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.