Photoaddition and photoisomerization reactions of the benzene ring
INTRODUCTION The thermal chemistry of benzene and its derivatives has been extensively studied for well over a century, and constitutes one of the best-trodden fields of organic chemistry: its characteristic feature is the strong tendency for substitution reactions leading to products in which the aromatic ring survives. In marked contrast, the photochemistry of the benzene ring had been scarcely investigated at all prior to the present decade. About twenty different types of photochemical reactions involving the benzene ring have now become known, and nearly all of these lead to non-aromatic products. Herein lies a major difference between the thermaland photo-chemistry of the benzene ring, a difference which provides a powerful stimulus for research into the nature of the photochemical processes involved, and an opportunity for the synthesis of molecules not readily accessible through thermal chemistry. Indeed, some of these molecules have quite exotic architecture. Our own interest in this field stems from the observation that liquid benzene can undergo photoisomerization to fulvene via the first optical transition1. The purpose of this lecture is to summarize both some new and previously known results on the photochemistry of benzene and its simple derivatives, and to present some mechanistic proposals which appear to collate much of the present information and may serve as a basis for future discussions and predictions.