Highly Selective Epoxidation of Cycloaliphatic Alkenes with Aqueous Hydrogen Peroxide Catalyzed by [PO4{WO(O2)2}4]3-/Imidazole

In the presence of imidazole as an additive, a phosphorus-containing tetranuclear peroxotungstate, THA(3)[PO4{WO(O-2)(2)}(4)] (I, THA=tetra-n-hexylammonium), could act as an efficient catalyst for epoxidation of cycloaliphatic alkenes with 30% aqueous hydrogen peroxide (H2O2). Compound I showed higher catalytic activity and selectivity to epoxide than other tung-states. By using the I/imidazole system, various kinds of cycloaliphatic alkenes could be highly selectively converted into the acid-sensitive epoxides including industrially important diepoxides in high to excellent yields under the almost stoichiometric conditions. The H-1 NMR spectroscopy showed that imidazole would work not only as a proton acceptor but also as a Lewis base to remarkably suppress the acid-catalyzed ring opening of epoxides.