Computational insights to the mechanism of alkene epoxidation by manganese-based catalysts in the presence of bicarbonate

The density functional theory (DFT) was applied to elucidate the electronic structure of a Mn catalyst, which is known to catalyze olefin epoxidation in the presence of H2O2 and catalytic amounts of bicarbonate buffer. We considered a Mn(II)-peroxycarbonate complex Mn(eta(2)-CO4) la as a starting point. It was shown that the coordination of sulfate anion to Mn(eta(2)-CO4) decreases the barrier for O=Mn bond formation from 9.7 to 2.3 kcal/mol. More importantly, it changes the nature of the transition state corresponding to the rate-determining barrier; in the reaction Mn(eta(2)-CO4) 1a -> OMn(eta(2)-OCO2) 4a it corresponds to the O-O bond cleavage, while in SO42- + Mn(eta(2)-CO4) 1a -> [(SO4)Mn(eta(2)-CO4)](2-) 1b -> [(SO4)OMn(eta(2)-OCO4)](2-) 4b it is associated with the rotation of the Mn-OCO2 bond. The solvent (water) molecules do not play significant role in activation of O-O peroxo bond of peroxycarbonate ligand or formation of active oxidant species of oxo-manganese complex 4. The epoxidation of ethylene by oxo-manganese complex 4b is a multi-step process and involves two major steps: one electron transfer from 4b to substrate and O-atom transfer. These steps of the reaction are found to proceed with 14.6 and 16.6 kcal/mol barriers, respectively. (C) 2009 Elsevier B.V. All rights reserved.