Development of polyoxometalate-based selective oxidation catalysts

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(1)Efficient epoxidation catalysts with hydrogen peroxide. (a) Peroxotungstate. The highly chemo-, regio-, and diastereoselective and stereospecific epoxidation of various allylic alcohols with only 1 equivalent of hydrogen peroxide in water could be efficiently catalyzed by an dinuclear peroxotungstate, [{W(=O)(O_2)_2(H_2O)}_2(μ-O)]^<2->. The peroxotungstate could be immobilized on ionic liquid-modified SiO_2. (b) Lacunary silicotungstate. The epoxidation with H_2O_2 catalyzed by lacunary silicodecatungstate ([γ-SiW_<10>O_<34>(H_2O)_2]^<4->) proceeded with 【greater than or equal】99% selectivity to epoxide, 【greater than or equal】99% efficiency of H_2O_2 utilization, high stereospecificity, and the easy recovery of the catalyst. A novel S-shaped disilicoeicosatungustate, [γ-SiW_<10>O_<32>(H_2O)_2)_2(μ-O)_2]^<4->, efficiently promoted the H_2O_2-based Baeyer-Villiger oxidations of cycloalkanones. (c) Di-vanadium or titanium-substituted silicotungstate. [γ-1,2-H_2SiV_2W_<10>O_<40>]^<4-> … More with the {VO-(μ-OH)_2-VO} core could catalyze epoxidation of olefins using only one equivalent H_2O_2 with the high epoxide yield and high efficiency of H_2O_2 utilization. Notably, this system showed unique stereospecificity, diastereoselectivity, and regioselectivity. A stereospecific epoxidation with hydrogen peroxide proceeded with a titanium-substituted silicotungstate cluster of [{γ-SiTi_2W_<10>O_<36>(OH)_2}_2(μ-O)_2]^<8->.(2)Bis (μ-oxo) bridged di-iron site on the lacunary POM as an efficient epoxidation catalyst with molecular oxygen. Bis (μ-oxo) bridged di-iron site on the lacunary POM has been used as catalysts for the oxygenation of alkenes in homogeneous reaction media using molecular oxygen as an oxygen donor. It is remarkable that selectivity to cyclooctene oxide and turnover number reached up to 98% and 10000, respectively, for the oxygenation of cyclooctene.(3)Microstructured POMs as a shape-selective catalyst. The Keggin-type POM, SiW_<12>O_<40>^<4->, and Cr(III) trinuclear cation, Cr_3O(OOCH)_6(H_2O)_3^+, assembled to create a channeled complex K_3[Cr_3O(OOCH)_6(H_2O)_3]SiW_<12>O_<40>・16H_2O with an opening of 0.5 x 0.8 nm. The complex reversibly absorbed water and polar organic molecules smaller than C2. The guest inclusion was highly selective and even a difference of one methylene group in the organic guest molecule was discriminated. Less