Photochemistry of heterocyclics. III. Photolysis of various 2-substituted benzimidazoles

Abstract

A variety of 2-substituted benzimidazoles have been irradiated with ultraviolet light to determine the effects of substituents on the photochemical behaviour of the benzimidazole ring system. Benzimidazol-2-amine gave [2,5'-bibenzimidazol]-2'-amine and [2,4'-bibenzimidazol]-2'-amine, showing that this derivative behaves in the same way as benzimidazole. 2-Chlorobenzimidazole loses chlorine forming a dimeric species and benzimidazol-2(3H)-one, which is itself inert to photolysis. 2-Trichloromethylbenzimidazole also loses chlorine to form a variety of products depending on the solvent. 2-Trifluoromethylbenzimidazole does not lose fluorine but forms a dimer and also undergoes imidazole ring cleavage. Benzimidazole-2-thiol also undergoes imidazole ring cleavage with loss of sulfur. The 2-phenyl group stabilizes the benzimidazole ring system to photolysis as does the 2-thiazolyl group; however, the latter suffers thiazole ring cleavage.