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Mechanism of [γ-H2SiV2W10O40]4--Catalyzed Epoxidation of Alkenes with Hydrogen Peroxide

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journal contribution
posted on 05.03.2007 by Yoshinao Nakagawa, Noritaka Mizuno
The mechanism of [γ-H2SiV2W10O40]4--catalyzed epoxidation of alkenes with hydrogen peroxide in acetonitrile/tert-butyl alcohol was investigated. The negative Hammett ρ+ (−0.88) for the competitive oxidation of p-substituted styrenes and the low XSO (XSO = (nucleophilic oxidation)/(total oxidation)) value of <0.01 for the [γ-H2SiV2W10O40]4--catalyzed oxidation of thianthrene-5-oxide reveal that the strong electrophilic oxidant species is formed on [γ-H2SiV2W10O40]4- (I). The preferable formation of trans-epoxide for the epoxidation of 3-substituted cyclohexenes shows the steric constraints of the active oxidant on I. The 51V NMR, 183W NMR, and CSI−MS spectroscopy show that the reaction of I with hydrogen peroxide leads to the reversible formation of a hydroperoxo species [γ-HSiV2W10O39OOH]4- (II). The successive dehydration of II forms III, which possibly has an active oxygen species of a μ-η22-peroxo group. The kinetic and spectroscopic studies show that the present epoxidation proceeds via III. The energy diagram of the epoxidation with density functional theory (DFT) supports the idea.