Cobalt-Catalyzed Epoxidation of Olefins. Dual Pathways for Oxygen Atom Transfer

Article abstract of DOI:10.1021/jo00229a022

Cobalt catalysis of olefin oxidation with tert-butyl hydroperoxide and iodosylbenzene as terminal oxidants is examined with Co^II(Ia), where the ligand Ia is the bis-salicylamide dianion 2,3-bis(3,5-dichloro-2-oxybenzamido)-2,3-dimethylbutane.The catalytic epoxidation of norbornene with tert-butyl hydroperoxide and with iodosulbenzene proceeds via different pathways, since the former is effectively inhibited by ionol (2,6-di-tert-butyl-p-cresol) whereas the latter is not.A radical chain mechanism is proposed for tert-butyl hydroperoxide in which the homolytic addition of tert-butylperoxy radicals to the double bond leads to epoxidation.Cobalt catalysis is associated with the Co^II/Co^III interconversion in the one-electron redox processes mediating the formation of the critical tert-butylperoxy radical.By contrast, there is no evidence for radical intermediates in the cobalt catalysis with iodosylbenzene which occurs readily with various types of olefins including 2-hexene, cyclooctene, β-methylstyrene, and even cyclohexene.A two-electron mechanism is proposed to account for the high reactivity, selectivity, and stereochemical retention in the catalytic epoxidation, despite the low solubility of Co^II(Ia) in the reaction aspects of the cobalt-catalyzed epoxidations with iodosylbenzene.