A new halide-free efficient reaction-controlled phase-transfer catalyst based on silicotungstate of [(C18H37)2(CH3)2N]3[SiO4H(WO5)3] for olefin epoxidation, oxidation of sulfides and alcohols with hydrogen peroxide

Abstract

A new reaction-controlled phase-transfer catalyst based on silicotungstate of [(C₁₈H₃₇)₂(CH₃)₂N]₃[SiO₄H(WO₅)₃] for oxidation of hydrocarbons is developed. The catalyst is a new heteropoly compound with silicon as heteroatom, which is different to the previously reported reaction-controlled phase transfer catalysts that were composed of quaternary ammonium heteropolyoxotungstates of [π-C₅H₅N(CH₂)₁₅CH₃]₃[PW₄O₁₆] and [π-C₅H₅N(CH₂)₁₅CH₃]₃[PW₄O₃₂] with phosphorus as heteroatom. The oxidation of various alkenes (such as linear terminal olefins, internal olefins, cyclic olefins and unactivated alkenes) to epoxides, sulfides to sulfoxides and sulfones, alcohols to carbonyl compounds, are successfully catalyzed by this recyclable and environmentally benign catalyst using H₂O₂ as oxidant and ethyl acetate as solvent. This catalyst is not only capable of catalyzing homogeneous oxidation of organic substrates with unique reaction-controlled phase-transfer character, but also avoids the use of toxic solvents. The catalyst could be easily recovered and reused after reaction, and the epoxidation of cyclohexene was performed twenty times without obvious loss in activity. The fresh catalyst and the used one were characterized by ICP, IR, UV-vis, ²⁹Si MAS NMR and ¹⁸³W NMR in detail.