Epoxidation of Olefins with Cationic (salen)MnIII Complexes. The Modulation of Catalytic Activity by Substituents

Article abstract of DOI:10.1021/ja00269a029

Cationic manganese(III) complexes of the salen ligand are effective catalysts for the epoxidation of various olefins with iodosylbenzene as the terminal oxidant.The presence of electron-withdrawing groups, such as 5,5'-dichloro or -dinitro substituents, enhances the catalytic activity of the (salen)Mn^III catalyst in measure with the electron-deficient character of the cationic complex as evaluated by the standard reduction potential E⁰.Various types of olefins, including substituted styrenes, stilbenes, and cyclic and acyclic alkenes, are epoxidized in 50-75percent yields within 15 min at ambient temperatures in acetonitrile.Stereospecific epoxidation in achieved with trans-olefins such as (E)-2-hexene and (E)-β-methylstyrene. cis-Olefins produce high yields of cis-epoxides which contain minor amounts of the corresponding trans isomer.Competition from allylic oxidation is minor with this catalyst system-cyclohexene being converted selectively to its epoxide accompanied by only traces of cyclohexenol.Competition studies indicate that the relative reactivity of olefins toward catalytic epoxidation with the cationic (salen)Mn^III complexes falls into an unusually narrow range, the difference between the most reactive, p-methoxystyrene, and the least reactive, 1-octene, being only a factor of 10.The effect of donor ligands such as pyridine and imidazole is discussed in the context of a radical-like behavior of an oxomanganese species as the reactive intermediate.The latter is supported by some preliminary studies of alkane oxdation using cyclohexane as a model.