Stepwise versus Concerted Reductive Elimination Mechanisms in the

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Stepwise versus Concerted Reductive Elimination Mechanisms in the Carbon−Iodide Bond Formation of (DPEphos)RhMeI2 Complex Jing-Lu Yu,† Shuo-Qing Zhang,† and Xin Hong* Department of Chemistry, Zhejiang University, Hangzhou 310027, People’s Republic of China

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S Supporting Information *

ABSTRACT: Reductive elimination is the key bond formation process of organometallic reactions. Goldberg and co-workers recently revealed an unprecedented competition of parallel stepwise reductive elimination pathways for the carbon−iodide bond formation of (DPEphos)RhMeI2 complex. To understand the controlling factors that differentiate the concerted and stepwise pathways, we performed density functional theory (DFT) calculations to elucidate the mechanistic details. The competing stepwise pathways were identified as the anionic and zwitterionic stepwise pathways. The anionic pathway involves the direct SN2 attack of the external iodide anion to the methyl group, leading to the observed carbon−iodide bond formation. Alternatively, heterolytic Rh−I bond cleavage generates the cationic (DPEphos)RhMeI+ intermediate, and the subsequent SN2 attack of the iodide anion to the methyl group occurs via the zwitterionic transition state. In comparison with the stepwise reductive elimination pathways, the classic concerted pathways require significantly higher barriers. This is due to the energy penalty associated with the orientation change of the methyl group during the classic three-centered reductive elimination. The energy required for this orientation change is highly related to the hybrization of carbon; thus, the selectivity for the stepwise reductive elimination pathways can be switched if the C(sp2) or C(sp) group participates in the carbon−iodide bond formation.

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INTRODUCTION

Goldberg et al. recently reported a detailed experimental study on the Me−I reductive elimination of (DPEphos)RhMeI2 complex (Scheme 1a).11 The kinetic analysis revealed an unprecedented competition of two parallel reductive elimination pathways (Scheme 1b). Pathway A, whose rate is independent of iodide concentration, is characterized as a zwitterionic stepwise process (Scheme 1c). This pathway involves a heterolytic cleavage of the Rh−I bond, followed by SN2 attack of the I anion at the methyl group of the cationic Rh(III) complex. Subsequent oxidative addition with iodobenzene generates the observed (DPEphos)RhPhI2 complex. The rate of pathway B has first-order dependence on the iodide concentration (Scheme 1b). This implies an anionic stepwise pathway involving external I anion as the nucleophile. Depending on the presence of the Rh−O bond, Goldberg and co-workers proposed two anionic stepwise pathways (B1 and B2; Scheme 1c). To identify the controlling factors of these competing reductive elimination pathways, here we report a density functional theory (DFT) computational study on the Me−I reductive elimination process of (DPEphos)RhMeI2 complex.

Reductive elimination is one of the most important elementary steps in organometallic chemistry.1 A wide array of transitionmetal-catalyzed transformations involve reductive elimination as the key product-forming step.2 Although the mechanisms of reductive elimination include both the concerted and stepwise pathways, the majority of related mechanistic studies have focused on the concerted reductive eliminations via transitionmetal catalysts, including Pd,3 Pt,4 Rh,5 and Au.6 Only limited examples of high-valent transition metals were identified to undergo the stepwise reductive eliminations. Goldberg et al. showed that alkylplatinum(IV) complexes undergo C−I, C−O, and C−N bond formations via an SN2-type process.7 Muñiz and co-workers found a similar mechanistic pathway in the C− N bond formation of alkylpalladium(IV) species.8 In addition to group 10 metals, Groves et al. reported that the epoxide formation of the alkylrhodium(III) intermediate occurs through the ionic stepwise process.9 We recently discovered that a stepwise C(sp2)−O reductive elimination pathway determines the solvent-dependent chemoselectivity of Ru(II)catalyzed decarboxylative C−H alkenylation of arenecarboxylic acids.10 Understanding the controlling factors that differentiate the concerted and stepwise reductive elimination pathways is critical toward the rational design of transition-metal-mediated C−X bond formations. © XXXX American Chemical Society

Received: October 7, 2018

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DOI: 10.1021/acs.organomet.8b00723 Organometallics XXXX, XXX, XXX−XXX

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Scheme 1. Goldberg’s Experimental Results of the Me−I Reductive Elimination of (DPEphos)RhMeI2 Complex and Proposed Mechanisms11

Article

COMPUTATIONAL METHODS

All density functional theory (DFT) calculations were performed using the Gaussian 09 program.12 The geometry optimizations were carried out with the B3LYP functional,13 with the LANL2DZ basis set14 for rhodium and iodine and the 6-31G(d) basis set for the other elements. The vibrational frequency calculations were conducted at the same level of theory to confirm whether each optimized stationary point is an energy minimum or a transition state, as well as to evaluate the zero-point vibrational energy and thermal corrections at 298 K. On the basis of the gas-phase optimized structures, the single-point energies and solvent effects in iodobenzene were calculated with the M06 functional,15 the SDD basis set16 for rhodium and iodine, and the 6-311+G(d,p) basis set for the other elements. The solvation energies were evaluated by self-consistent reaction field using the SMD implicit solvent model.17 To verify the key free energy profiles, we also performed geometry optimizations and vibrational frequency calculations with Grimme’s dispersion corrections18 in the solution phase. 3D diagrams of computed species were generated by CYLView.19 In order to adjust the Gibbs free energies from 1 atm to 1 mol/L, a correction of RT ln(cs/cg) (1.9 kcal/mol) is added to energies of all species. cs is the standard molar concentration in solution (1 mol/L), cg is the standard molar concentration in the gas phase (0.0446 mol/L), and R is the gas constant.

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RESULTS AND DISCUSSION Anionic Stepwise Pathway. On the basis of our computations, the anionic stepwise pathway is the most favorable pathway leading to the observed (DPEphos)RhPhI2 complex. The detailed process and free energy changes of the anionic stepwise pathway are shown in Figure 1. From the (DPEphos)RhMeI2 complex 1, the nucleophilic attack of the external iodide anion occurs via TS2, leading to iodomethane and the intermediate 3. This SN2-type Me−I bond formation requires a barrier of 22.7 kcal/mol and is endergonic by 17.0 kcal/mol. TS2 has a Rh−O bond of 2.78 Å, which corresponds to the proposed pathway B1 by Goldberg et al. (Scheme 1c).11

Figure 1. DFT-computed Gibbs free changes of the anionic stepwise reductive elimination pathway of (DPEphos)RhMeI2 complex 1 and the optimized structures of transition states. B


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Organometallics

Figure 2. DFT-computed Gibbs free changes of the anionic and zwitterionic stepwise reductive elimination pathways from 1 to 4 and the optimized structures of key transition states.

Figure 3. DFT-computed free energy changes of the anionic stepwise and concerted reductive elimination pathways from 1 to 5 and the optimized structures of the key transition states.

We cannot locate the SN2 attack transition state without the presence of a Rh−O bond. All the attempts for such a transition state led to TS2, which indicates that the hypothesized pathway B2 (Scheme 1c) is not operative in this reaction. These results also highlight the importance of the

Rh−O bond in stabilizing the leaving rhodium complex during the SN2 attack via TS2. From 3, the dissociation of iodide anion is exergonic, generating the (DPEphos)RhI intermediate 4. This tetracoordinated Rh(I) species 4 is unstable and isomerizes to the corresponding tricoordinated species 5. 5 C


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Organometallics

high-energy cationic intermediate 10. Alternatively, the neutral (DPEphos)RhMeI2 complex 1 can directly undergo the classic three-centered reductive elimination, with or without the presence of an Rh−O bond. The neutral concerted pathways are more favorable than the cationic concerted pathway but still require significant barriers. In comparison to the (DPEphos)RhMeI2 complex 1, TS14 is 37.9 kcal/mol higher in free energy and TS16 is 27.5 kcal/mol higher in free energy. On comparison of the concerted and stepwise pathways, the anionic stepwise pathway is about 5 kcal/mol more favorable than the concerted pathways. This is consistent with Goldberg’s experimental results that the concerted pathways are not competitive in the Me−I bond formation of (DPEphos)RhMeI2.11 We also performed the calculations with solution-phase optimized geometries and dispersion corrections, and the same trend was observed (Figures S3− S5). Inspired by previous mechanistic studies on the reductive elimination and oxidative addition of gold, palladium, and platinum complexes by Morokuma and Musaev,21a,b Yates,21c Datta,21d,e and Ananikov,21f we hypothesized that the directionality of the methyl group plays an important role in differentiating the concerted and stepwise reductive elimination pathways. In the ionic stepwise pathway, the orientation of the methyl group barely changes in TS2 (Figure 3), which maintains favorable orbital overlap with both rhodium and iodide. However, in the neutral concerted pathway via TS16, the orientation of the methyl group has to distort significantly for the C−I bond formation (Figure 3). In addition, the directionality of the sp3 methyl group is strong, which makes the neutral concerted process unfavorable due to the energy penalty associated with the orientation change. To explore the directionality of the R group, a series of constrained optimizations were conducted to obtain the energies associated with the orientation change of the R group (Figure 4a). The coordinating iodide anion is first removed from the (DPEphos)RhRI2 complexes to avoid C−I bond formation during the constrained optimizations, leading to the optimized geometry of cationic (DPEphos)RhRI+. Starting from the optimized geometry of (DPEphos)RhRI+, a series of constrained optimizations force the orientation of the R group to distort certain degrees from the original orientation. Thus, the computed energy differences reflect the ease of the orientation change of the R group. We found that C(sp3) groups (Me, Et, and iPr) showed significantly stronger directionality in comparison with C(sp2) and C(sp) groups (vinyl, phenyl, and alkynyl). The low directionality of the C(sp2) and C(sp) groups is due to the compensating π−metal interaction during the orientation change. The involvement of π orbital interaction is supported by NBO analysis of the distorted complexes (Figure S6). For the C(sp3) groups, the order of the directionality is related to the steric effects of the substituent. The iPr group is bulkier than the methyl group, which requires additional energy for the orientation change due to steric reasons. This concept of directionality is rooted in Goddard’s early studies on H−H, C−H and C−C reductive eliminations.22 We also performed a distortion/interaction analysis23 to further decompose the energy penalty associated with the orientation change (Figure 4b). The (DPEphos)RhRI+ complex is separated into two fragments, (DPEphos)RhI2+ and the corresponding R anion. The distortion energy (ΔEdist) is the energy associated with the geometric changes of these fragments during the orientation change. The interaction

further undergoes the oxidative addition with iodobenzene through TS7, generating the tetracoordinated (DPEphos)RhPhI2 intermediate 8.20 8 then isomerizes to the more stable (DPEphos)RhPhI2 complex 9, which is observed experimentally.11 The oxidative addition with iodobenzene is facile with a barrier of 14.4 kcal/mol (5 to TS7). Therefore, the Me−I bond formation via TS2 is the ratedetermining step of the anionic stepwise reductive elimination pathway. Our computational results corroborate the experimental kinetic studies that the rate of pathway B1 has firstorder dependence on the iodide concentration.11 In addition, the optimized structure of the (DPEphos)RhMeI2 complex 1 is quite consistent with the experimental X-ray crystal structure11 with a RMSD of 0.199 Å (Figure S2). Zwitterionic Stepwise Pathway. In addition to the anionic stepwise pathway, the zwitterionic stepwise pathway for Me−I bond formation was found to be competitive by Goldberg et al.11 The computed free energy changes of the proposed zwitterionic stepwise pathway are shown in Figure 2 (red pathway). From the (DPEphos)RhMeI2 complex 1, the heterolytic cleavage of the Rh−I bond is endergonic by 25.2 kcal/mol, leading to the high-energy cationic (DPEphos)RhMeI species 10. The SN2 attack of the iodide anion to 10 is quite facile via the zwitterionic transition state TS11, generating the tetracoordinated (DPEphos)RhI intermediate 4. 4 undergoes subsequent oxidative addition with iodobenzene to produce the observed (DPEphos)RhPhI2 (9), as in the anionic stepwise pathway (4 to 9, Figure 1). On comparison to the two competing reductive elimination pathways, the computations showed that the zwitterionic stepwise pathway is 3.9 kcal/mol less favorable than the anionic stepwise pathway (TS2 vs TS11, Figure 2). This 3.9 kcal/mol barrier difference corresponds to a 260-fold difference of rate constant at 80 °C, which is in reasonable alignment with Goldberg’s kinetic analysis. The anionic stepwise pathway has an experimental rate constant of (1.8 ± 0.2) × 10−2 M−1 s−1, and the rate constant of the zwitterionic stepwise pathway is (5 ± 2) × 10−4 s−1.11 The major contribution to the 26.3 kcal/ mol barrier of the zwitterionic stepwise pathway is the unfavorable heterolytic cleavage of the Rh−I bond. This Rh−I bond cleavage requires a free energy of 25.2 kcal/mol (1 to 10, Figure 2), making the zwitterionic stepwise pathway less competitive in comparison with the anionic stepwise pathway. The significant endergonicity of the heterolytic Rh−I bond cleavage suggests that the competition between the two stepwise pathways should be very sensitive to the polarity of the solvent. Concerted Pathway. We next investigated the possible concerted reductive elimination pathways, and the corresponding free energy changes are shown in Figure 3. There are three possible concerted reductive elimination pathways from the (DPEphos)RhMeI2 complex 1: the cationic pathway (labeled in red; Figure 3) and two neutral pathways (with a Rh−O bond labeled in green and without an Rh−O bond labeled in blue; Figure 3). The cationic concerted pathway proceeds via the heterolytic cleavage of the Rh−I bond, generating the cationic Rh(III) species 10 and the iodide anion. From 10, instead of the SN2 attack of the iodide anion (via TS11, Figure 2), the concerted reductive elimination occurs through TS12. This leads to the cationic Rh(I) intermediate 13, and the recoordination of an iodide anion produces the neutral Rh(I) intermediate 4. This cationic concerted pathway requires a significant barrier of 51.4 kcal/mol (1 to TS12), due to the D


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Organometallics

Substituent Effects on the Competition between the Reductive Elimination Pathways. Due to the sharp contrast in the directionalities among the C(sp3), C(sp2), and C(sp) substituents (Figure 4), we envisioned that the substituent can potentially switch the selectivity between the concerted and stepwise reductive elimination pathways. Table 1 includes the free energy barriers of the concerted and Table 1. Free Energy Barriers of the Stepwise and Concerted Reductive Elimination Pathways for Various (DPEphos)RhRI2 Complexes

a

Gibbs free energies in kcal/mol.

stepwise reductive elimination pathways, for a series of (DPEphos)RhRI2 complexes. The computed results indeed confirmed our hypothesis. For the reductive elimination involving C(sp3) substituents, the trend is similar, and the anionic stepwise pathway is the most favorable (entries 1−3, Table 1). It is noteworthy that the barrier of the anionic stepwise pathway is not sensitive to the steric repulsions of the alkyl substituent; the SN2 attack of the methyl substituent has a barrier of 22.7 kcal/mol (entry 1), and the corresponding barriers for ethyl and iPr groups are 20.8 and 21.6 kcal/mol, respectively. This low sensitivity of steric effects is in contrast to the SN2 attack of alkyl halides (Table S1), and the origin of this difference is currently under investigation in our laboratory. These calculations suggest that the anionic stepwise pathway is also operative in the corresponding Et−I and iPr−I bond formations, which can potentially be verified by future experiments. Importantly, the trend is reversed if the π orbital is present in the bond-forming carbon. All of the C(sp2) and C(sp)

Figure 4. (a) Energy penalty for the orientation change of the R group in the (DPEphos)RhMeI+ complex. (b) Distortion/interaction analysis of the energy penalty.

energy (ΔEint) reflects the change in rhodium−carbon interaction during the orientation change. The sum of distortion and interaction energy is the total electronic energy change (ΔE = ΔEdist + ΔEint). On the basis of the distortion/ interaction analysis, the major contribution to the total energy penalty of orientation change is the interaction energy, which is caused by the change in rhodium−carbon interaction. This analysis further supports the rationale of directionality. E


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substituents showed a strong preference for the neutral concerted reductive elimination pathway (entries 4−6, Table 1). These results support the above directionality analysis. The π orbitals participate in the Rh−R interaction during the concerted reductive elimination, and this favorable interaction does not exist in the C(sp3)−I reductive eliminations. In addition, the C(sp2) and C(sp) substituents have higher barriers for the anionic stepwise reductive eliminations in comparison to the C(sp3) substituents. This is understandable, since nucleophilic substitution generally occurs on saturated carbon.24 Therefore, the kinetic behavior of the C−I bond formation of (DPEphos)RhRI2 complex should be dramatically different if the alkyl groups are changed to the C(sp2) and C(sp) groups. Only the concerted reductive elimination pathway should be identified for these substituents. We also calculated the free energy changes of the entire pathway with the five substituents (entries 2−6, Table 1). With the alkyl and phenyl substituents, the rate-determining step of product formation is the C−I reductive elimination (Figure 1 and Figures S7−S9). The neutral concerted C(alkenyl)−I reductive elimination is very facile with a barrier of 17.4 kcal/mol, and the subsequent oxidative addition of LRhI species determines the overall rate (Figure S10). For the alkynyl C− I reductive elimination, this hypothetical transformation is not feasible due to the high endergonicity of C−I reductive elimination (Figure S11).

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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.8b00723. Calculations of catalyst initiation, additional conformers of intermediates and transition states, and coordinates and energies of DFT-computed stationary points (PDF) Cartesian coordinates of the calculated structures (XYZ)

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AUTHOR INFORMATION

Corresponding Author

*E-mail for X.H.: [email protected]. ORCID

Shuo-Qing Zhang: 0000-0002-7617-3042 Xin Hong: 0000-0003-4717-2814 Author Contributions †

J.-L.Y. and S.-Q.Z. contributed equally.

Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS Financial support from the National Scientific Foundation of China (21702182), the Chinese “Thousand Youth Talents Plan”, the “Fundamental Research Funds for the Central Universities”, and Zhejiang University is gratefully acknowledged. Calculations were performed on the high-performance computing system at the Department of Chemistry, Zhejiang University.

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CONCLUSIONS The concerted and stepwise reductive elimination mechanisms in the C−I bond formation of (DPEphos)RhMeI2 complex have been elucidated with DFT calculations. The C−I bond formation proceeds via two competing pathways, anionic and zwitterionic stepwise pathways. In the anionic pathway, the (DPEphos)RhMeI2 complex is directly attacked by the external iodide anion, leading to the C−I bond formation. Alternatively, the (DPEphos)RhMeI2 complex undergoes a heterolytic Rh−I cleavage, and the iodide anion attacks the cationic (DPEphos)RhMeI+ species through the zwitterionic transition state. From the (DPEphos)RhMeI2 complex, the anionic stepwise reductive elimination pathway has a barrier of 22.7 kcal/mol, while the barrier of the zwitterionic stepwise pathway is 26.6 kcal/mol. These computational results agree well with the experimental observations that the rate constant of the anionic stepwise pathway is about 2 orders of magnitude higher than that of the zwitterionic stepwise pathway. In addition, the possible concerted reductive elimination pathways require significantly higher barriers and are not operative for the C−I bond formation of (DPEphos)RhMeI2 complex. The strong directionality of the methyl group in the reductive formation step controls the preference for the stepwise pathway. For the concerted reductive elimination process, the orientation of the carbon substituent has to distort significantly to achieve orbital overlap with both rhodium and iodide. In contrast, the orientation of the carbon substituent changes to a limited extent during the stepwise reductive elimination. Therefore, the C(sp3) substituents, which have strong directionality during the C−I bond formation, favor the stepwise pathway. The C(sp2) and C(sp) substituents do not have this strong directionality and prefer the classic concerted reductive elimination pathways. The mechanistic dependence of the carbon substituent provides useful information for the rational design of transition-metal-mediated bond formations.

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REFERENCES

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