The influence of pressure on arene–ethene photocycloaddition reactions

Abstract

Pressures of as little as 500 bar have been shown to exert a substantial effect upon rates and product ratios of ortho and meta photocycloadditions of ethenes to the benzene ring. In all cases, rates are enhanced by pressure and calculated ‘pseudo-activation volumes’ for both modes of addition appear to be of similar magnitude to those of thermal cycloaddition processes. The pressure-induced discrimination between isomeric meta cycloadducts from the ethyl vinyl ether–benzene system is rationalised in terms of the mechanism involving partial charge separation in the S₁ arene and is backed up by calculated molar volumes of the adducts. Similar results are observed for the intramolecular photocycloaddition reactions of arene–ethene non-conjugated bichromophores.