Polyoxometalate catalysts: toward the development of green H2O2-based epoxidation systems

This paper describes the development of green, efficient H(2)O(2)-based epoxidation systems with three kinds of polyoxometalates: (i) a dinuclear peroxotungstate [W(2)O(3)(O(2))(4)(H(2)O)(2)](2-) (I), (ii) a divacant lacunary polyoxotungstate [gamma-SiW(10)O(34)(H(2)O)(2)]4 (II), (iii) and a divanadium-substituted polyoxotungstate [gamma-1,2-H(2)SiV(2)W(10)O(40)](4-) (III). The highly chemo-, regio-, and diastereoselective epoxidation of various allylic alcohols with only 1 equiv H(2)O(2) in water can be efficiently catalyzed by potassium salt of I (K-I). The catalyst K-I can be recycled with the retention of the catalytic performance. Protonation of a divacant lacunary polyoxotungstate [gamma-SiW(10)O(36)](8-) gives [gamma-SiW(10)O(34)(H(2)O)(2)](4-) (II) with two aquo ligands. The tetra-n-butylammonium salt of II (TBA-II) catalyzes epoxidation of common olefins including propylene with >or=99% selectivity to epoxide and >or=99% efficiency of H(2)O(2) utilization. The bis(mu-hydroxo)bridged dioxovanadium site in [gamma-1,2-H(2)SiV(2)W(10)O(40)](4-) (III) can also efficiently catalyze epoxidation of a variety of olefins with 1 equiv H(2)O(2). Notably, the system with III shows unique stereospecificity, diastereoselectivity, and regioselectivity for the epoxidation of cis/trans olefins, 3-substituted cyclohexenes, and nonconjugated dienes, respectively, which are quite different from those reported for epoxidation systems up to now. Furthermore, the heterogenization of the mentioned polyoxometalates can be achieved by using ionic liquid-modified SiO(2) as a support without loss of catalytic performance.