Mechanistic Insights into Asymmetric C–H Insertion Cooperatively

May 19, 2016 - Mechanistic Insights into Asymmetric C–H Insertion Cooperatively Catalyzed by a Dirhodium(II) Complex and Chiral Phosphoric Acid. Hui...
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Mechanistic Insights into Asymmetric C−H Insertion Cooperatively Catalyzed by a Dirhodium(II) Complex and Chiral Phosphoric Acid Hui Liu, Jun-Xin Duan, Deyu Qu, Li-Ping Guo, and Zhi-Zhong Xie* Department of Chemistry, School of Chemistry, Chemical Engineering and Life Sciences, Wuhan University of Technology, Wuhan 430070, People’s Republic of China S Supporting Information *

ABSTRACT: The reaction mechanism of methyl 2-diazo-2-phenylacetate (DIA) with 1phenylpyrrolidine (An) cooperatively catalyzed by a dirhodium(II) complex and chiral spirophosphoric acid (SPA) has been studied with the aid of DFT methods. The results show that the nucleophilic attack of An at the in situ generated carbenoid is preferred to give a metal-associated enol intermediate. In addition, a stronger interaction between the dirhodium(II) complex and molecules such as the enol and chiral spirophosphoric acid is better for the asymmetric C−H insertion. This is due to the fact that a stronger interaction can restrict the chiral spirophosphoric acid around the dirhodium(II) complex. In this case, the dissociated enol can be captured by chiral spirophosphoric acid immediately to give the major S-enantiomer product. Otherwise, the dimerization of enol molecules will occur followed by a self-catalyzed process to give the racemic C−H insertion product. This finding should be important for the design of other related dual catalyses in the future.

1. INTRODUCTION Carbene-induced C−H functionalization by transition-metalcatalyzed decomposition of diazo compounds is among the most efficient and reliable synthetic tools for the construction of C−C bonds. 1−11 Among them, the dirhodium(II) carboxylates has garnered considerable interest in both nonasymmetric and asymmetric functionalizations of C(sp3)− H bonds over the past few decades.1,2,6,7 On the basis of the well-documented concerted mechanism,12 asymmetric C−H insertion can be controlled efficiently by chiral dirhodium(II) catalysts.13 However, for the C(sp2)−H counterpart, few asymmetric insertions catalyzed by dirhodium(II) complex have been reported.14,15 The reason may be attributed to their stepwise mechanisms, in which an aromatic ring such as that in indole and aniline derivatives plays an important role in stabilizing the metal-associated and/or metal-free zwitterionic intermediate(s). In addition, this point can be strongly supported by Hu’s three-component reactions.16,17 However, this scenario could be altered by a cooperative dual catalysis. Recently, Zhou’s group18 discovered that the arylation of diazo compounds with aniline derivatives cooperatively catalyzed by the complex dirhodium(II) tetrakis(trifuoroacetate) (Rh2(TFA)4) and chiral spirophosphoric acid (SPA) could give a good yield (up to 95%) with high enantioselectivity (up to 97% ee). This led to a better mechanistic understanding of the dual catalysis becoming desirable for chemists in this field. The mechanism is broadly viewed as consisting of two key events: namely, the generation of the active intermediate(s) by the dirhodium complex and its subsequent enantioselective protonation by chiral phosphoric acid (PPA). Normally, dirhodium(II)-catalyzed X−H (X = C, O, S, N, etc.)19−27 insertions have been widely proposed to begin with the decomposition of the diazo compounds to yield © XXXX American Chemical Society

an active dirhodium(II) carbenoid intermediate (3; Scheme 1). Then, the X−H bond will undertake a nucleophilic attack at the Scheme 1. Previously Proposed Mechanism

carbene carbon to form a metal-associated ylide (5). However, for the subsequent conversion of 5, three possible intermediates such as free ylide (B), O-bound ylide (C), and enol (7) have all been proposed. In addition, the proton shift of these intermediates can be promoted significantly by a proton transfer catalyst. For instance, Yu et al.26 discovered that the [1,2]-H shift of B and C can be facilitated efficiently by a water molecule. We found that the [1,3]-H shift of enol 7 catalyzed by another enol molecule is crucial for the Rh(II)-catalyzed C− H insertion of indole.19 However, in the presence of PPA, the enantioselective protonation of these intermediate(s) (B, C, or 7) will be promoted significantly and lead to a final chiral carbon center.20−22,24,28 Notably, this was supported strongly Received: April 8, 2016

A

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determined at the B3LYP level.45 All 3D renderings of stationary points were generated using CYLview.46

by Zhou’s 31P NMR study, which ruled out the formation of a chiral rhodium(II) phosphate species.29 However, although the established mechanism is crucial for us to understand the dual catalysis, many questions remain unresolved. For instance, the reactions conducted by Zhou18 indicated that the dual catalysis was sensitive to the choice of both the dirhodium(II) complexes and SPAs. According to the mechanism proposed previously, this phenomenon cannot be well explained. However, their elegant work provided an important clue for us to assume that the coordination of the dirhodium(II) complex may play an important role in determining the outcomes. This is due to the fact that only the coordination is associated with the electronic and structural effects of the dirhodium(II) complex and SPA concurrently. Actually, this hypothesis is also consistent with the experimental observations that many molecules in the medium can coordinate to the axial site of the rhodium center.30−36 To confirm this point, two reactions as presented in Scheme 2 were selected as our subject in the present study. According

3. RESULTS AND DISCUSSION 3.1. Computational Model. Our study commenced with the proper choice of the computational model. Figure 1 shows

Scheme 2. Cooperative Catalysis by Rh2L4 and SPA in an Asymmetric C−H Insertion Reaction

to our DFT calculations, we could confirm that the enol molecule should be the only possible intermediate corresponding to the C−H insertion rather than the unstable B- and C-like ylides (Scheme 1). Importantly, the intensity of the coordination can regulate the ratio of the reaction pathways. To the best of our knowledge, this issue has not been addressed previously. Details can be found in the following sections.

Figure 1. Binding modes for SPA and dirhodium(II) complexes Rh2(TFA)4 (top) and Rh2(OAc)4 (bottom) along with the selected bond lengths (in Å). The energies in parentheses are relative to the sum of the energies of separated SPA and dirhodium(II) complex (in kcal/mol). For clarity, some of the hydrogen atoms are omitted.

that SPA can coordinate to Rh2(TFA)4 in three distinct manners: namely, SPAA-1, SPAA-2 and SPAA-3. In SPAA-1, Rh2(TFA)4 is out of the chiral cavity of SPA. In this case, the phenyl group can coordinate to the rhodium center with the shortest Rh···C distance of 2.60 Å. However, for SPAA-2 and SPAA-3, Rh2(TFA)4 is in the box of SPA. In analogy to SPAA1, SPAA-2 also has a π coordination (dRh···C = 2.63 Å). In addition, the PO−H moiety of SPA has a hydrogen-bonding interaction with the F atom of Rh2(TFA)4 (dPO−H···F = 1.96 Å). Notably, the binding mode for SPAA-3 has been reported by Sunoj et al.,20 in which the oxygen atom of the PO moiety of SPA coordinates to Rh2(TFA)4 (dRh···O = 2.24 Å) and the PO− H forms a hydrogen bond with the oxygen atom of the trifuoroacetate ligand. The formations of these adducts are all remarkably exergonic by 5.6, 5.7, and 7.0 kcal/mol for SPAA-1, SPAA-2, and SPAA-3, respectively, indicating that SPA can be confined around Rh2(TFA)4 in solution. However, as one axial site is occupied tightly, the separation between SPA and Rh2(TFA)4 will occur spontaneously. Using the dirhodium carbenoid as an example, Figure 1 shows that formation of the corresponding adducts SPAA3-1, SPAA3-2, and

2. COMPUTATIONAL DETAILS All calculations were performed with the Gaussian 09 program package. 37 The B3LYP method 38,39 was used for geometry optimization calculations. The LanL2DZ40,41 basis set with the associated effective core potential was used for Rh, and the standard Pople all-electron basis set 6-31G(d) was used to describe other atoms. Such a combination has been demonstrated to be reliable with high efficiency in searching for the stationary points. As soon as convergence was reached, frequency analysis was conducted at the same level of theory to verify the stationary points to be real minima (no imaginary frequency) or saddle points (only one imaginary frequency). In addition, an intrinsic reaction coordinate (IRC)42 analysis has been also carried out to confirm the transition state connecting the related reactant and product on the potential energy surface. The single-point energies for the B3LYP-optimized structures in solution have been computed by using the M06 method43 and SMD44 solvation model in chloroform. For these calculations, the LanL2DZ basis set was retained for Rh, while the 6-31G(d) basis set was augmented to 6-311+G(d,p) for other atoms. Reported energies are Gibbs free energies (298 K, 1 atm) determined by the sum of these single-point electronic energies and the thermal corrections B

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Figure 2. Computed free energy surface for the formation of enol catalyzed by Rh2(TFA)4 (in kcal/mol) along with the selected bond parameters (in Å).

will undertake a nucleophilic attack at the carbene carbon of A3, giving the metal-associated ylide A5. This process should be much more rapid, owing to the low energy barrier of only 3.9 kcal/mol (ATS4). Additionally, formation of A5 can drop the relative energy significantly by 10.7 kcal/mol (relative to A3). The reason can be attributed partially to the electrondeficient Rh2(TFA)4 being capable of stabilizing the developed negative charges in A5. On the other hand, the C−N bond length is shortened remarkably from 1.38 Å in free An to 1.33 Å in A5. This means that the N atom can donate its lone pair electrons to the neighboring carbon to form a CN bond. In this case, a large conjugation system is formed to delocalize the developed positive charges to a large extent (Figure 2). Thus, in A5, both the negative and positive charges can be stabilized well. Then, the proton migration from the An moiety to the adjacent ester group affording the enol intermediate was studied first. This pathway proceeding via ATS6 has an activation free energy of only 9.8 kcal/mol. In addition, formation of the metal-associated enol intermediate A7 can reduce the relative energy further by 3.5 kcal/mol. Both findings indicate that the formation of A7 should be facile. In A7, although the coordination of the enol to the rhodium center has a long bond distance (dRh···C(enol) = 2.58 Å), the direct dissociation of enol should be rather difficult owing to a binding energy as high as 11.8 kcal/mol, which is much higher than those for DIA and SPA. Notably, as discussed in the following section, this is very important in achieving a high ee value. Alternatively, as shown in Scheme 1, searches for other possible intermediates have been also conducted. Calculations indicate that both the B- and C-like ylides are unstable (Scheme 1). In contrast, the transition state ATS9, corresponding to the coordinative atom exchange between the carbon atom and the carbonyl oxygen of the ester group, can be located successfully (Figure 2). In ATS9, the Rh−C bond is broken heavily (dRh···C = 3.04 Å; Figure 2) and the original carbene carbon approaching the carbon at the meta position of the An moiety is observed with formation of a C−C bond distance of 1.80 Å (Figure 2). Through ATS9, the forward IRC calculations afford an unexpected O-bound substituted cyclopropane (A10). However, the estimated energy barrier for the formation of

SPAA3-3 is endergonic by 3.3, 1.1, and 6.0 kcal/mol, respectively. Additionally, in comparison to SPAA-1, SPAA-2, and SPAA-3, the labeled bond distances are all elongated significantly in SPAA3-1, SPAA3-2, and SPAA3-3. For instance, the values for dRh···C are elongated from 2.60 and 2.63 Å for SPAA-1 and SPAA-2 to 3.04 and 3.01 Å for SPAA3-1 and SPAA3-2 Å, respectively (Figure 1). The significant separation suggests that, once the reaction occurs, the influence of SPA could be less important. In addition, it could be the reason the relative free energies for the N−H insertion catalyzed by Rh2(TFA)4 have negligible variation in comparison to that by SPAA-3 for the formation of the enol intermediate.20 However, in comparison with the case for Rh2(TFA)4, the coordination of SPA to Rh2(OAc)4 is rather weaker. The relative energies for SPAB-1, SPAB-2, and SPAB-3 are 1.0, −1.0, and −2.4 kcal/mol (relative to the sum of the energies of separated Rh2(OAc)4 and SPA), respectively. This means that the binding energy between Rh2(TFA)4 and SPA is at least 4.6 kcal/mol greater than that between Rh2(OAc)4 and SPA. This can be attributed to the more electron deficient nature of Rh2(TFA)4 in comparison to Rh2(OAc)4. As shown in Figure 1, the separation of SPA from Rh2(OAc)4 is also preferred for the case of a carbenoid (see SPAB3-1, SPAB3-2, and SPAB3-3; Figure 1). This confirms further that only one axial site could be available for the catalysis. 3.2. Arylation of an Aniline Derivative Cooperatively Catalyzed by Rh2(TFA)4 and SPA. Thus, it is reasonable to use the simple Rh2(TFA)4 as the catalyst to determine the intermediate responsible for the subsequent enantioselective protonation. On the other hand, a previous study also showed that no major effect of axial ligation was noticed for the formation of the active intermediate.20 The energy profile for the reaction of diazoacetate (DIA) with 1-phenylpyrrolidine (An) catalyzed by Rh2(TFA)4 along with the important bond lengths are presented in Figure 2. As shown in Figure 2, the coordination of Rh2(TFA)4 to DIA has a binding energy (6.2 kcal/mol) similar to that of SPA. Subsequently, dinitrogen extrusion via the transition state ATS2 occurs to give the carbenoid A3. As expected, this process is facile with a moderate energy barrier of 10.5 kcal/ mol. Next, in line with Zhou’s18 experiment, the substrate An C

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relative energies for the metal-free complexes are obtained with the consideration of the DIA coordination to Rh2(TFA)4 (A1; Figure 2). Figure 3 shows that the S-type hydrogen-bonding complexes are always more stable than their corresponding R-type counterparts. For instance, the relative energies for S-SPAenol, S-SPAenol-1, and S-SPAenol-2 are −34.3, −34.4, and −32.7 kcal/ mol, which are lower than those of R-SPAenol, R-SPAenol-1, and R-SPAenol-2 by 5.1, 5.1, and 3.8 kcal/mol, respectively. In comparison with the relative energy for A7 (−37.5 kcal/mol), this means that formation of these S-type adducts does not greatly increase the relative energies. In addition, a similar trend can be also observed for the transition states. As shown in Figure 3, the transition states of S-SPAenol-TS, S-SPAenol-TS1, and S-SPAenol-TS2 have relative energies of −28.9, −29.4, and −19.7 kcal/mol, respectively, which are remarkably lower than those of R-SPAenol-TS, R-SPAenol-TS1, and R-SPAenol-TS2 by 4.6, 7.2, and 6.0 kcal/mol. These findings could be attributed to the phenyl group of SPA and the An moiety of enol adopting an eclipsed conformation in the R-type precursors, while the conformation is staggered in the S-type precursors. Apparently, the former will lead to greater steric hindrance than the latter (highlighted in red in Figure 3). Additionally, the O−H bond of the enol moiety has a greater deviation from the CC double bond plane to facilitate the hydrogen-bonding interaction of O−H···OP for the former. For instance, the corresponding dihedral angle values (θ) are 124.4 and 31.4° for R-SPAenol and R-SPAenol-TS, whereas S-SPAenol and S-SPAenol-TS have rather smaller values of 27.7 and 19.3°, respectively. Notably, the deviation of the O−H bond from the enol plane will destroy the delocalization of the lone pair electrons of oxygen atom through a three-center−four-electron (3c-4e) bond to some extent (Figure 3). Thus, formation of the S-enantiomer product should dominate, which is consistent with the experimental observations (Scheme 2). However, in view of the reaction proceeding via S-SPAenol-TS2 having the highest energy barrier of 13.0 kcal/mol, this barrier is 7.6 and 8.0 kcal/mol higher than those through S-SPAenol-TS and S-SPAenol-TS1, respectively. Thus, it is reasonable to expect that the reaction proceeding via SSPAenol-TS2 should be minor. From Figure 3, one can find that Rh2(TFA)4 is in the box of either S-SPAenol-2 or S-SPAenolTS2. This is due to the fact that the protonation requires the enol moiety to move more deeply into the box, which will lead to a greater repulsion between SPA and the bulky Rh2(TFA)4 moieties. In this case, a remarkable elongation of the dRh···O value from 2.33 Å in S-SPAenol-2 to 2.43 Å in S-SPAenol-TS2 can be observed (Figure 3), indicating that the stabilization of the enol moiety by Rh2(TFA)4 should be much weaker in SSPAenol-TS2 with respect to S-SPAenol-2. On the other hand, Figure 3 shows that the coordination of Rh2(TFA)4 to SPA has a negligible influence on the formation of the S-enantiomer product because of the similar activation free energies of 5.4 and 5.0 kcal/mol for S-SPAenol-TS and SSPAenol-TS1, respectively. However, coordination can increase the energy barriers for the formation of the R enantiomer from 4.9 kcal/mol (R-SPAenol-TS) to 7.1 kcal/mol (R-SPAenol-TS1). In addition, the energy difference between the R- and S-type transition states increases from 4.6 to 7.2 kcal/mol, respectively. This means that the coordination of Rh2(TFA)4 to SPA is favorable for achieving a high ee value. As shown in Figure 3, the bulky Rh2(TFA)4 locates at the edge of the phenyl group, which can increase the steric hindrance between the An moiety

A10 is 23.7 kcal/mol, which is 13.9 kcal/mol higher than that for A7. Apparently, this pathway cannot compete with the formation of A7. Additionally, using the fragment of A5 by truncating the Rh2(TFA)4 moiety was yielded automatically. In analogy to the formation of A10, the stabilization for the negative charges is poor. This can promote the neutralization of the anion and cation centers to occur spontaneously to afford the substituted cyclopropane. However, it should be noted that the Rh−C bond is partially cleaved in ATS9 (dRh···C = 3.04 Å). Thus, it is reasonable to expect that the complete cleavage of the Rh−C bond of A5 should be more difficult. As discussed above, calculations can determine that the enol molecule should be the intermediate responsible for the subsequent asymmetric protonation by SPA. Notably, although the SPA-induced asymmetric proton transfer for enol has been well documented, the influence of the dirhodium(II) complex on this reaction has not been detailed. For comparison, the other two cases with the coordination of Rh2(TFA)4 to either the phenyl group of SPA or the oxygen atom of the methoxide moiety of the enol have been also considered. Thus, in all, three distinct cases have been studied, which are all displayed in Figure 3. In addition, according to the orientation of the enol approaching SPA, species corresponding to the formation of an R-enantiomer product have a prefix of “R”, whereas for the Senantiomer product, they have a prefix of “S”. Notably, the

Figure 3. Optimized structures for the asymmetric protonation of an enol with the selected bond lengths (in Å), dihedral angles (θ in deg) corresponding to the deviation of the O−H bond from the enol plane, and the relative energies between brackets (in kcal/mol). For clarity, some of the hydrogen atoms are omitted. D

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Figure 4. Computed free energy surface for the formation of enol catalyzed by Rh2(OAc)4 (in kcal/mol) along with the selected bond parameters (in Å).

Figure 5. Computed free energy surface for the self-catalyzed protonation of enol (in kcal/mol) along with selected bond parameters (in Å).

and the bulky Rh2(TFA)4 in R-SPAenol-TS1 (highlighted in red in Figure 3). However, the bonded enol is far away from Rh2(TFA)4 in S-SPAenol-TS1. This can also well explain why SSPAenol-TS and S-SPAenol-TS1 have similar activation energies. 3.3. Arylation Catalyzed by SPA and Rh2(OAc)4. The energy profile for the formation of enol catalyzed by Rh2(OAc)4 is presented in Figure 4. Owing to the less electron deficient nature, the coordination of DIA to Rh2(OAc)4 is endergonic by 3.9 kcal/mol, and formation of the dirhodium(II) carbenoid intermediate B3 becomes more difficult with an overall barrier of 13.4 kcal/mol (BTS2) with respect to the case of Rh2(TFA)4 (Figure 2). In addition, the carbene carbon of B3 is also less electron deficient than that of A3. This leads to the activation free energy for the nucleophilic attack of An increasing remarkably from 3.9 kcal/mol (ATS4; Figure 2) to 10.5 kcal/mol (BTS4). However, because Rh2(OAc)4 is worse than the electron-deficient Rh2(TFA)4 in stabilizing the developed negative charges, the neutralization of the cation and anion centers of the ammonium ylide fragment in B5 will be enhanced through either proton transfer or cyclopropana-

tion. The computational energy barriers decease from 9.8 and 23.7 kcal/mol for ATS6 and ATS9 to 7.5 and 12.6 kcal/mol for BTS6 and BTS9, respectively. Importantly, the energy difference between the transition states for the proton transfer and the cyclopropanation is reduced from 13.9 kcal/mol for A5 to 5.1 kcal/mol for B5. This makes formation of the cyclopropane possible. Notably, different from the remarkably endergonic feature for the enol dissociation from Rh2(TFA)4 (11.8 kcal/mol; Figure 2), this process for Rh2(OAc)4 is spontaneous (exergonic by 2.7 kcal/mol; Figure 4). Since the coordination of SPA to Rh2(OAc)4 is rather weak (Figure 1) and formation of the enol is exergonic, it is reasonable to envisage that the dirhodium(II) complex may enter the second catalytic cycle. In addition, in this case, the major S-enantiomer product should be mainly obtained through the metal-free transition state S-SPAenol-TS. Thus, the combination of SPA and Rh2(OAc)4 is worse than that with Rh2(TFA)4 for the asymmetric arylation intrinsically. Since the interaction between SPA and Rh2(OAc)4 is rather weak, it is not certain whether SPA is around Rh2(OAc)4 as E

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been studied with the aid of DFT methods. Calculations confirm that the enol molecule should be the exact intermediate responsible for the subsequent C−H insertion reaction. Importantly, the stronger interaction between the dirhodium(II) complex and molecules such as enol and chiral spirophosphoric acid is better for the asymmetric C−H insertion. This is due to the fact that the stronger interaction can restrict the chiral spirophosphoric acid around the dirhodium(II) complex. As soon as the enol dissociation occurs, the chiral spirophosphoric acid can capture the enol molecule immediately to give the major S-enantiomer product. Otherwise, it is also energetically feasible for the enol dimer to give the racemic product through a self-catalyzed process. Apparently, the latter should be compressed for the asymmetric C−H insertion.

soon as the enol is formed. In this case, the formed enol intermediate may not be captured by SPA immediately. Alternatively, the protonation of enol can also happen in the presence of a proton transfer catalyst, such as alcohol,23 water,26 or the enol itself.19 However, we would assume herein that the proton shift catalyzed by the residual water should be neglected. This is because trace water is also active in reacting with B3 to give the O−H insertion product.26 Thus, we only focus on the self-catalyzed process for the enol in the present work. Figure 5 shows that this process is initiated by the dimerization of enol (11; endergonic by 5.7 kcal/mol). Subsequently, one enol molecule will donate its proton from the hydroxyl group to the carbon atom in the other enol molecule through the transition state TS12. In TS12, the dissociating O−H and the forming C−H bond distances are 1.29 and 1.35 Å, respectively. However, the O−H bond length for the proton acceptor is 0.99 Å. This indicates that the proton exchange is highly asynchronous. The computed activation free energy of 14.2 kcal/mol (relative to 11) indicates that this process is feasible under the experimental conditions. Interestingly, through TS12, IRC calculations confirm that the proton acceptor will transfer its H to the carbon atom at the meta position of the phenyl group of the enolate to form a C− H bond insertion product and an α,β-unsaturated ester pair (13; Figure 5). Notably, other possible proton transfer modes have been also examined, but all failed. The reason could be attributed to the negatively charged aromatic ring of the enolate fragment in TS12, which can have a relatively strong electrostatic interaction with the hydroxyl group (dOH···C = 2.24 Å; Figure 5). The formation of 13 is favored (exergonic by 12.1 kcal/mol relative to 11). In addition, the decomposition of 13 into the C−H insertion product 14 and the α,β-unsaturated ester 15 can drop the relative energy remarkably by 9.8 kcal/ mol. Four possible pathways for the transformation of 15 have been studied. Calculations show that the most favorable pathway is related to the regeneration of the enol (Figure 5), and the energy profile for the other three pathways is presented in the Supporting Information. However, the estimated energy barrier for the regeneration of the enol is as high as 20.9 kcal/ mol, indicating that this process should be rather difficult. In line with our calculations, it is clear that the self-catalyzed protonation of enol dimer can occur readily to give the C−H insertion product. However, it is disfavored significantly over the SPA-induced asymmetric protonation of the enol. This is consistent with the fact that using Rh2(OAc)4 as the catalyst can also achieve an ee value of 51%. However, in comparison with the high ee value of 92% for Rh2(TFA)4, we would hypothesize that the self-catalyzed protonation of the enol dimer should also happen for the case of Rh2(OAc)4. This is because the self-catalyzed protonation of the enol should be racemic, which will decrease the total ee value. In addition, the high energy barrier of 20.9 kcal/mol for the regeneration of the enol indicates that this step should be slow. Thus, under the experimental conditions, we would envisage that the conversion is not complete. This agrees well with a lower yield of 45% for Rh2(OAc)4 (Scheme 2).

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ASSOCIATED CONTENT

* Supporting Information S

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.6b00283. Calculated energies for all optimized stationary points and the complete energy profile for the self-catalyzed protonation of the enol dimer (PDF) All computed molecule Cartesian coordinates (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for Z.-Z.X.: [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 (61274135, 21103130, and 21476177) and the Fundamental Research Funds for the Central Universities (WUT:2016-IB-005 and 2015-IB-001). Parts of the calculations were performed on the cluster of the Key Laboratory of Theoretical and Computational Photochemistry, Ministry of Education.

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REFERENCES

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4. CONCLUSIONS In this paper, the reaction of methyl 2-diazo-2-phenylacetate with 1-phenylpyrrolidine cooperatively catalyzed by a dirhodium(II) complex and chiral spirophosphoric acid has F

DOI: 10.1021/acs.organomet.6b00283 Organometallics XXXX, XXX, XXX−XXX

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DOI: 10.1021/acs.organomet.6b00283 Organometallics XXXX, XXX, XXX−XXX