Photo-Aryl Coupling and Related Reactions. II. The Formation of Triphenylenes from Halogeno-o-terphenyls

Abstract

Abstract Electronic effects on the photochemical conversion of o-terphenyl to triphenylene were examined by comparing the ultraviolet irraiation reaction, carried out in a benzene solution using iodine as an oxidant, of three classes of o-terphenyl derivatives, 4- (3) and 4′-substituted (4) and 4,4″-disubstituted derivatives (5). While a strong electron-withdrawing group such as nitro group, hindered the cyclization reaction to occur, methoxy, fluoro, chloro and bromo compounds afforded the corresponding triphenylene derivatives. With bromo and iodo compounds, however, a complex mixture of the products resulted due to photochemical cleavage of a carbon-halogen bonding, which led to the formation of phenylation products and hence phenyltriphenylenes together with reduction products. Further, with iodo compounds, it was found that no oxidant was necessary to effect the cyclodehydrogenation reaction since iodine was liberated during the photolysis. Possible pathways for the formation of the photolysis products from bromo and iodo compounds were discussed in relation to the relative rates of carbon-halogen cleavage and cyclization reactions. Comparison among three classes of halogeno compounds indicated that, with the exception of iodides, the cyclization reaction occurred more readily with type 4 compound than type 3 compounds, while type 5 compounds reacted only with difficulty.