Epoxidation of Alkenes with Hydrogen Peroxide Catalyzed by Selenium-Containing Dinuclear Peroxotungstate and Kinetic, Spectroscopic, and Theoretical Investigation of the Mechanism

The dinuclear peroxotungstate with a SeO42- ligand, (TBA)(2)[SeO4{WO(O-2)(2)}(2)] (I; TBA=[(n-C4H9)(4)N](+)), could act as an efficient homogeneous catalyst for the selective oxidation of various kinds of organic substances such as olefins, alcohols, and amines with H2O2 as the sole oxidant. The turnover frequency (TOF) was as high as 210 h(-1) for the epoxidation of cyclohexene catalyzed by I with H2O2. The catalyst was easily recovered and reused with maintenance of the catalytic performance. The SeO42- ligand in I played an important role in controlling the Lewis acidity of the peroxotungstates, which significantly affects their electrophilic oxygen-transfer reactivity. Several kinetic and spectroscopic results showed that the present catalytic epoxidation included the following two steps: (i) formation of the subsequent peroxo species [SeWmOn](o-) (II; m=1 and 2) by the reaction of I with an olefin and (ii) regeneration of I by the reaction of II with H2O2. Compound I was the dominant species under steady-state turnover conditions. The reaction rate for the catalytic epoxidation showed a first-order dependence on the concentrations of olefin and I and a zero-order dependence on the concentration of H2O2. The rate of the stoichiometric epoxidation with I agreed well with that of the catalytic epoxidation with H2O2 by I. All of these kinetic and spectroscopic results indicate that oxygen transfer from I to the C=C double bond is the rate-determining step. The computational studies support that the oxygen-transfer step is the rate-determining step.