogen source, a sulfamate ester, and substrate are shown to impact isomeric product ratios. Linear regression math. modeling is used to define a relationship that equates both IR stretching parameters and Hammett σ+ values to the differential free energy of benzylic vs. tertiary C-H amination. This model has informed the development of a novel sulfamate ester, which affords the highest benzylic-to-tertiary site selectivity (9.5:1) obsd. for this system.
  17. 17
    Harrison, J. G.; Gutierrez, O.; Jana, N.; Driver, T. G.; Tantillo, D. J. Mechanism of Rh2(II)-Catalyzed Indole Formation: The Catalyst Does Not Control Product Selectivity. J. Am. Chem. Soc. 2016, 138, 487– 490,  DOI: 10.1021/jacs.5b11427
    Google Scholar
    17
    Mechanism of Rh2(II)-Catalyzed Indole Formation: The Catalyst Does Not Control Product Selectivity
    Harrison, Jason G.; Gutierrez, Osvaldo; Jana, Navendu; Driver, Tom G.; Tantillo, Dean J.
    Journal of the American Chemical Society (2016), 138 (2), 487-490CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Possible mechanisms for Rh-promoted indole formation from vinyl/azidoarenes were examd. computationally, and a mechanism is proposed in which the Rh catalyst promotes generation of a nitrene but is not directly involved in cyclization.
  18. 18
    Das, A.; Wang, C.-H.; Van Trieste, G. P.; Sun, C.-J.; Chen, Y.-S.; Reibenspies, J. H.; Powers, D. C. In Crystallo Snapshots of Rh2-Catalyzed C–H Amination. J. Am. Chem. Soc. 2020, 142, 19862– 19867,  DOI: 10.1021/jacs.0c09842
    Google Scholar
    18
    In Crystallo Snapshots of Rh2-Catalyzed C-H Amination
    Das, Anuvab; Wang, Chen-Hao; Van Trieste, Gerard P.; Sun, Cheng-Jun; Chen, Yu-Sheng; Reibenspies, Joseph H.; Powers, David C.
    Journal of the American Chemical Society (2020), 142 (47), 19862-19867CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    While X-ray crystallog. routinely provides structural characterization of kinetically stable pre-catalysts and intermediates, elucidation of the structures of transient reactive intermediates, which are intimately engaged in bond-breaking and -making during catalysis, is generally not possible. Here, we demonstrate in crystallo synthesis of Rh2 nitrenoids that participate in catalytic C-H amination, and we characterize these transient intermediates as triplet adducts of Rh2. Further, we observe the impact of coordinating substrate, which is present in excess during catalysis, on the structure of transient Rh2 nitrenoids. By providing structural characterization of authentic C-H functionalization intermediates, and not kinetically stabilized model complexes, these expts. provide the opportunity to define crit. structure-activity relationships.
  19. 19
    Das, A.; Reibenspies, J. H.; Chen, Y.-S.; Powers, D. C. Direct Characterization of a Reactive Lattice-Confined Ru2 Nitride by Photocrystallography. J. Am. Chem. Soc. 2017, 139, 2912– 2915,  DOI: 10.1021/jacs.6b13357
    Google Scholar
    19
    Direct Characterization of a Reactive Lattice-Confined Ru2 Nitride by Photocrystallography
    Das, Anuvab; Reibenspies, Joseph H.; Chen, Yu-Sheng; Powers, David C.
    Journal of the American Chemical Society (2017), 139 (8), 2912-2915CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Reactive metal-ligand (M-L) multiply bonded complexes are ubiquitous intermediates in redox catalysis and have thus been long-standing targets of synthetic chem. The intrinsic reactivity of mid-to-late M-L multiply bonded complexes renders these structures challenging to isolate and structurally characterize. Although synthetic tuning of the ancillary ligand field can stabilize M-L multiply bonded complexes and result in isolable complexes, these efforts inevitably attenuate the reactivity of the M-L multiple bond. A reactive Ru2 nitride intermediate direct characterization by photocrystallog. is reported. Photogeneration of reactive M-L multiple bonds within cryst. matrixes supports direct characterization of these crit. intermediates without synthetic derivatization.
  20. 20
    Das, A.; Maher, A. G.; Telser, J.; Powers, D. C. Observation of a Photogenerated Rh2 Nitrenoid Intermediate in C–H Amination. J. Am. Chem. Soc. 2018, 140, 10412– 10415,  DOI: 10.1021/jacs.8b05599
    Google Scholar
    20
    Observation of a Photogenerated Rh2 Nitrenoid Intermediate in C-H Amination
    Das, Anuvab; Maher, Andrew G.; Telser, Joshua; Powers, David C.
    Journal of the American Chemical Society (2018), 140 (33), 10412-10415CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Rh2-catalyzed C-H amination is a powerful method for nitrogenating org. mols. While Rh2 nitrenoids are often invoked as reactive intermediates in these reactions, the exquisite reactivity and fleeting lifetime of these species has precluded their observation. Here, we report the photogeneration of a transient Rh2 nitrenoid that participates in C-H amination. The developed approach to Rh2 nitrenoids, based on photochem. cleavage of N-Cl bonds in N-chloroamido ligands, has enabled characterization of a reactive Rh2 nitrenoid by mass spectrometry and transient absorption spectroscopy. We anticipate that photogeneration of metal nitrenoids will contribute to the development of C-H amination catalysis by providing tools to directly study the structures of these crit. intermediates.
  21. 21
    Das, A.; Chen, Y.-S.; Reibenspies, J. H.; Powers, D. C. Characterization of a Reactive Rh2 Nitrenoid by Crystalline Matrix Isolation. J. Am. Chem. Soc. 2019, 141, 16232– 16236,  DOI: 10.1021/jacs.9b09064
    Google Scholar
    21
    Characterization of a Reactive Rh2 Nitrenoid by Crystalline Matrix Isolation
    Das, Anuvab; Chen, Yu-Sheng; Reibenspies, Joseph H.; Powers, David C.
    Journal of the American Chemical Society (2019), 141 (41), 16232-16236CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The fleeting lifetimes of reactive intermediates in C-H functionalization chem. often prevent their direct characterization. For example, the crit. nitrenoid intermediates that mediate Rh2-catalyzed C-H amination have eluded characterization for more than 40 years. In the absence of structural characterization of these species, methodol. development is often computationally guided. Here authors report the first x-ray crystal structure of a reactive Rh2 nitrenoid, enabled by N2 elimination from an org. azide ligand within a single-crystal matrix. The resulting high-resoln. structure displays metrical parameters consistent with a triplet nitrene complex of Rh2. The demonstration of facile access to reactive metal nitrenoids within a cryst. matrix provides a platform for structural characterization of the transient species at the heart of C-H functionalization.
  22. 22
    Van Trieste, G. P.; Reibenspies, J. H.; Chen, Y.-S.; Sengupta, D.; Thompson, R. R.; Powers, D. C. Oxygen-atom transfer photochemistry of a molecular copper bromate complex. Chem. Commun. 2022, 58, 12608– 12611,  DOI: 10.1039/D2CC04403J
    Google Scholar
    22
    Oxygen-atom transfer photochemistry of a molecular copper bromate complex
    Van Trieste III, Gerard P.; Reibenspies, Joseph H.; Chen, Yu-Sheng; Sengupta, Debabrata; Thompson, Richard R.; Powers, David C.
    Chemical Communications (Cambridge, United Kingdom) (2022), 58 (90), 12608-12611CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    We report the synthesis and oxygen-atom transfer (OAT) photochem. of [Cu(tpa)BrO3]ClO4. In situ spectroscopy and in crystallo expts. indicate OAT proceeds from a Cu-O fragment generated by sequential Cu-O bond cleavage and OAT from BrOx to [Cu(tpa)]+. These results highlight synthetic opportunities in M-O photochem. and demonstrate the utility of in crystallo expts. to evaluating photochem. reaction mechanisms.
  23. 23
    Bukvic, A. J.; Burnage, A. L.; Tizzard, G. J.; Martínez-Martínez, A. J.; McKay, A. I.; Rees, N. H.; Tegner, B. E.; Krämer, T.; Fish, H.; Warren, M. R.; Coles, S. J.; Macgregor, S. A.; Weller, A. S. A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane. J. Am. Chem. Soc. 2021, 143, 5106– 5120,  DOI: 10.1021/jacs.1c00738
    Google Scholar
    23
    A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane
    Bukvic, Alexander J.; Burnage, Arron L.; Tizzard, Graham J.; Martinez-Martinez, Antonio J.; McKay, Alasdair I.; Rees, Nicholas H.; Tegner, Bengt E.; Kramer, Tobias; Fish, Heather; Warren, Mark R.; Coles, Simon J.; Macgregor, Stuart A.; Weller, Andrew S.
    Journal of the American Chemical Society (2021), 143 (13), 5106-5120CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    Using solid-state mol. organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy2PCH2CH2PCy2)(ηn:ηm-alkane)][BArF4] have been prepd. (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H-C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some addnl. Rh(I)···H-C interactions with the geminal C-H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomps. rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of [BArF4]- anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in soln. supramol. chem. and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.
  24. 24
    Cohen, M. D.; Schmidt, G. M. J. 383. Topochemistry. Part I. A survey. J. Chem. Soc. Resumed 1964, 1996– 2000,  DOI: 10.1039/jr9640001996
    Google Scholar
    There is no corresponding record for this reference.
  25. 25
    Ohashi, Y. Dynamical structure analysis of crystalline-state racemization. Acc. Chem. Res. 1988, 21, 268– 274,  DOI: 10.1021/ar00151a003
    Google Scholar
    25
    Dynamical structure analysis of crystalline-state racemization
    Ohashi, Yuji
    Accounts of Chemical Research (1988), 21 (7), 268-74CODEN: ACHRE4; ISSN:0001-4842.
    A review with many refs. The kinetics and mechanism of racemization, e.g., of a cyanoethyl-substituted cobaloxime, in the cryst. state were studied by x-ray crystallog. (dynamic structure anal.). The cyanoethyl-substituted cobaloximes were racemized by x-ray exposure without degrdn. of crystallinity.
  26. 26
    Reid, K. A.; Powers, D. C. In crystallo organometallic chemistry. Chem. Commun. 2021, 57, 4993– 5003,  DOI: 10.1039/D1CC01684A
    Google Scholar
    26
    In crystallo organometallic chemistry
    Reid, Kaleb A.; Powers, David C.
    Chemical Communications (Cambridge, United Kingdom) (2021), 57 (41), 4993-5003CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
    A review. X-ray crystallog. is an invaluable tool in design and development of organometallic catalysis, but application typically requires species to display sufficiently high soln. concns. and lifetimes for single cryst. samples to be obtained. In crystallo organometallic chem. relies on chem. reactions that proceed within the single-crystal environment to access cryst. samples of reactive organometallic fragments that are unavailable by alternate means. This highlight describes approaches to in crystallo organometallic chem. including (a) solid-gas reactions between transition metal complexes in mol. crystals and diffusing small mols., (b) reactions of organometallic complexes within the extended lattices of metal-org. frameworks (MOFs), and (c) intracryst. photochem. transformations to generate reactive organometallic fragments. Application of these methods has enabled characterization of catalytically important transient species, including σ-alkane adducts of transition metals, metal alkyl intermediates implicated in metal-catalyzed carbonylations, and reactive M-L multiply bonded species involved in C-H functionalization chem. Opportunities and challenges for in crystallo organometallic chem. are discussed.
  27. 27
    Das, A.; Van Trieste, G. P.; Powers, D. C. Crystallography of Reactive Intermediates. Comments Inorg. Chem. 2020, 40, 116– 158,  DOI: 10.1080/02603594.2020.1747054
    Google Scholar
    27
    Crystallography of Reactive Intermediates
    Das, Anuvab; Van Trieste, Gerard Pierre III; Powers, David C.
    Comments on Inorganic Chemistry (2020), 40 (3), 116-158CODEN: COICDZ; ISSN:0260-3594. (Taylor & Francis, Inc.)
    Synthetic manipulation of the coordination geometry and ligand donicity, as well as introduction of sterically encumbering ligands, have each emerged as powerful methods to tame the inherent reactivity of kinetically labile M-L multiple bonds. While these efforts have resulted in families of well-characterized complexes and provided crit. insights regarding structure and bonding, the synthetic derivatization required to stabilize M-L fragments of interest often obviates the substrate functionalization activity relevant to catalysis. Photochem. synthesis of reactive species provides a conceptually attractive strategy to generate reactive M-L fragments under conditions compatible with time-resolved or cryogenic steady-state characterization, and photogeneration has enabled observation of a no. of reactive M-L fragments. The suite of tools available to characterize photogenerated reactive species is often more limited than typical for kinetically stabilized complexes and structural characterization is typically not possible. Recently, photocrystallog. expts., in which reactive M-L multiply bonded intermediates are generated within single-crystal matrixes, have been advanced as a strategy to interrogate the structures of reactive intermediates in C-H functionalization. This Comment describes the historical antecedents to these expts., highlights examples of photocrystallog. characterization of reactive intermediates, and discusses future opportunities.
  28. 28
    Zheng, S.-L.; Wang, Y.; Yu, Z.; Lin, Q.; Coppens, P. Direct Observation of a Photoinduced Nonstabilized Nitrile Imine Structure in the Solid State. J. Am. Chem. Soc. 2009, 131, 18036– 18037,  DOI: 10.1021/ja9094523
    Google Scholar
    28
    Direct Observation of a Photoinduced Nonstabilized Nitrile Imine Structure in the Solid State
    Zheng, Shao-Liang; Wang, Yizhong; Yu, Zhipeng; Lin, Qing; Coppens, Philip
    Journal of the American Chemical Society (2009), 131 (50), 18036-18037CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
    The direct observation of a bent geometry for a nonstabilized nitrile imine in a metal-coordination crystal is reported. The photoinduced tetrazole ring rupture to release N2 appears to depend on the size of voids around the N3-N4 bond in the crystal lattice. The selective formation of the 1,3-addn. product when a reactive nitrile imine was photogenerated in water is obsd. The bent nitrile imine geometry agrees with the 1,3-dipolar structure, a transient reactive species that mediates the photoinduced 1,3-dipolar cycloaddn. in the aq. medium.
  29. 29
    Kawano, M.; Takayama, T.; Uekusa, H.; Ohashi, Y.; Ozawa, Y.; Matsubara, K.; Imabayashi, H.; Mitsumi, M.; Toriumi, K. Structure Analysis of Photo-induced Triplet Phenylnitrene Using Synchrotron Radiation. Chem. Lett. 2003, 32, 922– 923,  DOI: 10.1246/cl.2003.922
    Google Scholar
    29
    Structure analysis of photo-induced triplet phenylnitrene using synchrotron radiation
    Kawano, Masaki; Takayama, Terufumi; Uekusa, Hidehiro; Ohashi, Yuji; Ozawa, Yoshiki; Matsubara, Koudatsu; Imabayashi, Hidekazu; Mitsumi, Minoru; Toriumi, Koshiro
    Chemistry Letters (2003), 32 (10), 922-923CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
    The crystal structures of [(PhCH2)2NH2]+ [m-C6H4(N3)-(COO)]- before and after UV-irradn. were analyzed at 25 K by using an X-ray vacuum camera set up at the synchrotron lab. (SPring-8). The C-N (nitrene) bond distance in the triplet state of the photo-induced m-carboxyphenylnitrene is detd. to be 1.34(4) Å.
  30. 30
    Cole, J. M. Single-crystal X-ray diffraction studies of photo-induced molecular species. Chem. Soc. Rev. 2004, 33, 501– 513,  DOI: 10.1039/b205339j
    Google Scholar
    30
    Single-crystal x-ray diffraction studies of photoinduced molecular species
    Cole, Jacqueline M.
    Chemical Society Reviews (2004), 33 (8), 501-513CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
    This tutorial review gathers together the recent developments in single-crystal x-ray diffraction that are starting to enable one to quantify directly the nature of light-induced electronic perturbations in chem. structures. Such structural information is key to understanding many photoactivated chem. processes and phys. properties, and a description of the scientific impetus behind this incipient area of structural science, from academic and industrial perspectives, is given. Photoisomerism processes, solid-state photochem. reactions and spin-cross-over magnetic transitions, that have long-lived or irreversible light-induced states, are best understood by unravelling their three-dimensional structures measured in situ in their photoconverted state. A review of steady-state laser-induced single-crystal x-ray diffraction studies conducted, to date, and the exptl. methodologies used in order to realize such structures, is presented. The structural characterization of more transient photoinduced species (down to picosecond lifetimes) is paramount to a better understanding of the materials that undergo high-speed electronic switching, which make operative much of the electronics and optics industry, since there exists an inherent relationship between the excited-state structure and the phys. properties exhibited. Prime examples include excited-state structures of mol. conductors and luminescent materials with potential applications as mol. wires, light-emitting diodes, non-linear optics, triboluminescence and electroluminescence. Previously, only indirect and qual. interpretations of the nature of these excited-states could be formulated via spectroscopic techniques, but the developments in ms-ps time-resolved laser pump, x-ray probe single-crystal diffraction techniques, described herein, are overcoming this barrier, affording results that are entirely quant. via a three-dimensional structural representation. In this regard, a review of structures of transient species studied to date is presented along with a discussion of the key exptl. parameters that are required for a successful expt., in terms of the x-ray, laser and sample characteristics. The importance of auxiliary spectroscopic work and complementary theor. calcns. is also briefly discussed. The paper concludes with a future outlook on new possible x-ray sources that will facilitate such work and extend it to structural studies on even more ephemeral species in the future.
  31. 31
    Antoni, P. W.; Mackenroth, A. V.; Mulks, F. F.; Rudolph, M.; Helmchen, G.; Hashmi, A. S. K. Dibenzothiophenesulfilimines: A Convenient Approach to Intermolecular Rhodium-Catalysed C–H Amidation. Chem.─Eur. J. 2020, 26, 8235– 8238,  DOI: 10.1002/chem.202002371
    Google Scholar
    31
    Dibenzothiophenesulfilimines: A Convenient Approach to Intermolecular Rhodium-Catalysed C-H Amidation
    Antoni, Patrick W.; Mackenroth, Alexandra V.; Mulks, Florian F.; Rudolph, Matthias; Helmchen, Guenter; Hashmi, A. Stephen K.
    Chemistry - A European Journal (2020), 26 (37), 8235-8238CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
    A sulfilimine-based Group 9 transition-metal-catalyzed C-H amidation procedure was reported. Dibenzothiophene-based sulfilimines were shown to constitute a class of novel amidation reagents which enable the transfer of a wide range of N-sulfonyl and N-acyl moieties. It was demonstrated that sulfilimines, which were easily accessible from cheap reagents, were safe-to-handle and represent broadly applicable amidation reagents. The dibenzothiophene can be recycled after use. The C-H amidation was shown to proceed with high selectivity and gave the mono-amidated products, mostly in good to excellent yields.
  32. 32
    Desikan, V.; Liu, Y.; Toscano, J. P.; Jenks, W. S. Photochemistry of N-Acetyl-, N-Trifluoroacetyl-, N- Mesyl-, and N-Tosyldibenzothiophene Sulfilimines. J. Org. Chem. 2008, 73, 4398– 4414,  DOI: 10.1021/jo702654q
    Google Scholar
    32
    Photochemistry of N-Acetyl-, N-Trifluoroacetyl-, N- Mesyl-, and N-Tosyldibenzothiophene Sulfilimines
    Desikan, Vasumathi; Liu, Yonglin; Toscano, John P.; Jenks, William S.
    Journal of Organic Chemistry (2008), 73 (12), 4398-4414CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    Time-resolved IR (TRIR) spectroscopy, product studies, and computational methods were applied to the photolysis of sulfilimines derived from dibenzothiophene that were expected to release acetylnitrene, trifluoroacetylnitrene, mesylnitrene, and tosylnitrene. All three methods provided results for acetylnitrene consistent with literature precedent and analogous expts. with the benzoylnitrene precursor, i.e., that the ground-state multiplicity is singlet. In contrast, product studies clearly indicate triplet reactivity for trifluoroacetylnitrene, though TRIR expts. were more ambiguous. Product studies suggest that these sulfilimines are superior sources for sulfonylnitrenes, which have triplet grounds states, to the corresponding azides, and computational studies shed light on the electronic structure of the nitrenes.
  33. 33
    Morita, H.; Tatami, A.; Maeda, T.; Ju Kim, B.; Kawashima, W.; Yoshimura, T.; Abe, H.; Akasaka, T. Generation of Nitrene by the Photolysis of N-Substituted Iminodibenzothiophene. J. Org. Chem. 2008, 73, 7159– 7163,  DOI: 10.1021/jo800604t
    Google Scholar
    33
    Generation of Nitrene by the Photolysis of N-Substituted Iminodibenzothiophene
    Morita, Hiroyuki; Tatami, Atsushi; Maeda, Tetsuo; Ju Kim, Byung; Kawashima, Wataru; Yoshimura, Toshiaki; Abe, Hitoshi; Akasaka, Takeshi
    Journal of Organic Chemistry (2008), 73 (18), 7159-7163CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
    To evaluate the ability of dibenzothiophene N-substituted sulfilimines as photochem. nitrene sources, their photolyses in the presence of several trapping reagents, such as sulfides, olefins, and phosphorus compds., were performed. In the reactions, the corresponding imino-transfer compds., namely sulfilimines, aziridines, and iminophosphoranes, were formed in good yields, indicating dibenzothiophene N-tosyl and N-acylsulfilimines have a potent nature as nitrogen sources.
  34. 34
    Tian, X.; Song, L.; Hashmi, A. S. K. Synthesis of Carbazoles and Related Heterocycles from Sulfilimines by Intramolecular C–H Aminations. Angew. Chem., Int. Ed. 2020, 59, 12342– 12346,  DOI: 10.1002/anie.202000146
    Google Scholar
    34
    Synthesis of Carbazoles and Related Heterocycles from Sulfilimines by Intramolecular C-H Aminations
    Tian, Xianhai; Song, Lina; Hashmi, A. Stephen K.
    Angewandte Chemie, International Edition (2020), 59 (30), 12342-12346CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
    Biaryl and arylalkenyl sulfilimines such as 2-PhC6H4N:SPh2 were prepd. from biarylamines and alkenylanilines such as 1,1'-biphenyl-2-amine and Martin's sulfurane as nitrene equiv.; under blue LED irradn., sulfilimines underwent intramol. C-H amination reactions to yield carbazoles or fused indoles and indoles such as carbazole. The method avoids the prepn. and use of aryl azides. The method was used to prepd. the carbazole alkaloids clausine C and N.
  35. 35
    Breslow, R.; Gellman, S. H. Intramolecular nitrene carbon-hydrogen insertions mediated by transition-metal complexes as nitrogen analogs of cytochrome P-450 reactions. J. Am. Chem. Soc. 1983, 105, 6728– 6729,  DOI: 10.1021/ja00360a039
    Google Scholar
    35
    Intramolecular nitrene carbon-hydrogen insertions mediated by transition-metal complexes as nitrogen analogs of cytochrome P-450 reactions
    Breslow, Ronald; Gellman, Samuel H.
    Journal of the American Chemical Society (1983), 105 (22), 6728-9CODEN: JACSAT; ISSN:0002-7863.
    2,5-Diisopropylbenzenesulfonylimidoiodobenzene was converted to a sultam (I) by intramol. insertion catalyzed by 5-10 mol% of various metal complexes. Other products included 2-isopropenyl-5-isopropylbenzenesulfonamide and 2,5-diisopropylbenzenesulfonamide. The best catalysts for the process were Fe(III)-tetraphenylporphyrin chloride, which produced a 77% yield of the sultam, and di-Rh tetraacetate which produced an 86% yield of I. Poorer yields were obtained with Mn(III)-tetraphenylporphyrin chloride, with the FeCl3 complex of 1,4,8,11-tetraazacyclotetradecane, or with FeCl3 itself. The process was a N analog model for O insertion reactions catalyzed by cytochrome P 450 monooxygenases.
  36. 36
    Davies, H. M. L.; Manning, J. R. Catalytic C–H functionalization by metal carbenoid and nitrenoid insertion. Nature 2008, 451, 417– 424,  DOI: 10.1038/nature06485
    Google Scholar
    36
    Catalytic C-H functionalization by metal carbenoid and nitrenoid insertion
    Davies, Huw M. L.; Manning, James R.
    Nature (London, United Kingdom) (2008), 451 (7177), 417-424CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
    A review. Novel reactions that can selectively functionalize carbon-hydrogen bonds are of intense interest to the chem. community because they offer new strategic approaches for synthesis. A very promising 'carbon-hydrogen functionalization' method involves the insertion of metal carbenes and nitrenes into C-H bonds. This area has experienced considerable growth in the past decade, particularly in the area of enantioselective intermol. reactions. Here we discuss several facets of these kinds of C-H functionalization reactions and provide a perspective on how this methodol. has affected the synthesis of complex natural products and potential pharmaceutical agents.
  37. 37
    Mack, J. B. C.; Bedell, T. A.; DeLuca, R. J.; Hone, G. A. B.; Roizen, J. L.; Cox, C. T.; Sorensen, E. J.; Du Bois, J. Rhodium-Catalyzed C–H Amination: A Case Study of Selectivity in C–H Functionalization Reactions. J. Chem. Educ. 2018, 95, 2243– 2248,  DOI: 10.1021/acs.jchemed.7b00697
    Google Scholar
    37
    Rhodium-Catalyzed C-H Amination: A Case Study of Selectivity in C-H Functionalization Reactions
    Mack, James B. C.; Bedell, T. Aaron; DeLuca, Ryan J.; Hone, Graham A. B.; Roizen, Jennifer L.; Cox, Charles T.; Sorensen, Erik J.; Du Bois, J.
    Journal of Chemical Education (2018), 95 (12), 2243-2248CODEN: JCEDA8; ISSN:0021-9584. (American Chemical Society and Division of Chemical Education, Inc.)
    An advanced undergraduate org. chem. lab. expt. involving a rhodium-catalyzed intermol. hydrocarbon C-H oxidn. reaction is described. In the initial phase of this lab, students conduct a C-H amination reaction of ethylbenzene and isolate a benzylic amine product. In the second part of the lab, competition expts. are performed to compare the relative rates of C-H insertion with electronically disparate para-substituted ethylbenzene derivs. Reaction progress is monitored by thin-layer chromatog. and the outcome quant. assessed using 1H NMR spectroscopy. Data from competition expts. form the basis for a discussion of reaction mechanism including anal. of putative transition structures and relative potential energy barriers for C-H oxidn. Expts. have been designed such that instructors can tailor the level of detail and discussion from introductory to advanced.
  38. 38
    Warzecha, E.; Berto, T. C.; Berry, J. F. Axial Ligand Coordination to the C–H Amination Catalyst Rh2(esp)2: A Structural and Spectroscopic Study. Inorg. Chem. 2015, 54, 8817– 8824,  DOI: 10.1021/acs.inorgchem.5b01532
    Google Scholar
    38
    Axial Ligand Coordination to the C-H Amination Catalyst Rh2(esp)2: A Structural and Spectroscopic Study
    Warzecha, Evan; Berto, Timothy C.; Berry, John F.
    Inorganic Chemistry (2015), 54 (17), 8817-8824CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
    The compd. Rh2(esp)2 (esp = α,α,α',α'-tetramethyl-1,3-benzenediproponoate) is the most generally effective catalyst for nitrenoid amination of C-H bonds. However, much of its fundamental coordination chem. is unknown. The authors study the effects of axial ligand coordination to the catalyst Rh2(esp)2. The authors report here crystal structures, cyclic voltammetry, UV-visible, IR, Raman, and 1H NMR spectra for the complexes Rh2(esp)2L2 (I: 1 - 6) where L = dichloromethane (1), pyridine (2), 3-picoline (3), 2,6-lutidine (4), acetonitrile (5), and methanol (6). The compds. all show well-defined π* → σ* electronic transitions in the 16500 to 20500 cm-1 range, and Rh-Rh stretching vibrations at 304-322 cm-1. Taking these data into account the strength of axial ligand binding to Rh2(esp)2 increases in the series CH3OH ∼ 2,6-lutidine < CH3CN < 3-methylpyridine approx. pyridine. Quasi-reversible Rh24+/5+ redox waves are only obtained when either acetonitrile or no axial ligand is present. In the presence of pyridines, irreversible oxidn. waves are obsd., suggesting that these ligands destabilize the Rh2 complex under oxidative conditions.
  39. 39
    Berry, J. F. The role of three-center/four-electron bonds in superelectrophilic dirhodium carbene and nitrene catalytic intermediates. Dalton Trans. 2012, 41, 700– 713,  DOI: 10.1039/C1DT11434D
    Google Scholar
    39
    The role of three-center/four-electron bonds in superelectrophilic dirhodium carbene and nitrene catalytic intermediates
    Berry, John F.
    Dalton Transactions (2012), 41 (3), 700-713CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
    A review. Three-center/four-electron (3c/4e) bonds are important bonding motifs that dictate the electronic structure, and thereby the reactivity, of metal-metal bonded carbene and nitrene intermediate complexes that are crucial to the dirhodium-catalyzed functionalization of hydrocarbons. In this Perspective article, general features of the 3c/4e bond are presented and discussed in comparison to two-center/two-electron (2c/2e) bonds. Specifically, 3c/4e bonding interactions lead to longer distances between the atoms involved and measurably weaker bonds. Addnl., excited states derived from the 3c/4e bonding manifold are lower in energy than those derived from a 2c/2e manifold, signifying a greater degree of reactivity in the former case. Three coterminous 3c/4e Ru-Ru-N bonds are present in metal-metal/metal-ligand multiply bonded diruthenium terminal nitrido compds. This bonding situation results in an unusual superelectrophilic character of the nitride nitrogen atom, exemplified by its insertion into aryl C-H bonds via an electrophilic arom. substitution mechanism. The key catalytic intermediates in dirhodium-catalyzed C-H functionalization reactions, dirhodium carbene and dirhodium nitrene complexes, may also be described as superelectrophilic by virtue of 3c/4e Rh-Rh-C(or N) σ and π bonds. These 3c/4e bonding interactions set apart dirhodium carbene and nitrene intermediates from other, less electrophilic, carbene or nitrene species.
  40. 40
    Hush, N. S.; Reimers, J. R. Solvent Effects on the Electronic Spectra of Transition Metal Complexes. Chem. Rev. 2000, 100, 775– 786,  DOI: 10.1021/cr980409v
    Google Scholar
    40
    Solvent effects on the electronic spectra of transition metal complexes
    Hush, Noel S.; Reimers, Jeffrey R.
    Chemical Reviews (Washington, D. C.) (2000), 100 (2), 775-786CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
    A review with 79 refs. on the Zeng-Hush-Reimers solvent shift (ZHR-SS), a computational method for the calcn. of solvent effects on metal-to-ligand charge transfer (MLCT) transitions. The given examples are almost exclusively dealing with H2O as the solvent. The topics treated are: the (n,π*) spectra of azine solns., the solvation effects on MLCT transitions in Ru2+ complexes (Ru(NH3)5L), the photochem. of Fe(H2O)62+ complexes in water, metal-metal coupling in dimeric systems, and the electroabsorption (Stark) effect.
  41. 41
    Suslick, K. S.; Bautista, J. F.; Watson, R. A. Metalloporphyrin photochemistry with matrix isolation. J. Am. Chem. Soc. 1991, 113, 6111– 6114,  DOI: 10.1021/ja00016a029
    Google Scholar
    41
    Metalloporphyrin photochemistry with matrix isolation
    Suslick, Kenneth S.; Bautista, Jocelyn F.; Watson, Randall A.
    Journal of the American Chemical Society (1991), 113 (16), 6111-14CODEN: JACSAT; ISSN:0002-7863.
    The photochem. of a no. of metalloporphyrin oxoanion complexes was examd. by matrix isolation techniques, using both frozen solvent glasses and polymer films. After an extensive search for a noncoordinating, unreactive, glassing solvent, a 3:1 mixt. of 2,2-dimethylbutane and tert-butylbenzene was found to work well at temps. <70 K. Alternatively, the photochem. of metalloporphyrins was monitored in polymer films by the evapn. onto a sapphire window of metalloporphyrin solns. in PhMe contg. either PMMA or poly(α-methylstyrene). The polymer films have the added advantage of a greatly increased temp. range, providing diffusional isolation even at room temp. The photoredn. of the metal by homolytic α-bond cleavage and loss of the axial ligand appears to be a general mechanism for all metalloporphyrin complexes examd. The formation of metal-oxo species from photolysis of metalloporphyrin oxoanion complexes in soln. derives from secondary, thermal reactions.

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Cite this: Inorg. Chem. 2023, 62, 31, 12557–12564
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  • Abstract

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    Figure 1

    Figure 1. (a) In crystallo photogeneration and characterization of transient Rh2 nitrene from an organic azide photoprecursor. (b) Photoactivation of dibenzosulfilimines affords reactive nitrene fragments with concurrent extrusion of dibenzothiophene. (c) Here, we demonstrate the nitrene photochemistry of Rh2 sulfilimine complexes.

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    Figure 2

    Figure 2. Synthesis of N-coordinated bis-sulfilimine Rh2 complexes (9a–9d), and S-coordinated bis-sulfilimine Rh2 complex 9e, which were isolated by crystallization.

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    Figure 3

    Figure 3. Displacement ellipsoid plots of N-coordinated Rh2 sulfilimine complexes 9a–9d and S-coordinated 9e plotted at 50% probability. H atoms and solvents are removed for clarity. The crystalline sample used in this diffraction experiment was obtained from a concentrated CH2Cl2 solution layered with pentane at −20 °C. Selected metrical parameters: for 9a, Rh(1)–N(1) = 2.2992(2) Å and Rh(1)–Rh(2) = 2.3893(3) Å; for 9b. Rh(1)–N(1) = 2.297(4) Å and Rh(1)–Rh(2) = 2.3944(8) Å; for 9c, Rh(1)–N(1) = 2.3374(1) Å and Rh(1)–Rh(2) = 2.3921(3) Å; for 9d, Rh(1)–N(1) = 2.363(3) Å and Rh(1)–Rh(2) = 2.4026(5) Å; for 9e, Rh(1)–S(1) = 2.5438(5) Å and Rh(1)–Rh(2) = 2.4048(3) Å.

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    Figure 4

    Figure 4. Reaction of dibenzothiophene with 1 affords Rh2esp2(dbt)2 (10). Displacement ellipsoid plots of 10 are plotted at 50% probability. H-atoms and and solvent are removed for the sake of clarity. Selected metrical parameters: for 10, Rh(1)–S(1) = 2.5298(4) Å and Rh(1)–Rh(2) = 2.3993(3) Å.

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    Figure 5

    Figure 5. UV–vis spectra obtained during the titration of 1 with 6a. The well-anchored isosbestic points at 434 and 639 nm indicate the absence of steady-state intermediates in the conversion of 1 (black) to 9a (red).

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    Figure 6

    Figure 6. Photolysis of Rh2 sulfilimine 9d results in Rh2esp2(dbt)2 (10) and heterocycle 13d, which are the products expected of intramolecular NGT photochemistry. A well-anchored isosbestic point is observed at 675 nm for the UV–vis spectra collected during photolysis of compound 9d in CH2Cl2 (λ > 335 nm), which indicates the lack of a steady-state intermediate in the conversion of 9d to 10 and 13d.

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    Figure 7

    Figure 7. Photolysis of compounds 9a, 9b, and 9c with tetralin and ethylbenzene in CH2Cl2 results in the respective intermolecular aminated products.

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    Figure 8

    Figure 8. Solid-state photolysis of compound IR spectra collected during the photolysis (335 nm < λ < 610 nm) of a KBr pellet of 9d at 23 °C from 0 min (black) to 24 h (red). The disappearance of the peaks at 896 cm–1 is for the N═S cleavage. The disappearance of the peak at 719 cm–1 and slight red shift from 752 cm–1 to 744 cm–1 depicts the conversion from 9d to 10 during 24 h of photolysis.

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    Figure 9

    Figure 9. Solid-state cryogenic photolysis of compound 9d in a polystyrene film matrix. (a) Solid-state UV–vis spectra periodically collected during the photolysis over 4 h at 129 K where the colors black, red, and green represent UV–vis spectra at t = 0, 4.5 h, and the end of the thaw-freeze cycle, respectively. (b) Overlay of the UV–vis spectrum of independently prepared 10 (blue) and the thermally annealed spectrum (red) indicates the formation of 10 during the cryogenic photolysis of 9d.

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  • References

    This article references 41 other publications.

    1. 1
      Chiappini, N. D.; Mack, J. B. C.; Du Bois, J. Intermolecular C(sp3)–H Amination of Complex Molecules. Angew. Chem., Int. Ed. 2018, 57, 4956– 4959,  DOI: 10.1002/anie.201713225
      1
      Intermolecular C(sp3)-H Amination of Complex Molecules
      Chiappini, Nicholas D.; Mack, James B. C.; Du Bois, J.
      Angewandte Chemie, International Edition (2018), 57 (18), 4956-4959CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A general and operationally convenient method for intermol. amination of C(sp3)-H bonds is described. This technol. allows for efficient functionalization of complex mols., including numerous pharmaceutical targets. The combination of pivalonitrile as a solvent, Al2O3 as an additive, and Ph sulfamate as a nitrogen source affords differential reaction performance and substrate scope. Mechanistic data strongly implicate a pathway for catalyst decompn. that initiates with solvent oxidn., thus providing rationale for the marked influence of pivalonitrile on this reaction process.
    2. 2
      Espino, C. G.; Fiori, K. W.; Kim, M.; Du Bois, J. Expanding the Scope of C–H Amination through Catalyst Design. J. Am. Chem. Soc. 2004, 126, 15378– 15379,  DOI: 10.1021/ja0446294
      2
      Expanding the Scope of C-H Amination through Catalyst Design
      Espino, Christine G.; Fiori, Kristin Williams; Kim, Mihyong; Du Bois, J.
      Journal of the American Chemical Society (2004), 126 (47), 15378-15379CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Anal. of the mechanism for Rh-mediated C-H amination has led to the development of a remarkably effective dinuclear Rh catalyst derived from 1,3-benzenedipropionic acid. This unique complex, Rh2(esp)2, is capable of promoting both intra- and intermol. C-H oxidn. reactions, and in all cases is superior to Rh2(O2CtBu)4. For the first time, C-H insertion is described with urea and sulfamide substrates to give 1,2- and 1,3-diamine derivs., resp. In addn., intermol. amination of benzylic and secondary C-H bonds is shown to proceed efficiently even under conditions in which the starting alkane is employed as the limiting reagent.
    3. 3
      Espino, C. G.; Wehn, P. M.; Chow, J.; Du Bois, J. Synthesis of 1,3-Difunctionalized Amine Derivatives through Selective C–H Bond Oxidation. J. Am. Chem. Soc. 2001, 123, 6935– 6936,  DOI: 10.1021/ja011033x
      3
      Synthesis of 1,3-Difunctionalized Amine Derivatives through Selective C-H Bond Oxidation
      Espino, Christine G.; Wehn, Paul M.; Chow, Jessica; Du Bois, J.
      Journal of the American Chemical Society (2001), 123 (28), 6935-6936CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Cyclization of sulfamate esters via a Rh-catalyzed C-H bond oxidn./insertion reaction is described. Thus, oxathiazinanes, e.g. I, were prepd. from the stereoselective intramol. oxidative cyclization of sulfamate esters, e.g. II, using Rh2(OAc)4, PhI(OAc)2, and MgO in CH2Cl2. Nucleophilic ring opening of oxathiazinanes with water followed by oxidn. afforded β-amino acids. Thus, chiral oxathiazinane III underwent ring opening followed by oxidn. to afford (R)-(Benzyloxycarbonyl)-β-isoleucine in 81% yield.
    4. 4
      Nägeli, I.; Baud, C.; Bernardinelli, G.; Jacquier, Y.; Moraon, M.; Müllet, P. Rhodium(II)-Catalyzed CH Insertions with {[(4-Nitrophenyl)sulfonyl]imino}phenyl-λ3-iodane. Helv. Chim. Acta 1997, 80, 1087– 1105,  DOI: 10.1002/hlca.19970800407
      4
      Rhodium(II)-catalyzed CH insertions with {[(4-nitrophenyl)sulfonyl]imino}phenyl-λ3-iodane
      Nageli, Ivo; Baud, Corine; Bernardinelli, Gerald; Jacquier, Yvan; Moran, Mary; Muller, Paul
      Helvetica Chimica Acta (1997), 80 (4), 1087-1105CODEN: HCACAV; ISSN:0018-019X. (Verlag Helvetica Chimica Acta)
      The [Rh2(OAc)4]-catalyzed decompn. of {[(4-nitrophenyl)sulfonyl]imino}phenyl-λ3-iodane (NsN:IPh) resulted in formal insertions into CH bonds, activated by Ph or vinyl groups, or by O-substituents. Scope and limitations of the reaction were investigated. Yields ≤84% were achieved in the most favorable cases, and were enhanced by electron-releasing substituents and decreased by steric hindrance. Aziridination competed with allylic insertion with olefinic substrates. The insertion reaction proceeded with retention of configuration. With chiral Rh(II) catalysts, a modest asym. induction was obsd. A mechanism involving direct insertion by a Rh-complexed nitrene into the CH bond is proposed.
    5. 5
      Nguyen, Q.; Sun, K.; Driver, T. G. Rh2(II)-Catalyzed Intramolecular Aliphatic C–H Bond Amination Reactions Using Aryl Azides as the N-Atom Source. J. Am. Chem. Soc. 2012, 134, 7262– 7265,  DOI: 10.1021/ja301519q
      5
      Rh2(II)-Catalyzed intramolecular aliphatic C-H bond amination reactions using aryl azides as the N-atom source
      Nguyen, Quyen; Sun, Ke; Driver, Tom G.
      Journal of the American Chemical Society (2012), 134 (17), 7262-7265CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Rhodium(II) dicarboxylate complexes were discovered to catalyze the intramol. amination of unactivated primary, secondary, or tertiary aliph. C-H bonds using aryl azides as the N-atom precursor. While a strong electron-withdrawing group on the nitrogen atom is typically required to achieve this reaction, we found that both electron-rich and electron-poor aryl azides are efficient sources for the metal nitrene reactive intermediate.
    6. 6
      Park, Y.; Kim, Y.; Chang, S. Transition Metal-Catalyzed C–H Amination: Scope, Mechanism, and Applications. Chem. Rev. 2017, 117, 9247– 9301,  DOI: 10.1021/acs.chemrev.6b00644
      6
      Transition Metal-Catalyzed C-H Amination: Scope, Mechanism, and Applications
      Park, Yoonsu; Kim, Youyoung; Chang, Sukbok
      Chemical Reviews (Washington, DC, United States) (2017), 117 (13), 9247-9301CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. The recent advances in intra- and intermol. C-H amination reactions utilizing late transition metal-based catalysts have been reviewed.
    7. 7
      Paudyal, M. P.; Adebesin, A. M.; Burt, S. R.; Ess, D. H.; Ma, Z.; Kürti, L.; Falck, J. R. Dirhodium-catalyzed C–H arene amination using hydroxylamines. Science 2016, 353, 1144– 1147,  DOI: 10.1126/science.aaf8713
      7
      Dirhodium-catalyzed C-H arene amination using hydroxylamines
      Paudyal, Mahesh P.; Adebesin, Adeniyi Michael; Burt, Scott R.; Ess, Daniel H.; Ma, Zhiwei; Kuerti, Laszlo; Falck, John R.
      Science (Washington, DC, United States) (2016), 353 (6304), 1144-1147CODEN: SCIEAS; ISSN:0036-8075. (American Association for the Advancement of Science)
      Primary and N-alkyl arylamine motifs are key functional groups in pharmaceuticals, agrochems., and functional materials, as well as in bioactive natural products. However, there is a dearth of generally applicable methods for the direct replacement of aryl hydrogens with NH2/NH(alkyl) moieties. Here, we present a mild dirhodium-catalyzed C-H amination for conversion of structurally diverse monocyclic and fused aroms. to the corresponding primary and N-alkyl arylamines using NH2/NH(alkyl)-O-(sulfonyl)hydroxylamines as aminating agents; the relatively weak RSO2O-N bond functions as an internal oxidant. The methodol. is operationally simple, scalable, and fast at or below ambient temp., furnishing arylamines in moderate-to-good yields and with good regioselectivity. It can be readily extended to the synthesis of fused N-heterocycles.
    8. 8
      Roizen, J. L.; Zalatan, D. N.; Du Bois, J. Selective Intermolecular Amination of C–H Bonds at Tertiary Carbon Centers. Angew. Chem., Int. Ed. 2013, 52, 11343– 11346,  DOI: 10.1002/anie.201304238
      8
      Selective Intermolecular Amination of C-H Bonds at Tertiary Carbon Centers
      Roizen, Jennifer L.; Zalatan, David N.; Du Bois, J.
      Angewandte Chemie, International Edition (2013), 52 (43), 11343-11346CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      An effective process for generating tetrasubstituted amine derivs., e.g., I, through selective, intermol. tertiary C-H bond amination have been developed. The optimized reaction is performed with limiting amts. of substrate, 1 mol % of com. available [Rh2(esp)2], and inexpensive PhI(OAc)2 as the terminal oxidant. The identification of aryloxysulfonamides, in particular DfsNH2, as functional nitrogen sources has been an instrumental find. Competition studies with substrates possessing disparate C-H bond types reveal variations in product selectivity, which derive solely from the choice of sulfamate ester.
    9. 9
      Trowbridge, A.; Walton, S. M.; Gaunt, M. J. New Strategies for the Transition-Metal Catalyzed Synthesis of Aliphatic Amines. Chem. Rev. 2020, 120, 2613– 2692,  DOI: 10.1021/acs.chemrev.9b00462
      9
      New Strategies for the Transition-Metal Catalyzed Synthesis of Aliphatic Amines
      Trowbridge, Aaron; Walton, Scarlett M.; Gaunt, Matthew J.
      Chemical Reviews (Washington, DC, United States) (2020), 120 (5), 2613-2692CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review. In light of the ever-increasing importance of aliph. amines across the range of chem. sciences, this review aims to provide a concise overview of modern transition-metal catalyzed approaches to alkylamine synthesis and their functionalization. Selected examples of amine bond forming reactions include, hydroamination and hydroaminoalkylation, transition-metal catalyzed C(sp3)-H functionalization and transition-metal catalyzed visible-light-mediated light photoredox catalysis.
    10. 10
      van den Heuvel, N.; Mason, S. M.; Mercado, B. Q.; Miller, S. J. Aspartyl β-Turn-Based Dirhodium(II) Metallopeptides for Benzylic C(sp3)–H Amination: Enantioselectivity and X-ray Structural Analysis. J. Am. Chem. Soc. 2023, 145, 12377– 12385,  DOI: 10.1021/jacs.3c03587
      10
      Aspartyl β-Turn-Based Dirhodium(II) Metallopeptides for Benzylic C(sp3)-H Amination: Enantioselectivity and X-ray Structural Analysis
      van den Heuvel, Naudin; Mason, Savannah M.; Mercado, Brandon Q.; Miller, Scott J.
      Journal of the American Chemical Society (2023), 145 (22), 12377-12385CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Amination of C(sp3)-H bonds is a powerful tool to introduce N into complex org. frameworks in a direct manner. Despite significant advances in catalyst design, full site- and enantiocontrol in complex mol. regimes remain elusive using established catalyst systems. To address these challenges, the authors herein describe a new class of peptide-based dirhodium(II) complexes derived from aspartic acid-contg. β-turn-forming tetramers. This highly modular system can serve as a platform for the rapid generation of new chiral dirhodium(II) catalyst libraries, as illustrated by the facile synthesis of 38 catalysts. Critically, the authors present the 1st crystal structure of a dirhodium(II) tetra-aspartate complex, which unveils retention of the β-turn conformation of the peptidyl ligand; a well-defined H-bonding network is evident, along with a near-C4 symmetry that renders the Rh centers inequivalent. The utility of this catalyst platform is illustrated by the enantioselective amination of benzylic C(sp3)-H bonds, in which state-of-the-art levels of enantioselectivity up to 95.5:4.5 er were obtained, even for substrates that present challenges with previously reported catalyst systems. Addnl., the authors found these complexes to be competent catalysts for the intermol. amination of N-alkylamides via insertion into the C(sp3)-H bond α to the amide N, yielding differentially protected 1,1-diamines. Of note, this type of insertion also occurs on the amide functionalities of the catalyst itself in the absence of the substrate but did not appear to be detrimental to reaction outcomes when the substrate was present.
    11. 11
      Perry, R. H.; Cahill, T. J.; Roizen, J. L.; Du Bois, J.; Zare, R. N. Capturing fleeting intermediates in a catalytic C–H amination reaction cycle. Proc. Natl. Acad. Sci. U. S. A. 2012, 109, 18295– 18299,  DOI: 10.1073/pnas.1207600109
      11
      Capturing fleeting intermediates in a catalytic C-H amination reaction cycle
      Perry, Richard H.; Cahill, Thomas J., III; Roizen, Jennifer L.; Du Bois, Justin; Zare, Richard N.
      Proceedings of the National Academy of Sciences of the United States of America (2012), 109 (45), 18295-18299, S18295/1-S18295/6CODEN: PNASA6; ISSN:0027-8424. (National Academy of Sciences)
      The authors have applied an ambient ionization technique, desorption electrospray ionization MS, to identify transient reactive species of an archetypal C-H amination reaction catalyzed by a dirhodium tetracarboxylate complex. Using this anal. method, the authors have detected previously proposed short-lived reaction intermediates, including two nitrenoid complexes that differ in oxidn. state. Findings suggest that an Rh-nitrene oxidant can react with hydrocarbon substrates through a hydrogen atom abstraction pathway and raise the intriguing possibility that two catalytic C-H amination pathways may be operative in a typical bulk soln. reaction. As highlighted by these results, desorption electrospray ionization MS should have broad applicability for the mechanistic study of catalytic processes.
    12. 12
      Zalatan, D. N.; Du Bois, J. Understanding the Differential Performance of Rh2(esp)2 as a Catalyst for C–H Amination. J. Am. Chem. Soc. 2009, 131, 7558– 7559,  DOI: 10.1021/ja902893u
      12
      Understanding the Differential Performance of Rh2(esp)2 as a Catalyst for C-H Amination
      Zalatan, David N.; Du Bois, J.
      Journal of the American Chemical Society (2009), 131 (22), 7558-7559CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Catalytic amination of satd. C-H bonds is performed efficiently with the use of Rh2(esp)2. Efforts to identify pathways for catalyst degrdn. and/or arrest have revealed a single-electron oxidn. event that gives rise to a red-colored, mixed-valence dimer, [Rh2(esp)2]+. This species is fortuitously reduced by carboxylic acid, a byproduct generated in the reaction cycle with each turnover of the diacyloxyiodine oxidant. These findings have led to the conclusion that the high performance of Rh2(esp)2 is due in part to the superior kinetic stability of its one-electron oxidized form relative to other dimeric Rh complexes.
    13. 13
      Fiori, K. W.; Espino, C. G.; Brodsky, B. H.; Du Bois, J. A mechanistic analysis of the Rh-catalyzed intramolecular C–H amination reaction. Tetrahedron 2009, 65, 3042– 3051,  DOI: 10.1016/j.tet.2008.11.073
      13
      A mechanistic analysis of the Rh-catalyzed intramolecular C-H amination reaction
      Fiori, Kristin Williams; Espino, Christine G.; Brodsky, Benjamin H.; Du Bois, J.
      Tetrahedron (2009), 65 (16), 3042-3051CODEN: TETRAB; ISSN:0040-4020. (Elsevier Ltd.)
      A detailed mechanistic investigation of the intramol. dirhodium tetracarboxylate-catalyzed sulfamate ester C-H amination reaction is presented. These studies provide support for the formation of a sulfamate-derived iminoiodinane, which reacts rapidly with the rhodium catalyst to generate a nitrenoid-type oxidant. Reactivity patterns, Hammett anal., kinetic isotope measurement, and a cyclopropane clock expt. are indicative of a concerted, asynchronous transition structure in the product-detg. C-H insertion event.
    14. 14
      Fiori, K. W.; Du Bois, J. Catalytic Intermolecular Amination of C–H Bonds: Method Development and Mechanistic Insights. J. Am. Chem. Soc. 2007, 129, 562– 568,  DOI: 10.1021/ja0650450
      14
      Catalytic Intermolecular Amination of C-H Bonds: Method Development and Mechanistic Insights
      Fiori, Kristin Williams; Du Bois, J.
      Journal of the American Chemical Society (2007), 129 (3), 562-568CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Reaction methodol. for intermol. C-H amination of benzylic and 3° C-H bonds is described. This process uses the starting alkane as the limiting reagent, gives optically pure tetrasubstituted amines through stereospecific insertion into enantiomeric 3° centers, displays high chemoselectivity for benzylic oxidn., and enables the facile prepn. of isotopically enriched 15N-labeled compds. Access to substituted amines, amino alcs., and diamines is thereby made possible in a single transformation. Important information relevant to understanding the initial steps in the catalytic cycle, reaction chemoselectivity, the nature of the active oxidant, and pathways for catalyst inactivation has been gained through mechanistic anal.; these studies are also presented.
    15. 15
      Varela-Álvarez, A.; Yang, T.; Jennings, H.; Kornecki, K. P.; Macmillan, S. N.; Lancaster, K. M.; Mack, J. B. C.; Du Bois, J.; Berry, J. F.; Musaev, D. G. Rh2(II,III) Catalysts with Chelating Carboxylate and Carboxamidate Supports: Electronic Structure and Nitrene Transfer Reactivity. J. Am. Chem. Soc. 2016, 138, 2327– 2341,  DOI: 10.1021/jacs.5b12790
      15
      Rh2(II,III) Catalysts with Chelating Carboxylate and Carboxamidate Supports: Electronic Structure and Nitrene Transfer Reactivity
      Varela-Alvarez, Adrian; Yang, Tzuhsiung; Jennings, Heather; Kornecki, Katherine P.; Macmillan, Samantha N.; Lancaster, Kyle M.; Mack, James B. C.; Du Bois, J.; Berry, John F.; Musaev, Djamaladdin G.
      Journal of the American Chemical Society (2016), 138 (7), 2327-2341CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Dirhodium-catalyzed C-H amination is hypothesized to proceed via Rh2-nitrene intermediates in either the Rh2(II,II) or Rh2(II,III) redox state. Herein, we report joint theor. and exptl. studies of the ground electronic state (GES), redox potentials, and C-H amination of Rh2II,III(O2CCH3)4(L)n+ (1_L) (L = none, Cl-, and H2O), Rh2(esp)2+ (2), and Rh2(espn)2Cl (3) (esp = α,α,α',α'-tetramethyl-1,3-benzenedipropanoate and espn = α,α,α',α'-tetramethyl-1,3-benzenedipropanamidate). CASSCF calcns. on 1_L yield a wave function with two closely weighted configurations, (δ*)2(π1*)2(π2*)1 and (δ*)2(π1*)1(π2*)2, consistent with reported EPR g values Chem. Phys. Lett. 1986, 130, 20-23. In contrast, EPR spectra of 2 show g values consistent with the DFT-computed (π*)4(δ*)1 GES. EPR spectra and Cl K-edge XAS for 3 are consistent with a (π*)4(δ*)1 GES, as supported by DFT. Nitrene intermediates 2N_L and 3N_L are also examd. by DFT (the nitrene is an NSO3R species). DFT calcns. suggest a doublet GES for 2N_L and a quartet GES for 3N_L. CASSCF calcns. describe the GES of 2N as Rh2(II,II) with a coordinated nitrene radical cation, (π*)4(δ*)2(πnitrene,1)1(πnitrene,2)0. Conversely, the GES of 3N is Rh2(II,III) with a coordinated triplet nitrene, (π*)4(δ*)1(πnitrene,1)1(πnitrene,2)1. Quartet transition states (4TSs) are found to react via a stepwise radical mechanism, whereas 2TSs are found to react via a concerted mechanism that is lower in energy compared to 4TSs for both 2N_L and 3N_L. The exptl. (detd. by intramol. competition) and 2TS-calcd. kinetic isotopic effect (KIE) shows a KIE ∼ 3 for both 2N and 3N, which is consistent with a concerted mechanism.
    16. 16
      Bess, E. N.; DeLuca, R. J.; Tindall, D. J.; Oderinde, M. S.; Roizen, J. L.; Du Bois, J.; Sigman, M. S. Analyzing Site Selectivity in Rh2(esp)2-Catalyzed Intermolecular C–H Amination Reactions. J. Am. Chem. Soc. 2014, 136, 5783– 5789,  DOI: 10.1021/ja5015508
      16
      Analyzing Site Selectivity in Rh2(esp)2-Catalyzed Intermolecular C-H Amination Reactions
      Bess, Elizabeth N.; DeLuca, Ryan J.; Tindall, Daniel J.; Oderinde, Martins S.; Roizen, Jennifer L.; Du Bois, J.; Sigman, Matthew S.
      Journal of the American Chemical Society (2014), 136 (15), 5783-5789CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Predicting site selectivity in C-H bond oxidn. reactions involving heteroatom transfer is challenged by the small energetic differences between disparate bond types and the subtle interplay of steric and electronic effects that influence reactivity. Herein, the factors governing selective Rh2(esp)2-catalyzed C-H amination of isoamylbenzene derivs. are investigated, where modification to both the nitrogen source, a sulfamate ester, and substrate are shown to impact isomeric product ratios. Linear regression math. modeling is used to define a relationship that equates both IR stretching parameters and Hammett σ+ values to the differential free energy of benzylic vs. tertiary C-H amination. This model has informed the development of a novel sulfamate ester, which affords the highest benzylic-to-tertiary site selectivity (9.5:1) obsd. for this system.
    17. 17
      Harrison, J. G.; Gutierrez, O.; Jana, N.; Driver, T. G.; Tantillo, D. J. Mechanism of Rh2(II)-Catalyzed Indole Formation: The Catalyst Does Not Control Product Selectivity. J. Am. Chem. Soc. 2016, 138, 487– 490,  DOI: 10.1021/jacs.5b11427
      17
      Mechanism of Rh2(II)-Catalyzed Indole Formation: The Catalyst Does Not Control Product Selectivity
      Harrison, Jason G.; Gutierrez, Osvaldo; Jana, Navendu; Driver, Tom G.; Tantillo, Dean J.
      Journal of the American Chemical Society (2016), 138 (2), 487-490CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Possible mechanisms for Rh-promoted indole formation from vinyl/azidoarenes were examd. computationally, and a mechanism is proposed in which the Rh catalyst promotes generation of a nitrene but is not directly involved in cyclization.
    18. 18
      Das, A.; Wang, C.-H.; Van Trieste, G. P.; Sun, C.-J.; Chen, Y.-S.; Reibenspies, J. H.; Powers, D. C. In Crystallo Snapshots of Rh2-Catalyzed C–H Amination. J. Am. Chem. Soc. 2020, 142, 19862– 19867,  DOI: 10.1021/jacs.0c09842
      18
      In Crystallo Snapshots of Rh2-Catalyzed C-H Amination
      Das, Anuvab; Wang, Chen-Hao; Van Trieste, Gerard P.; Sun, Cheng-Jun; Chen, Yu-Sheng; Reibenspies, Joseph H.; Powers, David C.
      Journal of the American Chemical Society (2020), 142 (47), 19862-19867CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      While X-ray crystallog. routinely provides structural characterization of kinetically stable pre-catalysts and intermediates, elucidation of the structures of transient reactive intermediates, which are intimately engaged in bond-breaking and -making during catalysis, is generally not possible. Here, we demonstrate in crystallo synthesis of Rh2 nitrenoids that participate in catalytic C-H amination, and we characterize these transient intermediates as triplet adducts of Rh2. Further, we observe the impact of coordinating substrate, which is present in excess during catalysis, on the structure of transient Rh2 nitrenoids. By providing structural characterization of authentic C-H functionalization intermediates, and not kinetically stabilized model complexes, these expts. provide the opportunity to define crit. structure-activity relationships.
    19. 19
      Das, A.; Reibenspies, J. H.; Chen, Y.-S.; Powers, D. C. Direct Characterization of a Reactive Lattice-Confined Ru2 Nitride by Photocrystallography. J. Am. Chem. Soc. 2017, 139, 2912– 2915,  DOI: 10.1021/jacs.6b13357
      19
      Direct Characterization of a Reactive Lattice-Confined Ru2 Nitride by Photocrystallography
      Das, Anuvab; Reibenspies, Joseph H.; Chen, Yu-Sheng; Powers, David C.
      Journal of the American Chemical Society (2017), 139 (8), 2912-2915CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Reactive metal-ligand (M-L) multiply bonded complexes are ubiquitous intermediates in redox catalysis and have thus been long-standing targets of synthetic chem. The intrinsic reactivity of mid-to-late M-L multiply bonded complexes renders these structures challenging to isolate and structurally characterize. Although synthetic tuning of the ancillary ligand field can stabilize M-L multiply bonded complexes and result in isolable complexes, these efforts inevitably attenuate the reactivity of the M-L multiple bond. A reactive Ru2 nitride intermediate direct characterization by photocrystallog. is reported. Photogeneration of reactive M-L multiple bonds within cryst. matrixes supports direct characterization of these crit. intermediates without synthetic derivatization.
    20. 20
      Das, A.; Maher, A. G.; Telser, J.; Powers, D. C. Observation of a Photogenerated Rh2 Nitrenoid Intermediate in C–H Amination. J. Am. Chem. Soc. 2018, 140, 10412– 10415,  DOI: 10.1021/jacs.8b05599
      20
      Observation of a Photogenerated Rh2 Nitrenoid Intermediate in C-H Amination
      Das, Anuvab; Maher, Andrew G.; Telser, Joshua; Powers, David C.
      Journal of the American Chemical Society (2018), 140 (33), 10412-10415CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Rh2-catalyzed C-H amination is a powerful method for nitrogenating org. mols. While Rh2 nitrenoids are often invoked as reactive intermediates in these reactions, the exquisite reactivity and fleeting lifetime of these species has precluded their observation. Here, we report the photogeneration of a transient Rh2 nitrenoid that participates in C-H amination. The developed approach to Rh2 nitrenoids, based on photochem. cleavage of N-Cl bonds in N-chloroamido ligands, has enabled characterization of a reactive Rh2 nitrenoid by mass spectrometry and transient absorption spectroscopy. We anticipate that photogeneration of metal nitrenoids will contribute to the development of C-H amination catalysis by providing tools to directly study the structures of these crit. intermediates.
    21. 21
      Das, A.; Chen, Y.-S.; Reibenspies, J. H.; Powers, D. C. Characterization of a Reactive Rh2 Nitrenoid by Crystalline Matrix Isolation. J. Am. Chem. Soc. 2019, 141, 16232– 16236,  DOI: 10.1021/jacs.9b09064
      21
      Characterization of a Reactive Rh2 Nitrenoid by Crystalline Matrix Isolation
      Das, Anuvab; Chen, Yu-Sheng; Reibenspies, Joseph H.; Powers, David C.
      Journal of the American Chemical Society (2019), 141 (41), 16232-16236CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The fleeting lifetimes of reactive intermediates in C-H functionalization chem. often prevent their direct characterization. For example, the crit. nitrenoid intermediates that mediate Rh2-catalyzed C-H amination have eluded characterization for more than 40 years. In the absence of structural characterization of these species, methodol. development is often computationally guided. Here authors report the first x-ray crystal structure of a reactive Rh2 nitrenoid, enabled by N2 elimination from an org. azide ligand within a single-crystal matrix. The resulting high-resoln. structure displays metrical parameters consistent with a triplet nitrene complex of Rh2. The demonstration of facile access to reactive metal nitrenoids within a cryst. matrix provides a platform for structural characterization of the transient species at the heart of C-H functionalization.
    22. 22
      Van Trieste, G. P.; Reibenspies, J. H.; Chen, Y.-S.; Sengupta, D.; Thompson, R. R.; Powers, D. C. Oxygen-atom transfer photochemistry of a molecular copper bromate complex. Chem. Commun. 2022, 58, 12608– 12611,  DOI: 10.1039/D2CC04403J
      22
      Oxygen-atom transfer photochemistry of a molecular copper bromate complex
      Van Trieste III, Gerard P.; Reibenspies, Joseph H.; Chen, Yu-Sheng; Sengupta, Debabrata; Thompson, Richard R.; Powers, David C.
      Chemical Communications (Cambridge, United Kingdom) (2022), 58 (90), 12608-12611CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      We report the synthesis and oxygen-atom transfer (OAT) photochem. of [Cu(tpa)BrO3]ClO4. In situ spectroscopy and in crystallo expts. indicate OAT proceeds from a Cu-O fragment generated by sequential Cu-O bond cleavage and OAT from BrOx to [Cu(tpa)]+. These results highlight synthetic opportunities in M-O photochem. and demonstrate the utility of in crystallo expts. to evaluating photochem. reaction mechanisms.
    23. 23
      Bukvic, A. J.; Burnage, A. L.; Tizzard, G. J.; Martínez-Martínez, A. J.; McKay, A. I.; Rees, N. H.; Tegner, B. E.; Krämer, T.; Fish, H.; Warren, M. R.; Coles, S. J.; Macgregor, S. A.; Weller, A. S. A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane. J. Am. Chem. Soc. 2021, 143, 5106– 5120,  DOI: 10.1021/jacs.1c00738
      23
      A Series of Crystallographically Characterized Linear and Branched σ-Alkane Complexes of Rhodium: From Propane to 3-Methylpentane
      Bukvic, Alexander J.; Burnage, Arron L.; Tizzard, Graham J.; Martinez-Martinez, Antonio J.; McKay, Alasdair I.; Rees, Nicholas H.; Tegner, Bengt E.; Kramer, Tobias; Fish, Heather; Warren, Mark R.; Coles, Simon J.; Macgregor, Stuart A.; Weller, Andrew S.
      Journal of the American Chemical Society (2021), 143 (13), 5106-5120CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      Using solid-state mol. organometallic (SMOM) techniques, in particular solid/gas single-crystal to single-crystal reactivity, a series of σ-alkane complexes of the general formula [Rh(Cy2PCH2CH2PCy2)(ηn:ηm-alkane)][BArF4] have been prepd. (alkane = propane, 2-methylbutane, hexane, 3-methylpentane; ArF = 3,5-(CF3)2C6H3). These new complexes have been characterized using single crystal X-ray diffraction, solid-state NMR spectroscopy and DFT computational techniques and present a variety of Rh(I)···H-C binding motifs at the metal coordination site: 1,2-η2:η2 (2-methylbutane), 1,3-η2:η2 (propane), 2,4-η2:η2 (hexane), and 1,4-η1:η2 (3-methylpentane). For the linear alkanes propane and hexane, some addnl. Rh(I)···H-C interactions with the geminal C-H bonds are also evident. The stability of these complexes with respect to alkane loss in the solid state varies with the identity of the alkane: from propane that decomps. rapidly at 295 K to 2-methylbutane that is stable and instead undergoes an acceptorless dehydrogenation to form a bound alkene complex. In each case the alkane sits in a binding pocket defined by the {Rh(Cy2PCH2CH2PCy2)}+ fragment and the surrounding array of [BArF4]- anions. For the propane complex, a small alkane binding energy, driven in part by a lack of stabilizing short contacts with the surrounding anions, correlates with the fleeting stability of this species. 2-Methylbutane forms more short contacts within the binding pocket, and as a result the complex is considerably more stable. However, the complex of the larger 3-methylpentane ligand shows lower stability. Empirically, there therefore appears to be an optimal fit between the size and shape of the alkane and overall stability. Such observations are related to guest/host interactions in soln. supramol. chem. and the holistic role of 1°, 2°, and 3° environments in metalloenzymes.
    24. 24
      Cohen, M. D.; Schmidt, G. M. J. 383. Topochemistry. Part I. A survey. J. Chem. Soc. Resumed 1964, 1996– 2000,  DOI: 10.1039/jr9640001996
      There is no corresponding record for this reference.
    25. 25
      Ohashi, Y. Dynamical structure analysis of crystalline-state racemization. Acc. Chem. Res. 1988, 21, 268– 274,  DOI: 10.1021/ar00151a003
      25
      Dynamical structure analysis of crystalline-state racemization
      Ohashi, Yuji
      Accounts of Chemical Research (1988), 21 (7), 268-74CODEN: ACHRE4; ISSN:0001-4842.
      A review with many refs. The kinetics and mechanism of racemization, e.g., of a cyanoethyl-substituted cobaloxime, in the cryst. state were studied by x-ray crystallog. (dynamic structure anal.). The cyanoethyl-substituted cobaloximes were racemized by x-ray exposure without degrdn. of crystallinity.
    26. 26
      Reid, K. A.; Powers, D. C. In crystallo organometallic chemistry. Chem. Commun. 2021, 57, 4993– 5003,  DOI: 10.1039/D1CC01684A
      26
      In crystallo organometallic chemistry
      Reid, Kaleb A.; Powers, David C.
      Chemical Communications (Cambridge, United Kingdom) (2021), 57 (41), 4993-5003CODEN: CHCOFS; ISSN:1359-7345. (Royal Society of Chemistry)
      A review. X-ray crystallog. is an invaluable tool in design and development of organometallic catalysis, but application typically requires species to display sufficiently high soln. concns. and lifetimes for single cryst. samples to be obtained. In crystallo organometallic chem. relies on chem. reactions that proceed within the single-crystal environment to access cryst. samples of reactive organometallic fragments that are unavailable by alternate means. This highlight describes approaches to in crystallo organometallic chem. including (a) solid-gas reactions between transition metal complexes in mol. crystals and diffusing small mols., (b) reactions of organometallic complexes within the extended lattices of metal-org. frameworks (MOFs), and (c) intracryst. photochem. transformations to generate reactive organometallic fragments. Application of these methods has enabled characterization of catalytically important transient species, including σ-alkane adducts of transition metals, metal alkyl intermediates implicated in metal-catalyzed carbonylations, and reactive M-L multiply bonded species involved in C-H functionalization chem. Opportunities and challenges for in crystallo organometallic chem. are discussed.
    27. 27
      Das, A.; Van Trieste, G. P.; Powers, D. C. Crystallography of Reactive Intermediates. Comments Inorg. Chem. 2020, 40, 116– 158,  DOI: 10.1080/02603594.2020.1747054
      27
      Crystallography of Reactive Intermediates
      Das, Anuvab; Van Trieste, Gerard Pierre III; Powers, David C.
      Comments on Inorganic Chemistry (2020), 40 (3), 116-158CODEN: COICDZ; ISSN:0260-3594. (Taylor & Francis, Inc.)
      Synthetic manipulation of the coordination geometry and ligand donicity, as well as introduction of sterically encumbering ligands, have each emerged as powerful methods to tame the inherent reactivity of kinetically labile M-L multiple bonds. While these efforts have resulted in families of well-characterized complexes and provided crit. insights regarding structure and bonding, the synthetic derivatization required to stabilize M-L fragments of interest often obviates the substrate functionalization activity relevant to catalysis. Photochem. synthesis of reactive species provides a conceptually attractive strategy to generate reactive M-L fragments under conditions compatible with time-resolved or cryogenic steady-state characterization, and photogeneration has enabled observation of a no. of reactive M-L fragments. The suite of tools available to characterize photogenerated reactive species is often more limited than typical for kinetically stabilized complexes and structural characterization is typically not possible. Recently, photocrystallog. expts., in which reactive M-L multiply bonded intermediates are generated within single-crystal matrixes, have been advanced as a strategy to interrogate the structures of reactive intermediates in C-H functionalization. This Comment describes the historical antecedents to these expts., highlights examples of photocrystallog. characterization of reactive intermediates, and discusses future opportunities.
    28. 28
      Zheng, S.-L.; Wang, Y.; Yu, Z.; Lin, Q.; Coppens, P. Direct Observation of a Photoinduced Nonstabilized Nitrile Imine Structure in the Solid State. J. Am. Chem. Soc. 2009, 131, 18036– 18037,  DOI: 10.1021/ja9094523
      28
      Direct Observation of a Photoinduced Nonstabilized Nitrile Imine Structure in the Solid State
      Zheng, Shao-Liang; Wang, Yizhong; Yu, Zhipeng; Lin, Qing; Coppens, Philip
      Journal of the American Chemical Society (2009), 131 (50), 18036-18037CODEN: JACSAT; ISSN:0002-7863. (American Chemical Society)
      The direct observation of a bent geometry for a nonstabilized nitrile imine in a metal-coordination crystal is reported. The photoinduced tetrazole ring rupture to release N2 appears to depend on the size of voids around the N3-N4 bond in the crystal lattice. The selective formation of the 1,3-addn. product when a reactive nitrile imine was photogenerated in water is obsd. The bent nitrile imine geometry agrees with the 1,3-dipolar structure, a transient reactive species that mediates the photoinduced 1,3-dipolar cycloaddn. in the aq. medium.
    29. 29
      Kawano, M.; Takayama, T.; Uekusa, H.; Ohashi, Y.; Ozawa, Y.; Matsubara, K.; Imabayashi, H.; Mitsumi, M.; Toriumi, K. Structure Analysis of Photo-induced Triplet Phenylnitrene Using Synchrotron Radiation. Chem. Lett. 2003, 32, 922– 923,  DOI: 10.1246/cl.2003.922
      29
      Structure analysis of photo-induced triplet phenylnitrene using synchrotron radiation
      Kawano, Masaki; Takayama, Terufumi; Uekusa, Hidehiro; Ohashi, Yuji; Ozawa, Yoshiki; Matsubara, Koudatsu; Imabayashi, Hidekazu; Mitsumi, Minoru; Toriumi, Koshiro
      Chemistry Letters (2003), 32 (10), 922-923CODEN: CMLTAG; ISSN:0366-7022. (Chemical Society of Japan)
      The crystal structures of [(PhCH2)2NH2]+ [m-C6H4(N3)-(COO)]- before and after UV-irradn. were analyzed at 25 K by using an X-ray vacuum camera set up at the synchrotron lab. (SPring-8). The C-N (nitrene) bond distance in the triplet state of the photo-induced m-carboxyphenylnitrene is detd. to be 1.34(4) Å.
    30. 30
      Cole, J. M. Single-crystal X-ray diffraction studies of photo-induced molecular species. Chem. Soc. Rev. 2004, 33, 501– 513,  DOI: 10.1039/b205339j
      30
      Single-crystal x-ray diffraction studies of photoinduced molecular species
      Cole, Jacqueline M.
      Chemical Society Reviews (2004), 33 (8), 501-513CODEN: CSRVBR; ISSN:0306-0012. (Royal Society of Chemistry)
      This tutorial review gathers together the recent developments in single-crystal x-ray diffraction that are starting to enable one to quantify directly the nature of light-induced electronic perturbations in chem. structures. Such structural information is key to understanding many photoactivated chem. processes and phys. properties, and a description of the scientific impetus behind this incipient area of structural science, from academic and industrial perspectives, is given. Photoisomerism processes, solid-state photochem. reactions and spin-cross-over magnetic transitions, that have long-lived or irreversible light-induced states, are best understood by unravelling their three-dimensional structures measured in situ in their photoconverted state. A review of steady-state laser-induced single-crystal x-ray diffraction studies conducted, to date, and the exptl. methodologies used in order to realize such structures, is presented. The structural characterization of more transient photoinduced species (down to picosecond lifetimes) is paramount to a better understanding of the materials that undergo high-speed electronic switching, which make operative much of the electronics and optics industry, since there exists an inherent relationship between the excited-state structure and the phys. properties exhibited. Prime examples include excited-state structures of mol. conductors and luminescent materials with potential applications as mol. wires, light-emitting diodes, non-linear optics, triboluminescence and electroluminescence. Previously, only indirect and qual. interpretations of the nature of these excited-states could be formulated via spectroscopic techniques, but the developments in ms-ps time-resolved laser pump, x-ray probe single-crystal diffraction techniques, described herein, are overcoming this barrier, affording results that are entirely quant. via a three-dimensional structural representation. In this regard, a review of structures of transient species studied to date is presented along with a discussion of the key exptl. parameters that are required for a successful expt., in terms of the x-ray, laser and sample characteristics. The importance of auxiliary spectroscopic work and complementary theor. calcns. is also briefly discussed. The paper concludes with a future outlook on new possible x-ray sources that will facilitate such work and extend it to structural studies on even more ephemeral species in the future.
    31. 31
      Antoni, P. W.; Mackenroth, A. V.; Mulks, F. F.; Rudolph, M.; Helmchen, G.; Hashmi, A. S. K. Dibenzothiophenesulfilimines: A Convenient Approach to Intermolecular Rhodium-Catalysed C–H Amidation. Chem.─Eur. J. 2020, 26, 8235– 8238,  DOI: 10.1002/chem.202002371
      31
      Dibenzothiophenesulfilimines: A Convenient Approach to Intermolecular Rhodium-Catalysed C-H Amidation
      Antoni, Patrick W.; Mackenroth, Alexandra V.; Mulks, Florian F.; Rudolph, Matthias; Helmchen, Guenter; Hashmi, A. Stephen K.
      Chemistry - A European Journal (2020), 26 (37), 8235-8238CODEN: CEUJED; ISSN:0947-6539. (Wiley-VCH Verlag GmbH & Co. KGaA)
      A sulfilimine-based Group 9 transition-metal-catalyzed C-H amidation procedure was reported. Dibenzothiophene-based sulfilimines were shown to constitute a class of novel amidation reagents which enable the transfer of a wide range of N-sulfonyl and N-acyl moieties. It was demonstrated that sulfilimines, which were easily accessible from cheap reagents, were safe-to-handle and represent broadly applicable amidation reagents. The dibenzothiophene can be recycled after use. The C-H amidation was shown to proceed with high selectivity and gave the mono-amidated products, mostly in good to excellent yields.
    32. 32
      Desikan, V.; Liu, Y.; Toscano, J. P.; Jenks, W. S. Photochemistry of N-Acetyl-, N-Trifluoroacetyl-, N- Mesyl-, and N-Tosyldibenzothiophene Sulfilimines. J. Org. Chem. 2008, 73, 4398– 4414,  DOI: 10.1021/jo702654q
      32
      Photochemistry of N-Acetyl-, N-Trifluoroacetyl-, N- Mesyl-, and N-Tosyldibenzothiophene Sulfilimines
      Desikan, Vasumathi; Liu, Yonglin; Toscano, John P.; Jenks, William S.
      Journal of Organic Chemistry (2008), 73 (12), 4398-4414CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      Time-resolved IR (TRIR) spectroscopy, product studies, and computational methods were applied to the photolysis of sulfilimines derived from dibenzothiophene that were expected to release acetylnitrene, trifluoroacetylnitrene, mesylnitrene, and tosylnitrene. All three methods provided results for acetylnitrene consistent with literature precedent and analogous expts. with the benzoylnitrene precursor, i.e., that the ground-state multiplicity is singlet. In contrast, product studies clearly indicate triplet reactivity for trifluoroacetylnitrene, though TRIR expts. were more ambiguous. Product studies suggest that these sulfilimines are superior sources for sulfonylnitrenes, which have triplet grounds states, to the corresponding azides, and computational studies shed light on the electronic structure of the nitrenes.
    33. 33
      Morita, H.; Tatami, A.; Maeda, T.; Ju Kim, B.; Kawashima, W.; Yoshimura, T.; Abe, H.; Akasaka, T. Generation of Nitrene by the Photolysis of N-Substituted Iminodibenzothiophene. J. Org. Chem. 2008, 73, 7159– 7163,  DOI: 10.1021/jo800604t
      33
      Generation of Nitrene by the Photolysis of N-Substituted Iminodibenzothiophene
      Morita, Hiroyuki; Tatami, Atsushi; Maeda, Tetsuo; Ju Kim, Byung; Kawashima, Wataru; Yoshimura, Toshiaki; Abe, Hitoshi; Akasaka, Takeshi
      Journal of Organic Chemistry (2008), 73 (18), 7159-7163CODEN: JOCEAH; ISSN:0022-3263. (American Chemical Society)
      To evaluate the ability of dibenzothiophene N-substituted sulfilimines as photochem. nitrene sources, their photolyses in the presence of several trapping reagents, such as sulfides, olefins, and phosphorus compds., were performed. In the reactions, the corresponding imino-transfer compds., namely sulfilimines, aziridines, and iminophosphoranes, were formed in good yields, indicating dibenzothiophene N-tosyl and N-acylsulfilimines have a potent nature as nitrogen sources.
    34. 34
      Tian, X.; Song, L.; Hashmi, A. S. K. Synthesis of Carbazoles and Related Heterocycles from Sulfilimines by Intramolecular C–H Aminations. Angew. Chem., Int. Ed. 2020, 59, 12342– 12346,  DOI: 10.1002/anie.202000146
      34
      Synthesis of Carbazoles and Related Heterocycles from Sulfilimines by Intramolecular C-H Aminations
      Tian, Xianhai; Song, Lina; Hashmi, A. Stephen K.
      Angewandte Chemie, International Edition (2020), 59 (30), 12342-12346CODEN: ACIEF5; ISSN:1433-7851. (Wiley-VCH Verlag GmbH & Co. KGaA)
      Biaryl and arylalkenyl sulfilimines such as 2-PhC6H4N:SPh2 were prepd. from biarylamines and alkenylanilines such as 1,1'-biphenyl-2-amine and Martin's sulfurane as nitrene equiv.; under blue LED irradn., sulfilimines underwent intramol. C-H amination reactions to yield carbazoles or fused indoles and indoles such as carbazole. The method avoids the prepn. and use of aryl azides. The method was used to prepd. the carbazole alkaloids clausine C and N.
    35. 35
      Breslow, R.; Gellman, S. H. Intramolecular nitrene carbon-hydrogen insertions mediated by transition-metal complexes as nitrogen analogs of cytochrome P-450 reactions. J. Am. Chem. Soc. 1983, 105, 6728– 6729,  DOI: 10.1021/ja00360a039
      35
      Intramolecular nitrene carbon-hydrogen insertions mediated by transition-metal complexes as nitrogen analogs of cytochrome P-450 reactions
      Breslow, Ronald; Gellman, Samuel H.
      Journal of the American Chemical Society (1983), 105 (22), 6728-9CODEN: JACSAT; ISSN:0002-7863.
      2,5-Diisopropylbenzenesulfonylimidoiodobenzene was converted to a sultam (I) by intramol. insertion catalyzed by 5-10 mol% of various metal complexes. Other products included 2-isopropenyl-5-isopropylbenzenesulfonamide and 2,5-diisopropylbenzenesulfonamide. The best catalysts for the process were Fe(III)-tetraphenylporphyrin chloride, which produced a 77% yield of the sultam, and di-Rh tetraacetate which produced an 86% yield of I. Poorer yields were obtained with Mn(III)-tetraphenylporphyrin chloride, with the FeCl3 complex of 1,4,8,11-tetraazacyclotetradecane, or with FeCl3 itself. The process was a N analog model for O insertion reactions catalyzed by cytochrome P 450 monooxygenases.
    36. 36
      Davies, H. M. L.; Manning, J. R. Catalytic C–H functionalization by metal carbenoid and nitrenoid insertion. Nature 2008, 451, 417– 424,  DOI: 10.1038/nature06485
      36
      Catalytic C-H functionalization by metal carbenoid and nitrenoid insertion
      Davies, Huw M. L.; Manning, James R.
      Nature (London, United Kingdom) (2008), 451 (7177), 417-424CODEN: NATUAS; ISSN:0028-0836. (Nature Publishing Group)
      A review. Novel reactions that can selectively functionalize carbon-hydrogen bonds are of intense interest to the chem. community because they offer new strategic approaches for synthesis. A very promising 'carbon-hydrogen functionalization' method involves the insertion of metal carbenes and nitrenes into C-H bonds. This area has experienced considerable growth in the past decade, particularly in the area of enantioselective intermol. reactions. Here we discuss several facets of these kinds of C-H functionalization reactions and provide a perspective on how this methodol. has affected the synthesis of complex natural products and potential pharmaceutical agents.
    37. 37
      Mack, J. B. C.; Bedell, T. A.; DeLuca, R. J.; Hone, G. A. B.; Roizen, J. L.; Cox, C. T.; Sorensen, E. J.; Du Bois, J. Rhodium-Catalyzed C–H Amination: A Case Study of Selectivity in C–H Functionalization Reactions. J. Chem. Educ. 2018, 95, 2243– 2248,  DOI: 10.1021/acs.jchemed.7b00697
      37
      Rhodium-Catalyzed C-H Amination: A Case Study of Selectivity in C-H Functionalization Reactions
      Mack, James B. C.; Bedell, T. Aaron; DeLuca, Ryan J.; Hone, Graham A. B.; Roizen, Jennifer L.; Cox, Charles T.; Sorensen, Erik J.; Du Bois, J.
      Journal of Chemical Education (2018), 95 (12), 2243-2248CODEN: JCEDA8; ISSN:0021-9584. (American Chemical Society and Division of Chemical Education, Inc.)
      An advanced undergraduate org. chem. lab. expt. involving a rhodium-catalyzed intermol. hydrocarbon C-H oxidn. reaction is described. In the initial phase of this lab, students conduct a C-H amination reaction of ethylbenzene and isolate a benzylic amine product. In the second part of the lab, competition expts. are performed to compare the relative rates of C-H insertion with electronically disparate para-substituted ethylbenzene derivs. Reaction progress is monitored by thin-layer chromatog. and the outcome quant. assessed using 1H NMR spectroscopy. Data from competition expts. form the basis for a discussion of reaction mechanism including anal. of putative transition structures and relative potential energy barriers for C-H oxidn. Expts. have been designed such that instructors can tailor the level of detail and discussion from introductory to advanced.
    38. 38
      Warzecha, E.; Berto, T. C.; Berry, J. F. Axial Ligand Coordination to the C–H Amination Catalyst Rh2(esp)2: A Structural and Spectroscopic Study. Inorg. Chem. 2015, 54, 8817– 8824,  DOI: 10.1021/acs.inorgchem.5b01532
      38
      Axial Ligand Coordination to the C-H Amination Catalyst Rh2(esp)2: A Structural and Spectroscopic Study
      Warzecha, Evan; Berto, Timothy C.; Berry, John F.
      Inorganic Chemistry (2015), 54 (17), 8817-8824CODEN: INOCAJ; ISSN:0020-1669. (American Chemical Society)
      The compd. Rh2(esp)2 (esp = α,α,α',α'-tetramethyl-1,3-benzenediproponoate) is the most generally effective catalyst for nitrenoid amination of C-H bonds. However, much of its fundamental coordination chem. is unknown. The authors study the effects of axial ligand coordination to the catalyst Rh2(esp)2. The authors report here crystal structures, cyclic voltammetry, UV-visible, IR, Raman, and 1H NMR spectra for the complexes Rh2(esp)2L2 (I: 1 - 6) where L = dichloromethane (1), pyridine (2), 3-picoline (3), 2,6-lutidine (4), acetonitrile (5), and methanol (6). The compds. all show well-defined π* → σ* electronic transitions in the 16500 to 20500 cm-1 range, and Rh-Rh stretching vibrations at 304-322 cm-1. Taking these data into account the strength of axial ligand binding to Rh2(esp)2 increases in the series CH3OH ∼ 2,6-lutidine < CH3CN < 3-methylpyridine approx. pyridine. Quasi-reversible Rh24+/5+ redox waves are only obtained when either acetonitrile or no axial ligand is present. In the presence of pyridines, irreversible oxidn. waves are obsd., suggesting that these ligands destabilize the Rh2 complex under oxidative conditions.
    39. 39
      Berry, J. F. The role of three-center/four-electron bonds in superelectrophilic dirhodium carbene and nitrene catalytic intermediates. Dalton Trans. 2012, 41, 700– 713,  DOI: 10.1039/C1DT11434D
      39
      The role of three-center/four-electron bonds in superelectrophilic dirhodium carbene and nitrene catalytic intermediates
      Berry, John F.
      Dalton Transactions (2012), 41 (3), 700-713CODEN: DTARAF; ISSN:1477-9226. (Royal Society of Chemistry)
      A review. Three-center/four-electron (3c/4e) bonds are important bonding motifs that dictate the electronic structure, and thereby the reactivity, of metal-metal bonded carbene and nitrene intermediate complexes that are crucial to the dirhodium-catalyzed functionalization of hydrocarbons. In this Perspective article, general features of the 3c/4e bond are presented and discussed in comparison to two-center/two-electron (2c/2e) bonds. Specifically, 3c/4e bonding interactions lead to longer distances between the atoms involved and measurably weaker bonds. Addnl., excited states derived from the 3c/4e bonding manifold are lower in energy than those derived from a 2c/2e manifold, signifying a greater degree of reactivity in the former case. Three coterminous 3c/4e Ru-Ru-N bonds are present in metal-metal/metal-ligand multiply bonded diruthenium terminal nitrido compds. This bonding situation results in an unusual superelectrophilic character of the nitride nitrogen atom, exemplified by its insertion into aryl C-H bonds via an electrophilic arom. substitution mechanism. The key catalytic intermediates in dirhodium-catalyzed C-H functionalization reactions, dirhodium carbene and dirhodium nitrene complexes, may also be described as superelectrophilic by virtue of 3c/4e Rh-Rh-C(or N) σ and π bonds. These 3c/4e bonding interactions set apart dirhodium carbene and nitrene intermediates from other, less electrophilic, carbene or nitrene species.
    40. 40
      Hush, N. S.; Reimers, J. R. Solvent Effects on the Electronic Spectra of Transition Metal Complexes. Chem. Rev. 2000, 100, 775– 786,  DOI: 10.1021/cr980409v
      40
      Solvent effects on the electronic spectra of transition metal complexes
      Hush, Noel S.; Reimers, Jeffrey R.
      Chemical Reviews (Washington, D. C.) (2000), 100 (2), 775-786CODEN: CHREAY; ISSN:0009-2665. (American Chemical Society)
      A review with 79 refs. on the Zeng-Hush-Reimers solvent shift (ZHR-SS), a computational method for the calcn. of solvent effects on metal-to-ligand charge transfer (MLCT) transitions. The given examples are almost exclusively dealing with H2O as the solvent. The topics treated are: the (n,π*) spectra of azine solns., the solvation effects on MLCT transitions in Ru2+ complexes (Ru(NH3)5L), the photochem. of Fe(H2O)62+ complexes in water, metal-metal coupling in dimeric systems, and the electroabsorption (Stark) effect.
    41. 41
      Suslick, K. S.; Bautista, J. F.; Watson, R. A. Metalloporphyrin photochemistry with matrix isolation. J. Am. Chem. Soc. 1991, 113, 6111– 6114,  DOI: 10.1021/ja00016a029
      41
      Metalloporphyrin photochemistry with matrix isolation
      Suslick, Kenneth S.; Bautista, Jocelyn F.; Watson, Randall A.
      Journal of the American Chemical Society (1991), 113 (16), 6111-14CODEN: JACSAT; ISSN:0002-7863.
      The photochem. of a no. of metalloporphyrin oxoanion complexes was examd. by matrix isolation techniques, using both frozen solvent glasses and polymer films. After an extensive search for a noncoordinating, unreactive, glassing solvent, a 3:1 mixt. of 2,2-dimethylbutane and tert-butylbenzene was found to work well at temps. <70 K. Alternatively, the photochem. of metalloporphyrins was monitored in polymer films by the evapn. onto a sapphire window of metalloporphyrin solns. in PhMe contg. either PMMA or poly(α-methylstyrene). The polymer films have the added advantage of a greatly increased temp. range, providing diffusional isolation even at room temp. The photoredn. of the metal by homolytic α-bond cleavage and loss of the axial ligand appears to be a general mechanism for all metalloporphyrin complexes examd. The formation of metal-oxo species from photolysis of metalloporphyrin oxoanion complexes in soln. derives from secondary, thermal reactions.

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