Copper-catalysed reactions of t-butyl perbenzoate with cis- and trans-p-Menth-2-ene and other cyclic olefins

Abstract

Each of the olefins cis-p-menth-2-ene (23), its trans-isomer (24), 3-methylcyclohexene (10) and 4- methylcyclohexene (ll), when treated with t-butyl perbenzoate and cupric octanoate catalyst in benzene, affords a mixture of allylic benzoates which can be analysed by gas chromatography after reduction with lithium aluminium hydride. In most cases the reaction proceeds with a high degree of stereo- and regio-selectivity, the major products being those containing a disubstituted double bond and bearing a benzoate group in a trans relationship to an alkyl substituent. Evidence is presented in support of a free-radical chain mechanism involving preferential abstraction of allylic hydrogen atoms occupying quasi-axial positions, followed by ligand transfer from cupric carboxylate via a cyclic transition state. When catalysed by the bipyridyl complex of cupric octanoate the reaction is less selective and appears to proceed by a different mechanism.