Mechanism of the olefin epoxidation catalyzed by molybdenum diperoxo complexes: Quantum-chemical calculations give an answer to a long-standing question

Quantum-chemical calculations at the B3LYP level have been carried our to e lucidate the reaction mechanism of the epoxidation of ethylene with the mol ybdenum diperoxo complex MoO(O-2)(2)OPH3. All relevant transition states an d intermediates which belong to the reaction pathways suggested by Mimoun a nd by Sharpless were optimized. The calculations show that there is no reac tion channel from the ethylene complex tb the putative metalla-2,3-dioxolan e intermediate as suggested by Mimoun. There is a transition state for the direct formation of the five-membered cyclic intermediate from ethylene and the diperoxo complex. However, the subsequent extrusion of a C2H4O species from the metalla-2,3-dioxolane does not yield the epoxide but acetaldehyde . The calculations show that the reaction of MoO(O-2)(2)OPH3 with ethylene can directly lead to the epoxide as suggested by Sharpless; The activation energy for the latter process is 15.2 kcal/mol, which is lower than the bar rier for the formation of the metalla-2,3-dioxolane (23.7 kcal/mol). Calcul ations with the ligand OPMe3 instead of OPH3 show an even larger preference of the pathway leading to the epoxide than the formation of the five-membe red ring. The calculations strongly support the mechanism suggested by Shar pless, while the Mimoun mechanism leads to carbonyl compounds as reaction p roducts. Examination of the electronic structure of the transition state of the epoxide formation with the Charge Decomposition Analysis shows that th e reaction should be considered as nucleophilic attack of the olefin toward the sigma* orbital of the peroxo bond.