Mechanism of photochemical meta-cycloaddition of alkenes to the benzene ring: a novel type of molecular interaction

Abstract

The regioselectivity of photochemical meta-cycloaddition of alkenes to alkylbenzenes changes progressively from 2,6- to 3,5- with increasing size of the alkyl group(s) while retaining endostereospecificity: this finding implies a novel type of weak but orienting intermolecular bonding interaction (intermolecular hyperconjugation) between S₁ benzene and α-hydrogens on the alkene which can predominate over steric effects of bulky alkyl groups.