Theoretical Study on CuCl-Catalyzed Coupling of Thiol Esters with

Article pubs.acs.org/JPCA

Theoretical Study on CuCl-Catalyzed Coupling of Thiol Esters with Organostannane Lingjun Liu, Jing Sun, Yuxia Liu, Ping Li, and Siwei Bi* School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, Shandong Province, People’s Republic of China S Supporting Information *

ABSTRACT: The reaction of CuCl-catalyzed coupling of thiol ester with organostannane has been theoretically investigated using density functional calculations. This reaction takes place with CuCl as a catalyst, giving the product ketone and an organotin sulfide. The relatively low overall activation barrier calculated (26.0 kcal/mol) supports the experimental fact that this reaction proceeds under mild reaction conditions. The conversion of the resulting Cu(I) thiolate intermediate (D) to the benzoisothiazole (E) was found to proceed via an one-step process. The relatively high overall activation barrier calculated (35.8 kcal/mol) supports the experimental fact that this reaction proceeds under harsher reaction conditions. The influence of hybridization of the carbon atom bonded to sulfur on the reaction was also discussed.

1. INTRODUCTION Palladium-catalyzed carbon−carbon cross-coupling reactions have been developed for creating new molecules,1 in which copper complexes were used as cocatalysts.2 Of the various copper cocatalysts, copper(I) carboxylate cofactors were found to be essential for the desulfitative carbon−carbon bond forming processes that take place between thioorganic substrates and organostannane reagents or boronic acids.3 Thioorganic compounds do not react directly with boronic acids, neither in the presence nor in the absence of palladium or nickel catalysts. Addition of a stoichiometric quantity of a copper(I) carboxylate cofactor renders the palladium-catalyzed system highly effective.3 These reactions with palladium as catalysts and copper as cocatalysts represent one typical class for carbon−carbon bond forming processes, and were described as the “first generation” system (eq 1). Mechanistic study on the Pd-catalyzed/copper(I) carboxylate-mediated desulfitative coupling of thioorganics and boronic acids has been reported recently.4 It was found that the requisite copper(I) carboxylate plays multiple important roles, such as enhancing the transmetalation process and providing a vital carboxylate, etc. The “second-generation” system is palladium-free, uses only catalytic quantities of Cu(I) cofactors and takes place under aerobic reaction conditions (eq 2).5 But this system requires a sacrificial second equivalent of organostannanes or boronic acids. © 2012 American Chemical Society

In 2011, Liebeskind and co-workers reported a new class of carbon−carbon bond forming reactions that were catalyzed only by Cu(I) under anaerobic conditions.6 This study suggested that Cu(I) could be rendered catalytically viable in the presence of thiolate to design a small molecule chemical analogue of the Received: September 5, 2012 Revised: November 7, 2012 Published: November 7, 2012 11736

dx.doi.org/10.1021/jp308810n | J. Phys. Chem. A 2012, 116, 11736−11744

The Journal of Physical Chemistry A

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A precise estimate of ΔG in solution is too difficult to calculate. Here we do not discuss the entropy corrections;15 instead, throughout this paper we discuss the calculated enthalpy (ΔH) values with consideration of the solvent effect. All the calculations were performed with the Gaussian 03 software package.16

metallothionein system in which a N−O reactant serves the same conceptual purpose of the S−S reactant of the biological system. The advantage of this system is palladium-free and without squandering an extra equivalent of organostannanes reagents or boronic acids (eq 3). In view of the system having so many advantages, understanding the mechanisms and the factors that govern these reactions are important in light of increasing relevance of copper in many catalytic processes. In this study, eq 3 was chosen for the purpose of exploring its reaction mechanisms with the help of the density functional theory (DFT) calculations. Pd-catalyzed cross-coupling reactions have attracted extensively theoretical studies. In contrast, theoretical studies on Cu-catalyzed cross-coupling reactions are limited.7 We hope this study would present further understanding for this system and is helpful for developing efficient desulfitative crosscoupling processes between thioorganic substrates and organostannane reagents or boronic acids.

3. RESULTS AND DISCUSSION Liebeskind’s group proposed the catalytic cycle for desulfitative cross-coupling processes between thioorganic substrates and organostannane reagents or boronic acids (Figure 1).6 The

2. COMPUTATIONAL DETAILS In our calculations, we optimized all molecular geometries at the Becke3LYP (B3LYP) level of density functional theory (DFT).8 To identify all stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency) and to provide enthalpic energies at 298.15 K, we calculated frequencies at the same level of theory. All the transition states were checked by intrinsic reaction coordinate (IRC) analysis.9 To describe Cu, Sn, S, and Cl atoms, we use the quasi-relativistic pseudopotentials of Hay and Wadt with a double-valence basis set (LanL2DZ),10 and to describe C, O, N, and H atoms, we use standard 6-31G(d,p) basis sets.11 Polarizations functions were added for Cu (ζf = 0.117), Sn (ζd = 0.183), S (ζd = 0.421), and Cl (ζd = 0.514).12 The solvent effect was examined by performing single-point self-consistent reaction field (SCRF) calculations based on the polarizable continuum model (PCM)13 for all the gas-phase optimized species. The atomic radii used for the PCM calculations were specified using the UAKS keyword. In the coupling reactions studied here, dimethylformamide (DMF) is the solvent in experiments. In Gaussian 03, the PCM parameters are not available for DMF. We instead used the parameters of MeCN for PCM calculations because the dielectric constant of DMF (ε = 37.8) is very close to that of MeCN (ε = 35.7). The natural bond orbital (NBO) program14 was also used to obtain Wiberg bond indexes and NBO charges for selected species. All the transition metal complexes involved in this work are found in the singlet state. To test the validity of the B3LYP method used in this work, we carried out the enthalpic activation barriers for a few selected reaction steps using the B3PW91 method. The results calculated in kcal/mol using B3LYP and B3PW91 functionals are as follows. 2→TS2−3 (19.2, 20.9); 7→TS7−8 (9.3, 7.8); 7→TS7−9 (35.1, 38.8) (Figures 2, 3, and 5). The calculations show that both functionals give similar results.

Figure 1. Catalytic cycle for Cu-catalyzed desulfitative coupling of thiol esters with boron or tin reagents suggested by Liebeskind’s group.

catalytic cycle begins with coordination of CuI to the thiol ester− oxime (A) to afford intermediate B. Transmetalation from boron or tin to CuI leads to intermediate C where the organocopper R2 is in close proximity with the thiol ester carbonyl carbon. The subsequent carbon−carbon bond forming step leads to the product ketone and the CuI thiolate intermediate (D). The last step produces the benzoisothiazole (E) and regenerates the catalyst CuIX. Experiments confirmed that the desulfitative carbon−carbon cross-coupling (leading to ketone and D) performed at 60 °C is facile, but the product benzoisothiazole (E) was obtained under a harsher reaction condition (100 °C). Experiments also demonstrated that different commercially available CuIX sources (X = Cl, I, and thiophene-2-carboxylate) were all effective for the new desulfitative carbon−carbon cross-coupling reactions.6 To probe the detailed reaction mechanisms and present further understanding of such reactions, we employed in this study the Cu(I)-catalyzed desulfitative cross-coupling reaction of thioorganic substrates with organostannane reagents. To reduce the computer cost, CuCl (used in experiments) was chosen as the catalyst in this work, and the butyl group in the tin reagent was modeled by a methyl group in our calculations. The model reaction is shown in eq 4. The errors incurred from the simplification in the tin reagent are expected to be small because the butyl groups are present in every species calculated and only the relative energies among the different species calculated are important in our discussion. Previous theoretical study indicates that such a modeling of the reagent is reasonable.4 11737



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Figure 2. Solvation-corrected enthalpic energy profile for the transmetalation between the organotin reagent (R2) and the coordinated CuCl (1). The calculated relative enthalpic energies are given in kcal/mol.

Figure 3. Solvation-corrected enthalpic energy profile for the process of ketone formation. The calculated relative enthalpic energies are given in kcal/mol.

for formation of both the product ketone and the CuI thiolate intermediate D and whether this transformation can be achieved catalytically with CuCl as the catalyst. Figure 2 presents the solvation-corrected enthalpic energy profile involving a transmetalation process where the chloride is substituted by the organic group −C6H4OMe. Figure 3 presents the solvationcorrected enthalpic energy profile involving ketone formation and regeneration of the catalyst CuCl. Figure 4 illustrates the selected B3LYP optimized structures shown in Figures 2 and 3. Coordination of CuCl and Transmetalation with Organostannane (R2). Liebeskind’s group suggested in Figure 1 that coordination of the thiol ester (R1) with the catalyst CuCl gives the six-membered chelate complex B. Our calculation results

3.1. Formation of the Product Ketone. In this section, we mainly focus on probing what are the overall activation processes 11738



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Figure 4. Selected B3LYP optimized structures involved in the mechanism of the model reaction (eq 4), together with selected bond distances given in Å. For the purpose of clarity, hydrogen atoms are omitted.

thermodynamically with an enthalpy change of 9.4 kcal/mol. Removal of Cl−SnMe3 from 3 gives the intermediate 4 with a small energy increase (1.5 kcal/mol). Ketone Formation and Regeneration of the Catalyst CuCl. As shown in Figure 3, the organo-copper intermediate 4 undergoes oxidative addition of the OC−S bond, followed by reductive elimination to produce the ketone product and the sixmembered metallacyclic species (D), both of which afford the complex 5 via Cu←(η2-aryl) coordination. Our calculations suggested that both the oxidative addition and the reductive elimination occur in one step. Examining the structural feature of the transition state TS4−5, one can see that this transition state is just related to the oxidative addition of the OC−S bond. The distance of OC···S is calculated to be 2.22 Å, much longer than that in 4 (1.86 Å), and those of OC···Cu and S···Cu are 2.12 and 2.35 Å, respectively, much shorter than those in 4, indicating the OC−S bond is being broken and Cu−S and Cu−C bonds are being formed in TS4−5. The distance of OC···C(aryl) is calculated to be 2.98 Å, confirming that the carbon atom of the OC−S moiety is far away from the aryl carbon and indicating that the Cu−C(aryl) bond remains nearly intact. The reason for nonexistence of the oxidative addition product bearing both Cu−CO and Cu−C(aryl) bonds can be attributed to the following two factors. (1) Reductive elimination involving coupling of sp2 carbons is normally much easier than that involving sp3 carbons because of the involvement of π electrons (orbitals) during the C−C coupling.19 (2) A Cu(III) metal center is relatively unstable. This is quite different from other metal centers. For example, a recent computational study on the C(sp2)−C(sp2) coupling at other late transition metals showed that the relevant intermediates having Pd(IV), Pd(II), Pt(IV), Pt(II), Rh(III), Ir(III), Ru(II), and Os(II) centers were found to be local minima.20 Step 5 → 6 is an isomerization process involving slippage of the η2 coordination mode to the next carbon−carbon unsaturated bond. Intermediate 6 was found to be more stable than intermediate 5. In the step 6 → 7, the product ketone is substituted by the resulting ClSnMe3, leading to 7 with the Cl of

indicated that only the nitrogen atom coordinates to Cu to give a N→Cu dative bond in the resulting intermediate 1 (Figure 2). In other words, the sulfur atom does not coordinate with Cu(I). The N−Cu bond length and the S···Cu distance are computed to be 1.96 and 3.17 Å, respectively and the bond angle N−Cu−Cl moiety is nearly linear with an angle of 176.9°. The proposed chelate complex bearing both sulfur and nitrogen coordination cannot be located despite trying the structural optimization in many ways. The nonpresence of the S→Cu dative bond is a result of the delocalization in the S−CO moiety, which weakens the coordination ability of sulfur. In addition, we also examined coordination possibility between the CO group and Cu(I) because Cu(I) has a tendency to coordinate with a double-bond unit in an η2 fashion.17 However, our calculation results suggest such a coordination cannot be present in the binding between Cu(I) and R1, probably due to unfavorable steric environment. In this work, we set the CuCl-coordinated substrate (1) as referencing the zero point. The process 1 to 4 is related to a transmetalation process, leading to the chloride being substituted by the organic group −C6H4OMe. Combination of the intermediate 1 with the organostannane (R2) leads to the adduct 2. The major interaction between them is found to be the Cu···C interaction (2.20 Å in 2), where the C atom is the phenyl C bonded to Sn. Transmetalation from tin or boron with copper is an assumed essential step in a wide variety of Cu-catalyzed bond-forming processes.18 The transmetalation step (2 to 3) affords the organocopper complex 3. The binding between the N-coordinated organo-copper species and the resulting Cl−SnMe3 in 3 is found to be a van der Waals interaction because the Cl−SnMe3 is calculated to be far away from the rest of the structure. In the transition state TS2−3, the bond distances Cu−C1 and Sn−C1 are 1.95 and 3.35 Å, indicating the Cu−C1 is nearly formed and the Sn−C1 bond is almost cleaved. Therefore, TS2−3 is believed to be a late transition state. The activation barrier for step 2 → 3 is calculated to be 19.2 kcal/mol, indicating the transmetalation process is kinetically accessible. But this step is disfavored 11739



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bond forming and breaking involved in the transformation 1-f2 + R2 → 4-f1 + ClSnMe3 are similar in energy. Thus, the enthalpy change from 1 to 4 comes mainly from the Cu←N bond energy difference in 1 and 4. That is, ΔH1−4 = ΔH1 + ΔH2 + ΔH3 + ΔH4 ≈ ΔH1 + ΔH4 = 8.8 kcal/mol. On the basis of the calculated EDA results, we concluded that the energy rise from 1 to 4 is due to the stronger ClCu←N coordination in 1 as compared to aryl-Cu←N coordination in 4. This is understandable because the high electronegativity of Cl renders Cu(I) bearing larger positive charge and hence enables Cu(I) having high ability to accept the lone pair of the nitrogen atom. The following evidence supports the above argument: (a) the Cu−N (1.96 Å) in 1 is shorter than the one in 4 (1.99 Å) (Figure 4), (b) the Cu−N Wiberg bond index in 1 (0.7551) is greater than that in 4 (0.6467), and (c) the nbo charge on Cu in 1 (+0.5845) is larger than that in 4 (+0.4009). As for the relatively low activation barrier for oxidative addition of OC−S to Cu, the sp2 hybridization of the two carbon atoms plays an essential role. For the purpose of understanding the role of the hybridization of the carbon bonded to sulfur, we comparatively investigated the situation of Me−S oxidative addition by Cu(I) in the final section. The overall enthalpic energy difference from 1 to 8 is calculated to be −22.6 kcal/mol, indicating this reaction is significantly favorable thermodynamically. The significant energy decrease can be ascribed to the C−C bond coupling, leading to the formation of ketone. In summary, CuCl can act as a catalyst to catalyze the reaction, leading to the formation of the products, the ketone and the organotin sulfide, under mild conditions. 3.2. Formation of the Products, Benzoisothiazole (E) and MeOSnMe3. According to the proposal by Liebeskind’s group, the intermediate Cu(I) thiolate (D) leads to the product benzoisothiazole (E) and regenerates the catalyst CuIX when the reaction temperature was elevated to 100 °C from 60 °C (Figure 1). In this study, two paths were proposed for regenerating the catalyst CuCl and producing the product E. Corresponding energy profiles are displayed in Figure 5. The

ClSnMe3 coordinating to Cu. The similar energies indicate that both of them are in equilibrium. Interestingly, we found the formed six-membered metallacyclic species (D) can further react with the resulting ClSnMe3. As shown in Figure 3, complex 7 undergoes a σ bond metathesis process, producing the organotin sulfide and regenerating the catalyst CuCl, both of which form the intermediate 8 via Cu←N coordination. The enthalpy change and the activation barrier for the conversion from 7 to 8 are −5.1 and +9.3 kcal/mol, respectively, indicating generation of the organotin sulfide and regeneration of the catalyst CuCl are favorable thermodynamically and kinetically. It can be found from Figures 2 and 3 that the overall enthalpic activation energy is 26.0 kcal/mol (2 to TS4−5). The relatively low barrier indicates the reaction can occur under mild reaction conditions, in accordance with the experimental fact that the product ketone can be obtained under the reaction condition of about 60 °C.6 The overall barrier can be simply attributed to two aspects of energy increase. One is the energy increase caused by substitution of the chloride with the organic group −C6H4OMe (1 to 4). The enthalpic energy increase from 1 to 4 is 9.4 kcal/mol. The other is related to the activation of OC-S (4 to TS4−5) with a barrier of 15.1 kcal/mol. For further understanding the reason for the energy increase from 1 to 4, the hypothetical energydecomposition analysis21 was carried out at the same level of theory using acetonitrile as solvent. Scheme 1 shows the proposed Scheme 1. Energy Decomposition Processes and Data (kcal/mol)

energy decomposition process where the fragments 1-f1 and 1-f2 are directly derived from 1 and 4-f1 and 4-f2 from 4. The enthalpic energies of all the fragments were obtained by carrying out single point calculations. ΔH1 and ΔH4 correspond to the ClCu←N dative bond energies in 1 and 4, respectively. ΔH2 is an isomeric enthalpy change from 1-f1 to 4-f1. ΔH3 is related to the enthalpy change of the process from Cu−Cl to Cu−C (that is, 1-f2 + R2 → 4-f1 + ClSnMe3). Calculation results reveal that ΔH2 (0.0 kcal/mol) and ΔH3 (0.6 kcal/mol) are very small, indicating the geometries of 1-f1 and 4-f1 are very similar and the

Figure 5. Solvation-corrected enthalpic energy profile for S−N bond formation and CuCl regeneration. The calculated relative enthalpic energies are given in kcal/mol. 11740



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Figure 6. Selected B3LYP optimized structures involved in Figure 5, together with selected bond distances given in Å. For the purpose of clarity, hydrogen atoms are omitted.

geometric structures, together with selected bond distances, were displayed in Figure 6. Path a is related to a stepwise mechanism, corresponding to the step D → E + CuIX (Figure 1). This path involves Cl−SnMe3 dissociation (7 to D), S−N bond formation (D to 10), and a σ-bond metathesis process between Cu−O and Cl−Sn (10 to 9). Path b is found to be an one-step process, directly affording 9 in which the two products coordinate with the regenerated catalyst CuCl. In path a, the overall activation barrier is very high (47.9 kcal/mol, 7 to TSD‑10). The step D to 10 can be viewed as N−OMe oxidative addition followed by S−N reductive elimination. The Cu(III) intermediate formed by N−OMe oxidative addition is not present as a result of the fact that the Cu(III) formed is not stable. The copper center in 10 coordinates with the sulfur rather than the nitrogen. The Cu···S and Cu···N distances are calculated to be 2.22 Å and 3.20 Å, respectively. In path b, the intermediate 7 could directly convert to the intermediate 9. Examining the transition states TS7−9 and TSD‑10, one can see their geometries are similar except the coordinated ClSnMe3, which means the coordinated ClSnMe3 has nearly no contribution to the activation barrier from 7 to TS7−9. As calculated, the barriers from 7 to TS7−9 (35.8 kcal/mol) and from D to TSD‑10 (36.3 kcal/mol) are similar. However, the Cu−OMe unit was not located in path b in the presence of the coordinated ClSnMe3, implying the σ-bond metathesis process Cu−OMe + Cl−SnMe3 → CuCl + O−SnMe3 is barrierless and thus affording the direct conversion from 7 to 9. Because the barrier (35.8 kcal/mol) in path b is lower than the one (47.9 kcal/mol) in path a, we suggest that the reaction for regenerating the catalyst CuCl would undergo the one-step process (path b) rather than the stepwise mechanism (path a). The conversion from 7 to 9 is significantly favorable thermodynamically with the enthalpy change calculated to be −35.1 kcal/mol. In view of the fact that the activation barrier (35.8 kcal/mol) is obviously higher than the one (26.0 kcal/mol) for forming the ketone, harsher reaction conditions are needed for conversion from 7 to 9, which is consistent with the experimental fact that the reaction temperature was elevated to 100 °C from 60 °C. The kinetically unfavorable step 7 to 9, as compared to the step 7 to 8, is due to the fact that in TS7−9 a strong N−O bond is needed to break. The catalytic cycles are illustrated in Figure 7. Figure 7a shows that the reaction can be catalytically achieved under mild reaction conditions, leading to formation of the ketone and regeneration of the catalyst CuCl. Both 7 and 8 are in equilibrium with the latter being dominant under the mild reaction conditions. Figure 7b shows that the reaction can further proceed under harsher reaction conditions, leading to final conversion of 7 and 8

Figure 7. Catalytic cycles for CuCl-catalyzed desulfitative coupling of eq 4: (a) catalytic cycle under mild conditions; (b) catalytic cycle under harsher conditions.

into the products benzoisothiazole (E) and Me3SnOMe, along with regeneration of the catalyst CuCl. 3.3. Influence of Hybridization of the Carbon Attached to Sulfur. In this section, our attention is to probe how the hybridization mode of the carbon atom attached to the sulfur affects the reaction. The model reaction is shown in eq 5, where a methyl is bonded to the sulfur atom. Compared to the model reaction studied above, the hybridization of the carbon atom attached to sulfur is changed into sp3. The calculated energy profile for the CuCl-catalyzed model reaction is separately 11741



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Figure 8. Solvation-corrected enthalpic energy profile for Cu−C bond formation through transmetalation. The calculated relative enthalpic energies are given in kcal/mol.

Figure 9. Solvation-corrected enthalpic energy profile for the process of ketone formation. The calculated relative enthalpic energies are given in kcal/mol.

displayed in Figures 8 and 9, where the CuCl-coordinated substrate (I) is set as referencing the zero point. Figure 8 shows the energy profile for the process of Cu−C bond formation through a transmetalation process. Figure 9 shows the energy profile for the process of ketone formation. The process for regenerating the catalyst CuCl is the same as the process from 7 to 9 as discussed above. Thus, this process is no longer described in this section. B3LYP optimized structures involved in the mechanism of the model reaction (eq 5) are shown in Figure 10.

Similar to the model in eq 4, the model in eq 5 also undergoes Cu←N coordination and the transmetalation process (I to IV). 11742



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Figure 10. Selected B3LYP optimized structures involved in the mechanism of the model reaction (eq 5), together with selected bond distances given in Å. For the purpose of clarity, hydrogen atoms are omitted.

4. CONCLUSIONS

The activation barrier for the transmetalation step (II to TSII−III) is calculated to be 22.1 kcal/mol, close to the corresponding step (19.2 kcal/mol) from 2 to TS2−3 (Figure 2) because the methyl attached to sulfur does not participate in the transmetalation process. IV to V is just a coordination isomerism where the coordination mode changes from Cu←N to Cu←S. The intermediate V was located by IRC calculations from the transition state TSV−VI. In Figure 9, an oxidative addition step occurs (V to VI), where the C(sp3)−S bond is cleaved by the metal center, giving a Cu(III) intermediate (VI). The transient presence of the unstable Cu(III) intermediate is due to the relatively difficult C(sp3)−C(sp2) coupling compared to the case of C(sp2)−C(sp2) coupling as described above. Via a very small barrier of 0.3 kcal/mol, VI easily converts to the intermediate VII, in which the product is produced and the six-membered metallacyclic species is generated. The similar energies of VII and 7 indicate there exists an equilibrium between them. Examining the overall reaction mechanism shown in Figures 8 and 9, one can see this reaction is significantly favorable thermodynamically with a enthalpy change of −22.8 kcal/mol (I to VII). Nevertheless, this reaction is found to be inaccessible kinetically under the experimental conditions. Our calculation results show that the overall activation process in the reaction mainly involves the transmetalation process and the C(sp3)−S activation (II to TSV−VI). The overall activation barrier is calculated to be 44.6 kcal/mol. Clearly, the kinetic inaccessibility for the reaction is caused by the difficult C(sp3)−S activation with an activation barrier of 30.2 kcal/ml (V to VI). This activation barrier is significantly higher than the C(sp2)−S activation as shown in Figure 3 (4 to TS4−5, 15.1 kcal/mol). The reason for the high barrier of C(sp3)−S activation is attributed to its high C−S antibonding σ bond, which leads to the weak backdonaiton from the metal. In other words, the methyl group is less stabilized in the oxidative addition transition state compared to the case of C(sp2) group. In summary, the difficult C(sp3)−S activation leads to the reaction inaccessible.

The reaction of CuCl-catalyzed coupling of thiol esters with organostannane has been theoretically investigated with the aid of density functional calculations. For formation of the product ketone, the reaction was found to be catalytically achieved along with generation of an organotin sulfide (8) under the mild conditions. This mechanism for this conversion is as follows: (a) coordination of CuCl to the nitrogen of the reactant R1, (b) transmetalation between Cu−Cl and Sn−C, giving the organocopper intermediate, (c) C(sp2)−S oxidative addition, directly forming the product ketone and, (d) generation of the organotin sulfide and regeneration of the catalyst CuCl. Furthermore, our calculation results show that the intermediate 7 produced under mild reaction conditions can directly convert to the final products via a one-step process under harsher reaction conditions. When the hybridization of the carbon atom bonded to sulfur is changed from C(sp2) to C(sp3), the reaction was found to be kinetically inaccessible due to the difficult oxidative addition of the C(sp3)−S bond by Cu(I).

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ASSOCIATED CONTENT

S Supporting Information *

Complete ref 16 and tables giving Cartesian coordinates and electronic energies for all of the calculated structures. This material is available free of charge via the Internet at http://pubs. acs.org.

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AUTHOR INFORMATION

Corresponding Author

*E-mail: [email protected]. Phone: +86-537-4458308. Fax: +86537-4456305. Notes

The authors declare no competing financial interest. 11743



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ACKNOWLEDGMENTS We are grateful for helpful discussion with Prof. Zhenyang Lin in the Hong Kong University of Science and Technology. This work was supported by the National Natural Science Foundation of China (No. 21173126 and 21003082).

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