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

Abstract

The protonation of a number of pyrrolo[1,2-a]imidazole derivatives in trifluoroacetic acid was studied by PMR spectroscopy. The 5,7-unsubstituted compounds form a mixture of two forms of cations, the structures of which correspond to the addition of a proton to the C₅ and C₇ atoms of the two-ring system with predominance (60–90%) of the 5-C cation. The introduction of a CH₃ group into the 5 position changes the direction of protonation to favor predominant (95%) formation of the 7-C cation. The 7-methyl derivatives of pyrroloimidazole are protonated exclusively at the C₅ atom. It is demonstrated that the basicity of the pyrroloimidazoles considerably surpasses the basicity of indolicine derivatives. The comparative proton-acceptor capacity of these systems is compared with the energy indexes and the reactivity indexes calculated by the simple MO LCAO method.