Protonation of pyrrolo[1,2-a]benzimidazole derivatives

The protonation of a number of pyrrolo1,2-a]benzimidazole derivatives in trifluoroacetic acid was studied by PMR spectroscopy. 1,3-Unsubstituted compounds are protonated exclusively at the C₁ atom. Under similar conditions, pyrrolobenzimidazoles that have a methyl group in the 1 position form a mixture of two protonated forms, which correspond to the addition of a proton to C₁ and C₃, respectively. The relative percentage of the C₃-protonated form decreases successively (from 81 to 18%) on passing from the 3-unsubstituted compound to the corresponding 3-phenyl and 3-methyl derivatives. The basicity constants of the pyrrolobenzimidazoles decrease symbatically with an increase in the relative percentage of this form. The relative proton-acceptor capacity of indolicine, pyrrolo1,2-a]-imidazole, and pyrrolo1,2-a]benzimidazole were examined on the basis of the protonation data and the reactivity indexes, calculated by the simple Hückel MO method.Translated from Khimiya Geterotsiklicheskih Soedinenii, No. 8, pp. 1132–1137, August, 1972.