Investigation of the protonation of 5-azaindole derivatives by pmr spectroscopy

The protonation of some 5-azaindoles and 5-azaindolines by trifluoroacetic acid in media with different dielectric constants was studied by PMR spectroscopy. Protonation occurs at the nitrogen atom of the pyridine ring. The structures of the monocations of 5-azaindole, 5-azaindoline, and their 1-phenyl derivatives correspond to a considerable contribution of the quinoid structure with transfer of positive charge to the nitrogen atom of the pyrrole fragment of the molecule. On the basis of an investigation of the chemical shifts of the protons of 1-phenyl-5-azaindole and 1-phenyl-5-azaindoline on the trifluoroacetic acid concentration in methylene chloride, acetonitrile, and deuteroacetone, a protonation mechanism in which transfer of a proton from the donor to the acceptor in slightly polar media occurs through the formation of a hydrogen-bonded complex of the base with the acid is proposed.Translated from Khimiya Geterotsiklicheskikh Soedinenii, No. 6, pp. 767–772, June, 1973.