Mechanistic insights into the different chemoselectivities of Rh2(II)-catalyzed ring expansion of cyclobutanol-substituted aryl azides and C–H bond amination of cyclopentanol-substituted aryl azides: a DFT study

Dafang Gao and Xiaoguang Bao*
College of Chemistry, Chemical Engineering and Materials Science, Soochow University, 199 Ren-Ai Road, Suzhou Industrial Park, Suzhou, Jiangsu 215123, China. E-mail: xgbao@suda.edu.cn

Received 1st February 2018 , Accepted 5th March 2018

First published on 6th March 2018


The mechanism of Rh2(II)-catalyzed ring expansion of ortho-cyclobutanol-substituted aryl azides to access N-heterocycles was investigated by DFT calculations. After the generation of the key Rh2(II)-N-arylnitrene intermediate (A), computational results suggest that it is more feasible to undergo proton transfer from the O–H group of the ortho-cyclobutanol moiety to the nitrene site in the singlet state, along with four-membered ring expansion via [1,2] migration in a concerted manner, to form the key intermediate (B). Subsequently, the [1,3] migration step driven by rearomatization could follow to yield the final benzazepinone product. For the substrate, ortho-cyclopentanol-substituted aryl azide, H-atom abstraction (HAA) from the proximal sp3 C–H bond in the triplet state is more favorable than competitive HAA/proton transfer from the O–H group after the formation of the rhodium N-aryl nitrene intermediate. Subsequently, the C–H bond amination product, indoline, could be obtained via a radical rebound step. The origin of the different chemoselectivities for the ortho-cyclobutanol-substituted and ortho-cyclopentanol-substituted aryl azides was discussed.


Introduction

Transition-metal-catalyzed nitrene transfer reactions have attracted increasing research interest in organic synthesis to construct C–N bonds in a straightforward manner.1 Aliphatic/aromatic C–H bond amination, promoted by a growing number of transition metal complexes with various nitrene precursors as the nitrogen source, is arguably the most extensively studied topic.2 Aziridination of alkenes employing nitrene precursors has become a practical protocol in synthesizing aziridine derivatives.3 Moreover, catalytic nitrene transfer involved difunctionalization of alkenes,1c,4 cascade reactions,5 and cycloaddition reactions6 are also advanced. Nevertheless, developing chemoselective nitrene transfer reactions in the presence of multiple reactive sites is highly desirable.7 Recently, the group of Driver reported a novel reactivity mode of rhodium N-aryl nitrenes towards the synthesis of medium-sized N-heterocycles, benzazepinone (2), from ortho-cyclobutanol-substituted aryl azides (1).8 Interestingly, the sp3 C–H bond amination product, indoline (4), was obtained from the reaction of ortho-cyclopentanol-substituted aryl azides (3).8
image file: c8qo00113h-u1.tif

Mechanistically, transition-metal–nitrene intermediates have been generally proposed to play a critical role in nitrene transfer reactions.9 After the generation of the rhodium N-aryl nitrene intermediate (A), the following three possible pathways could be proposed (Scheme 1). In path a, H-atom abstraction (HAA)/proton transfer from the O–H bond of the o-cyclobutanol group might occur, followed by a ring-expansion step to afford the intermediate B. Subsequently, the [1,3] migration step driven by rearomatization could follow to yield the final benzazepinone product. Alternatively, after the formation of A, the selective ring expansion of the o-cyclobutanol group followed by proton transfer could also generate B (path b).8 Another competitive route might be the HAA/proton transfer from the proximal sp3 C–H bond of the o-cyclobutanol group to produce the C–H amination product with different chemoselectivities (path c). For the substrate, o-cyclopentanol-substituted aryl azide, the aforementioned mechanistic pathways are also applicable.


image file: c8qo00113h-s1.tif
Scheme 1 Possible mechanistic pathways leading to different chemoselective products after the generation of the Rh2(II)-N-arylnitrene intermediate.

Although a number of theoretical studies on transition-metal-catalyzed nitrene transfer reactions have been reported,9 the Rh2(II)-catalyzed chemoselective ring expansion of 1 to form 2 via the Rh2(II)-N-arylnitrene intermediate requires further clarification. In this work, the detailed mechanism for the Rh2(II)-catalyzed ring expansion of 1 to produce 2 was investigated by DFT calculations. In addition, the origin of the different chemoselectivities for 3, which undergoes sp3 C–H bond amination to afford 4, was also discussed.

Computational methods

The M06 density functional method,10 which has shown good performance in Rh2(II)-catalyzed nitrene involved reactions,11 was employed in this work to carry out all the computations. The LANL2DZ basis set in conjunction with the LANL2DZ pseudopotential12 was used for the Rh atom. The 6-31G(d,p) basis set13 was used for the other atoms in the geometry optimizations. Vibrational frequency analyses at the same level of theory were performed on all optimized structures to characterize stationary points as local minima or transition states. Furthermore, intrinsic reaction coordinate (IRC) computations were carried out to confirm that transition states connect appropriate reactants and products. The gas-phase Gibbs free energies for all species were obtained at 298.15 K and 1 atm at their respective optimized structures.

To consider solvation effects, single-point energy computations using the SMD model14 with toluene as a solvent were performed based on the optimized gas-phase geometries of all species. The basis sets, SDD15 for Rh and 6-311++G(d,p) for other atoms, were utilized for single-point energy calculations on stationary points. The solution-phase Gibbs free energy was determined by adding the solvation single-point energy and the gas-phase thermal correction to the Gibbs free energy obtained from the vibrational frequency analysis. Unless otherwise specified, the solution-phase Gibbs free energy was used in the present discussion. The Gaussian 09 suite of programs16 was used throughout. The CYLview software was employed to show the 3D structures.17

Results and discussion

A mechanistic study of the Rh2(II)-catalyzed ring expansion of the cyclobutanol-substituted aryl azide

In order to reduce the computational cost, the two esp ligands employed in the Rh2(esp)2 catalyst were replaced with four acetate ligands. Initially, the cyclobutanol-substituted aryl azide substrate 1 could coordinate to the Rh2(II) carboxylate catalyst model to form the complex INT1. The transition state (TS) of the dissociation of N2 from INT1 to generate the key Rh2(II)-N-arylnitrene intermediate is located as TS1, in which the breaking N–N bond distance is lengthened to 1.70 Å (Fig. 1). The predicted energy barrier for the elimination of N2 is 26.3 kcal mol−1 relative to separated 1 and the Rh2(OAc)4 catalyst.18 The triplet state of the formed Rh2(II)-N-arylnitrene intermediate (3INT2) is 4.7 kcal mol−1 lower in energy than the corresponding singlet state (1INT2). Spin density analysis for 3INT2 shows that 1.45 e is located on the N atom of the nitrene moiety. The two singly occupied molecular orbitals (SOMO) of 3INT2 are shown in Fig. 2. One of the singly occupied electrons resides in the π orbital (SOMO-1) and is delocalized with the connected phenyl group. The other singly occupied electron is located in the σ orbital (SOMO-2). The N-centered diradical character of 3INT2 implies that the intramolecular HAA from either the O–H bond or the proximal sp3 C–H bond of the o-cyclobutanol group might be possible.
image file: c8qo00113h-f1.tif
Fig. 1 Energy profiles (in kcal mol−1) for the formation of the key Rh2(II)-N-arylnitrene intermediate with the substrate 1. Bond lengths are shown in Å.

image file: c8qo00113h-f2.tif
Fig. 2 The two singly occupied molecular orbitals (SOMO) of the key Rh2(II)-N-arylnitrene intermediate (3INT2).

Next, the proposed path a shown in Scheme 1 was investigated computationally. For the triplet state of the formed Rh2(II)-N-arylnitrene intermediate (3INT2), the optimized TS structure of the intramolecular HAA from the O–H bond of the o-cyclobutanol moiety was shown as 3TS2 in Fig. 3, in which the O⋯H distance is lengthened to 1.31 Å while the N⋯H distance is shortened to 1.14 Å. The computational result predicts that the energy barrier of HAA from the hydroxyl group in the triplet state is 13.3 kcal mol−1 relative to 3INT2. After the H-atom transfer step, the O-centered radical intermediate metalloimine (3INT3) is yielded. The formed 3INT3 could readily undergo the proximal C–C bond breaking to afford a more stable carbon-centered radical (3INT4), being exothermic by 10.9 kcal mol−1 relative to 3INT2. Subsequently, the terminal carbon-centered radical might form the ring-expansion intermediate 1INT5, which is much more exothermic by 50.5 kcal mol−1 relative to 3INT2.


image file: c8qo00113h-f3.tif
Fig. 3 Energy profiles (in kcal mol−1) for the HAA/proton transfer from the O–H group of the o-cyclobutanol moiety. Bond lengths are shown in Å.

For the singlet state of the formed Rh2(II)-N-arylnitrene intermediate (1INT2), the corresponding intramolecular HAA from the O–H group of the o-cyclobutanol moiety via the triplet state is actually converted to a proton transfer step. The located TS structure (1TS4) is shown in Fig. 3, in which the O⋯H distance is lengthened to 1.18 Å while the N⋯H distance is shortened to 1.30 Å. It should be noted that the C1⋯C2 distance is also lengthened to 1.83 Å while the C1⋯C3 distance is shortened to 2.24 Å along with the proton transfer step. Thus, the proton transfer from the hydroxyl group in the singlet state could simultaneously result in the ring-expansion intermediate, leading to 1INT5 in a concerted manner. The predicted energy barrier of proton transfer from the hydroxyl group in the singlet state is 12.3 kcal mol−1 relative to 3INT2, indicating that the formation of 1INT5 in a concerted manner is more favorable than the stepwise route via the triplet state.19

It should be noted that the aromaticity of the aryl ring is destroyed in 1INT5. Under the driving force of rearomatization, the C–C bond is relatively easy to break and the TS of C–C bond cleavage is actually located as the [1,3] migration step (1TS5), in which the C2⋯C3 distance is lengthened to 2.22 Å while the C2⋯N distance is shortened to 2.24 Å. Thus, after the formation of the ring-expansion intermediate 1INT5, the [1,3] migration step could follow to yield the final benzazepinone product.

In addition, the direct ring expansion of the four-membered ring moiety of 3INT2/1INT2 without the HAA/proton transfer step was also considered computationally (path b). The predicted energy barrier for the ring expansion in the triplet state after the formation of the Rh2(II)-N-arylnitrene intermediate is 37.9 kcal mol−1 relative to 3INT2 (Fig. 4). The significantly higher energy barrier indicates that the direct ring-expansion mechanism is highly unlikely compared with the route of proton transfer from the hydroxyl group along with the ring-expansion. The corresponding ring expansion via the singlet state was located as 1TS7 and the computed energy barrier is also much higher than the route of proton transfer from the hydroxyl group.


image file: c8qo00113h-f4.tif
Fig. 4 Energy profiles (in kcal mol−1) for the intramolecular HAA/proton transfer from the proximal C–H bond of the o-cyclobutanol group and the direct ring-expansion pathway. Bond lengths are shown in Å.

The insertion of the nitrene moiety of metal–nitrene complexes into C–H bonds to afford amination products has been well documented.2 The potential C–H amination routes via HAA from the proximal sp3 C–H bond of the o-cyclobutanol moiety were explored computationally and both singlet and triplet states of the formed Rh2(II)-N-arylnitrene intermediate were considered. In order to undergo HAA from the proximal sp3 C–H bond of the o-cyclobutanol moiety, the structural conformer INT2′ is located, in which the proximal C–H bond approaches to the N atom of the nitrene moiety. Subsequently, a stepwise HAA route via the triplet state is investigated and the optimized TS structure (3TS8) is shown in Fig. 4. In 3TS8, the C⋯H distance is lengthened to 1.33 Å while the N⋯H distance is shortened to 1.27 Å. After the formation of the diradical intermeidate 3INT7, a radical rebound step could follow to afford the intramolecular C–H amination product. The calculated energy barrier for the triplet state along path c is 16.8 kcal mol−1 relative to 3INT2, which is 4.5 kcal mol−1 higher in energy than that of the proton transfer from the hydroxyl group alongside the ring-expansion via path a. Meanwhile, the TS for the C–H amination via the singlet state, leading to the amination product in a concerted manner, is located as 1TS9. However, the predicted energy barrier is even higher than that of the corresponding triplet state. Thus, the intramolecular HAA from the proximal C–H bond of the o-cyclobutanol moiety (path c) is less favorable. Overall, computational results suggest that it is more likely for the Rh2(II)-N-arylnitrene intermediate to undergo the intramolecular proton transfer from the O–H group of the o-cyclobutanol moiety accompanying the ring-expansion in a concerted manner to generate the critical 1INT5 intermediate. Subsequently, under the driving force of rearomatization, the [1,3] migration step could follow to afford the final product.

image file: c8qo00113h-u2.tif

Experimental results showed that the substrate with an alkyl cis-substituent on the four-membered ring, 5, stereoselectively produces the benzazepinone derivative with a single diastereomer (6), suggesting the [1,2] migration is concerted.8 In addition, one may propose that the proximal C–H bond with an alkyl substituent would be more ready to undergo the C–H bond breaking compared with the unsubstituted substrate. Computational studies were carried out to rationalize the formation of the stereoselective benzazepinone derivative instead of the C–H amination product. Likewise, for the substrate 5′ (alkyl = Me), the above mentioned HAA/proton transfer steps from both the hydroxyl group and the proximal tertiary C–H bond were explored after the formation of the corresponding Rh2(II)-N-arylnitrene intermediate (Fig. S1). The proton transfer from the hydroxyl group in the singlet state is also found to accompany with the ring expansion (1TS4-a), leading to the 1INT5-a intermediate in a concerted manner. The energy barrier for this step is computed to be 7.2 kcal mol−1 relative to 3INT2-a (Fig. 5). The corresponding HAA from the hydroxyl group in the triplet state is shown as 3TS2-a and the predicted energy barrier is 12.5 kcal mol−1. Thus, the proton transfer from the hydroxyl group in the singlet state is also more favorable than the HAA in the triplet state due to the considerably lower energy barrier. In addition, the calculated ΔG for the competing HAA from the proximal tertiary C–H bond of 5′ in the triplet state is 9.2 kcal mol−1 relative to 3INT2-a, which is 2 kcal mol−1 higher than the proton transfer from the O–H group in the singlet state (Fig. 6). The proton transfer from the C–H bond in the singlet state is predicted to have an even higher energy barrier. Thus, the proton transfer from the hydroxyl group triggered concerted ring expansion pathway via the singlet state is also found to be the most favorable pathway for 5′ after the generation of the rhodium N-aryl nitrene intermediate. The presence of the alkyl substituent could lead to the ring expansion to form the 1INT5-a intermediate more ready to occur. Computational results demonstrate that both the ring expansion step to afford 1INT5-a and a subsequent [1,3] migration step to form the benzazepinone derivative (6′) proceed in a concerted manner. Therefore, the intermediate of 1INT5-a and hence the final benzazepinone derivative could be stereoselectively obtained, which is consistent with the experimental results.


image file: c8qo00113h-f5.tif
Fig. 5 Energy profiles (in kcal mol−1) for the HAA/proton transfer from the O–H group of the o-cyclobutanol moiety with a methyl cis-substituent on the four-membered ring. Bond lengths are shown in Å.

image file: c8qo00113h-f6.tif
Fig. 6 Energy profiles (in kcal mol−1) for the pathways of direct ring-expansion and intramolecular HAA/proton transfer from the C–H bond of the o-cyclobutanol group with a methyl cis-substituent on the four-membered ring. Bond lengths are shown in Å.

A mechanistic study of the Rh2(II)-catalyzed C–H bond amination of the cyclopentanol-substituted aryl azide

The three proposed mechanistic pathways in Scheme 1 were also considered for the substrate, o-cyclopentanol-substituted aryl azide (3). After the formation of the initial complex with the catalyst, an energy barrier of 27.9 kcal mol−1 is predicted for the dissociation of N2 from 3 to form the Rh–nitrene intermediate. Similarly, the triplet state of the generated Rh–nitrene intermediate (3INT10) is found to be the ground state with ΔES-T = 6.1 kcal mol−1 (Fig. S2). For the intramolecular proton transfer from the O–H group of the o-cyclopentanol moiety via the singlet state, the located TS structure (1TS13) is shown in Fig. 7, in which the O⋯H distance is lengthened to 1.60 Å while the N⋯H distance is shortened to 1.06 Å. Similar to 1TS4, the C1⋯C2 distance is also lengthened to 1.73 Å while the C1⋯C3 distance is shortened to 2.37 Å accompanying with the proton transfer step. The concerted proton transfer from the hydroxyl group in the singlet state could lead to the ring-expansion intermediate 1INT13. The predicted energy barrier of proton transfer from the hydroxyl group in the singlet state is 17.6 kcal mol−1 relative to 3INT10. For the HAA from the O–H group in the triplet state, the corresponding TS (3TS11) was optimized, in which the O⋯H distance is lengthened to 1.31 Å while the N⋯H distance is shortened to 1.14 Å. The resulted O-centered radical intermediate (3INT11) could undergo the proximal C–C bond breaking to yield a carbon-centered radical (3INT12) and further convert into the ring-expansion intermediate 1INT13. The predicted energy barrier for HAA from the hydroxyl group in the triplet state is 18.0 kcal mol−1 relative to 3INT10, which is much close to that in the singlet state.
image file: c8qo00113h-f7.tif
Fig. 7 Energy profiles (in kcal mol−1) for the HAA/proton transfer from the O–H group of the o-cyclopentanol moiety. Bond lengths are shown in Å.

Next, the competing C–H amination via HAA from the proximal sp3 C–H bond of the o-cyclopentanol moiety was explored computationally. From the triplet ground state of the formed Rh2(II)-N-arylnitrene intermediate 3INT10, the TS of stepwise HAA was located as 3TS17, as shown in Fig. 8, in which the C⋯H distance is lengthened to 1.31 Å while the N⋯H distance is shortened to 1.29 Å. The formed diradical intermediate 3INT15 could produce the intramolecular C–H amination product by a radical rebound step. The calculated energy barrier for the HAA from the C–H bond in the triplet state is 14.1 kcal mol−1 relative to 3INT10, which is ca. 3.5 kcal mol−1 lower in energy than that of the HAA/proton transfer from the hydroxyl group in either the triplet or the singlet state. In addition, the C–H amination via the singlet state was also considered. The optimized TS is shown as 1TS18. However, the predicted energy barrier is 22.1 kcal mol−1 relative to 3INT10, which is much higher than that of the corresponding triplet state. The energy barriers for the direct ring-expansion pathway (path b) from either the triplet or the singlet state of the Rh–nitrene intermediate are also very high (Fig. 8). Thus, computational results suggest that the intramolecular HAA from the proximal C–H bond of the o-cyclopentanol moiety in the triplet state followed by a radical rebound step to result in the amination product is more favorable than the HAA/proton transfer from the hydroxyl group, which is in accordance with the experimental results.20

image file: c8qo00113h-u3.tif


image file: c8qo00113h-f8.tif
Fig. 8 Energy profiles (in kcal mol−1) for the intramolecular HAA/proton transfer from the C–H bond of the o-cyclopentanol group and the direct ring-expansion pathway. Bond lengths are shown in Å.

The bond dissociation energy (BDE) analyses were carried out to shed light on the origin of the different chemoselectivities for the o-cyclobutanol-substituted and o-cyclopentanol-substituted aryl azides. The BDE values of the C–H bond of cyclobutane and cyclopentane are 100 and 97.6 kcal mol−1, respectively.21 According to the results of the isodesmic reactions (1) and (2), the predicted BDE values of the proximal C–H bonds for 7 and 8 are 100.2 and 98.8 kcal mol−1, respectively. According to previous computational results, the predicted ΔG values of the favored intramolecular HAA from the C–H bond of o-cyclobutanol and o-cyclopentanol groups are 16.8 and 14.1 kcal mol−1, respectively. The higher energy barrier (2.7 kcal mol−1) for HAA from the substrate with a four-membered ring than with a five-membered ring could be partially attributed to the slightly larger BDE value of the C–H bond in the four-membered ring than in the five-membered ring. In addition, the accessible nitrene moiety for the C–H bonds to undergo HAA reaction could not be ignored. Structural examinations of 3INT2′ and 3TS8 show that the marked bond angle in Scheme 2 has to bend considerably from 121° to 116° to undergo the intramolecular HAA from the C–H bond of the o-cyclobutanol group. However, the corresponding bond angle deviation is only 2° from 3INT10′ to 3TS17 for the intramolecular HAA from the C–H bond of the o-cyclopentanol group due to the larger ring size. Thus, the considerably larger bond angle changes could result in an extra energy penalty, which is the second reason to account for the relatively higher energy barrier for the intramolecular HAA from the C–H bond of the o-cyclobutanol group.


image file: c8qo00113h-s2.tif
Scheme 2 Bond angle changes for the intramolecular HAA from C–H bonds of o-cyclobutanol and o-cyclopentanol groups, respectively.22

The BDE value of the O–H bond for methanol is 104.2 kcal mol−1.21 The BDE values of the O–H bond for 7 and 8 are calculated to be 102.0 and 104.4 kcal mol−1, respectively, according to the results of the isodesmic reactions (3) and (4). The calculated ΔG values of the favored intramolecular proton transfer from the O–H group of o-cyclobutanol and o-cyclopentanol moieties are 12.3 and 17.6 kcal mol−1, respectively. The considerably lower energy barrier for proton transfer from the hydroxyl group attached to the four-membered ring than to the five-membered ring (ΔΔG = 5.3 kcal mol−1) could not be solely attributed to the slightly lower BDE value of the O–H bond attached to the four-membered ring than to the five-membered ring. It should be noted that the favored proton transfer from the hydroxyl group of the o-cyclobutanol moiety is associated with the four-membered ring expansion. The release of ring tension of the four-membered ring could provide extra driving force in the process of cleavage of the O–H bond. For the five-membered ring substrate, however, such driving force of the release of ring tension via proton transfer from the O–H group would be much less significant. Therefore, the intramolecular proton transfer from the hydroxyl group is more ready to take place for the o-cyclobutanol moiety than for the o-cyclopentanol moiety.

Conclusions

In summary, for the mechanism of the Rh2(II)-catalyzed ring expansion of the cyclobutanol-substituted aryl azide, computational results suggest that it is more feasible for the Rh2(II)-N-arylnitrene intermediate to undergo the intramolecular proton transfer from the O–H group of the o-cyclobutanol moiety in the singlet state. The cleavage of the O–H bond is associated with the four-membered ring expansion via [1,2] migration in a concerted manner to generate the critical 1INT5 intermediate. Subsequently, under the driving force of rearomatization, the [1,3] migration step could follow to afford the final benzazepinone product. For the substrate, o-cyclopentanol-substituted aryl azide, the H-atom abstraction (HAA) from the proximal sp3 C–H bond in the triplet state is more favorable than the competitive HAA/proton transfer from the O–H group after the formation of the corresponding metal N-aryl nitrene intermediate. Subsequently, the C–H bond amination product, indoline, could be obtained via a radical rebound step.

Since the BDE of the O–H bond is even slightly higher than that of the C–H bond for the o-cyclobutanol group, the origin of the chemoselectivity for the o-cyclobutanol-substituted aryl azide could not be rationalized by the BDE differences. Because the favored pathway of proton transfer from the O–H group of the o-cyclobutanol moiety is associated with the four-membered ring expansion, the release of ring strain of the four-membered ring could provide extra diving force for the cleavage of the O–H bond. In contrast, the HAA from the C–H bond of the o-cyclobutanol group has to result in considerably larger bond angle deviation to approach the nitrene moiety due to the smaller ring size. Thus, such a destabilizing factor could lead to more energy penalty in the HAA from the C–H bond of the o-cyclobutanol group. Consequently, the proton transfer from the O–H group is more ready to occur and the chemoselective benzazepinone product is formed. The advantage of O–H breaking for the o-cyclopentanol group, however, is less important because the ring strain of the five-membered ring is less than the four-membered ring. In addition, the destabilizing factor for C–H bond breaking of the o-cyclopentanol group also becomes less important due to the larger ring size. Thus, the origin of the chemoselectivity for the o-cyclopentanol-substituted aryl azide to afford the sp3 C–H bond amination product could be mainly attributed to the considerably smaller BDE value of the C–H bond than that of the O–H bond.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We are grateful to the National Natural Science Foundation of China (21642004) and the Priority Academic Program Development of Jiangsu Higher Education Institutions (PAPD) for financial support.

References

  1. For selected reviews, see: (a) P. Müller and C. Fruit, Chem. Rev., 2003, 103, 2905 CrossRef PubMed; (b) T. G. Driver, Org. Biomol. Chem., 2010, 8, 3831 RSC; (c) G. Dequirez, V. Pons and P. Dauban, Angew. Chem., Int. Ed., 2012, 51, 7384 CrossRef CAS PubMed; (d) B. Hu and S. G. DiMagno, Org. Biomol. Chem., 2015, 13, 3844 RSC; (e) B. Darses, R. Rodrigues, L. Neuville, M. Mazurais and P. Dauban, Chem. Commun., 2017, 53, 493 RSC.
  2. For selected reviews, see: (a) M. M. Díaz-Requejo and P. J. Pérez, Chem. Rev., 2008, 108, 3379 CrossRef PubMed; (b) B. J. Stokes and T. G. Driver, Eur. J. Org. Chem., 2011, 4071 CrossRef CAS; (c) F. Collet, C. Lescot and P. Dauban, Chem. Soc. Rev., 2011, 40, 1926 RSC; (d) J. L. Roizen, M. E. Harvey and J. Du Bois, Acc. Chem. Res., 2012, 45, 911 CrossRef CAS PubMed; (e) M. L. Louillat and F. W. Patureau, Chem. Soc. Rev., 2014, 43, 901 RSC; (f) K. Shin, H. Kim and S. Chang, Acc. Chem. Res., 2015, 48, 1040 CrossRef CAS PubMed; (g) Y. Park, Y. Kim and S. Chang, Chem. Rev., 2017, 117, 9247 CrossRef CAS PubMed.
  3. (a) J. A. Halfen, Curr. Org. Chem., 2005, 9, 657 CrossRef CAS; (b) I. D. Watson, L. Yu and A. K. Yudin, Acc. Chem. Res., 2006, 39, 194 CrossRef CAS PubMed; (c) J. W. W. Chang, T. M. U. Ton and P. W. H. Chan, Chem. Rec., 2011, 11, 331 CrossRef CAS PubMed; (d) S. A. Cramer and D. M. Jenkins, J. Am. Chem. Soc., 2011, 133, 19342 CrossRef CAS PubMed.
  4. J. Ciesielski, G. Dequirez, P. Retailleau, V. Gandon and P. Dauban, Chem. – Eur. J., 2016, 22, 9338 CrossRef CAS PubMed.
  5. (a) A. R. Thornton and S. B. Blakey, J. Am. Chem. Soc., 2008, 130, 5020 CrossRef CAS PubMed; (b) A. R. Thornton, V. I. Martin and S. B. Blakey, J. Am. Chem. Soc., 2009, 131, 2434 CrossRef CAS PubMed; (c) A. H. Stoll and S. B. Blakey, Chem. Sci., 2010, 2, 112 RSC; (d) K. Chen, Z. Z. Zhu, J. X. Liu, X. Y. Tang, Y. Wei and M. Shi, Chem. Commun., 2016, 52, 350 RSC.
  6. (a) M. R. Fructos, E. Alvarez, M. M. Díaz-Requejo and P. J. Pérez, J. Am. Chem. Soc., 2010, 132, 4600 CrossRef CAS PubMed; (b) Q. Wu, J. Hu, X. Ren and J. Zhou, Chem. – Eur. J., 2011, 17, 11553 CrossRef CAS PubMed.
  7. (a) J. A. Alderson, A. M. Phelps, R. J. Scamp, N. S. Dolan and J. M. Schomaker, J. Am. Chem. Soc., 2014, 136, 16720 CrossRef CAS PubMed; (b) N. S. Dolan, R. J. Scamp, T. Yang, J. F. Berry and J. M. Schomaker, J. Am. Chem. Soc., 2016, 138, 14658 CrossRef CAS PubMed; (c) C. Weatherly, J. M. Alderson, J. F. Berry, J. E. Hein and J. M. Schomaker, Organometallics, 2017, 36, 1649 CrossRef CAS.
  8. W. Mazumdar, N. Jana, B. T. Thurman, D. J. Wink and T. G. Driver, J. Am. Chem. Soc., 2017, 139, 5031 CrossRef CAS PubMed.
  9. (a) X. Lin, C. Zhao, C. M. Che, Z. Ke and D. L. Phillips, Chem. – Asian J., 2007, 2, 1101 CrossRef CAS PubMed; (b) L. Maestre, W. M. C. Sameera, M. M. Díaz-Requejo, F. Maseras and P. J. Pérez, J. Am. Chem. Soc., 2013, 135, 1338 CrossRef CAS PubMed; (c) X. Zhang, H. Xu and C. Zhao, J. Org. Chem., 2014, 79, 9799 CrossRef CAS PubMed; (d) K. Hou, D. A. Hrovat and X. Bao, Chem. Commun., 2015, 51, 15414 RSC; (e) X. Zhang, H. Xu, X. Liu, D. L. Phillips and C. Zhao, Chem. – Eur. J., 2016, 22, 7288 CrossRef CAS PubMed; (f) J. Wang, C. Zhao, Y. Weng and H. Xu, Catal. Sci. Technol., 2016, 6, 5292 RSC.
  10. (a) Y. Zhao and D. G. Truhlar, Acc. Chem. Res., 2008, 41, 157 CrossRef CAS PubMed; (b) Y. Zhao and D. G. Truhlar, Theor. Chem. Acc., 2008, 120, 215 CrossRef CAS.
  11. J. G. Harrison, O. Gutierrez, N. Jana, T. G. Driver and D. J. Tantillo, J. Am. Chem. Soc., 2016, 138, 487 CrossRef CAS PubMed.
  12. P. J. Hay and W. R. Wadt, J. Chem. Phys., 1985, 82, 299 CrossRef CAS.
  13. (a) P. C. Hariharan and J. A. Pople, Theor. Chim. Acta, 1973, 28, 213 CrossRef CAS; (b) W. J. Hehre, R. Ditchfield and J. A. Pople, J. Chem. Phys., 1972, 56, 2257 CrossRef CAS.
  14. A. V. Marenich, C. J. Cramer and D. G. Truhlar, J. Phys. Chem. B, 2009, 113, 6378 CrossRef CAS PubMed.
  15. (a) M. Dolg, U. Wedig, H. Stoll and H. J. Preuss, Chem. Phys., 1987, 86, 866 CAS; (b) D. Andrae, U. Haussermann, M. Dolg, H. Stoll and H. Preuss, Theor. Chim. Acta, 1990, 77, 123 CrossRef CAS.
  16. M. J. Frisch, G. W. Trucks, H. B. Schlegel, G. E. Scuseria, M. A. Robb, J. R. Cheeseman, G. Scalmani, V. Barone, B. Mennucci, G. A. Petersson, H. Nakatsuji, M. Caricato, X. Li, H. P. Hratchian, A. F. Izmaylov, J. Bloino, G. Zheng, J. L. Sonnenberg, M. Hada, M. Ehara, K. Toyota, R. Fukuda, J. Hasegawa, M. Ishida, T. Nakajima, Y. Honda, O. Kitao, H. Nakai, T. Vreven, J. A. Montgomery, J. E. Peralta, F. Ogliaro, M. Bearpark, J. J. Heyd, E. Brothers, K. N. Kudin, V. N. Staroverov, R. Kobayashi, J. Normand, K. Raghavachari, A. Rendell, J. C. Burant, S. S. Iyengar, J. Tomasi, M. Cossi, N. Rega, N. J. Millam, M. Klene, J. E. Knox, J. B. Cross, V. Bakken, C. Adamo, J. Jaramillo, R. Gomperts, R. E. Stratmann, O. Yazyev, A. J. Austin, R. Cammi, C. Pomelli, J. W. Ochterski, R. L. Martin, K. Morokuma, V. G. Zakrzewski, G. A. Voth, P. Salvador, J. J. Dannenberg, S. Dapprich, A. D. Daniels, O. Farkas, J. B. Foresman, J. V. Ortiz, J. Cioslowski and D. J. Fox, Gaussian 09, Revision C.01, Gaussian, Inc., Wallingford, CT, 2010 Search PubMed.
  17. C. Y. Legault, CYLview, 1.0b, Universitéde Sherbrooke, Montreal, 2009; http://www.Cylview.org Search PubMed.
  18. In the absence of the Rh2(OAc)4 catalyst, the dissociation of N2 from 1 is predicted to have a much higher energy barrier (38.6 kcal mol−1). Thus, the presence of the Rh(II) catalyst could facilitate the elimination of N2 from the aryl azide and hence the key Rh2(II)-N-arylnitrene intermediate is formed.
  19. To undergo the proton transfer from the hydroxyl group to the nitrene site in the singlet state, a spin–flip process via the minimum energy crossing point (MECP) is required from the triplet ground state 3INT2. The MECP was located in a range of 3.0–4.3 kcal mol−1 higher in energy than 3INT2.
  20. Calculations using the full size Rh2(esp)2 catalyst model were carried out to evaluate the role of steric effects, if any, caused by the esp ligand on the critical transition states. Due to the nitrene moiety lying in the axial direction, the bulky esp ligands have negligible effects on the key HAA/proton transfer steps. Computational results using the full size model are consistent with the truncated model (see Fig. S3 and S4).
  21. The BDE data are available via the Internet Bond-energy Databank (iBond 2.0) http://ibond.chem.tsinghua.edu.cn or http://ibond.nankai.edu.cn CrossRef CAS PubMed; X.-S. Xue, P.-J. Ji, B. Zhou and J.-P. Cheng, Chem. Rev., 2017, 117, 8622 CrossRef CAS PubMed.
  22. See Fig. S5 for the mentioned bond angles in Scheme 2, which are shown in 3D structures.

Footnote

Electronic supplementary information (ESI) available: Fig. S1–S5, Cartesian coordinates and energies of stationary points in the reactions. See DOI: 10.1039/c8qo00113h

This journal is © the Partner Organisations 2018