A critical step in olefin epoxidation catalyzed by methyltrioxorhenium (MTO, CH3ReO3) is the formation of peroxorhenium complexes via activation of H2O2. Our recent combined experimental-computational study revealed that the sequential and reversible formations of CH3ReO2(?2-O2), A , and CH3ReO(?2-O2)2(H2O), B , are strongly accelerated by H2O in semi-aqueous solvents. Here, we elucidate the precise role of H2O in the rate law as well as in the reaction mechanism. The water concentration dependence of both the ligand exchange reactions, and the following epoxidation of cyclohexene, were both studied experimentally and computationally. Density functional theory calculations predict the participation of two solvent H2O molecules in the transition states associated with the proton transfer from coordinated H2O2 to the oxo ligand of MTO and A  and to the hydroxo ligand of the intermediates during the formation of B . This new two water-assisted mechanism yields activation parameters in excellent agreement with experimental measurements. Furthermore, calculations of deuterium isotope effects predict fractionation factors in good agreement with experiment for the two water-assisted mechanism.

Elucidating the precise role of water in olefin epoxidation catalyzed by methyltrioxorhenium

248th National Meeting (2014)

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