Mechanistic Insight into Asymmetric N–H Insertion Cooperatively

Article pubs.acs.org/Organometallics

Mechanistic Insight into Asymmetric N−H Insertion Cooperatively Catalyzed by a Dirhodium Compound and a Spiro Chiral Phosphoric Acid Xu-Chao Wang,†,‡ Xian-Shuang Song,† Li-Ping Guo,† Deyu Qu,† Zhi-Zhong Xie,*,† Francis Verpoort,*,† and Jun Cao*,§ †

Department of Chemistry, School of Chemistry, Chemical Engineering and Life Science, and ‡School of Material Science and Engineering, Wuhan University of Technology, Wuhan 430070, People’s Republic of China § College of Chemistry, Beijing Normal University, Beijing 100875, People’s Republic of China S Supporting Information *

ABSTRACT: The insertion of a carbenoid into an N−H bond of an amine cooperatively catalyzed by a dirhodium catalyst and a spiro chiral phosphoric acid has been investigated in detail using density functional theory methods. Calculations indicate that the reaction begins with the nucleophilic amine attacking at the carbenoid, forming a metal-associated ammonium ylide first followed by a rapid proton transfer to afford a metal-associated enamine intermediate. Subsequently, the enamine intermediate dissociates from the metal and yield a more stable sevenmembered-ring conformation via an intramolecular hydrogen-bond exchange. Formation of the enamine intermediate requires an overall barrier of 5.7 kcal/mol and is exergonic by 5.1 kcal/mol. Calculations demonstrated that, although the conversion of the achiral enamine into the N−H insertion product can be facilitated efficiently by the dirhodium catalyst through a two-step process, it can be compressed to a large extent. This is due to the more competitive decomposition of the diazoacetate catalyzed by the dirhodium catalyst, which can give a carbenoid for the next catalytic cycle. Meanwhile, formation of the carbenoid is considerably exergonic, which can promote the direct [1,3]-proton shift of enamine. However, in the presence of the spiro chiral phosphoric acid, the asymmetric proton induction of enamine is greatly favored, requiring an activation free energy of 6.0 kcal/mol to afford the major R product. This agrees well with the experimental observation.

1. INTRODUCTION Rhodium-catalyzed carbene insertion into a N−H bond is an important developing synthetic strategy to create new C−N bonds under mild conditions, which can be widely employed in the synthesis of α-amino acids and their derivatives, alkaloids, nitrogen-containing heterocycles, and so on.1−4 Over the past few decades, considerable efforts have been made in this field, especially in the asymmetric versions of N−H insertion.5−10 However, although various chiral dirhodium catalysts have been investigated, no significant progress has been achieved.10−14 Only very recently has the situation been altered by employing an additional proton transfer catalyst.5,7,15 For instance, Zhou et al.5 reported that the asymmetric N−H insertion cooperatively catalyzed by the dirhodium(II) carboxylates and chiral spiro phosphoric acids (SPAs) could give a high enantioselectivity (up to 95% ee), suggesting that when an appropriate cocatalyst is employed, the dirhodium(II) catalyst is also promising as its Cu(I) counterpart in asymmetric N−H insertion.16−19 Despite the great experimental achievements, to the best of our knowledge, a detailed mechanistic understanding of N−H insertion has not been established. According to the generally accepted mechanism, the most likely process should involve the sequential formations of carbenoid (CB), C-bound ammonium ylide A, free ammonium ylide B or O-bound ammonium ylide C, and the N−H insertion product D (see Scheme 1).4,18,20 © 2014 American Chemical Society

However, recent achievements in the analogous Rh-catalyzed O−H21−23 and S−H24 insertion reactions suggest that this may be questionable. In 2009, Yu et al.23 studied the Rh-catalyzed carbene insertion into the O−H bond of water. They found that an alternative free enol was the most stable intermediate in comparison to the A-, B-, and C-like oxonium ylides (see Scheme 1). Later, Davies et al.22 proposed that free enol could Scheme 1

Received: May 24, 2014 Published: July 29, 2014 4042

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be obtained through the [1,4]-proton shift of the A-like oxonium ylide. Very recently, we studied the reaction of CB with primary allyl alcohol.21 Those results indicated that the orientation of the carbonyl oxygen of the acetate group of CB played a crucial role in determining the reaction proceeding via either an enol or free oxonium ylide pathway. As the carbonyl oxygen orients toward the approaching alcohol, the free enol can be obtained readily via the pathway proposed by Davies22 and the [1,3]-proton shift of free enol can occur relatively easily by using a two- or three-alcohol cluster as catalyst. In contrast, formation of an alcohol-stabilized free oxonium ylide is energetically feasible. Only in this case is the [1,2]-proton shift of free oxonium ylide proposed by Yu energetically feasible to give the O−H insertion product.23 In addition, Zhu and Yu reported an combined experimental and theoretical investigation into the enantioselective S−H insertion reaction, which was cooperatively catalyzed by a dirhodium(II) compound and chiral spiro phosphoric acid.24 Calculations indicated that the free ylide could be formed easily. In addition, in the presence of SPA, it could be converted quickly into a free enol intermediate. In comparison to the [1,2]-proton shift of free ylide, the S product obtained through the SPA-induced [1,3]proton shift of enol is more energetically favored. According to these results, this could suggest that both the ammonium ylide and free enamine pathways should be taken into account for a comprehensive mechanistic understanding of the analogous Rh(II)-catalyzed N−H insertion reaction mechanism. In the present work, we attempt to elucidate the Rh(II)catalyzed N−H insertion reaction mechanism and show why SPA can induce the asymmetric N−H insertion. The reaction depicted in Scheme 2 is selected as our subject.5 In light of our

Scheme 3

chiral induction of enamine could indeed be realized with an energy barrier of only 6.0 kcal/mol to form the major R product, which is favored by 4.3 kcal/mol over that to give the S product. This trend is in good agreement with the experimental observations.5

Scheme 2

2. COMPUTATIONAL METHODS All calculations have been performed using the DFT method implemented in the commercial Gaussian 09 program package.25 All stationary points, including the reactant, intermediate, transition states, and product, have been optimized with the B3LYP26,27 functional without any constraints. The effective core potential (ECP) of LanL2DZ28,29 was used to describe Rh. For the remaining atoms the 6-31G* Pople basis set was used. For convenience, such a basis set combination is referred to as BSI. Frequency analysis has been computed at the B3LYP/BSI level to identify all the stationary points as minima or first-order saddle points. Further, intrinsic reaction coordinate (IRC)30 calculations have been employed to confirm the transition structures connecting the corresponded reactants and products. The solvent effect has also been considered with the SMD solvation model,31 and CHCl3 was used as the solvent. The effective core potential (ECP) of LanL2DZ was still used to describe Rh. For the remaining atoms, we augmented the 6-31G* Pople basis set to 6-311+ +G**. For convenience, such a basis set combination is referred to as BSII. Without additional statement, the single-point energies based on B3LYP/BSI-optimized geometries in solution have been computed at the M06/BSII/SMD//B3LYP/BSI level.32−34

calculations, the formation of the ammonium ylides (B and C) is disfavored and the [1,2]-proton shift for both species suffers from the inaccessible overall barrier to give the N−H insertion product at room temperature. On the other hand, the formation of the enamine intermediate is more favorable kinetically as well as thermodynamically. However, different from the case for enol,21 the [1,3]-proton shift of enamine is complicated (see Scheme 3). Calculations indicate that this process can be facilitated efficiently by the dirhodium catalyst. However, the decomposition of diazoacetate (DIA) catalyzed by the dirhodium is more competitive. Consequently, the metal-assisted two-step [1,3]-proton shift could be compressed to some extent. However, both reactions are considerably exergonic. In addition, the exerted heat is sufficient to overcome the energy barrier corresponding to the direct [1,3]-proton shift of enamine. In turn, there exist two energetically feasible pathways corresponding to the [1,3]proton shift of enamine. However, in the presence of SPA, the

3. RESULTS AND DISCUSSION For comparison, the previously proposed mechanisms proceeding via either free or O-bound (having oxygen as the coordinative atom) ammonium ylides in the gas phase and CHCl3 are discussed first. Then a detailed study of the mechanism summarized in Scheme 3, namely via the enamine 4043

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3.1. Formation of Free and O-Bound Ammonium Ylides and their [1,2]-Proton Shift. The DFT computed free energy surfaces in the gas phase and in CHCl3 are presented in Figure 2. The optimized structures for the key stationary points along with the important bond parameters are given in Figure 3. In this pathway the reaction begins with the hydrogenbonding interaction between CB and AM. In the hydrogenbond intermediate A1, AM donates its two N−H bonds to the ligated oxygen atom (O3) and the methoxide (O2) with N1− H1···O2 Me and N1−H2···O3 bond distances of 2.31 and 2.06 Å, respectively. The formation of A1 is endergonic by 5.3 kcal/ mol (exothermic by 8.9 kcal/mol). Obviously, the pronounced difference between the relative Gibbs free energy and enthalpy is caused by the penalty of entropy (see Figure 2). As expected, the C-bound (having carbon anion as the coordinated atom) ammonium ylide A3 can be obtained easily through the nucleophilic attack of N1 at C1. The characterized transition structure ATS2 was confirmed to be a first-order saddle point with only one imaginary frequency of 91.0i cm−1. Indeed, IRC analysis indicates that ATS2 is the exact transition structure to connect A1 and A3. As shown in Figure 3, the forming N1−C1 bond has a relatively long distance of 2.37 Å in ATS2. Meanwhile, in comparison with A1, the CB and AM moieties both have minor geometrical variations; e.g., the lengths of the Rh1−C1 and N1−C3 bonds have been found to slightly increase from 2.03 and 1.36 Å in A1 to 2.09 and 1.38 Å in ATS2, respectively. Consequently, relative to A1, a low barrier of 3.2 kcal/mol (+8.5 kcal/mol relative to the sum of CB and AM) is achieved. In A3, the N1−C1 bond (1.52 Å) is formed, whereas both the 1 N −C3 (1.50 Å) and the Rh1−C1 (2.26 Å) bonds are significantly elongated with respect to the corresponding values in A1. Thermodynamically, the formation of A3 is favorable, which is exergonic by 9.1 kcal/mol relative to A1.

pathway, is carried out. Finally, the proton transfer of the enamine catalyzed by SPA is discussed. In the present work, a simplied dirhodium catalyst23,35 and CbzNH2 have been adopted to reduce the computational costs (see Figure 1), whereas for the asymmetric [1,3]-proton shift of

Figure 1. Simplified model molecules optimized at the B3LYP/BSI level (in Å).

the enamine catalyzed by SPA, no simplification has been made, to ensure that the steric effect is considered sufficiently. As shown in Figure 1, the C1−C2 bond in CB has a length of 1.49 Å, which is close to a regular C−C single bond. Hence, one would expect that the rotation of the acetate along the C1− C2 axis should have a low energy barrier, meaning that on the approach of the amine substrate (AM) to CB from either its Re face or Si face, the C2O1 and C2−O2 Me species are both able to orient toward it. Thus, in principle, there exist four attacking manners for AM at CB. However, according to the mechanism summarized in Scheme 3, the achiral enamine intermediate with a ring conformation suggests that calculations conducted either at the Re face or at the Si face will not change the mechanistic understanding. Therefore, without additional statement, calculations herein only focus on the reaction occurring at the Re face.

Figure 2. Computed free energy surface for the N−H insertion via free and O-bound ammonium ylides in CHCl3 at the M06/BSII/SMD//B3LYP/ BSI level. For comparison, the corresponding values computed in the gas phase at the B3LYP/BSI level are also presented. 4044

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Figure 3. Optimized stationary points for the N−H insertion via free and O-bound ammonium ylides at the B3LYP/BSI level (distances given in Å).

Figure 4. Computed free energy profile for the formation of the enamine intermediate in CHCl3 at the M06/BSII/SMD//B3LYP/BSI level. For comparison, the corresponding values computed in the gas phase at the B3LYP/BSI level are also presented.

The free ammonium ylide A4 is formed through the decomposition of A3. At the M06/BSII/SMD//B3LYP/BSI

level, the free ammonium ylide (A4) dissociation from A3 is endergonic by 18.8 kcal/mol, indicating that the formation of 4045

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Figure 5. DFT-optimized structures for the formation of enamine (distances given in Å).

A4 is disfavored. In addition, a later study of the [1,2]-proton shift also supports that the free ammonium ylide is unable to serve as the intermediate to give the N−H insertion product. Previous studies, through DFT calculations23 and ultrafast time-resolved IR spectroscopic studies,36 showed that either water or alcohol can assist the proton shift in the O−H insertion reaction. Accordingly, the analogous [1,2]-proton shift of the ammonium ylide catalyzed by AM was carried out. The transition state ATS6 corresponds to the [1,2]-proton shift of A4, as depicted in Figure 2, and relative to A3, an overall barrier of 40.0 kcal/mol is obtained. In light of this, it is clear that the [1,2]-proton shift of A4 cannot be facilitated efficiently by the AM molecule to give the N−H insertion product. Nonetheless, similar to the [1,3]-proton shift of enol,21 the [1,2]-proton shift of A4 catalyzed by a two-AM cluster has been studied as well. However, results indicate that during the proton shift the two-AM cluster is broken. This can be explained by the hydrogen-bonding interaction between AM molecules being weaker than that of either water or alcohol. As shown in Figure 1, the C−N bond has a length of 1.37 Å, which is much shorter than that of a regular single bond (∼1.47 Å). This indicates that the lone-pair electrons populate on the N atom participating in the bonding with the adjacent C atom, which decreases the electron density on the N atom significantly. This is disfavored for the hydrogen-bonding interaction.

As shown in Figure 3, a possible alternative intermediate, the O-bound ammonium ylide A7, can be viewed as the recombination of A4 and Rh2L4 (with O1 as the coordinated atom). However, relative to A3, the formation of A7 is highly endergonic by 16.1 kcal/mol and the subsequent AM-catalyzed [1,2]-proton shift of A7 also has an inaccessible barrier as high as 38.3 kcal/mol (see Figure 2), suggesting that the O-bound ammonium ylide cannot serve as the reactive intermediate responsible for the N−H insertion as well. 3.2. Formation of the Enamine Intermediate. The free energy surface for the formation of the enamine intermediate B7 is displayed in Figure 4. In this pathway, the reaction is also initiated from the formation of the hydrogen-bonded intermediate B1. Different from the case for A1, here one of the N−H bonds in AM is donated to the carbonyl oxygen (O1) in B1. The relative energy of B1 is lower than that of A1 by 1.8 kcal/mol, signifying that CB prefers to bind the incoming AM in this manner. The subsequent nucleophilic attack is also easy, which is due to a low overall barrier of 5.7 kcal/mol, resulting in the C-bound ammonium ylide B3. Owing to the fact that the C2O1 group is capable of acting as a proton acceptor, the proton transfer from the ammonium cation to O1 can occur through the transition state BTS4. At BTS4, the shifting H1 is shared well by N1 and O1 atoms with short N1···H1 (1.27 Å) and O1···H1 (1.28 Å) distances (Figure 5). Additionally, in comparison with B3, the elongation of the 4046

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Figure 6. Computed free energy profile for possible pathways of the [1,3]-proton shift of enamine in CHCl3 at the M06/BSII/SMD//B3LYP/BSI level. For comparison, the corresponding values computed in the gas phase at the B3LYP/BSI level are also presented.

Rh1−C1 bond distance from 2.27 to 2.32 Å in BTS4 is also not remarkable and, as a consequence, the proton transfer has a low energy barrier of only 2.5 kcal/mol to give the Rh(II)-enamine B5. At the resulting B5, the C1−C2 bond length is shortened to 1.41 Å, as a weakened C−C double bond (relative to the regular value of ∼1.36 Å). Meanwhile, a remarkable elongation of the Rh1···C1 distance to 2.45 Å is also observed. This is in good agreement with the soft Lewis base of the C1−C2 double bond having a weak interaction with the hard Lewis acid of Rh(II) intrinsically. As a consequence, a moderate value of 8.9 kcal/mol is obtained corresponding to the free enamine (B6) dissociation from the dirhodium catalyst. Calculations indicate that B6 can transform to a more stable isomer, B7, through intramolecular hydrogen-bond exchange (between N1···H1 and O4···H1). B7 has a seven-membered-ring conformation, which is favored over B6 by 0.6 kcal/mol. For comparison, the direct decomposition of the Rh(II)ammonium ylide B3 to give an alternative free ammonium ylide has been also studied. However, calculations at both the B3LYP/6-31G* and MP2/6-311++G** levels indicate that the free ammonium ylide in this case is unstable, which will result in B6 automatically. This can be attributed to the instability of the free ammonium ylide, including the ammonium cation and carbon anion in close proximity.37 In the presence of a proton stabilizer, the ammonium tends to transfer its proton to reduce the positive charges on it. 3.3. [1,3]-Proton Shift of Enamine. According to the calculations above, formation of the enamine intermediate B7 is by far favored kinetically and thermodynamically and its conversion into the N−H insertion product is now studied. As shown in Figure 6, three possible pathways corresponding to the [1,3]-proton shift reaction have been investigated. The selected optimized stationary points are collected in Figure 7 along with the key bond parameters. The AM-catalyzed [1,3]-proton shift of B7 was first explored, for which the related transition state BTS9 is presented in Figure 7. The transition state BTS9 features an asynchronous

O−H bond cleavage (1.59 Å) and a C−H bond formation (1.96 Å), in which the AM molecule acts as a proton shuttle to transfer the proton from O1 to C1. A Mulliken charge analysis reveals that there exists a considerable charge separation in BTS9; e.g., the enamine anion is negatively charged by −0.61e. Thus, ideally the ∠O1C2C1N1 dihedral angle should tend to be planar to maximize the delocalization of the negative charges through the formation of a three-center−four-electron bond (O1, C2, and C1). However, the computed value of 33.8° implies a significant geometrical distortion. This can be addressed to the enamine anion, which is responsible to a large extent for the [1,3]-proton shift of enamine bearing an inaccessible overall barrier as high as 39.8 kcal/mol relative to B5. Inspired by previous calculations on the Au(I)-catalyzed [1,2]-hydrogen shift step for the intramolecular transformation of enynyl acetate,38 the effect of the acetoxy group in the [1,3]proton shift of enamine has also been considered. The acetoxy atom O4 in BTS10 can act as a proton stabilizer and relay to transfer H1 from O1 to C1 concertedly. As shown in Figure 7, the shifting H1 is bonded to O4 with two relatively long O1···H1 (2.33 Å) and C1···H1 (2.26 Å) bond distances. Calculations indicate that BTS10 has an imaginary frequency of 230i cm−1 corresponding to the coupling of swing of H1 between O1 and C1 and the O4−H1 stretch. In comparison to BTS9, the relative energy for BTS10 is considerably lower by 13.1 kcal/mol. However, relative to B5, the obtained energy barrier of 26.7 kcal/mol is still rather high. Additionally, the [1,3]-proton shift of B7 catalyzed by Rh2L4 has also been studied. As shown in Figure 6, it is initiated by the complexation of B7 and Rh2L4, in which the O1 acts as the coordinative atom (B11). At the B3LYP/BSI level, the Rh1···O1 distance is estimated to be 2.32 Å and is very close to the corresponding value of 2.31 Å for the adsorption of water on the dirhodium catalyst.35 This implies that the adopted B3LYP/ BSI method can give a reasonable description for geometrical optimizations in the present work. 4047

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Figure 7. Optimized stationary points for the [1,3]-proton shift of enamine at the B3LYP/BSI level (distances given in Å).

C1···H1 bond distance is shortened remarkably to 1.65 Å relative to that of 2.00 Å in B13, indicating that extra stabilization on the shifting H1 is obtained. As a consequence, this step requires an activation free energy of 2.7 kcal/mol. Relative to B5, an overall barrier of 21.5 kcal/mol is achieved. It is much lower than those corresponding to the metal-free processes discussed above. However, when the decomposition of diazoacetate (DIA) mediated by Rh2L4 is considered, the situation will be changed and the metal-assisted [1,3]-proton shift of enamine could be compressed to a large extent. As shown in the Supporting Information, the transition state TSCB corresponds to the decomposition of DIA.23,35 At the same computational level, the relative free energy of TSCB is 0.9 kcal/mol lower than that of BTS14. This indicates that the metal-assisted decomposition of DIA is more competitive kinetically. Additionally, the decomposition of DIA to give CB and dinitrogen is considerably exergonic by 27.5 kcal/mol (32.6 kcal/mol relative to the energy reference). From the viewpoint of theory, the amount of exerted heat is adequate to overcome the energy barrier (BTS10) to give the final N−H insertion product. Thus, as suggested by one reviewer, both the metal-assisted and direct

Formation of B11 is endergonic by 2.4 kcal/mol (+10.7 kcal/ mol more in comparison to B5). The conversion of B11 into the final N−H insertion product B15 is a two-step process starting with the shift of H1 to the acetoxy O4 atom via transition state BTS12. In comparison to the corresponding values in BTS10, BTS12 has a shorter O1···H1 (2.24 Å) and longer C1···H1 (2.35 Å) distance. Frequency calculations predict that its imaginary vibration mode (264i cm−1) also corresponds to the coupling of swing of H1 between O1 and C1 and the O4−H1 stretch. While different from BTS10, BTS12 does not lead to C1−H1 bond formation directly. In contrast, it gives the intermediate B13 first with a O4−H1···C1 distance of 2.00 Å. Calculations indicate that the metal-assisted proton shift from O1 to O4 is greatly favored kinetically and bears a moderate barrier of 8.8 kcal/mol relative to B11. This is due to Rh2L4 playing an important role in stabilizing the developing negative charges in BTS12, which is evidenced by the shortened Rh1···O1 distance from 2.32 Å in B11 to 2.23 Å in BTS12 (see Figure 7). The second step corresponds to the proton transfer from O4 to C1. At the located transition structure BTS14, the O4−H1 bond cleavage is minor at a distance of 1.11 Å. Meanwhile, the 4048

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Figure 8. Optimized stationary points for the SPA-induced asymmetric [1,3]-proton shift of enamine along with the relative Gibbs free energies calculated at the M06/BSI/SMD//B3LYP/BSI level. For clarity, most of the hydrogen atoms in the transition structures have been omitted.

point energies for these structures are calculated at the M06/ BSI/SMD//B3LYP/BSI level. One would expect that the accuracy for the energy difference between (R)-TSEM‑SPA and (S)-TSEM‑SPA will be decreased to some extent and the energy order should be the same. Relative to the sum of SPA and EM, the computed relative energies for (R)-TSEM‑SPA and (S)-TSEM‑SPA are 6.0 and 10.3 kcal/mol, respectively (see Figure 8). This demonstrates that the proton transfer of EM can be facilitated asymmetrically by SPA and indeed give the major R product. The trend is consistent with the experimental observations. From Figure 8, one can observe that the naphthyl and CbzNH moieties have a staggered conformation in (R)TSEM‑SPA and an eclipsed conformation in (S)-TSEM‑SPA. Obviously, the steric hindrance caused by the two large groups in (S)-TSEM‑SPA is greater than that in (R)-TSEM‑SPA. This may explain why SPA can induce the asymmetric N−H insertion. For comparison, the direct [1,3]-proton shift of EM has been also studied at the same level. A transition state similar to BTS10, BTS10′, has been located. The obtained activation free energy of 19.7 kcal/mol is significantly higher than that associated with the SPA-induced [1,3]-proton shift of EM. Thus, in the presence of SPA, the non-enantioselective N−H insertion can be compressed exclusively.

[1,3]-proton shift of enamine are energetically feasible to give the N−H insertion product. In addition, one would expect that the latter pathway would be the major one. 3.4. Asymmetric [1,3]-Proton Shift of Enamine Induced by SPA. According to above calculations, it is clear that the carbonyl oxygen atom of the acetate group and the ligated oxygen atom play an important role in inducing the incoming AM to form B1 dominantly, which can then readily afford the enamine intermediate B7. The origin of its asymmetric [1,3]-proton shift induced by SPA is now explored. In previous work, Yamanaka39 discovered that the chiral phosphoric acid is a bifunctional proton transfer catalyst, which prefers to accept the proton by the PO moiety and donate its proton by the PO−H moiety simultaneously. Recently, Yu et al.24 also found that, for free ylide and enol, the proton shift for both induced by SPA was indeed in this manner for the S−H insertion reaction. In analogy to their findings, the asymmetric proton transfer of enamine has been studied. However, it should be noted that, to consider the steric hindrance sufficiently, the larger seven-membered-ring enamine (EM) is adopted (see Figure 8). As shown in Figure 8, the two located transition structures (R)-TSEM‑SPA and (S)-TSEM‑SPA are responsible for the formation of the R and S products, respectively. We observed that the [1,3]-proton shift of EM catalyzed by SPA is asynchronous. In both species, the O1−H1 bond is nearly intact with a length of 1.02 Å, whereas the PO6−H3 bond has a slight cleavage with the identical O6···H3 distance of 1.15 Å. This suggests that the process is initiated from SPA donating its proton to EM. Again, it should be noted that we had failed to obtain the relative energies for these species at the M06/BSII/ SMD//B3LYP/BSI level. This may due to the fact that the transition structures consist of 108 atoms. Instead, the single-

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CONCLUSION According to the calculations at the M06/BSII/SMD//B3LYP/ BSI level, the reaction mechanism for the insertion of Rh(II) carbenoid into the N−H bond of an amine has been elucidated in detail. Calculations show that the incoming amine prefers to coordinate to the ligated oxygen atom and the carbonyl oxygen atom of the ester group of the dirhodium carbenoid simultaneously. In this case, the subsequent nucleophilic attack 4049

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of the nitrogen atom of amine at the carbene carbon affords a C-bound carbenoid followed by a rapid proton transfer to give a metal-associated enamine. Then, the enamine dissociates from the metal and undergoes an intramolecular hydrogenbond exchange to give a seven-membered-ring conformation. This process has an overall barrier of 5.7 kcal/mol and is exergonic by 5.1 kcal/mol. Calculations show that Rh2L4 can facilitate efficiently the [1,3]-proton shift of enamine to give the final N−H insertion product. However, the possibility of this process can be decreased significantly owing to the dirhodium catalyst preferring to be involved in the decomposition of diazoacetate to give the carbenoid for the next catalytic cycle. In addition, formation of the carbenoid is considerably exergonic and the amount of exerted heat is adequate to conquer the energy barrier corresponding to the direct [1,3]-proton shift of enmaine. Finally, calculations indicate that the spiro chiral phosphoric acid can promote the [1,3]-proton shift of enamine remarkably. In addition, this will lead to the major R product, owing to the smaller steric hindrance in the transition structure in comparison to that to give the S product.

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ASSOCIATED CONTENT

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AUTHOR INFORMATION

S Supporting Information *

An xyz file giving computed molecule Cartesian coordinates for convenient visualization of Cartesian coordinates and a table giving energies for all optimized stationary points. This material is available free of charge via the Internet at http://pubs.acs.org. Corresponding Authors

*E-mail for Z.-Z.X.: [email protected]. *E-mail for F.V.: [email protected]. *E-mail for J.C.: [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS The authors acknowledge generous financial support by the Natural Science Foundation of China (21103130 and 61274135) and the Fundamental Research Funds for the Central Universities (WUT:2014-Ia-016, 2012-Ia-019 and 2010-IV-004). F.V. acknowledges the Chinese Central Government for an “Expert of the State” position in the program of “Thousand Talents” and the Wuhan University of Technology for financial support. All calculations were performed at the the cluster of the Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education.

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REFERENCES

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dx.doi.org/10.1021/om5005572 | Organometallics 2014, 33, 4042−4050