Mechanism and Origins of Chemo-and Regioselectivities of Pd

Article Cite This: Organometallics XXXX, XXX, XXX−XXX

Mechanism and Origins of Chemo- and Regioselectivities of Pd-Catalyzed Intermolecular σ‑Bond Exchange between Benzocyclobutenones and Silacyclobutanes: A Computational Study Zheng-Yang Xu,†,‡ Shuo-Qing Zhang,† Ji-Ren Liu,† Pan-Pan Chen,† Xin Li,† Hai-Zhu Yu,§ Xin Hong,*,† and Yao Fu*,‡ †

Department of Chemistry, Zhejiang University, Hangzhou 310027, China Hefei National Laboratory for Physical Sciences at the Microscale, CAS Key Laboratory of Urban Pollutant Conversion, Anhui Province Key Laboratory of Biomass Clean Energy, iChEM, Department of Chemistry, University of Science and Technology of China, Hefei 230026, China § Department of Chemistry and Centre for Atomic Engineering of Advanced Materials, Anhui University, Hefei 230601, China ‡

S Supporting Information *

ABSTRACT: The palladium/isocyanide catalyst is able to facilitate the intermolecular σ-bond exchange between benzocyclobutenones and silacyclobutanes. This reaction cleaves the C−C bond of the benzocyclobutenones and the C−Si bond of the silacyclobutanes, providing a unprecedented access to the eight-membered silacycles with remarkable chemo- and regioselectivities. We studied the mechanism and origins of the chemo- and regioselectivities with density functional theory (DFT) calculations. The reaction proceeds via two sequential oxidative additions (first with benzocyclobutenone and second with silacyclobutane) and two subsequent reductive eliminations (first, C−Si bond formation; second, C−C bond formation). The oxidative addition abilities of the substrates and the trans effect of silyl group synergistically control the chemoselectivity toward the heteroexchange. The homoexchange of benzocyclobutenones is unfavorable because the oxidative addition ability of benzocyclobutenone is not strong enough to facilely generate the Pd(IV) intermediate. For the homoexchange of silacyclobutanes, the strong trans effect of the silyl group increases the energy of the Pd(IV) intermediate, leading to the high overall barrier of the subsequent reductive elimination step. The regioselectivity of the C−C bond activation of benzocyclobutenone is controlled by the interaction between substrate and palladium, the favorable aryl−palladium interaction directs the catalyst to selectively cleave the C(aryl)−C(carbonyl) bond of benzocyclobutenone.

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Huang,13 and Kantchev,14 are mainly focused on Rh- and Nicatalyzed C−C activation. Fruitful advances have also been achieved in the mechanistic understandings of C−C bond activation of cyclopropanes, by the works from Houk and Wender,15 Yu,16 Cheong,17 Wang,18 and others.19 For palladium catalysis, the activation mode of C−C and C−Si bonds, and especially the controlling factors of the distinctive chemo- and regioselectivities, still remain elusive. Understanding these fundamental mechanistic questions is critical toward future design of synthetic transformations based on C−C and C−Si bond activations, and the general mechanistic picture of transition metal catalyzed cleavage of low-polar σ-bonds. To elucidate the mechanism and origins of chemo- and regioselectivities of Pd-catalyzed C−C and C−Si bond activations, here we report the computational study on the Pd/isocyanide-catalyzed intermolecular σ-bond exchange

INTRODUCTION C−C and C−Si single bonds ubiquitously exist in organic compounds, and selective activation and functionalization of these low-polar σ-bonds can revolutionize the synthetic strategy of constructing small molecules.1,2 Through the judicious catalyst design, remarkable success has been achieved in the transition metal catalyzed cleavage of strained σ-bonds,3 especially for the cyclobutanone derivatives.3−7 Recently, Murakami and co-workers reported an intriguing Pd-catalyzed intermolecular σ-bond exchange between benzocyclobutenones and silacyclobutanes (Scheme 1).8,9 This reaction elegantly exchanges the C−C and C−Si σ-bonds, providing a unprecedented expediate access to the eight-membered silacycles with exceptional chemo- and regioselectivities (Scheme 1). Despite the noteworthy success of synthetic transformations, the mechanistic understandings on the Pd-catalyzed C−C and C−Si bond activations are quite limited.10 Previous computational studies on the transition metal catalyzed C−C bond activation of benzocyclobutenones, contributed by Liu,11 Li,12 © XXXX American Chemical Society

Received: December 21, 2017

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DOI: 10.1021/acs.organomet.7b00903 Organometallics XXXX, XXX, XXX−XXX

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Organometallics

(n − m) × 4.3 kcal/mol). This approach has been validated by a number of computational and experimental studies. Wang and co-workers also found improved descriptions of free energy changes in a number of metal-catalyzed reactions using this empirical approach.29 Yu and co-workers discovered that the entropy corrections are overestimated by about half in several cycloaddition reactions.30 In addition, to correct the Gibbs free energies from 1 atm to 1 mol/L, a correction of RT ln(cs/cg) (about 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 gas phase (0.0446 mol/L), and R is the gas constant.

Scheme 1. Chemo- and Regioselectivities of Pd-Catalyzed Intermolecular Σ-Bond Exchange between Benzocyclobutenones and Silacyclobutanes8

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RESULTS AND DISCUSSION Proposed Catalytic Cycle. On the basis of Murakami’s stoichiometric experiments with Pd(t-octyl isocyanide)28 and previous mechanistic studies of catalytic transformations involving transition metal catalyzed C−C and C−Si bonds,11−19 the proposed catalytic cycles of Pd-catalyzed intermolecular σ-bond exchange between benzocyclobutenone and silacyclobutane are shown in Scheme 2. Palladium catalyst A undergoes the first between benzocyclobutenones and silacyclobutanes. The mechanism involves two oxidative additions and two subsequent reductive eliminations. The sequential oxidative additions occur first with benzocyclobutenone and then silacyclobutane, cleaving the C−C and C−Si bonds and generating the Pd(IV) intermediate. This Pd(IV) intermediate undergoes subsequent reductive eliminations to form the C−Si and C−C bonds, allowing the intermolecular σ-bond exchange. The intrinsic oxidative addition abilities of the substrates and the trans effect of silyl group synergistically control the chemoselectivity, leading to the preference of the heteroexchange. We also discovered that the favorable aryl-palladium interaction directs the catalyst to selectively cleave the C(aryl)−C(carbonyl) bond of benzocyclobutenone. These mechanistic insights will facilitate the development of new synthetic transformations involving transition metal catalyzed C−C and C−Si bond cleavages.

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Scheme 2. Proposed Catalytic Cycles of Pd-Catalyzed Intermolecular Σ-Bond Exchange between Benzocyclobutenone and Silacyclobutane Involving Intermediates A−F

COMPUTATIONAL METHODS

All density functional theory (DFT) calculations were performed by Gaussian 09 program.20 The geometry optimizations were conducted using the B3LYP functional,21 with LANL2DZ basis set22 for palladium and 6-31G(d) basis set for the other atoms (including the keyword 5D). To confirm whether each optimized stationary point is an energy minimum or a transition state, as well as evaluate the zeropoint vibrational energy and thermal corrections at 298 K, the vibrational frequencies were computed at the same level of theory as for the geometry optimizations. The single-point energies and solvent effects were computed with the M06 functional23 using the SDD basis set24 for palladium and the 6-311+G(d,p) basis set for the other atoms, based on the gas-phase optimized structures. The solvation energies were evaluated by a self-consistent reaction field (SCRF) using the SMD implicit solvent model.25 We have thoroughly examined the conformational space of each intermediate and transition state, and only the most stable conformers are included in the discussions. Fragment distortion and interaction energies26 were computed at the M06/6-311+G(d,p)-SDD level without the inclusion of solvation energy corrections. The 3D diagrams of molecules were generated using CYLView.27 To correct the overestimation of entropy contributions to the free energy changes in solution, an empirical approach is applied following the procedure proposed by Martin and co-workers.28 For each component change in a reaction at 298 K and 1 atm, a correction of 4.3 kcal/mol is applied to the reaction free energy (i.e., a reaction from m- to n-components has an additional free energy correction for

oxidative addition to cleave the C(aryl)−C(carbonyl) bond of benzocyclobutenone, generating five-membered palladacycle intermediate B. Subsequent oxidative addition with silacyclobutane breaks the C(methylene)−Si bond and leads to Pd(IV) intermediate C. From C, two sequential reductive eliminations allow the formations of the new C−Si and C−C bonds, producing the observed eight-membered silacycle product through ninemembered ring Pd(II) intermediate D. Alternatively, the sequence of the oxidative additions may reverse, and the formation of intermediate C can involve five-membered ring intermediate E. The sequence of the reductive eliminations from C can also reverse, leading to the product formation pathway via the intermediate F. In addition, the sequential oxidative addition and reductive elimination (B/E to D/F) can occur in a concerted σ-bond metathesis fashion, which circumvents Pd(IV) intermediate C. B


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generate eight-membered silacycle product 5 (Scheme 3b). These experiments suggest the existence of the intermediate B in the catalytic cycle (Scheme 2). In addition, no reaction was observed when mixing Pd(t-octyl isocyanide)2 complex 1 and silacyclobutane 4 (Scheme 3c). To explore the reaction mechanism, we used the experimental catalyst and substrates for the model reaction in computations (Scheme 3d). Reaction Mechanism. The Gibbs free energy changes of the most favorable pathway for the intermolecular σ-bond exchange are shown in Figure 1, and the optimized structures of selected intermediates and transition states are included in Figure 2. From Pd(t-octyl isocyanide)2 1,31 the oxidative addition of benzocyclobutenone 2 occurs via bis-ligated threemembered ring transition state TS6, leading to five-membered palladacycle intermediate 3. This initial oxidative addition from 1 to 3 is exergonic by 1.8 kcal/mol, requiring a barrier of 21.5 kcal/mol. From 3, the dissociation of one t-octyl isocyanide ligand allows the subsequent oxidative addition of silacyclobutane to occur via monoligated transition state TS8, generating pentacoordinated Pd(IV) intermediate 9. 9 then coordinates with an additional ligand to 10, and the reductive elimination of 10 via TS11 generates nine-membered ring intermediate 12 with the C−Si bond formation. The final reductive elimination through TS13 produces observed silacycle product 5 and regenerates active catalyst 1. The catalytic cycle has the five-membered palladacycle intermediate 3 as the resting state. The rate-determining step is the C−Si reductive elimination step via TS11, with a 26.7 kcal/mol overall barrier as compared to the resting state 3. We have carefully examined the conformers and number of ligand for all the intermediates and transition states, and the unfavorable ones are included in Figures S2 and S3. Our computational results agree well with Murakami’s stoichiometric experiments. The initial oxidative addition of benzocyclobutenone 2 has a low kinetic barrier of 21.5 kcal/mol (via TS6) and is exergonic by 1.8 kcal/mol to generate 3.

Murakami’s stoichiometric experiments with Pd(t-octyl isocyanide)2 provided valuable mechanistic information (Scheme 3).8 Pd(t-octyl isocyanide)2 complex 1 undergoes Scheme 3. Murakami’s Stoichiometric Experiments with Pd (a−c) and Model Reactions Used for Computation (d)8a

a

L = t-octyl isocyanide.

facile oxidative addition with benzocyclobutenone 2 under room temperate, quantitatively producing five-membered palladacycle species 3 (Scheme 3a). 3 reacts with the silacyclobutane 4 to

Figure 1. DFT-computed Gibbs free changes of the favorable pathway of Pd-catalyzed intermolecular σ-bond exchange between benzocyclobutenone 2 and silacyclobutane 4. C


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Figure 2. DFT-optimized structures of selected intermediates and transition states for Pd-catalyzed intermolecular σ-bond exchange between benzocyclobutenone 2 and silacyclobutane 4. All the C−H bonds and the ligand structure are hidden for simplicity. L = t-octyl isocyanide.

five-membered Pd(II) intermediate 15 is 1.4 kcal/mol higher in free energy as compared to 1. The low oxidative addition barrier with silacyclobutane is consistent with the general understandings that the silacyclobutanes can undergo facile oxidative addition.2,32 However, the oxidative addition of silacyclobutane is endergonic by 1.4 kcal/mol, suggesting that 1 and 15 are in a fast equilibrium favoring 1. Therefore, the predominant form of the reaction mixture between 1 and 4 is still the separated reactants, not Pd(II) intermediate 15. This explains the experimental observations that no reaction was detected when mixing Pd(t-octyl isocyanide)2 complex 1 and silacyclobutane 4. While the generation of 15 is facile, the subsequent oxidative addition with benzocyclobutenone is very difficult. TS17 is 38.2 kcal/mol higher in free energy than Pd(t-octyl isocyanide)2 complex 1 (Figure 3a),33 making this pathway unlikely. Comparing the secondary oxidative addition transition states (TS8 vs TS17, Figure 4), we found that two effects contribute

This parallels with the experimental results that Pd(t-octyl isocyanide)2 complex 1 reacts with benzocyclobutenone 2 to produce intermediate 3 quantitatively under room temperate (Scheme 3a). In addition, the reaction between 3 and silacyclobutane 4 requires a 26.7 kcal/mol barrier (3 to TS11) to produce eight-membered silacycle product 5. This barrier is 5.2 kcal/mol higher as compared to the initial oxidative addition of benzocyclobutenone via TS6 (26.7 kcal/mol vs 21.5 kcal/mol), which is consistent with the experimental observations that the reaction between complex 3 and silacyclobutane 4 required heating (Scheme 3b). In addition to the favorable reaction pathway, the intermolecular σ-bond exchange can occur in a number of other fashions. Switching the sequence of oxidative additions, the reaction can proceed via the initial oxidative addition with the silacyclobutane 4 (Figure 3a). This Pd(0)−Pd(II) process is very facile with a kinetic barrier of 18.2 kcal/mol and generated D


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Figure 3. DFT-computed Gibbs free changes of the alternative pathways (a and b) of Pd-catalyzed intermolecular σ-bond exchange between benzocyclobutenone 2 and silacyclobutane 4. Gibbs free energies in kcal/mol are given in parentheses. L = t-octyl isocyanide.

phenyl π-orbital proximately to the silyl group. Thus, the C−Si reductive elimination via TS11 does not require significant geometric distortion of the benzocyclobutenone moiety, and the corresponding transition state is more stable (Figure 5). In addition, we have also explored the possible metathesis pathways, which are both unfeasible due to the high overall barriers (Figure S6). Origins of Chemoselectivities. The Pd/t-octyl isocyanide-catalyzed intermolecular σ-bond exchange between benzocyclobutenone and silacyclobutane showed exceptional chemoselectivity toward the C−C and C−Si heteroexchange silacycle product (Scheme 4). In the heteroexchange pathway, the reaction may produce the aldehyde product through β-hydride elimination (Scheme 4a). Also, the possible homoexchanges of the C−C or C−Si bonds can lead to a number of additional eight-membered ring products (Scheme 4b,c). Considering the intrinsic oxidative addition abilities of benzocyclobutenone and silacyclobutane, perhaps the most important question is why the C−Si homoexchange does not occur. On the basis of the above mechanistic understandings, we next explored the origins of chemoselectivities. Figure 6 showed the free energy changes of the C−Si homoexchange pathway with two silacyclobutanes. Indeed, both oxidative additions with silacyclobutanes (TS14 and TS23) are facile, leading to Pd(IV) intermediate 24.36 However, the subsequent C−Si reductive elimination via TS26 requires an overall barrier of 29.4 kcal/mol as compared to Pd(t-octyl isocyanide)2 complex 1. This reductive elimination makes the C−Si homoexchange pathway 4.5 kcal/mol less favorable as compared to the heteroexchange pathway (TS26, Figure 6, vs TS11, Figure 1). These computational results are consistent with the experimental observations that the homoexchange

to the preference of the specific sequence of oxidative additions (Pd(0) to Pd(II) with benzocyclobutenone and Pd(II) to Pd(IV) with silacyclobutane). TS8 has the t-octyl isocyanide ligand trans to the acyl group, while TS17 has the same ligand trans to the silyl group.34 The silyl group is significantly stronger than the acyl group in terms of the trans effect,35 making the coordination of isocyanide much weaker in TS17. This weakening of coordination is supported by the elongation of Pd−isocyanide distance in TS17 (highlighted in red, Figure 4a), as well as the calculated energy for ligand dissociation of the two transition states (Figure S5). In addition to the trans effect, the oxidative addition ability of silacyclobutane is intrinsically stronger than that of benzocyclobutenone. Even without the presence of additional ligands, silacyclobutane oxidative addition transition state TS21 is 8.8 kcal/mol more stable than benzocyclobutenone oxidative addition transition state TS22 (Figure 4b). Therefore, these two effects together make the secondary oxidative addition significantly unfavorable for the benzocyclobutenone, and the two sequential oxidative additions occur initially with benzocyclobutenone and subsequently with silacyclobutane. Alternatively, Pd(IV) intermediate 10 can undergo a C−C reductive elimination via TS18, which leads to the reversal of the sequence of reductive eliminations (Figure 3b). This pathway requires a dramatically high barrier, TS18 is 44.8 kcal/mol higher in free energy than resting state 3. TS18 requires the participation of the acyl π-orbital in the reductive elimination, and the acyl moiety has to rotate dramatically to force the perpendicular π-orbital to participate in C−C bond formation (Figure 5). This geometric distortion of the benzocyclobutenone moiety, especially the highlighted dihedral angle (Figure 5), increases the barrier for the C−C reductive elimination via TS18. In contrast, the structure of Pd(IV) intermediate 10 aligns the E


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TS26, which is consistent with the change of highlighted Pd−isocyanide distance in the two transition states (highlighted in red, Figure 7), as well as the energy required to dissociate the corresponding ligand (Figure S8). The same trans effects also exist in the corresponding Pd(IV) intermediates. Therefore, Pd(IV) intermediate 25 in the C−Si homoexchange pathway is 20.4 kcal/mol higher in free energy as compared to Pd(t-octyl isocyanide)2 1 (Figure 6), while the free energy of Pd(IV) intermediate 10 in the heteroexchange pathway is only 12.9 kcal/mol higher than that of 1 (Figure 1). We also explored the possibility of C−C homoexchange between two benzocyclobutenones. Figure 8 showed the free energy changes of the competing C−C homoexchange pathway and the heteroexchange pathway. From five-membered ring Pd(II) intermediate 7, the secondary oxidative addition of benzocyclobutenone requires a 20.1 kcal/mol barrier (7 to TS30), making the C−C homoexchange pathway (labeled in red, Figure 8) much less favorable than the heteroexchange pathway (labeled in black, Figure 8). This is understandable considering the oxidative addition abilities of the benzocyclobutenone and silacyclobutane. The benzocyclobutenone requires higher barrier for oxidative addition than silacyclobutane, thus two sequential Pd-mediated oxidative additions of benzocyclobutenone are unlikely. These computational results are consistent with the experimental observations that the reaction selectively produces the heteroexchange product.8 We next explored the chemoselectivity between C−C reductive elimination and β-hydride elimination, and the free energy changes of these competing pathways are shown in Figure 9. From 12, the C−C reductive elimination can occur via the bis-ligated pathway (black, Figure 9) or the monoligated pathway (blue, Figure 9). The additional ligand coordination stabilizes reductive elimination transition state TS13, and this bis-ligated C−C reductive elimination requires a barrier of 9.2 kcal/mol as compared to intermediate 12. Alternatively, the β-hydride elimination via TS34 and subsequent C−H reductive elimination via TS36 delivers aldehyde product 37 (red pathway, Figure 9). Comparing the competing C−C reductive elimination and β-hydride elimination pathways, the reductive elimination pathway is 16.6 kcal/mol more favorable (TS13 vs TS34), which suggests a strong chemoselectivity toward the silacycle product. This is consistent with the experimental selectivity (Scheme 1).8 The chemoselectivity toward C−C reductive elimination results from two major effects, the unfavorable ligand dissociation and the substrate distortion during the β-hydride elimination. In the β-hydride elimination, one isocyanide ligand has to dissociate to accommodate the forming Pd−H bond, which requires significant energy penalty. This energy penalty is also reflected by the 11.5 kcal/mol free energy difference between the Pd(II) intermediates (12 vs 32, Figure 9). Moreover, significant substrate distortion is required in TS34, and this contributes to the higher intrinsic barrier of the β-hydride elimination as compared to the C−C reductive elimination (14.3 kcal/mol from 32 to TS34, and 6.1 kcal/mol from 32 to TS33, Figure 9). Origins of Regioselectivities. In addition to the questions of chemoselectivities, a key issue of the transformations involving benzocyclobutenone is the regioselectivity of the C−C bond activation. Figure 10 showed the free energy changes of the two competing C−C bond activations with benzocyclobutenone 2, as well as the optimized structures of the transition states. The C(sp3)−C(carbonyl) bond activation via TS38

Figure 4. Origins of the energy differences between the second oxidative addition transition states with benzocyclobutenone 2 and silacyclobutane 4. All the C−H bonds and the ligand structure are hidden for simplicity. L = t-octyl isocyanide.

Figure 5. Origins of the energy differences between the first reductive elimination transition states from intermediate 10. All the C−H bonds and the ligand structure are hidden for simplicity. L = t-octyl isocyanide.

between the two silacyclobutanes are not observed.8 The energies and structures of the additional conformers of the intermediates and transition states are included in the Figure S7. Comparing these reductive elimination transition states, we found that the trans effect is the leading factor that differentiates the two pathways. TS26 has both the t-octyl isocyanide ligands trans to silyl group, while one of the t-octyl isocyanide ligands is trans to acyl group in TS11 (Figure 7). The strong trans effect of the silyl group weakens the isocyanide coordinations in F


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Organometallics Scheme 4. Chemoselectivities of Pd-Catalyzed Intermolecular Σ-Bond Exchange between Benzocyclobutenone 2 and Silacyclobutane 4

Figure 6. DFT-computed Gibbs free changes of C−Si homoexchange pathway with two silacyclobutanes. L = t-octyl isocyanide.

TS6. These computational results agree well with the experimental results that the C−C bond activation of benzocyclobutenone only occurs on the C(aryl)−C(carbonyl) bond.8

requires a barrier of 26.4 kcal/mol as compared to Pd(t-octyl isocyanide)2 1, which is 4.9 kcal/mol less favorable compared with the competing C(aryl)-C(carbonyl) bond activation via G


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barriers. In TS6, the palladium catalyst interacts with the π orbitals of the substrate aryl moiety, and this significantly stabilizes the transition state and makes the C(aryl)−C(carbonyl) bond activation favorable. In TS38, the orbital interactions involving the π orbitals do not exist between the palladium and the methylene group, leading to the unfavorable C(sp3)−C(carbonyl) bond activation.

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CONCLUSIONS The mechanism and origins of chemo- and regioselectivities of Pd-catalyzed intermolecular σ-bond exchange between benzocyclobutenones and silacyclobutanes have been elucidated by DFT calculations. The catalytic cycle consists of two sequential oxidative additions and two subsequent reductive eliminations. The initial oxidative addition with benzocyclobutenone occurs to generate the five-membered palladacycle intermediate and cleaves the C(aryl)−C(carbonyl) bond of benzocyclobutenone. The secondary oxidative addition with silacyclobutane leads to the Pd(IV) intermediate, and this intermediate undergoes the sequential C−Si and C−C reductive eliminations to produce the observed eight-membered silacycle product. The postintermediate of the first oxidative addition with benzocyclobutenone is the resting state of the catalytic cycle, and the C−Si reductive elimination of the Pd(IV) intermediate is the ratedetermining step with a 26.7 kcal/mol overall barrier as compared to the resting state. The chemoselectivity toward the heteroexchange between benzocyclobutenones and silacyclobutanes is controlled by the oxidative addition ability of the substrates and the trans effect of silyl group. The silacyclobutane has an intrinsically stronger oxidative addition ability, and thus the oxidative additions involving silacyclobutane are all quite facile. This disfavors the homoexchange of benzocyclobutenones, which requires two consequetive oxidative additions of benzocyclobutenone. Despite the facile oxidative additions with silacyclobutane, the strong trans effect of the silyl group makes Pd(IV) intermediate unstable in the C−Si

Figure 7. DFT-optimized structures of the C−Si reductive elimination transition states TS11 and TS26. Gibbs free energies are compared to those of Pd(t-octyl isocyanide)2 complex 1. L = t-octyl isocyanide.

To understand the controlling factors of the regioselectivity of C−C bond activation, we performed distortion/interaction analysis26 on the two competing transition states (Figure 10). Each transition state structure is separated into two distorted fragments, the Pd(t-octyl isocyanide)2 catalyst moiety and the benzocyclobutenone substrate moiety. The distortion energy is the energy required to distort the ground state structure to the corresponding distorted geometry for these moieties (ΔEdist‑cat and ΔEdist‑sub). The interaction energy represents the strength of the interaction between the catalyst and substrate fragments in the transition states and is calculated by the difference between the activation energy and the total distortion energy (ΔEint = ΔEact − ΔEdist‑cat − ΔEdist‑sub). The distortion/interaction analysis reveals that the interaction between the catalyst and substrate is the leading factor responsible for the regioselectivity. ΔE int (TS38) is −26.3 kcal/mol, while ΔE int (TS6) is −31.6 kcal/mol. This is consistent with the difference between the corresponding activation energies as well as the free energy

Figure 8. DFT-computed Gibbs free changes of C−C homoexchange and the heteroexchange pathways. Gibbs free energies are compared to those of Pd(t-octyl isocyanide)2 complex 1. L = t-octyl isocyanide. H


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Figure 9. DFT-computed Gibbs free changes of C−C reductive elimination and β-hydride elimination pathways from 12. Gibbs free energies are compared to those of Pd(t-octyl isocyanide)2 complex 1. L = t-octyl isocyanide.

Figure 10. DFT-computed competing Pd(t-octyl isocyanide)2-mediated C−C bond activations of benzocyclobutenone 2. Energies are in kcal/mol.

ORCID

homoexchange pathway. Therefore, the reductive elimination of this high-energy Pd(IV) intermediate requires a fairly high overall barrier comparing with the resting state, making the C−Si homoexchange pathway unfeasible. For the C−C bond activation of benzocyclobutenone, the favorable aryl−Pd interaction stabilizes the transition state of the C(aryl)−C(carbonyl) bond activation, leading to the observed regioselectivity.

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Shuo-Qing Zhang: 0000-0002-7617-3042 Hai-Zhu Yu: 0000-0003-3010-1331 Xin Hong: 0000-0003-4717-2814 Yao Fu: 0000-0003-2282-4839 Notes

The authors declare no competing financial interest.

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ASSOCIATED CONTENT

ACKNOWLEDGMENTS Financial support from and the NSFC (X.H., 21702182; Y. F., 21325208, 21572212, and 21672001), MOST (Y.F., 2017YFA0303500), CAS (Y.F., XDB20000000), the Chinese “Thousand Youth Talents Plan” (X. H.), “Fundamental Research Funds for the Central Universities” (X.H. and Y.F.), Zhejiang University (X.H.), and PCSIRT (Y.F.) is gratefully acknowledged. Calculations were performed on the high-performance computing system at the Department of Chemistry, Zhejiang University.

S Supporting Information *

The Supporting Information is available free of charge on the ACS Publications website at DOI: 10.1021/acs.organomet.7b00903. Calculations of catalyst initiation; additional conformers of intermediates and transition states (PDF) Coordinates and energies of DFT-computed stationary points (XYZ)

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AUTHOR INFORMATION

Corresponding Authors

*E-mail: [email protected]. *E-mail: [email protected].

REFERENCES

(1) For reviews on C−C bond activation, see (a) Rybtchinski, B.; Milstein, D. Angew. Chem., Int. Ed. 1999, 38, 870−883. (b) Takahashi, I


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Organometallics

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Mechanism and Origins of Chemo-and Regioselectivities of Pd

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