Epoxidation of Olefins with Cationic (salen)MnIII Complexes. The Modulation of Catalytic Activity by Substituents
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Source and publish data:
Journal of the American Chemical Society p. 2309 - 2320 (1986)
Update date:2022-08-11
Topics:
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Authors:
Srinivasan, K.
Michaud, P.
Kochi, J. K.
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)MnIII 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)MnIII 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.
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Full text of DOI:10.1021/ja00269a029