Model systems for cytochrome P450 dependent mono-oxygenases. Part 4. The epoxidation of alkenes by peroxyacids in the presence of cobalt complexes

Abstract

The possible involvement of peroxyacids in oxidations by cytochrome P450 dependent mono-oxygenases has led us to screen the effect of a wide selection of transition metal complexes on the peroxyacid epoxidation of alkenes. This has been achieved by examining the effect that these metal complexes exhibit on the relative reactivities of styrene and 3-chlorostyrene towards epoxidation by 3- chloroperoxybenzoic and peroxybenzoic acids. Cobalt, and to a lesser extent rhodium compounds, produce the largest change in the relative rates. A stepwise radical mechanism for the cobalt(II)-peroxyacid epoxidations has been established using Hammett correlations of data obtained from the epoxidation of 3- and 4-substituted styrenes, stereochemical studies with cis- and trans-alkenes, and ¹⁸O labelling experiments. An oxy-radical, from the one-electron reduction of the peroxyacid by cobalt(II), reacts with the alkene to give a β-hydroxyalkyl radical, which on further oxidation is cyclised to the epoxide. Alternative concerted two-electron processes and a mechanism involving an alkene radical cation from the one-electron oxidation of the alkene by cobalt(III) have been eliminated.