450963 Rate-Enhancing Roles of Water Molecules in Methyltrioxorhenium-Catalyzed Olefin Epoxidation By Hydrogen Peroxide

Sunday, November 13, 2016: 3:30 PM
Franciscan A (Hilton San Francisco Union Square)
Bryan R Goldsmith, Chemical Engineering, UC Santa Barbara, Santa Barbara, CA, Taeho Hwang, University of California, Santa Barbara, Santa Barbara, CA, Stefan Seritan, Department of Chemistry and Biochemistry, University of California, Santa Barbara, Santa Barbara, CA, Baron Peters, Chemical Engineering, University of California Santa Barbara, Santa Barbara, CA and Susannah L. Scott, Departments of Chemical Engineering and Chemistry, University of California, Santa Barbara, Santa Barbara, CA

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 (CH3ReO22-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.

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