title: Mechanism of [gamma-H2SiV2W10O40](4-)-catalyzed epoxidation of alkenes with hydrogen peroxide.
abstract: The mechanism of [gamma-H2SiV2W10O40]4--catalyzed epoxidation of alkenes with hydrogen peroxide in acetonitrile/tert-butyl alcohol was investigated. The negative Hammett rho+ (-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 [gamma-H2SiV2W10O40]4--catalyzed oxidation of thianthrene-5-oxide reveal that the strong electrophilic oxidant species is formed on [gamma-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 [gamma-HSiV2W10O39OOH]4- (II). The successive dehydration of II forms III, which possibly has an active oxygen species of a mu-eta2:eta2-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.
authors.affiliation: Department of Applied Chemistry, School of Engineering, The University of Tokyo, 7-3-1 Hongo, Bunkyo-ku, Tokyo 113-8656, Japan.
authors.forename: Yoshinao
authors.initials: Y
authors.lastname: Nakagawa
authors.forename: Noritaka
authors.initials: N
authors.lastname: Mizuno
mesh_terms: D000475:Alkenes / Q000138:chemical synthesis* / Q000737:chemistry; D002384:Catalysis; D006861:Hydrogen Peroxide / Q000737:chemistry*; D007700:Kinetics; D008956:Models, Chemical; D015394:Molecular Structure; D017646:Organosilicon Compounds / Q000138:chemical synthesis / Q000737:chemistry*; D010084:Oxidation-Reduction; D011789:Quantum Theory; D017640:Silicates / Q000737:chemistry*; D012997:Solvents / Q000737:chemistry; D013237:Stereoisomerism; D013997:Time Factors; D017973:Tungsten Compounds / Q000737:chemistry*; D014639:Vanadium / Q000737:chemistry*
chemical_list: D000475:Alkenes; D017646:Organosilicon Compounds; D017640:Silicates; D012997:Solvents; D017973:Tungsten Compounds; D014639:Vanadium; D006861:Hydrogen Peroxide