Photochemistry of heterocyclics. II. Photolysis of benzimidazole and its 2-alkyl derivatives

Abstract

Photolysis of benzimidazole in various solvents with free access to air gives the unsymmetrical dehydro dimers, 2,4'- and 2,5'-bibenzimidazole. By contrast, 2-alkylbenzimidazoles do not give dimers but produce a mixture of acyl derivatives with improved yields in acidic solvent, e.g. 2-methylbenz-imidazole gives a mixture of 1-acetyl-, 4-acetyl- and 5-acetyl-2-methylbenzimidazole. A mechanism is suggested wherein a common path is followed in the initial stages. A photoexcited species first abstracts a hydrogen atom and then reacts with a ground-state substrate molecule to give a 2,3-benzimidazoline dimer. Subsequent reactions depend on the nature of the group at the 2-position of the benzimidazole-if hydrogen oxidation to dehydro dimer occurs but with an alkyl group a mixture of acylated benzene ring and o-phenylenediamine derivatives is produced by hydrolytic fission. Quantum yields of 1.52- 1.90 provide support for this scheme.