Investigation of the Reaction Mechanism for the Epoxidation of Alkenes with Hydrogen Peroxide Catalyzed by a Protonated Tetranuclear Peroxotungstate with NMR Spectroscopy, Kinetics, and DFT Calculations

Abstract

AbstractFor the epoxidation of cyclooctene with hydrogen peroxide (H2O2), the catalytic activity of a dinuclear peroxotungstate, [{WO(O2)2}2(μ‐O)]2–, is strongly dependent on additives, and HClO4 is the most effective. The reaction of [{WO(O2)2}2(μ‐O)]2– with HClO4 (0.5 equiv.) gives a protonated tetranuclear peroxotungstate, [H{W2O2(O2)4(μ‐O)}2]3– (I). The diastereoselectivity for epoxidation of 3‐methyl‐1‐cyclohexene shows that steric constraints of the active site of I are comparable to those of di‐ and tetranuclear peroxotungstates with XO4n– ligands (X = SeVI, AsV, PV, SVI, and SiIV). The lowest XSO [XSO = (nucleophilic oxidation)/(total oxidation)] value of 0.13 for the I‐catalyzed oxidation of thianthrene 5‐oxide among peroxotungstates reveals that I has the most electrophilic active oxygen species. Kinetic and spectroscopic results show that an inactive species (I′) is reversibly formed by the reaction of I with water. Reaction rates for the catalytic epoxidation show first‐order dependences on concentrations of alkene and I and a zero‐order dependence on concentrations of H2O2. All these results indicate that oxygen transfer from I to a C=C double bond in an alkene is the rate‐determining step. Computational studies support the proposed reaction mechanism.