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

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.