Olefin Epoxidation with Hydrogen Peroxide Catalyzed by Lacunary Polyoxometalate [gamma-SiW_(10)O_(34)(H_2O)_2]~(4-)

摘要

The tetra-n-butylammonium(TBA)salt of the divacant Keggin-type polyoxometalate [TBA]_4[gamma-SiW_(10)O_(34)-(H_2O)_2](I)catalyzes the oxygen-transfer reactions of olefins,allylic alcohols,and sulfides with 30% aqueous hydrogen peroxide.The negative Hammett rho~+(-0.99)for the competitive oxidation of p-substituted styrenes and the low value of(nucleophilic oxidation)/(total oxidation),X_(SO)=0.04,for I-cata-lyzed oxidation of thianthrene 5-oxide(SSO)reveals that a strongly electro-philic oxidant species is formed on I.The preferential formation of trans-ep-oxide during epoxidation of 3-methyl-1-cyclohexene demonstrates the steric constraints of the active site of I.The I-catalyzed epoxidation proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide.~(29)Si and ~(183)W NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diper-oxo species,[TBA]_4[gamma-SiW_(10)O_(32)(O_2)_2](II),with retention of a gamma-Keggin type structure.Whereas the isolated compound II is inactive for stoichiometric epoxidation of cyclooctene,epoxidation with II does proceed in the presence of hydrogen peroxide.The reaction of II with hydrogen peroxide would form a reactive species(HI),and this step corresponds to the induction period observed in the catalytic epoxidation.The steric and electronic characters of HI are the same as those for the catalytic epoxidation by I.Kinetic,spectroscopic,and mechanistic investigations show that the present epoxidation proceeds via III.