Mechanism of the MeReO3-Catalyzed Deoxygenation of Epoxides

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Mechanism of the MeReO3‑Catalyzed Deoxygenation of Epoxides Siwei Bi,*,† Jiayong Wang,† Lingjun Liu,† Ping Li,† and Zhenyang Lin*,‡ †

School of Chemistry and Chemical Engineering, Qufu Normal University, Qufu 273165, Shandong Province, People’s Republic of China ‡ Department of Chemistry, The Hong Kong University of Science and Technology, Clear Water Bay, Kowloon, Hong Kong, People’s Republic of China S Supporting Information *

ABSTRACT: The reaction mechanisms for the MTO-catalyzed deoxygenation of epoxides and diols were investigated with the aid of density functional theory (DFT) calculations. The DFT results indicate that the reaction starts with a [2σ+2π] addition of epoxide to MTO to give a five-membered-ring rhena-2,5-dioxolane intermediate, followed by H2 addition, proton transfer, and extrusion of olefin to regenerate the catalyst. The experimental observation for formation and subsequent disappearance of diol appearing in the catalytic reaction is explained as follows. Diol was produced by the hydrolysis of epoxide with the coproduct water through the five-membered-ring rhena-2,5-dioxolane intermediate. Then the diol produced undergoes catalytic conversion to olefin by reacting with H2 under the catalytic conditions.

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INTRODUCTION Recently, there has been considerable interest in developing new and efficient processes for conversion of abundant renewable resources derived from biomass to useful chemicals.1 Diols and polyols are typical resources derived from biomass and are of significant importance for generating synthetically versatile unsaturated hydrocarbon products. In the conversion of diols or polyols to unsaturated hydrocarbons such as olefins, deoxygenation is the key issue. The reverse processes of the conversion, for example, epoxidation or dihydroxylation of olefins, have been well documented.2 However, studies of deoxygenation of epoxides and diols are limited.3 Stepwise deoxydehydration (DODH) reactions by various reagents converting vicinal diols to olefins were studied in 1980s.4 In 1996, Cook and Andrews reported catalytic DODH of diols using Cp*ReO3 as the catalyst and PPh3 as the reductant.5 Later, Gable and his co-workers developed more robust rhenium tris(pyrazolyl)borate catalysts for these DODH reactions.6 In 2009, Bergman and co-workers reported formic acid-mediated deoxygenation reactions of polyols to olefins.7 Also in 2009, Abu-Omar and co-workers reported MeReO3catalyzed deoxygenation of epoxides and diols shown in eq 1 using H2 as the reductant.8 In 2010, Nicholas and co-workers reported MeReO3-catalyzed DODH of glycols by sulfite.9

versatile catalyst for a wide variety of oxygen transfer reactions, due to its ability to donate an oxygen atom to various substrates.10 Molecular dihydrogen is an attractive reductant for polyol DODH reactions because of generation of environmentally friendly water. According to the proposed reaction mechanism shown in Scheme 1,8 MTO is first reduced by Scheme 1. Mechanism Postulated by Abu-Omar and Coworkers

dihydrogen followed by dissociation of a water molecule to afford methyldioxorhenium (MDO). Coordination of an epoxide molecule to MDO followed by ring-opening of the epoxide gives a four-membered-ring metallacyclic intermediate. Finally, olefin extrusion and regeneration of the catalyst MTO complete the catalytic cycle. For theoretical simplicity, we employed the reaction shown in eq 2 to theoretically study the deoxygenation mechanism with the aid of density functional theory calculations. To the best of our knowledge, there are no early reports of theoretical studies on deoxygenation of

In this work, we are interested in the MeReO3-catalyzed deoxygenation reactions of epoxides and diols reported by AbuOmar and co-workers. MeReO3 (abbreviated as MTO) is a © XXXX American Chemical Society

Received: June 1, 2012

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dx.doi.org/10.1021/om300485w | Organometallics XXXX, XXX, XXX−XXX

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epoxides (or DODH of diols) to alkenes. A new alternative mechanism will be proposed on the basis of our density functional theory (DFT) calculations.

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Table 1. Relative Enthalpies (kcal/mol) of Reactants, Transition States, and Products Calculated for the [2+3] Addition of H2 to Various Metal Oxides

COMPUTATIONAL DETAILS

Molecular geometries of all the complexes were optimized without constraints via DFT calculations using the Becke3LYP (B3LYP)11 functional. Frequency calculations at the same level of theory have also been performed to identify all stationary points as minima (zero imaginary frequencies) or transition states (one imaginary frequency) and to provide enthalpic energies at 423.15 K and 20 atm (the experimental conditions). Intrinsic reaction coordinates12 were calculated for the transition states to confirm that such structures indeed connect two relevant minima. The 6-31G(d,p) basis set13 was used for C, H, and O atoms, while the quasi-relativistic pseudopotentials of Hay and Wadt with a double-valence basis set (LanL2DZ)14 were used to describe the transition metal atoms.15 To examine how the results of calculations are affected by using other pseudopotentials, we employed the Stuttgart−Dresden−Bonn relativistic effective core potentials, SDDALL, to obtain the energetics associated with the [2+2] and [2+3] addition pathways of MTO + H2. The results show that both the LanL2DZ and SDDALL pseudopotentials give similar results. In the [2+2] and [2+3] additions of MTO + H2, LanL2DZ gives activation enthalpies of 59.2 and 44.4 kcal/mol and reaction enthalpies of 14.9 and 7.1 kcal/mol, respectively. SDDALL gives activation enthalpies of 61.3 and 43.5 kcal/mol and reaction enthalpies of 12.0 and 7.3 kcal/mol, respectively. All calculations were performed with the Gaussian 03 software package.16 This methodology has proven successful for the evaluation of potential energy surfaces in the reactions of MnO4− + H2,17a OsO4 + H2,17b OsO4 + C2H4,18 CrCl2O2 + C2H4,19 and the MTO-catalyzed transformation of hydrotrioxides (ROOOH) to dihydrogen trioxide (HOOOH).20 The gas-phase enthalpic energies are used to evaluate the potential energy surfaces presented in this work.17−20

reaction

reactants

transition states

products

MnO4− + H2 TcO4− + H2 ReO4− + H2

0.0 0.0 0.0 0.0 0.0

11.7 27.6 38.4 8.6 19.4

−47.7 −5.6 18.9 −60.9 −27.8

RuO4 + H2 OsO4 + H2

Figure 1. Enthalpic energy profiles calculated for the [2+2] and [3+2] additions of MTO to H2. The relative enthalpic energies are given in kcal/mol.

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RESULTS AND DISCUSSION Addition of H2 to MTO. As postulated by Abu-Omar and co-workers (Scheme 1), the first step of the MTO-catalyzed Scheme 2

deoxygenation of epoxides is the reduction of MTO with dihydrogen. We first calculated the barriers for the addition of H2 to various MO4− (M = Mn, Tc, and Re) and MO4 (M = Ru and Os). It should be pointed out here that the results of these calculations are for the purpose of comparison and identifying the periodic trend. Early studies showed that these addition reactions proceed via a [2+3] mechanism rather than a [2+2] one.17 Therefore, only the [2 +3] pathway was examined (Scheme 2). The relative enthalpies of reactants, transition states, and products are listed in Table 1. The enthalpic data for

Figure 2. Enthalpic energy profiles calculated for the [2+3] additions of H2 to MTO assisted by epoxide (a) and THF (b). The relative enthalpic energies are given in kcal/mol. B


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Figure 3. Enthalpic energy profiles for H2 addition to both MTO and the coordinated epoxide. The relative enthalpic energies are given in kcal/mol.

Figure 5. Enthalpic energy profiles for [2+3] addition of H2 to the intermediate 2. The relative enthalpic energies are given in kcal/mol.

reactions were observed between third-row transition metal oxo compounds, such as OsO4 and ReO4−, and H2 at 25 °C.17a Since there are no previous theoretical calculations reported on the addition of H2 to MTO, we here provide our calculation results on the [2+2] and [2+3] additions for the reaction. The enthalpic energy profiles are shown in Figure 1. Consistent with what was found for the addition of H2 to other transition metal oxides, the [2+3] mechanism is preferred kinetically and thermodynamically over the [2+2] mechanism (Figure 1). The addition of H2 to MTO via the [2+3] mechanism is also found to be endothermic, and the barrier is calculated to be inaccessibly high (43.5 kcal/mol), consistent with the notion that a Re(VII) center is not easily reduced by H2 with a ligand environment having oxo and alkoxy ligands. It was well established that addition of olefins to OsO4 occurs via a [2+3] mechanism21 and that such addition is accelerated by ligation of tertiary amines to the osmium in the form of OsO4(L)n.21,22 In 2005, the Mayer group found that an additional ligand such as OH− reduces the [2+3] addition barrier for the reaction of osmium tetroxide with molecular dihydrogen.17b With these early results, we also carried out

Figure 4. Enthalpic energy profiles calculated for [2σ+2π] addition of a coordinated epoxide substrate molecule to MTO. The relative enthalpic energies are given in kcal/mol.

the addition reactions of H2 to MnO4− and H2 to OsO4 were taken from the literature.17 The results given in Table 1 are well consistent with the well-known periodic trend that the oxidizing ability of transition metals decreases down a group and increases on moving from left to right across a row of the periodic table. Interestingly, the calculated results show that the addition of H2 to ReO4− is both kinetically and thermodynamically unfavorable. This result indicates that the rhenium metal center prefers to remain in the highest oxidation state in the ligand environment present. Indeed, experimental observations indicated that the addition reactions of H2 to MnO4− and RuO4 occur rapidly at 25 °C [k1 ≈ (3−6) × 10−2 M−1 s−1], while no C


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Figure 6. Enthalpic energy profiles for formation of a water molecule in the intermediate 4 through proton transfer. The relative enthalpic energies are given in kcal/mol.

calculations on the reaction of MTO + H2 by introducing epoxide (substrate) or THF (solvent) as a ligand (Figure 2). The reaction barriers still remain inaccessibly high (Figure 2), indicating that both epoxide and THF do not efficiently accelerate the addition reaction of H2 to MTO. Addition of H2 simultaneously to MTO and the coordinated epoxide was also examined (Figure 3). Two isomeric H2 addition products are afforded based on different H2 addition modes. Very high reaction barriers (70.1 and 58.2 kcal/mol, respectively) were obtained (Figure 3), suggesting that such H2 addition modes are also not feasible. In conclusion, direct addition of H2 to MTO is found to be very unfavorable kinetically, even with the involvement of epoxide or solvent. Clearly, mechanisms different from the one given in Scheme 1 are expected for the MTO-catalyzed deoxygenation of epoxides. Alternative Mechanism for the MTO-Catalyzed Deoxygenation of Epoxides. The results discussed above suggest that direct addition of H2 to MTO to initiate the catalytic reaction is not feasible in view of the inaccessible barriers calculated. It is therefore reasonable to consider that reaction between MTO and a coordinated epoxide substrate molecule starts the catalytic reaction. [2σ+2π] Addition of Epoxide to MTO. Figure 4 shows two pathways calculated for the [2σ+2π] addition of a coordinated epoxide to MTO, depending on which C−O bond of the epoxide, H2C−O or PhHC−O, is involved in the [2σ+2π] addition. The calculation results indicate that the [2σ+2π] pathway (1 to 2) via TS1−2 involving the PhHC−O bond is kinetically much more favorable than that (1 to 2′) via TS1−2′ involving the H2C−O bond, a result of the weaker bond of PhHC−O versus H2C−O in the epoxide substrate molecule. Related experimental observations of [2σ+2π] addition of a coordinated epoxide to MTO have been reported.23 Interestingly, the [2σ+2π] addition products 2 and 2′ are

Figure 7. Enthalpic energy profiles for styrene extrusion via a concerted mechanism. (a) H2O remains coordinated in the transition state. (b) H2O dissociates prior to styrene extrusion. The relative enthalpic energies are given in kcal/mol.

D


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Figure 8. Enthalpic energy profiles for styrene extrusion via a stepwise mechanism. (a) H2O remains coordinated in the transition state. (b) H2O dissociates prior to styrene extrusion. The relative enthalpic energies are given in kcal/mol.

Scheme 3

change the oxidation state of the rhenium metal center and, therefore, gives lower addition barriers. These results further support the claim given above that the rhenium metal center

thermodynamically more stable than the epoxide-coordinated MTO. Unlike the [2+3] addition of H2 to MTO or the epoxide-coordinated MTO, the [2σ+2π] addition does not E


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Figure 9. Enthalpic energy profiles calculated for two possible pathways of MTO-catalyzed hydrolysis of epoxide. The relative enthalpic energies are given in kcal/mol.

most favorable. This is due to the fact that the ORe− O(axial) structural moiety has a small angle and is optimal for the H2 addition. Formation of H2O. The most favorable pathway for the addition of H2 to the intermediate 2 gives the intermediate 4, in which there are two OH groups next to each other. It is worth noting that 4 is more stable than 2 + H2, which is in contrast to the notion mentioned above that the rhenium metal center prefers to remain in the highest oxidation state in the ligand environment present. The addition here breaks one π bond of one ReO unit and changes a Re−O σ bond to a Re←O dative bond. However, the addition of H2 to MTO breaks two π bonds involving two ReO units. Clearly, the results here suggest that once the diolate is formed, H2 addition is facilitated. From 4, a coordinated water molecule can be generated by proton transfer from one hydroxyl oxygen to the other. Two possible pathways exist. One is the proton transfer from the coordinated OH group in the axial position to the equatorial OH ligand (path A, Figure 6), and the other corresponds to the proton transfer from the equatorial OH ligand to the coordinated OH group in the axial position (path B, Figure 6).

prefers to remain in the highest oxidation state in the ligand environment present. Addition of H2 to Intermediate 2. As discussed above, [2σ+2π] addition of the coordinated epoxide substrate molecule to MTO initiates the catalytic reaction and gives the intermediate 2 in the most favorable pathway. The intermediate 2 adopts a trigonal-bipyramidal structure in which the two oxo ligands are lying on the equatorial plane and the methyl ligand occupies one of the axial positions. The step immediately following is expected to be the addition of H2 to the intermediate 2. In the intermediate 2, the oxygen atoms are environmentally different. Three possible [2+3] addition pathways were calculated. Figure 5a shows the energy profile calculated for the pathway in which H2 is added to the intermediate 2 across a ORe−O(axial) moiety. Figure 5b shows the profile calculated for the pathway in which H2 is added to the intermediate 2 across the ORe−O(equatorial) moiety. Figure 5c shows the profile calculated for the pathway in which H2 is added to the intermediate 2 across the ORe O moiety. The results show that the pathway in which H2 is added to the intermediate 2 across a ORe−O(axial) moiety is the F


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The enthalpic energy profiles for two possible pathways of the MTO-catalyzed hydrolysis are illustrated in Figure 9. In Figure 9a, from the intermediate 2, a water adduct (I) was formed through hydrogen bonding. Then, the two protons from the water molecule sequentially migrate to the diolate ligand in the intermediate 2. The overall barrier for the MTOcatalyzed hydrolysis is 25.6 kcal/mol. In Figure 9b, the sequence of proton migration is switched. The overall barrier is 24.7 kcal/mol, comparable to the barrier calculated for the pathway shown in Figure 9a. Conversion of Diol to Olefin. Experimentally, it was also found that styrenediol formed from the catalyzed hydrolysis of epoxide was converted to styrene. The calculation results in Figure 9 show that I and III, as well as I′ and IV′, have similar stability, indicating that I and III, as well as I′ and IV′, are in fast equilibrium because the overall barriers are smaller than the barrier of the rate-determining step (Figure 5a; 3 to TS3−4: 34.1 kcal/mol) of the whole catalytic reaction. Experiments confirmed that when styrenediol, instead of epoxide, was used as a substrate, the catalytic reaction with molecular dihydrogen also produced styrene. Experimental studies also showed that MTO reacted with diols to give metalla-2,5-dioxolanes,24 consistent with our proposed catalytic cycle (Scheme 3). In summary, our calculations show that the overall activation barrier is 34.1 kcal/mol. This barrier is significantly high, explaining the experimental observation that this reaction does not occur under ambient pressure and temperature. Figure 10 presents the lowest complete enthalpic energy profiles calculated for the deoxygenation of epoxides. Scheme 4 summarizes what we found regarding formation of diol and its further reaction with H2 under the experimental catalytic conditions. Comments on a Related Experimental Observation. Experimentally, it was observed that under the catalytic conditions the catalyst MTO changes color from colorless to red. On the basis of this experimental observation, MDO was believed to be formed and proposed as a species present in the catalytic cycle (Scheme 1). However, our proposed mechanism does not involve the formation of MDO because our calculations indicate that the barrier calculated for the formation of MDO by reduction of MTO with H2 was substantially high (43.5 kcal/mol). We believe that the redcolored species does not correspond to MDO. Instead, the red species likely corresponds to the d2 species 5 appearing in our proposed catalytic cycle. The calculation results (Figures 6 and 10) show that complexes 6 and 5 are in fast equilibrium, with complex 5 being dominant. It should also be pointed out that in a paper published in 200025 MDO was considered as a colorless species, further suggesting that MDO is not the colored species observed experimentally.

Figure 10. Complete lowest enthalpic energy profiles calculated for the deoxygenation of epoxides, giving water and olefin. The relative enthalpic energies are given in kcal/mol.

Scheme 4

One can see from Figure 6 that path A is both kinetically and thermodynamically more favorable than path B. TS5′‑6′ is significantly higher in energy than TS5−6. In the step 5′ → 6′, a drastic structural (bonding) change occurs. However, in the step 5 → 6, the bonding interactions between the metal center and the ligands change only slightly. Complex 5 is markedly more stable than complex 4 by 8.1 kcal/mol because the former contains hydrogen bonding while the latter does not. Extrusion of the Product Styrene. The product styrene is produced from the five-membered-ring metallacyclic intemediate 6, accompanied by regeneration of the catalyst MTO. Figure 7 shows the direct extrusion of the product. We can see that the pathway when H2O remains coordinated is more favorable than that when H2O dissociates prior to the styrene extrusion. We also studied an alternative mechanism via the formation of a four-membered-ring intermediate (Figure 8). Clearly, the energy calculations indicate that this stepwise alternative mechanism is not favorable. Scheme 3 summarizes the favorable catalytic cycle for the catalytic reaction. Formation of Styrenediol. Experimentally, it was found that styrenediol was formed at the beginning and subsequently disappeared at the later stage of the catalytic reaction. It is a reasonable assumption that styrenediol was produced via hydrolysis of epoxide with the coproduct water. The activation barrier for hydrolysis of epoxide was calculated to be 42.1 kcal/ mol in the absence of the catalyst MTO, indicating that formation of styrenediol is not feasible without the involvement of MTO.

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CONCLUSIONS In 2009, Abu-Omar and co-workers reported MeReO3catalyzed deoxygenation of epoxides and diols and proposed a mechanism starting with addition of H2 to MTO. Our calculations showed that addition of H2 to MTO via the [2+3] mechanism is significantly endothermic and the barrier is inaccessibly high. In this work, we proposed an alternative mechanism with the aid of DFT calculations. In the alternative mechanism (Scheme 3), MTO and epoxide react first to give a five-membered-ring rhenium diolate intermediate (2). To the intermediate, addition of H2 via a [2+3] mechanism gives an G


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Juliette, J. J. J. Am. Chem. Soc. 1996, 118, 2625. (d) Gable, K. P.; Zhuravlev, F. A. J. Am. Chem. Soc. 2002, 124, 3970. (e) Gable, K. P.; Abu-Baker, A.; Zientara, K.; Wainwright, A. M. Organometallics 1999, 18, 173. (f) Gable, K. P.; Ross, B. ACS Symp. Ser. 2006, 921 (Feedstocks for the Future), 143. (7) Arceo, E.; Marsden, P.; Bergman, R. G.; Ellman, J. A. Chem. Commun. 2009, 3357. (8) Ziegler, J. E.; Zdilla, M. J.; Evans, A. J.; Abu-Omar, M. M. Inorg. Chem. 2009, 48, 9998. (9) Vkuturi, S.; Chapman, G.; Ahmad, I.; Nicholas, K. M. Inorg. Chem. 2010, 49, 4744. (10) (a) Stankovic, S.; Espenson, J. H. J. Org. Chem. 2000, 65, 5528. (b) Adam, W.; Saha-Möller, C. R.; Weichold, O. J. Org. Chem. 2000, 65, 2897. (c) Wang, Y.; Espenson, J. H. J. Org. Chem. 2000, 65, 104. (d) Saladino, R.; Neri, V.; Cardona, F.; Goti, A. Adv. Synth. Catal. 2004, 346, 639. (e) Santos, A. M.; Pedro, F. M.; Yogalekar, A. A.; Lucas, I. S.; Romão, C. C.; Kühn, F. E. Chem.Eur. J. 2004, 10, 6313. (f) Zauche, T. H.; Espenson, J. H. Inorg. Chem. 1998, 37, 6827. (g) Owens, G. S.; Arias, J.; Abu-Omar, M. M. Catal. Today 2000, 55, 317. (h) Kaple, K. P. In Advances in Organometallic Chemistry; Academic Press Inc.: San Diego, CA, 1997; Vol. 41, p 127. (i) Herrmann, W. A.; Kühn, F. E. Acc. Chem. Res. 1997, 30, 169. (j) Kühn, F. E.; Herrmann, W. A. In Structure and Bonding; Meunier, B., Ed.; Springer-Verlag: Heidelberg, 2000; Vol. 97, p 213. (k) Mathew, T. M.; du Plessis, A. J. K.; Prinsloo, J. J. J. Mol. Catal. A 1999, 148, 157. (l) Kühn, F. E.; Scherbaum, A.; Herrmann, W. A. J. Organomet. Chem. 2004, 689, 4149. (11) (a) Becke, A. D. J. Chem. Phys. 1993, 98, 5648. (b) Miehlich, B.; Savin, A.; Stoll, H.; Preuss, H. Chem. Phys. Lett. 1989, 157, 200. (c) Lee, C.; Yang, W.; Parr, G. Phys. Rev. B 1988, 37, 785. (d) Stephens, P. J.; Devlin, F. J.; Chabalowski, C. F. J. Phys. Chem. 1994, 98, 11623. (12) (a) Fukui, K. J. Phys. Chem. 1970, 74, 4161. (b) Fukui, K. Acc. Chem. Res. 1981, 14, 363. (13) (a) Hehre, W. J.; Ditchfield, R.; Pople, J. A. J. Chem. Phys. 1972, 56, 2257. (b) Hariharan, P. C.; Pople, J. A. Theor. Chim. Acta 1973, 28, 213. (c) 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. (d) Clark, T.; Chandrasekhar, J.; Spitznagel, G. W.; Schleyer, P. v. R. J. Comput. Chem. 1983, 4, 294. (14) (a) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 270. (b) Wadt, W. R.; Hay, P. J. J. Chem. Phys. 1985, 82, 284. (c) Hay, P. J.; Wadt, W. R. J. Chem. Phys. 1985, 82, 299. (15) Selected examples of the successful application of B3LYP/ LanL2DZ in calculations of osmium compounds: (a) Torrent, M.; Sola, M.; Frenking, G. Chem. Rev. 2000, 100, 439. (b) Liang, B.; Andrews, L. J. Phys. Chem. A 2002, 106, 4042. (c) Deubel, D. V. J. Am. Chem. Soc. 2004, 126, 996. (d) Gobetto, R.; Nervi, C.; Romanin, B.; Salassa, L.; Milanesio, M.; Croce, G. Organometallics 2003, 22, 4012. (e) Grey, J. K.; Butler, I. S.; Reber, C. Inorg. Chem. 2004, 43, 5103. (f) Ferrando-Miguel, G.; Gerard, H.; Eisenstein, O.; Caulton, K. G. Inorg. Chem. 2002, 41, 6440. (g) Castarlenas, R.; Esteruelas, M. A.; Gutierrez-Puebla, E.; Jean, Y.; Lledos, A.; Martin, M.; Onate, E.; Tomas, J. Organometallics 2000, 19, 3100. (16) Frisch, M. J.; et al. Gaussian 03, revision B05; Gaussian, Inc.: Pittsburgh, PA, 2003. (17) (a) Collman, J. P.; Slaughter, L. M.; Eberspacher, T. S.; Strassner, T.; Brauman, J. I. Inorg. Chem. 2001, 40, 6272. (b) Dehestani, A.; Lam, W. H.; Hrovat, D. A.; Davidson, E. R.; Borden, W. T.; Mayer, J. M. J. Am. Chem. Soc. 2005, 127, 3423. (18) Torrent, M.; Deng, L.; Ziegler, T. Inorg. Chem. 1998, 37, 1307. (19) Limberg, C.; Köppe, R. Inorg. Chem. 1999, 38, 2106. (20) Bergant, A.; Cerkovnik, J.; Plesničar, B.; Tuttle, T. J. Am. Chem. Soc. 2008, 130, 14086. (21) (a) Delmonte, A. J.; Haller, J.; Houk, K. N.; Sharpless, K. B.; Singleton, D. A.; Strassner, T.; Thomas, A. A. J. Am. Chem. Soc. 1997, 119, 9907 , and references therein. (b) Dapprich, S.; Ujaque, G.; Feliu, M.; Lledós, A.; Musaev, D. G.; Morokuma, K. J. Am. Chem. Soc. 1996, 118, 11660. (c) Pidum, U.; Boehme, C.; Frenking, G. Angew. Chem.

oxo-hydroxy species (4). A proton transfer in 4 results in formation of a rhenium diolate intermediate (6) having a water ligand. Subsequent extrusion of olefin from the rhenium diolate intermediate (6) completes the reaction and regenerates the catalyst MTO. Formation of diol at the beginning of the reaction observed experimentally is related to the hydrolysis of epoxide with the coproduct water through the rhenium diolate intermediate (2) via two possible paths (Scheme 3). Our calculation results confirmed that the rhenium diolate intermediate (2) + H2O and the hydrolysis products (MTO + diol) are in fast equilibrium, resulting in the eventual conversion of diol to olefin.

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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]; [email protected]. Notes

The authors declare no competing financial interest.

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ACKNOWLEDGMENTS This work was supported by the National Natural Science Foundation of China (Grant Nos. 21173126 and 21003082) and the Research Grants Council of Hong Kong (HKUST603711 and HKU1/CRF/08).

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