DFT mechanistic studies on the epoxidation of cyclohexene by non-heme tetraaza manganese complexes

Herein, we report density functional calculations on the epoxidation of cyclohexene with H2O2 activated by (Me2EBC)MnCl2 (Me2EBC stands for 4,11-dimethyl-1,4,8,11-tetraazabicyclo[6.6.2]hexadecane). The computed energy landscapes for different interaction modes of cyclohexene with the Mn-IV-hydroperoxo complex and the Mn-V-oxo species support recent experimental findings by Busch and co-workers [J. Am. Chem. Soc. 127, 17170 (2005)], according to which the Mn-IV-hydroperoxo species is the active complex for olefin epoxidation. Thus, the dominant olefin epoxidation pathway is via direct transfer of the distal protonated oxygen of the hydroperoxo adduct without changes in the oxydation state of its tetravalent metal centre, i.e., the mechanism commonly observed in the uncatalyzed epoxidation by peracids. The homolytic decomposition of the O-OH bond in the active manganese complex leading to the Mn-V-oxo species is found to be the only epoxidation pathway that could possibly compete with the O-beta transfer from the hydroperoxo adduct. However, the generated MnV-oxo is shown to be a rather poor oxidant resulting in low yields of the target epoxy cyclohexane.