A Computational Study of the MoO2(acac)2 Catalyzed Epoxidation of Ethylene with Hydrogen Peroxide and t-Butyl Hydroperoxide

Abstract

The cis-MoO2(acac)2 catalyzed epoxidation of ethylene by H2O2 or tert-butyl hydroperoxide (TBHP) was studied by density functional theory at ωB97x–D using the 6-31G(d,p) and 6-311G(2d,p) basis sets and the PCM model to include the effect of acetonitril solvation. Three initial MoO2(acac)2⋅H2O2 adducts were identified. An associative pathway with a barrier of 26.7 kcal/mol is favored for H2O2 which inserts first into an acac ligand coordination site of the catalyst. TBHP favors instead a step-wise mechanism in which a catalyst's Mo=O bond inserts into the oxidant upon which ethylene gets epoxidized by the hepta-coordinate MoO(OH)(acac)2 (TBHP-κ2O,O’) intermediate with an activation energy of 33.8 kcal/mol. The reaction barriers for H2O2 and TBHP are both within the typical temperature range (70 °C) used for Mo catalyzed epoxidations. The results indicate that the mechanism depends largely on the nature of the oxidant and the dioxo molybdenum catalysts used.