Rate-Enhancing Roles of Water Molecules in Methyltrioxorhenium-Catalyzed Olefin Epoxidation By Hydrogen Peroxide

Olefin epoxidation catalyzed by methyltrioxorhenium (MTO, CH₃ReO₃) is strongly accelerated in the presence of H₂O. The participation of H₂O in each of the elementary steps of the catalytic cycle, involving the formation of the peroxo complexes (CH₃ReO₂(η²-O₂), A, and CH₃ReO(η²-O₂)₂(H₂O), B), as well as in their subsequent epoxidation of cyclohexene, was examined in aqueous acetonitrile. Experimental measurements demonstrate that the epoxidation steps exhibit only weak [H₂O]-dependence, attributed by DFT calculations to hydrogen-bonding between uncoordinated H₂O and a peroxo ligand. The primary cause of the observed H₂O acceleration is the strong co-catalytic effect of water on the rates at which A and B are regenerated, and consequently on the relative abundances of the three interconverting Re-containing species at steady-state. Proton transfer from weakly coordinated H₂O₂ to the oxo ligands of MTO and A, resulting in peroxo complex formation, is directly mediated by solvent H₂O molecules. Computed activation parameters and kinetic isotope effects, in combination with proton-inventory experiments, suggest a proton shuttle involving one or (most favorably) two H₂O molecules in the key ligand exchange steps to form A and B from MTO and A, respectively.