The tetra-n-butylammonium (TBA) salt of the divacant Keggin-type polyoxometalate [TBA]4[ﾎｳ-SiW10O34(H2O)2] (I) catalyzes the oxygen-transfer reactions of olefins, allylic alcs., and sulfides with 30% aq. hydrogen peroxide. The neg. Hammett ﾏ� (-0.99) for the competitive oxidn. of p-substituted styrenes and the low value of (nucleophilic oxidn.)/(total oxidn.), XSO = 0.04, for I-catalyzed oxidn. of thianthrene 5-oxide (SSO) reveals that a strongly electrophilic oxidant species is formed on I. The preferential formation of trans-epoxide during epoxidn. of 3-methyl-1-cyclohexene demonstrates the steric constraints of the active site of I. The I-catalyzed epoxidn. proceeds with an induction period that disappears upon treatment of I with hydrogen peroxide. 29Si and 183W NMR spectroscopy and CSI mass spectrometry show that reaction of I with excess hydrogen peroxide leads to fast formation of a diperoxo species, [TBA]4[ﾎｳ-SiW10O32(O2)2] (II), with retention of a ﾎｳ-Keggin type structure. Whereas the isolated compd. II is inactive for stoichiometric epoxidn. of cyclooctene, epoxidn. with II does proceed in the presence of hydrogen peroxide. The reaction of II with hydrogen peroxide would form a reactive species (III), and this step corresponds to the induction period obsd. in the catalytic epoxidn. The steric and electronic characters of III are the same as those for the catalytic epoxidn. by I. Kinetic, spectroscopic, and mechanistic investigations show that the present epoxidn. proceeds via III. [on SciFinder(R)]
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