Stereochemical control in meta photocycloaddition of alkenes to the benzene ring: the anomalous behaviour of cyclohexene

Abstract

The intermolecular meta photocycloaddition of cycloalkenes to benzenes occurs with marked endo stereoselectivity, although the exo process would be favoured on purely steric grounds. Additions normally occur 2,6 to monoalkylbenzenes, but increasing the size of the alkyl group in the order Me, Prⁱ, Bu^t leads to an increasing tendency for 3,5-addition without significant loss of the endo stereoselectivity. These effects are attributed to a weak but orienting interaction between the α-CH in the alkene and the S₁ state of the aromatic species: this is termed ‘intermolecular hyperconjugation’. In the bichromophoric system 14b, models show that only an endo orientation is feasible, but that this is subject to steric hindrance resulting from unfavourable orientations of the α-methylene groups in the cyclohexenyl ring: hence the low quantum efficiency of the meta addition process, in contrast with the relatively efficient endo process in the cyclopentene 14a where there is no analogous steric effect. In compounds 15a and 15b, only an exo orientation between the cycloalkene moiety and the aromatic ring is sterically feasible. Accordingly, the meta cycloadditions are stereospecif ically exo, and occur with comparable quantum yields for the cyclohexene and cyclopentene derivatives. The anomalously low reactivity of cyclohexene in meta photocycloaddition to benzene applies only to the normally favoured endo process, and is attributable to sterically unfavoured ‘chair’ conformations of the cycloalkene.