Intermediates in the Epoxidation of Alkenes by Cytochrome P-450 Models. 1. cis-Stilbene as a Mechanistic Probe

Article abstract of DOI:10.1021/ja00209a025

Product yields were determined for the reactions of cis-stilbene with C6F5IO in CH2Cl2 with iron(III) chloride <(F20TPP)Fe^III(Cl)>, iron(III) chloride <(Cl8TPP)Fe^III(Cl), and manganese(III) chloride <(Cl8TPP)Mn^III(Cl)> as catalysts.Because they provided less cis-stilbene destruction and best yields of cis-stilbene oxide, the above catalysts and solvent were chosen from a number of systems investigated. cis-Stilbene destruction was found to exceed the C6F5IO oxidant employed.Aside from cis-stilbene, the products cis-stilbene oxide, trans stilbene, trans-stilbene oxide, diphenylacetaldehyde (DPhA), deoxybenzoin (DEB), and PhCHO were quantified.These products do not arise from cis-stilbene oxide.In the absence of O2, yields of PhCHO decreased, yields of trans-stilbene and cis-stilbene oxide increased, and destruction of cis-stilbene (to unaccountable products) decreased.Formation of PhCHO and destruction of cis-stilbene are explained by 1e^- oxidation of cis-stilbene to a carbocation radical (1).Reaction of 1 with O2 results in the formation of PhCHO and radycal polymerization of the alkene.In the absence of O2, electron capture by 1 yields cis- and trans-stilbene.The products DPhA, DEB, and cis-stilbene oxide are proposed to be formed from the acyclic carbocation 2.From 2, phenyl migration provides DPhA, proton migration yields DEB, and cyclization yields epoxide.The observation that DPhA is produced in greater yield compared to DEB is explained in terms of an intrinsically greater migratory aptitude for phenyl versus hydrogen in the cationic intermediate 2b.We conclude that the acyclic carbocation 2 is a pivotal structure in the epoxidation reaction and the formation of other side products.