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

Abstract

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(η(2)-O2), A, and CH3ReO(η(2)-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.

Peroxotantalate‐Based Ionic Liquid Catalyzed Epoxidation of Allylic Alcohols with Hydrogen Peroxide

“…[1] Some expensive and environmentally unacceptable oxidants such as peracids and hazardous organic solvents are still widely used in epoxidation reactions. [1] Some expensive and environmentally unacceptable oxidants such as peracids and hazardous organic solvents are still widely used in epoxidation reactions.…”

“…Epoxidation of olefins and allylic alcohols is an important reaction in organic synthesis as the epoxy compounds are at ype of important reaction intermediate, for example,i nt he manufacture of polyethers, b-haloalcohols, glycols, surfactants, etc. [1] Some expensive and environmentally unacceptable oxidants such as peracids and hazardous organic solvents are still widely used in epoxidation reactions. The use of H 2 O 2 as an oxidant is much more promising from economica nd environmental points of view as the only byproduct (H 2 O) is environmentally benign.…”

Molecular Epoxidation Reactions Catalyzed by Rhenium, Molybdenum, and Iron Complexes

“…published a new study of the oxidation mechanism, emphasizing the important role of water. It apparently enhances the formation of both the monoperoxo and bisperoxo complexes via hydrogen bonding, leading to an acceleration of the epoxidation of cyclohexene …”

Kinetic studies of fluorinated aryl molybdenum(ii) tricarbonyl precursors in epoxidation catalysis

“…12,13 MTO was originally established as an epoxidation catalyst by Herrmann et al in 1991. 14 In contrast to MTO where extensive work has provided a commonly accepted mechanism, 14,15 mechanisms of molybdenum-based epoxidation reactions are still disputed. This is not surprising, however, since the structural variety of compounds applicable in epoxidation catalysis is far greater than in the case of Re.…”