The mechanism of [γ-H2SiV2W10O 40]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-H 2SiV2W10O40]4--catalyzed oxidation of thianthrene-5-oxide reveal that the strong electrophilic oxidant species is formed on [γ-H2SiV2W10O 40]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-HSiV2W 10O39OOH]4- (II). The successive dehydration of II forms III, which possibly has an active oxygen species of a μ-η2:η2-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.
ASJC Scopus subject areas
- Physical and Theoretical Chemistry
- Inorganic Chemistry