Catalytic Olefin Epoxidation and Dihydroxylation with Hydrogen Peroxide in Common Ionic Liquids: Comparative Kinetics and Mechanistic Study

Abstract

Methylrhenium trioxide (MTO) catalyzes the epoxidation of alkenes and allylic alcohols with urea hydrogen peroxide (UHP) in room-temperature ionic liquids (RTILs) that are based on N, N'-dialkylimidazolium or N-alkylpyridinium cations. The ionic liquids must be halide-free because halides catalyze the disproportionation of hydrogen peroxide to molecular oxygen and water. The kinetics and thermodynamics of the reaction of MTO with H2O2 in different ionic liquids have been investigated. The rate constant for the formation of the catalytically active diperoxorhenium complex, dpRe, is highly dependent on the concentration of water in the ionic solvent. Also, the rate constants for olefin epoxidation by the peroxorhenium complexes of MTO have been measured by UV-visible and 2H NMR spectroscopies. 2H NMR experiments conducted with [D3]dpRe confirmed the speciation of the catalytic system and asserted the validity of the UV-vis kinetics. The dpRe is more reactive in RTILs than its analogous monperoxo species, mpRe. The rate of olefin epoxidation is unaffected by the nature of the ionic liquid's cation; however, a discernable kinetic effect was noted for coordinating anions such as nitrate.