A DFT Study on the Mechanism of the Coupling Reaction between

Apr 6, 2010 - Cai-Hong Guo†‡, Jiang-Yu Song†, Jian-Feng Jia†, Xian-Ming Zhang† and Hai-Shun Wu*† ... All possible pathways are examined, a...
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Organometallics 2010, 29, 2069–2079 DOI: 10.1021/om100020s

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A DFT Study on the Mechanism of the Coupling Reaction between Chloromethyloxirane and Carbon Dioxide Catalyzed by Re(CO)5Br Cai-Hong Guo,†,‡ Jiang-Yu Song,† Jian-Feng Jia,† Xian-Ming Zhang,† and Hai-Shun Wu*,† †

School of Chemistry and Materials Science, Shanxi Normal University, Linfen 041004, People’s Republic of China, and ‡Chemical Engineering Department, Taiyuan University of Technology, Taiyuan 030024, People’s Republic of China Received January 7, 2010

The detailed mechanism of the experimentally observed formation of five-membered cyclic carbonates in the coupling reaction between chloromethyloxirane and CO2 catalyzed by the Re(CO)5Br complex is revealed by means of density functional theory (DFT) calculations. All possible pathways are examined, and their corresponding energetics are demonstrated. Our calculations indicate that the real active catalyst is the unsaturated complex Re(CO)4Br rather than free radical species (Re(CO)5 or Br radicals). The preferred mechanism (path I) for the catalytic production of cyclic carbonates can be divided into three main stages involving epoxide oxidative addition, carbon dioxide insertion, and reductive elimination of cyclic carbonate, none of which contains significantly large barriers. Our results provide support for Jiang’s proposal that the reaction proceeds through the reactive oxametallacyclobutane 2b. Furthermore, we have found CO dissociation from 2b is an essential step, which facilitates CO2 coordination and insertion leading to metallacarbonate 6. The whole CO2 insertion is predicted to be an endergonic process. From the unstable metallacyclic species 6, the cyclic carbonate reductive elimination occurs via a three-center transition-state structure. Reductive elimination is a very facile step as compared with epoxide oxidative addition and carbon dioxide insertion. As for the ring-opening of epoxide, the activation energy barrier from 1d to 2b is 36.2 kcal/mol in supercritical CO2, which is slightly higher, by 2.2 kcal/mol, than that of the CO2 multistep insertion (2b f 6). Thus, each of them can be the rate-determining step with variation in the reaction conditions (temperature and pressure). The high-energy barriers along the alternative reaction pathway indicate path II is not competitive. Our present theoretical study provides a clear profile for the cycloaddition of carbon dioxide with chloromethyloxirane catalyzed by Re(CO)5Br. 1. Introduction Catalytic transformation of CO2 into useful organic compounds has attracted much attention due to the economic and environmental benefits arising from the utilization of renewable sources and the growing concern about the greenhouse effect.1-3 As an attractive C1 building block in organic synthesis, CO2 is highly functional, abundant, cheap, nontoxic, and nonflammable.1 One of the most widely applied technologies for CO2 chemical fixation is its coupling with epoxides to form five-membered cyclic carbonates.2 These carbonates are of interest for a wide variety of applications, including aprotic polar solvents, fine chemical intermediates, starting materials for the synthesis of polymers, engineering plastics, and biomedical fields.4-6

During the past few decades, much effort has been devoted to develop highly efficient and practical catalysts for the CO2/epoxide coupling reactions.7-23 However, most of the

*To whom correspondence should be addressed. Fax and Tel: þ86 357 2052468. E-mail: [email protected]. (1) Arakawa, H.; Aresta, M.; Armor, J. N.; et al. Chem. Rev. 2001, 101, 953. (2) Darensbourg, D. J.; Holtcamp, M. W. Coord. Chem. Rev. 1996, 153, 155. (3) Gibson, D. H. Chem. Rev. 1996, 96, 2063. (4) Clements, J. H. Ind. Eng. Chem. Res. 2003, 42, 663. (5) Peppel, W. J. Ind. Eng. Chem. 1958, 50, 767. (6) Shaikh, A.-A. G.; Sivaram, S. Chem. Rev. 1996, 96, 951.

(7) Kihara, N.; Hara, N.; Endo, T. J. Org. Chem. 1993, 58, 6198. (8) Yano, T.; Matsui, H.; Koike, T.; Ishiguro, H.; Fujihara, H.; Yoshihara, M.; Maeshima, T. Chem. Commun. 1997, 1129. (9) Yamaguchi, K.; Ebitani, K.; Yoshida, T.; Yoshida, H.; Kaneda, K. J. Am. Chem. Soc. 1999, 121, 4526. (10) Yasuda, H.; He, L.-N.; Sakakura, T. J. Catal. 2002, 209, 547. (11) Nomura, R.; Ninagawa, A.; Matsuda, H. J. Org. Chem. 1980, 45, 3735. (12) Aida, T.; Inoue, S. J. Am. Chem. Soc. 1983, 105, 1304. (13) Kruper, W. J.; Deller, D. V. J. Org. Chem. 1995, 60, 725. (14) Kim, H. S.; Kim, J. J.; Lee, B. G.; Jung, O. S.; Jang, H. G.; Kang, S. O. Angew. Chem., Int. Ed. 2000, 39, 4096. (15) Paddock, R. L.; Nguyen, S. T. J. Am. Chem. Soc. 2001, 123, 11498. (16) Shen, Y.-M.; Duan, W.-L.; Shi, M. J. Org. Chem. 2003, 68, 1559. (17) Lu, X.-B.; Liang, B.; Zhang, Y.-J.; Tian, Y.-Z.; Wang, Y.-M.; Bai, C.-X.; Wang, H.; Zhang, R. J. Am. Chem. Soc. 2004, 126, 3732. (18) Sun, J.; Wang, L.; Zhang, S. J.; Li, Z. X.; Zhang, X. P.; Dai, W. B.; Mori, R. J. Mol. Catal. A: Chem. 2006, 256, 295. (19) Doskocil, E. J.; Bordawekar, S. V.; Kaye, B. G.; Davis, R. J. J. Phys. Chem. B 1999, 103, 6277. (20) Peng, J. J.; Deng, Y. Q. New J. Chem. 2001, 25, 639. (21) Kawanami, H.; Sasaki, A.; Matsui, K.; Ikushima, Y. Chem. Commun. 2003, 896. (22) Kim, Y. J.; Varma, R. S. J. Org. Chem. 2005, 70, 7882. (23) Park, D. W.; Mun, N. Y.; Kim, K. H.; Kim, I.; Park, S. W. Catal. Today 2006, 115, 130.

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catalysts, such as metal halides,7 metal oxides,8-10 organometallic compounds,11-17 Lewis acids or bases,18 and zeolites,19 usually suffer from low catalyst activity or need harsh reaction conditions (high pressure and high temperature) or need additional co-solvent. Thus, developing new catalysts for this coupling is highly desirable. Recently, Jiang et al.24 reported the first example of re-catalyzed coupling of CO2 with epoxides at 383.15 K to give cyclic carbonates under solvent-free conditions, and their observations showed that Re(CO)5Br is an efficient, simple catalyst with a high yield of products (up to 97%). Also, they proposed a possible reaction mechanism, which is closely related to that of the Ni-catalyzed reactions.25 Nonetheless, the comprehensive understanding of the reaction mechanism by experimental methods presents several challenges. The most significant challenge is to isolate or trap reaction intermediates. To our knowledge, although the cycloaddition of CO2 with epoxides affording the cyclic carbonates catalyzed by Re(CO)5Br has been successfully achieved, the details of the reaction mechanism are still ambiguous so far because none of the intermediates were detected. Is Re(CO)4Br suggested by Jiang et al. indeed the catalytically active species? Note that the bromine radical and rhenium pentacarbonyl radical could also be present in the reaction media. Moreover, what would be the reaction pathways? Which pathway is preferable? How does each of the catalytic steps take place? Which step is rate determining in the whole catalytic cycle? To answer the questions raised above and gain insight into the mechanism of the title coupling reaction, we have investigated the full catalytic cycle of the Re(CO)5Br-catalyzed reaction of CO2 with epoxides using the B3LYP density functional method. It is noted that several transition-metalmediated coupling reactions involving CO2 and epoxides have been computed in recent years. However, their conclusions are hardly transferable to our present reaction system. For example, the mechanism of Zn(II)-catalyzed copolymerization of CO2 with cyclohexene oxide was investigated with the hybrid molecular orbital method ONIOM.26 Heterobimetallic Ru-Mn complexes were first synthesized to mediate the coupling reaction of CO2 with ethylene oxide, and the corresponding reaction mechanism was studied by means of the B3LYP level of density functional theory.27 Using alkylmethylimidazolium chloride ([Cnmim]Cl, n = 2, 4, 6) as catalysts, Sun et al.28 studied the coupling mechanism of CO2 with propylene oxide at the B3PW91/6-31G(d,p) level of density functional theory. More recently, we have elucidated the mechanistic details of the coupling reaction of propylene oxide with carbon dioxide catalyzed by copper(I) cyanomethyl using the B3LYP/6-311G** method.29 In this work, all possible reaction pathways associated with the formation of cyclic carbonate reported in ref 24 have been explored and calculated at the B3LYP density functional theory. Chloromethyloxirane is chosen as a prototype of epoxides for study due to its high conversion and relative simplicity. The structures of all intermediates and transition (24) Jiang, J.-L.; Gao, F.; Hua, R.; Qiu, X. J. Org. Chem. 2005, 70, 381. (25) De Pasquale, R. J. J. Chem. Soc., Chem. Commun. 1973, 157. (26) Liu, Z.; Torrent, M.; Morokuma, K. Organometallics 2002, 21, 1056. (27) Man, M. L.; Lam, K. C.; Sit, W. N.; Ng, S. M.; Zhou, Z.; Lin, Z.; Lau, C. P. Chem.;Eur. J. 2006, 12, 1004. (28) Sun, H.; Zhang, D. J. J. Phys. Chem. A 2007, 111, 8036. (29) Guo, C.-H.; Wu, H.-S.; Zhang, X.-M.; Song, J.-Y.; Zhang, X. J. Phys. Chem. A 2009, 113, 6710.

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states involved in the processes have been located. To take the effect of entropy into account, we use the free energies (ΔG) of activation and reaction and the corresponding enthalpies (ΔH) to analyze the reaction mechanisms. On the basis of the discussion of each elementary step, a detailed mechanism has been proposed. According to our computations, we have clarified the rate-controlling step of the whole reaction and provided a reasonable explanation for experimental observations. This work aims to shed light on the underlying catalytic reaction mechanism in detail, since better mechanistic insight should help the development of more powerful catalysts or obtaining tailor-made products.

2. Computational Details All calculations were performed with the Gaussian 03 program package.30 The geometries of the reactants, complexes, transition states, intermediates, and products were fully optimized without any symmetry constraints at the B3LYP level of theory.31-34 The harmonic vibrational frequencies were also calculated at the same level to characterize the nature of the stationary points as true minima with no imaginary frequencies or transition states with only one imaginary frequency. Especially, the lone imaginary frequency of each transition state displayed the desired displacement orientation, and the validity of each reaction path was further examined by the intrinsic reaction coordinate calculations (IRC).35,36 The Re atom was described using the LANL2DZ basis set including a double-ζ valence basis set with the Hay and Wadt effective core potential (ECP).37,38 The 6-31G(d,p) split-valence polarized basis set39 was used for C, H, O, Cl, and Br atoms. This B3LYP method has been confirmed to be appropriate for rhenium carbonyl complexes in a number of recent studies,40-42 as indicated by the excellent explanation for the experimental observations. To match with the experimental conditions in ref 24, all thermodynamic data reported in this paper were calculated under an actual reaction temperature of 383.15 K and pressure of 60 atm. Zero-point energy corrections (ZPE), derived from the frequency calculations, were added to the total energies of each species in the catalytic cycle. NBO43 analysis has been carried out on some structures for interpretation purposes. To take into account condensed-phase effects, single-point self-consistent reaction field (SCRF) calculations based on the polarizable continuum model (PCM)44 were performed on the gas-phase-optimized geometries for intermediates and transition states along paths I and II. A GEPOL cavity with an average (30) Frisch, M. J.; Trucks, G. W.; Schlegel, H. B.; Scuseria, G. E.; Robb, M. A.; Cheeseman, J. R.; et al. Gaussian 03, Revision B.05; Gaussian, Inc.: Pittsburgh, PA, 2003. (31) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (32) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (33) Lee, C.; Yang, W.; Parr, R. G. Phys. Rev. B 1988, 37, 785. (34) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F.; Frisch, M. J. J. Phys. Chem. 1994, 98, 11623. (35) Gonzalez, C.; Schlegel, H. B. J. Chem. Phys. 1989, 90, 2154. (36) Gonzalez, C.; Schlegel, H. B. J. Phys. Chem. 1990, 94, 5523. (37) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (38) Wadt, W. R; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (39) Francl, M. M.; Pietro, W. J.; Hehre, W. J.; Binkley, J. S.; Gordon, M. S.; DeFrees, D. J.; Pople, J. A. J. Chem. Phys. 1982, 77, 3654. (40) Kirgan, R.; Simpson, M.; Moore, C.; Day, J.; Bui, L.; Tanner, C.; Rillema, D. P. Inorg. Chem. 2007, 46, 6464. (41) Howell, S. L.; Scott, S. M.; Flood, A. H.; Gordon, K. C. J. Phys. Chem. A 2005, 109, 3745. (42) Bergamo, M.; Beringhelli, T.; D’Alfonso, G.; Mercandelli, P.; Sironi, A. J. Am. Chem. Soc. 2002, 124, 5117. (43) Reed, E.; Curtiss, L. A.; Weinhold, F. Chem. Rev. 1988, 88, 899. (44) (a) Miertus, S.; Scrocco, E.; Tomasi, J. Chem. Phys. 1981, 55, 117. (b) Miertus, S.; Tomasi, J. Chem. Phys. 1982, 65, 239.

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Scheme 1. Forms of the Catalyst from Re(CO)5Br

area of the tesserae of 0.4 A˚2 has been employed in our calculations.45 The value of the dielectric constant (ε=1.49) and density (F=0.817 g/cm3) of the scCO2 solution, which were presented in ref 46, were used to simulate carbon dioxide as solvent.

3. Results and Discussion 3.1. Initial Considerations. To understand the detailed mechanism of chloromethyloxirane with carbon dioxide catalyzed by Re(CO)5Br, insight into possible active forms of the catalyst from Re(CO)5Br is necessary (see Scheme 1). Conceivably, the 16-electron unsaturated complex Re(CO)4Br is an active catalyst species generated through the decarbonylation of Re(CO)5Br. It should be noted that there are two pathways for CO dissociation from Re(CO)5Br, that is, the axial one and the equatorial one (see Figure S1 in the Supporting Information). The bond dissociation energy for the equatorial CO dissociation leading to the C2v singlet state (A1) is 19.0 kcal/mol, while that for the axial CO dissociation leading to the C4v singlet state (A2) is 49.7 kcal/mol. Obviously, the loss of the equatorial CO from Re(CO)5Br is energetically favorable, which is similar to the only favored pathway for the CO dissociation from HCo(CO)4.47 It is noteworthy that the equatorial CO dissociation is much easier than the axial one, which is in line with the Re-C bond length (2.017 vs 1.945 A˚) in Re(CO)5Br. Moreover, the calculated Wiberg bond index of the equatorial Re-C bond in Re(CO)5Br is smaller than the axial one. From the above results, it is concluded that the formation of A1 has precedence over A2, as was reported previously.48 Thus, the C2v singlet state (A1) of Re(CO)4Br is to be considered as the catalytic species. Here, we should mention that the other three possible symmetry structures of the intermediate produced via the equatorial CO dissociation from Re(CO)5Br have been calculated. Our calculations show that optimization of the Cs symmetry structure by keeping its geometry like Re(CO)5Br leads to intermediate A3, and optimization of the other Cs symmetry structure by arrangement of the axial Br ligand also goes to intermediate A3. Note that A3 has almost the same energy as A1. From the geometrical parameters of A3, it can be seen that isomer A3 has essentially the same symmetry as A1. All of our attempts to optimize the C3v structure of Re(CO)4Br failed. Nonconvergence was found for the C3v structure by arrangement of the three equatorial CO ligands. Thus, the existence of the expected C3v isomer during the CO dissociation of Re(CO)5Br can be ruled out. Apart from Re(CO)4Br (A1), the homolytic cleavage of the (CO)5Re-Br bond via thermolysis of Re(CO)5Br, with the (45) Pomelli, C. S.; Tomasi, J. Theor. Chim. Acta 1998, 99, 34. (46) Pomelli, C. S.; Tomasi, J.; Sola, M. Organometallics 1998, 17, 3164. (47) Huo, C.-F.; Li, Y.-W.; Wu, G.-S.; Beller, M.; Jiao, H. Phys. Chem. A 2002, 106, 12161. (48) Lichtenberger, D. L.; Brown, T. L. J. Am. Chem. Soc. 1977, 100, 366.

Figure 1. Relative free energies (kcal/mol) for the optimized isomers of chloromethyloxirane.

generation of bromine radical (B) and rhenium pentacarbonyl fragment (C) (see Scheme 1), needs to be explored. The Re(CO)5 radical, which was first detected by mass spectrometry,49 has been extensively studied both experimentally and theoretically over the past several decades.50-53 In our computations, we found that the Re(CO)5 radical has a square-pyramidal C4v doublet structure (see Figure S1 in the Supporting Information), which is in agreement with the results by Andrews53 and Huber.50 As was expected, the formation of B and C is predicted to be largely endergonic by 62.7 kcal/mol. This relatively high energy requirement indicated that homolysis of the (CO)5Re-Br bond into two radicals (B þ C) difficultly occurs. In other words, the formation of these free radicals is less favorable than that of 16-electron unsaturated complex Re(CO)4Br (A1). Therefore, it is not necessary to examine the reaction pathways promoted by free radicals (B or C). In the following section, two kinds of reaction mechanisms (paths I and II) for the coupling reaction of CO2 and chloromethyloxirane with Re(CO)4Br (A1) as catalyst will be comprehensively investigated. The structures and relative free energies of intermediates and transition states involved in the postulated paths are discussed first, and then the relative free energy profiles are constructed. In light of these results, a comparison of proposed mechanisms is addressed. Considering the orientation of the chlorine atom in the chloromethyl group, three isomers for the chloromethyloxirane are located. As illustrated in Figure 1, the lowest free energy structure is isomer a, which is more stable than b and c by 1.8 and 0.6 kcal/mol, respectively. Apart from the lowest energy, the smallest spatial steric hindrance is another crucial factor for choosing a as the reactant. 3.2. Complete Mechanism I (Path I). Initially, we postulate the path I mechanism (see Scheme 2), which is somewhat in accordance with the proposal by Jiang et al.24 A. Chloromethyloxirane Coordination and Oxidative Addition. In light of the C2v-symmetric conformation of Re(CO)4Br, there are six orientations for chloromethyloxirane coordination via the Oepoxide atom. The Newman projection for the optimized structures of chloromethyloxirane coordination complexes and relative free energies are given in Figure 2. Obviously, complexes 1a and 1f are the most stable precursors, lying 4.5 kcal/mol below the initial reactants in Gibbs free energy. It is interesting to point out that structures 1b and 1c are not suitable for the ring-opening of the epoxide, partly due to the greater steric hindrance of the chloromethyl (49) Junk, G. A.; Svec, H. J. J. Chem. Soc. A 1970, 2102. (50) Huber, H.; Kundig, E. P.; Ozin, G. A. J. Am. Chem. Soc. 1974, 96, 5585. (51) Meckstroth, W. K.; Walters, R. T.; Waltz, W. L.; Wojcicki, A.; Dorfman, L. M. J. Am. Chem. Soc. 1982, 104, 1842. (52) Yang, H.; Snee, P. T.; Kotz, K. T.; Payne, C. K.; Frei, H.; Harris, C. B. J. Am. Chem. Soc. 1999, 121, 9227. (53) Andrews, L.; Zhou, M.; Wang, X.; Bauschlicher, C. W. J. Phys. Chem. A 2000, 104, 8887.

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Guo et al. Scheme 3. Possible Oxidative Additions to Different Rhenium Carbonyl Complexes

Scheme 4. Mechanism II of the Coupling Reaction between Chloromethyloxirane and CO2 Catalyzed by Re(CO)4Br Figure 2. Bond parameters (in A˚) and the Newman projection for the optimized structures involved in the chloromethyloxirane coordination process. The relative free energies (in parentheses) are given in kcal/mol relative to Re(CO)4Br þ chloromethyloxirane. Scheme 2. Mechanism I of the Coupling Reaction between Chloromethyloxirane and CO2 Catalyzed by Re(CO)4Br

and ligand Br. Complexes 1a and 1d are the precursors for the cleavage of the CCH2-O bond, while complexes 1e and 1f are the precursors for the cleavage of the CCHR-O bond. Which precursor favors epoxide oxidative addition? Does the CCH2-O bond dissociate more easily than the CCHR-O bond? All these questions will be discussed in the subsequent section. Jiang and co-workers hinted that the epoxide oxidative addition might occur with 16-electron unsaturated complex Re(CO)4Br.24 In our calculations, four different modes of the oxidative additions of epoxide were taken into account and calculated (Scheme 3). In Scheme 3, the first and second ringopening modes are with respect to the cleavage of the CCH2-O bond, leading to oxametallacyclobutane intermediates 2a and 2b. The latter two modes are correlated with the CCHR-O bond cleavage, resulting in intermediates 2c and 2d. The optimized structures and selected parameters of the

critical points are illustrated in Figure 3, and the related free energy profiles are shown in Figure 4. First, the breaking of the CCH2-O bond by Re(CO)4Br has been examined. One pathway is the transfer of CCH2 nearly to the bromine atom; the other is the shift of CCH2 close to the CCO atom. Taking the most stable complex 1a as the starting point, the four-membered-ring intermediate 2a is formed via the transition state TS(1a/2a). The analysis of the vibrational modes indicates that the imaginary frequency is associated with the CCH2-Oepoxide bond stretching motion. In 2a, the Re-Br bond (2.698 A˚) is slightly elongated by 0.052 A˚, whereas the Br ligand is largely tilted away from the axial site due to the proximity of the CCH2 atom to the bromine atom. With respect to 2a, 2b is found 8.7 kcal/mol lower in Gibbs free energy. We have located an authentic transition state, TS(1d/2b), which connects 1d and 2b. Both 2a and 2b adopt the geometry derived from a pentagonal bipyramid. At this stage, one would raise a query about the Re(CO)4Br attack selectively to the CCH2 atom instead of the CCHR atom. Herein, we have calculated the other oxidative addition form, which is associated with the cleavage of the CCHR-O bond of the epoxide. The oxametallacyclobutane 2c is produced through the transition state TS(1e/2c). In 2c, the CCHR atom is adjacent to the bromine atom. As compared with 2c, intermediate 2d is predicted to be 7.0 kcal/mol lower in Gibbs free energy. The authentic transition state

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Figure 3. Bond parameters (in A˚) of the critical points involved in the ring-opening process of chloromethyloxirane.

Figure 4. Free energy profiles (kcal/mol, enthalpies in parentheses) for the ring-opening process of chloromethyloxirane.

TS(1f/2d), which connects 1f and 2d, has been located. From the energy profiles in Figure 4, process 1a f 2a is calculated to be endergonic by 38.2 kcal/mol with a higher activation barrier of 45.4 kcal/mol; process 1d f 2b is endergonic by

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26.1 kcal/mol with a lower energy barrier of 37.2 kcal/mol; process 1e f 2c is endergonic by 42.2 kcal/mol with a relatively high barrier of 48.3 kcal/mol; process 1f f 2d is endergonic by 35.7 kcal/mol with a higher barrier of 43.4 kcal/mol. It is found that the formation of intermediate 2b is more favored than the other three intermediates. Furthermore, species 2b is the most stable structure among the four different oxametallacyclobutane intermediates (2a, 2b, 2c, and 2d). Thus, the three other oxidative addition channels are predicted to be even more energy demanding and thus are excluded. Here, one should note that the CCHR-O bond is not easily broken by Re(CO)4Br, which is partly due to the larger steric hindrance of the chloromethyl group on the CCHR atom. This tendency is expected to become more obvious with augmentation of the substitutional group on CCHR. It is concluded that the CCH2-Oepoxide bond is more easily activated by the rhenium center of Re(CO)4Br. Therefore, the epoxide oxidative addition (1d f 2b) proceeds with an endothermicity of 12.3 kcal/mol and a moderate activation barrier of 36.1 kcal/mol relative to the initial reactants. It is evident that the Re(I) center has a prominent role in activating epoxides to afford oxametallacyclotutanes. The next step of the catalytic cycle will proceed along intermediate 2b. B. CO Dissociation and CO2 Insertion. It is worth noting that the conformation of intermediate 2b has no vacant site in the axial position of the Re(CO)4Br fragment, which will disfavor the insertion of CO2 into the Re-O bond to form metallacarbonate species. CO dissociation was considered as the key step for the regioselectivity, which is similar to the alkylation process of HCo(CO)3-catalyzed propene hydroformylation investigated by Huo et al.54 The 16-electron unsaturated species 3 is generated via CO dissociation from intermediate 2b. Four structures of 3 have been calculated because there are four nonequivalent CO’s in 2b. Isomer 3a, considered the result of the removal of the equatorial CO ligand close to Oepoxide from 2b, has almost the same energy as 3b, deduced from the loss of the other equatorial CO close to Oepoxide. Removing another equatorial CO from 2b leads to 3c, which is 2.7 kcal/mol higher in energy than 3a. As shown in Figure 5, isomer 3d, which is considered the result of the removal of the axial CO ligand from 2b, is less stable by 4.7 kcal/mol than 3a. From the above-mentioned relative energies, one can exclude the possibility of the formation of 3c and 3d during the CO dissociation of 2b. On account of the small energy difference between 3a and 3b, it is difficult to choose which isomer is preferred in the whole catalytic cycle. Thus, we have tested the reactivity of 3a and 3b, respectively. Our calculations indicate that 3a, which is less stable by 0.9 kcal/mol than 3b, is slightly favored for the reaction to proceed due to a lower free energy barrier. For the sake of space-saving, detailed results for the reaction proceeding from 3b are given in Figures S2 and S3 (see Supporting Information), and the subsequent discussion will consider only the isomer 3a. The structures of the critical points are displayed in Figure 5, and the corresponding free energy profile is represented in Figure 6. Our computations show that the dissociation of CO from 2b leading to 3a costs 6.9 kcal/mol. The vacant coordination site generated might then be occupied by an incoming carbon dioxide. The end-on-coordination carbon dioxide complex 4 was located as the product of CO2 coupled with 3a. The (54) Huo, C.-F.; Li, Y.-W.; Beller, M.; Jiao, H. Organometallics 2003, 22, 4665.

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Figure 5. Optimized structures with selected structural parameters (bond distances in A˚ and bond angles in deg) for the species involved in CO2 insertion.

Figure 6. Free energy profile (kcal/mol, enthalpies in parentheses) for CO2 insertion and cyclic carbonate reductive elimination.

formation of 4 is slightly endergonic, by 3.3 kcal/mol. It is noted that the whole substitution process (2b f 4) is predicted to have an endothermicity of 14.7 kcal/mol. Once complex 4 is formed, the next step should properly correspond to the CO2 insertion. The C-O double bond of CO2 is activated through the transition state TS(4/5). As depicted in Figure 5, the Re-Oepoxide bond distance in TS(4/5) is elongated relative to that in 4 (2.108 vs 1.963 A˚), which implies that the interaction between rhenium and

oxygen atoms begins to weaken. When the Re-Oepoxide bond distance is elongated to 2.168 A˚, the insertion reaction of carbon dioxide occurs to produce species 5. The imaginary vibration mode indicates that the attack of the CCO2 atom on Oepoxide is accompanied by a simultaneous cleavage of the Re-Oepoxide bond. In 5, two C-O bonds of CO2 are elongated (1.265/1.191 vs 1.173/1.166 A˚) and the O-C-O bond angle is 138.4°. It is implied that the carbon dioxide molecule was activated in this step (4 f 5). The energy barrier from 4 to TS(4/5) is 13.1 kcal/mol, which indicates that carbon dioxide insertion proceeds easily. From the geometry of 5, one can see that the OCO2 atom connecting with the Re center remains in the axial position, which is not suitable for the subsequent ring-closure of the cyclic carbonate. Therefore, the following step is the geometry-shrinking process from 5 to 6. The authentic transition state TS(5/6) was located. Its imaginary frequency is 164.0i cm-1, which is associated with the carboxyl rotation. The most significant change in TS(5/6) is the longer distance between Oepoxide and Re (2.857 A˚). Note that the two CCO2-OCO2 bond lengths in 6 are 1.329 and 1.198 A˚, which are 0.064 and 0.007 A˚ longer than those in 5; the OCO2-CCO2-OCO2 bond angle in 6 is decreased to 127.1°. It is indicated that the CCO2 atom undergoes a sp-sp2 hybridization change to delocalize the electron density on the Oepoxide atom during the whole course of CO2 insertion. This isomerization process (5 f 6) is predicted to be endergonic by 7.7 kcal/mol with a low free energy barrier of 13.1 kcal/mol. As illustrated in Figure 5, the six-membered metallacycle intermediate 6 has a conformation

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Figure 7. Bond parameters (in A˚) of the critical points involved in cyclic carbonate elimination along path I.

in which the OCO2 atom deviates from the axial coordination site and faces the CCH2 atom. These interesting geometrical features provide conditions for the later interaction between OCO2 and CCH2 atoms. Interestingly, the profile in Figure 6 shows that intermediate 5 is slightly more stable than the transition state TS(4/5), by only 0.1 kcal/mol. In view of this small energy difference between TS(4/5) and 5, we have searched for the TS between 4 and 6. Unfortunately, all trials to find a TS connecting 4 and 6 gave TS(4/5) or TS(5/6). IRC calculations reveal that TS(4/5) indeed connects 4 with 5. In order to identify the relative stabilities of TS(4/5) and 5, we have performed more reliable single-point energy calculations on them at the CCSD/6-31(d,p) level. The CCSD-calculated Gibbs free energies show species 5 is more stable than TS(4/5) by 1.4 kcal/mol in the gas phase and 2.2 kcal/mol in scCO2 solution. With these results it is safe to say that intermediate 5 represents a very shallow minimum on the potential energy surface. Hereby, the carbon dioxide insertion from 4 to 6 should be considered as a nearly concerted process via two consecutive transition states, TS(4/5) and TS(5/6). From Figure 6, one can find that the whole CO2 insertion step (2b f 6) is computed to be an endergonic process under the stoichiometric conditions. Therefore, optimization of CO2 concentration or partial pressure is essential for accelerating the reaction. C. Cyclic Carbonate Elimination and Catalyst Regeneration. Following the formation of unstable metallacarbonate 6, the elimination reaction occurs via the rotational attack of the CCH2 atom on the nearby OCO2 atom. The optimized structures are presented in Figure 7. It is shown that this process proceeds via the three-center transition state TS(6/7) to the cyclic carbonate adduct 7. The analysis of the imaginary vibrational mode indicates that the transition state TS(6/7) can produce a five-membered-ring geometry. The IRC calculation shows that the eletrophiclic attack of the CCH2 atom from Re(CO)3Br to the OCO2 atom leads to the cleavage of the Re-O and Re-C bonds and the formation of the OCO2-CCH2 bond. In TS(6/7), the Re-O and Re-C bond distances are 2.060 and 2.687 A˚, which are 0.068 and 0.396 A˚ longer than those in 6, respectively; while the OCO2-CCH2 bond distance is decreased to 2.102 A˚. In adduct 7, one can see the cyclic carbonate is attached to the Re(CO)3Br

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fragment through the OCO2 atom in a weak electrostatic interaction (the distance between Re and OCO2 is 2.355 A˚). As depicted in Figure 6, the cyclic carbonate elimination process (6 f 7) is highly exergonic, by 39.8 kcal/mol, with a low free energy barrier of 13.8 kcal/mol. The released energy from this process can compensate for energy consumption in the epoxide oxidative addition and CO2 insertion. From the kinetic point of view, the reductive elimination of cyclic carbonate proceeds very easily compared with the ring-opening of epoxide and CO2 insertion. It is noteworthy that species 7 is a coordinatively unsaturated complex. To recover the catalyst, the CO coordination reaction occurs via the transition state TS(7/8). Its imaginary frequency is 48.9i cm-1; the corresponding imaginary vibrational mode indicates that TS(7/8) can regenerate the catalyst Re(CO)4Br. Note that this CO coordination process is calculated to be moderately exoergic, by 18.9 kcal/mol, with a very low energy barrier of 7.2 kcal/mol. It is implied that the regeneration of catalyst Re(CO)4Br takes place easily. In 8, the product molecule (4-chloromethyl-[1,3]dioxolan-2-one) is coordinated to the Re atom via the OCO2 atom in a very weak electrostatic interaction (the distance between Re and OCO2 is 2.372 A˚). 3.3. Alternative Reaction Mechanism (Path II). In contrast with the path I mechanism discussed above, there exists another possible mechanism, with the coordination/activation of CO2 as the first step and the second step is the ring-opening of epoxide as well as ring-closure of cyclic carbonate. According to this idea, we have examined path II for the title coupling reaction. A. Carbon Dioxide Activation. The first step in path II is associated with the CO2 activation by Re(CO)4Br. The optimized structures and relative free energy profile are displayed in Figures 8 and 9, respectively. The nucleophilic attack of the OCO2 atom on the Re(I) center induced by the nucleophilic attack of Br- on the CCO2 atom results in the activation of CO2. This scene is very clear by observing the vivid transition vectors corresponding to the imaginary frequency of TS(9/10) (70.9i cm-1). IRC calculations reveal that TS(9/10) is indeed associated with complex 10, in which the mode of CO2 coordination is very similar to the μ2-η2 complex CpFe(CO)(PPh3)(CO2)Re(CO)4(PPh3).55 The formation of intermediate 10 is endergonic by 37.5 kcal/mol, requiring surpassing a higher energy barrier of 40.7 kcal/mol relative to the initial reactants. These data indicate that this step is less favored than the activation process of the epoxide (1d f 2b). Our calculations show that the unstable μ2-η2 complex 10 can isomerize into the μ2-η3 species 11 easily. This tautomerization process (10 f 11) is calculated to be exoergic by 11.7 kcal/mol, and the corresponding activation energy barrier is only 3.5 kcal/mol. Note that the CO2 activation product 11 is coordinatively saturated. To continue with the next step of the catalytic cycle, the CO dissociation from 11 to 12 should proceed with an endothermicity of 35.5 kcal/mol. From Figure 8, one can see that 16e species 12 has an empty metal orbital, which is available for epoxide coordination. B. Chloromethyloxirane Ring-Opening and Cyclic Carbonate Elimination. The optimized structures and the relative energy profile are given in Figures 10 and 11, respectively. The coordination of chloromethyloxirane through σ-donation (55) Gibson, D. H.; Ye, M.; Richardson, J. F. J. Am. Chem. Soc. 1992, 114, 9716.

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Figure 8. Optimized structures with selected structural parameters (bond distances in A˚ and bond angles in deg) for the species involved in the process of CO2 activation.

Figure 9. Free energy profile (kcal/mol, enthalpies in parentheses) for the process of CO2 activation.

of the lone pair of Oepoxide to the Re center of 12 forms complex 13. Species 13 was assumed to act as a precursor for the ringopening of chloromethyloxirane. We have located one authentic transition state, TS(13/14). The imaginary vibration mode indicates that the torsion of the CH2 group of the epoxide to the OCO2 atom leads to the cleavage of the CCH2-Oepoxide bond and the formation of the CCH2-OCO2 bond. The activation energy barrier from 13 to TS(13/14) is 36.8 kcal/mol. The ringopening of chloromethyloxirane is predicted to be endothermic by 10.5 kcal/mol. Subsequently, the ring-closure of the cyclic carbonate takes place through the transition state TS(14/15), which lies only 4.3 kcal/mol higher than the seven-membered intermediate 14. This step is thermodynamically downhill because the cyclic carbonate coordinated complex 15 is predicted to be more stable than 14 by 3.0 kcal/mol. Then, the 15 f 16 rearrangement, involving the shift of the Br atom to the vacant coordination site of the Re center, may occur via the transition state TS(15/16). Our calculations predict this transformation (15 f 16) is exoergic by 39.3 kcal/mol, with a small energy barrier of 5.4 kcal/mol. Concerning the regeneration of the catalyst and the reductive elimination

of the product (4-chloromethyl-[1,3]dioxolan-2-one), the CO coordination into the fragment Re(CO)3Br of 16 was investigated. We tried to locate the transition state for this CO coordination step many times, but we failed. The adduct 17 is located as the product complex, which lies 10.2 kcal/mol below the initial reactants in Gibbs free energy. Although the energy barrier of the whole reductive elimination of cyclic carbonate is quite low, the reaction would not proceed via this path due to little probability of the formation of 11. 3.4. Comparison of Path I and Path II Mechanisms. The above calculations are conducted in the gas phase. To examine the solvent effects of the scCO2 solution, we have performed SCRF calculations on the species involved in paths I and II. The detailed results for the solvent effects can be found in Table S1 in the Supporting Information. The calculated solvation energies were incorporated in the relative energies in the gas phase, and then the relative energies in scCO2 solution were obtained. According to the results, the solvation-corrected relative free energy profiles for paths I and II are constructed and displayed in Figure 12. A direct comparison of the energies for path I and path II reveals some interesting insights into the whole catalytic cycle. As we can see, path II is a relatively high energy pathway in contrast with path I, and most of the intermediates involved in path II are more variable than that involved in path I. For the first step of the catalytic cycle, the epoxide oxidative addition catalyzed by Re(CO)4Br was found to be obviously favored over CO2 activation in scCO2, and intermediate 2b is calculated to be more stable than 11 by 4.2 kcal/mol. Note that the energy required for the CO dissociation from 2b to 3a is 12.1 kcal/mol, while the CO dissociation energy from 11 to 12 is 27.5 kcal/mol. It is indicated that the CO dissociation in path I is more favored for subsequent reaction. Furthermore, the activation energy barrier for the ringopening of the epoxide in path II is in the vicinity of 16 kcal/mol higher than that for the CO2 insertion in path I

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Figure 10. Optimized structures with selected structural parameters (bond distances in A˚ and bond angles in deg) for the species involved in the ring-opening of chloromethyloxirane and ring-closure of cyclic carbonate along path II.

Figure 11. Free energy profile (kcal/mol, enthalpies in parentheses) for the ring-opening of chloromethyloxirane and cyclic carbonate reductive elimination along path II.

(35.4 vs 19.6 kcal/mol). This conclusion is in agreement with our previously theoretical study56 of a similar reaction system showing that the first two stages of the whole catalytic reaction along path II are not accessible kinetically. As for (56) Guo, C.-H.; Zhang, X.-M.; Jia, J.-F.; Wu, H.-S. J. Mol. Struct. (THEOCHEM) 2009, 916, 125.

the reductive elimination of cyclic carbonate, path II via 14 f 15 f 16 looks more complicated and intermediates involved are not stable. These facts demonstrate that path II promoted by Re(CO)4Br is not favored. It is clearly seen that path I is favorable both thermodynamically and kinetically. In the epoxide oxidative addition step, the activation free energies are 37.2 (in vacuum) and 36.2 (in scCO2) kcal/mol. For the carbon dioxide insertion, which is a multistep process, the activation energies from 4 to 6 are 26.1 (in vacuum) and 19.6 (in scCO2) kcal/ mol. In the cyclic carbonate elimination step, the activation free energies are 13.8 (in vacuum) and 15.7 (in scCO2) kcal/ mol. As for the regeneration of the catalyst step, the activation free energies are 7.2 (in vacuum) and 6.4 (in scCO2) kcal/ mol. On the basis of these data, we can see that the energy barrier of the CO2 insertion process (4 f 6) is decreased largely in scCO2, but for other steps, the energy barriers change a little unexpectedly. These data imply that carbon dioxide is activated very well in scCO2 and the scCO2 solution accelerates the CO2 insertion in the Re-O bond in 3a. However, we noted that the scCO2 solution has a minor effect on the thermodynamics of other steps. Therefore, the high reaction yield should be attributed to factors such as the rapid diffusion, the weak catalyst solvation, the high miscibility of chloromethyloxirane, and especially the high

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Figure 12. Solvation-corrected relative free energy profiles (in kcal/mol) for the entire catalytic cycle of the coupling reaction between CO2 and chloromethyloxirane catalyzed by Re(CO)4Br in scCO2 solution (dashed black line, path I; dashed green line, path II).

concentration of CO2 molecules. Noteworthily, compared with the energy barrier of 66.9 kcal/mol in the uncatalyzed coupling reaction (see Figure S4 in the Supporting Information), each step in path I has a moderate barrier that is accessibly overcome under experimental condition (high temperature and pressure and in the scCO2 solution). It is calculated that the former steps (1d f 2b and 3a f 6) are two endothermic processes, while the latter steps (6 f 7 and 7 f 8) are two exothermic processes with relatively low energy barriers in scCO2. It is worth noting that the effective barrier for the conventional mechanism (path I) seems quite high and not much lower than the alternative mechanism (path II). This may be because the B3LYP method overestimates the energies of transition states. On one hand, since this reaction was carried out in supercritical carbon dioxide solvent, in which the translation and rotation movements are considerably suppressed, compared to those in an ideal gas, the decrease in entropy and the thermal energy changes of the solution reaction are overestimated when two molecules form an adduct. Thus, the calculated activation free energies of epoxide oxidative addition and carbon dioxide insertion are both larger than the true value. The above reason for overvalue is very similar to the recent theoretical study by Sakaki et al. examining the Ru-catalyzed hydrogenation of carbon dioxide to formic acid.57 On the other hand, the DFT-B3LYP methodology gives overestimations of the barrier heights for some transition metal reaction systems.58 As for step 4 f 5, the CCSD-calculated barriers for the CCO2-Oepoxide bond formation process are 11.2 kcal/mol in vacuum and 5.9 kcal/mol in scCO2, indicating that the

(57) Ohnishi, Y.; Nakao, Y.; Sato, H.; Sakaki, S. Organometallics 2006, 25, 3352. (58) P apai, I.; Schubert, G.; Mayer, I.; Gabor, B.; Michele, A. Organometallics 2004, 23, 5252.

B3LYP energy barrier represented by the transition state is overestimated.

4. Conclusions At the B3LYP density functional and the LANL2DZ: 6-31G(d,p) compound basis set (consisting of the LANL2DZ basis set for Re atom and 6-31G(d,p) basis set for the remaining atoms), the entire catalytic cycle of Re(CO)4Brcatalyzed cycloaddition of CO2 and chloromethyloxirane has been investigated. All species involved in the catalytic cycle have been fully characterized to be energy minimum structures for the intermediates or saddle point structures for the transition states. The solvent effect of scCO2 has been considered by using the PCM model. By comparing the energetics of paths I and II, one can see that path I is a feasible reaction route for the formation of cyclic carbonate. As discussed above, the preferred path involves three major elementary steps: (i) epoxide coordination and ring-opening, (ii) CO2 insertion, and (iii) cyclic carbonate elimination with catalyst regeneration. First, the chloromethyloxirane is activated by the Re(I) center, leading to the unstable oxametallacyclobutane 2b. From 2b, CO dissociation requiring an energy of 12.1 kcal/mol takes place in scCO2. The 16e unsaturated species 3a is a crucial intermediate along the reaction path because it facilitates the formation of metallacyclic intermediate 6 in the CO2 insertion process. Then, the reductive elimination of cyclic carbonate occurs easily via the three-center transition-state structure with an exoergicity of 35.9 kcal/mol. The overall reaction is both exothermic and exoergic. On the basis of the constructed catalytic cycle in Scheme 2, it is very interesting to determine the rate-determining step of the entire reaction. The ring-opening of epoxide from 1d to 2b and the CO2 multistep insertion (including CO dissociation) from 2b to 6 have close activation free energies (36.2 vs 34.0 kcal/mol). Therefore, each of them can be the rate-determining step

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with variation in the reaction conditions (temperature and pressure). Noticeably, most of the activation energies along path I are larger than 30 kcal/mol, which is in agreement with the fact that high temperature is required to generate a sufficient concentration of 2b and 6. Our study indicates that the mechanism of the title coupling reaction in the presence of Re(CO)5Br has been modified and the barrier to be surmounted has been reduced remarkably. It is found that the reactivity of catalyst Re(CO)5Br is attributable to species Re(CO)4Br possessing electrophilicity high enough to ring-open the epoxide. Finally, the success of the complete cycle remains dependent on the success of the epoxide oxidative addition and CO2 insertion since all obtained energy profiles for both processes are endothermic. The present theoretical study provides a comprehensive mechanism for the cycloaddition of CO2 and chloromethyloxirane catalyzed by Re(CO)5Br. Also, some new points

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have been clarified. (1) The high concentration of the CO2 molecule and high temperature are helpful for the formation of metallacarbonate intermediate 6. (2) The key intermediates in the whole catalytic cycle are predicted to be labile, so no reaction intermediate can be observed or captured by experimental methods to date.

Acknowledgment. This work was supported by the Natural Science Foundations of China (20871077). Supporting Information Available: Figures S1-S4, table with total electronic energies and zero-point energies as well as thermal corrections to enthalpies and Gibbs free energies (383.15 K, 60 atm), and the optimized Cartesian coordinates for all species presented in the catalytic cycle. This material is available free of charge via the Internet at http:// pubs.acs.org.