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

Abstract

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 MnIV-hydroperoxo complex and the MnV-oxo species support recent experimental findings by Busch and co-workers [J. Am. Chem. Soc. 127, 17170 (2005)], according to which the MnIV-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 MnV-oxo species is found to be the only epoxidation pathway that could possibly compete with the Oβ 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.Key words: epoxidation, density functional theory, permanganic acid, peracid.