Mechanism of the Divanadium-Substituted Polyoxotungstate [γ-1, 2

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Inorg. Chem. 2009, 48, 1871-1878

Mechanism of the Divanadium-Substituted Polyoxotungstate [γ-1,2-H2SiV2W10O40]4- Catalyzed Olefin Epoxidation by H2O2: A Computational Study Aleksey E. Kuznetsov, Yurii V. Geletii, Craig L. Hill, Keiji Morokuma, and Djamaladdin G. Musaev* Cherry L. Emerson Center for Scientific Computation and Department of Chemistry, Emory UniVersity, 1515 Dickey DriVe, Atlanta, Georgia 30322 Received July 22, 2008

The mechanisms of olefin epoxidation by hydrogen peroxide catalyzed by [γ-1,2-H2SiV2W10O40]4-, 1, were studied using the density functional (B3LYP) approach in conjunction with large basis sets. The role of solvent is taken into account via both including an explicit water molecule into the calculations and using the polarizable continuum model (PCM) with acetonitrile as a solvent (numbers given in parentheses). The countercation effect (using one molecule of Me4N+ as a countercation (1CC)) is also taken into account (numbers given in brackets). It was shown that the formation of the vanadium-hydroperoxo species 2(H2O) with an {OV-(µ-OOH)(µ-OH)-VO}(H2O) core from 1 and H2O2 is a very facile process. The resulting complex 2(H2O) may eliminate a water molecule and form complex 2. From the intermediates 2 and 2(H2O), reaction may proceed via two distinct pathways: “hydroperoxo” and “peroxo”. The water-assisted “hydroperoxo” pathway starts with coordination of olefin (C2H4) to 2(H2O) and proceeds with a 36.8(25.5)[31.7][(21.6)] kcal/mol rate-determining barrier at the O-atom transfer transition state TS2[TS21CC]. The “water-free peroxo” and “water-assisted peroxo” pathways start with rearrangement of 2 and 2(H2O) to vanadium-peroxo species 3 and 3(H2O), respectively, with an {OV-(η2-O2)-VO} core, and follow the O-atom transfer from catalyst to olefin. The 2 f 3 and 2(H2O) f 3(H2O) hydroperoxo f peroxo rearrangement processes require 16.8(13.0)[13.0][(11.1)] and 14.2(9.0)[1.3][(7.2)] kcal/mol of energy, respectively. The calculated overall energy barriers are 28.1(19.1)[23.8][(17.2)] and 25.4(11.0)[10.6][(13.0)] kcal/mol for “water-free peroxo” and “water-assisted peroxo” pathways, respectively. On the basis of these data we predict that the [γ-1,2-H2SiV2W10O40]4-catalyzed olefin epoxidation by H2O2 most likely occurs via a “water-assisted peroxo” pathway.

I. Introduction Catalytic epoxidation of olefins has received considerable attention because of importance of epoxides as raw materials for the production of paints, epoxy resins, and surfactants.1,2 Over the years, a number of environmentally hazardous epoxidation processes that utilize a variety of catalysts and oxidants have been developed. The use of hydrogen peroxide as an oxidant offers an environmentally and economically attractive alternative;3,4 it is cheap, has the second highest active oxygen content (after the oxygen molecule), and produces water as an only by-product. In the literature, several catalysts with high-valent early transition metals such as Ti(IV), W(VI), V(V), * To whom correspondence should be addressed. E-mail: dmusaev@ emory.edu. (1) Sheldon, R. A.; Kochi, J. K. Metal-Catalyzed Oxidations of Organic Compounds; Academic Press: New York, 1981. (2) Tullo, A. Chem. Eng. News. 2004, 82, 15.

10.1021/ic801372j CCC: $40.75 Published on Web 01/28/2009

 2009 American Chemical Society

and Re(VII) have been reported to be effective for the epoxidation of alkenes with H2O2,5 among which the recently reported Vanadium(V)-catalyst is very promising. Indeed, the divanadium-substituted polyoxotungstate [γ-1,2-H2SiV2W10O40]4- (I) catalyst that contains an {OV-(µ-OH)2-VO} core catalyzes the epoxidation of unactivated aliphatic terminal C3-C10 alkenes including propene with very high yields, with g99% selectivity and g87% efficiency of H2O2 utilization.6-11 The epoxidation of cis- and trans-2-octenes gave cis-2,3epoxyoctane (90% yield) and trans-2,3-epoxyoctane (6% yield), respectively. This system requires only 1 equiv of hydrogen peroxide with respect to the alkene and produces the epoxide with high yield, stereospecificity, diastereoselectivity, and regioselectivity. It was shown the following: (1) no epoxidation of alkenes proceeded in the absence of I or when tertbutylhydroperoxide (TBHP) was used as an oxidant;6 (2) 51V Inorganic Chemistry, Vol. 48, No. 5, 2009

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Kuznetsov et al. Scheme 1. Schematic Presentation of the Proposed “hydroperoxo”, “water-free peroxo”, and “water-assisted peroxo” Pathways of the Reaction [γ-1,2-H2SiV2W10O40]4- + H2O2 + Olefin f [γ-1,2-H2SiV2W10O40]4- + H2O + Epoxidea

a

For simplicity, the [γ-SiW10O40]4- unit was omitted.

and 183W NMR studies of the compound I recovered after epoxidation show no formation of tungstate species such as [R-SiW12O40]4-, [γ-SiW10O34(H2O)2]4-, [W2O3(O2)4(H2O)2]2-, and [HnWO2(O2)2](2-n)-;10 (3) mono- and trivanadium-substituted compounds, [R-SiVW11O40]5- and [R-1,2,3-SiV3W9O40]7-, are catalytically inactive, suggesting that the V-O-W and V)O centers are not the active sites in catalysis;6,9,10 (4) the silicotungstates [γ-SiW12O40]4- with the same structure as I are catalytically inactive, suggesting that the W atoms in I are not the active sites;9,10 (5) the esterification of a bis(µ(3) (a) Sanderson, W. R. Pure Appl. Chem. 2000, 72, 1289–1304. (b) Lane, B. S.; Burgess, K. Chem. ReV. 2003, 103, 2457–2760. (c) Ishii, Y.; Yamawaki, K.; Ura, T.; Yamada, H.; Yishida, T.; Ogawa, M. J. Org. Chem. 1988, 53, 3587–3593. (d) Neumann, R.; Gara, M. J. Am. Chem. Soc. 1995, 117, 5066–5074. (e) Mizuno, N.; Nozaki, C.; Kiyoto, I.; Misono, M. J. Am. Chem. Soc. 1998, 120, 9267–9272. (f) Hill, C. L.; Prosser-McCartha, C. M. Coord. Chem. ReV. 1995, 143, 407– 455. (g) Kozhevnikov, I. V. Chem. ReV. 1998, 98, 171–198. (h) White, M. C.; Doyle, A. G.; Jacobsen, E. N. J. Am. Chem. Soc. 2001, 123, 7194–7195. (i) Que, L., Jr. Science 1991, 253, 273–274. (j) Pestovsky, O.; Stoian, S.; Bominaar, E. L.; Shan, X.; Munck, E.; Que, L., Jr.; Bakac, A. Angew. Chem., Int. Ed. 2005, 44, 6871–6874. (k) Bukowski, M. R.; Koehntop, K. D.; Stubna, A.; Bominaar, E. L.; Halfen, J. A.; Muenck, E.; Nam, W.; Que, L., Jr. Science 2005, 310, 1000–1002. (l) MacBeth, C. E.; Golombek, A. P.; Young, V. G., Jr.; Yang, C.; Kuczera, K.; Hendrich, M. P.; Borovik, A. S. Science 2000, 289, 938– 941. (m) Battioni, P.; Renaud, J. P.; Bartoli, J. F.; Reinaartiles, M.; Fort, M.; Mansuy, D. J. Am. Chem. Soc. 1988, 110, 8462–8470. (n) Notari, B. AdV. Catal. 1996, 41, 253–334. (o) Romao, C. C.; Ku¨hn, F. E.; Herrmann, W. A. Chem. ReV. 1997, 97, 3197–3246. (p) Oesz, K.; Espenson, J. H. Inorg. Chem. 2003, 42, 8122–8124. (q) Vasbinder, M. J.; Espenson, J. H. Organometallics 2004, 23, 3355–3358. (r) Venturello, C.; Alneri, E.; Ricci, M. J. Org. Chem. 1983, 48, 3831– 3833. (s) Duncan, D. C.; Chambers, R. C.; Hecht, E.; Hill, C. L. J. Am. Chem. Soc. 1995, 117, 681–691. (t) De Vos, D. E.; Meinershagen, J. L.; Bein, T. Angew. Chem., Int. Ed. 1996, 35, 2211–2213. (u) Neumann, R. Prog. Inorg. Chem. 1998, 47, 317–370. (v) Neumann, R.; Dahan, M. Nature 1997, 388, 353–355. (w) Neumann, R.; Dahan, M. J. Am. Chem. Soc. 1998, 120, 11969–11976. (x) Nishiyama, Y.; Nakagawa, Y.; Mizuno, N. Angew. Chem., Int. Ed. 2001, 19, 3751– 3753. (y) Okun, N. M.; Anderson, T. M.; Hill, C. L. J. Am. Chem. Soc. 2003, 125, 3194–3195. (z) Weinstock, I. A.; Barbuzzi, E. M. G.; Wemple, M. W.; Cowan, J. J.; Reiner, R. S.; Sonnen, D. M.; Heintz, R. A.; Bond, J. S.; Hill, C. L. Nature 2001, 414, 191–195. (aa) Hill, C. L. In ComprehensiVe Coordination Chemistry II; Wedd, A. G., Ed.; Elsevier Science: New York, 2004; Vol. 4, pp 679; (ab) Neumann, R. In Modern Oxidation Methods; Ba¨ckvall, J.-E., Ed.; John Wiley & Sons: New York, 2004.

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hydroxo) dioxovanadium site in [γ-H2SiV2W10O40]4- with alcohols is sterically controlled; the secondary and tertiary alcohol esters were hardly formed (equilibrium constant