Dipole-mediated regioselectivity in the [2+2]-photocycloaddition of double bonds to triplet benzenes

The regioselectivity of the intramolecular [2+2] photocycloaddition of 2-substituted 4-butenoxybenzaldehydes and 4-butenoxybenzonitriles and of two 4-butenoxybenzocycloalkanones has been investigated. para-(3-Buten-1-oxy)benzaldehydes with ortho methyl and trifluoromethyl substituents undergo regioselective syn cycloaddition, while 4-butenoxy tetralone, chromanone, and 2-methoxybenzaldehyde undergo mainly anti cycloaddition. ortho Substituted p-butenoxybenzonitriles and o-fluoro-p-butenoxybenzaldehyde produce both regioisomers. These results provide more evidence that the CO dipole largely determines selectivity, something the cyano groupʹs alignment prevents. Computations augment the conclusion that the observed regioselectivity results primarily from interactions between the intrinsic molecular dipole of the triplet state and the charge transfer induced dipole in the exciplex state.