Tautomeric and protonation equilibria of Schiff bases of salicylaldehyde with aminopyridines

The equilibria of the Schiff bases of salicylaldehyde with 2-aminopyridine, 2,3-diaminopyridine, 2,6-diaminopyridine and 3-aminomethylpyridine were studied by means of spectroscopic methods. In non-polar solvents enolimines were predominantly present. In polar solvents rapid tautomeric interconversion of enolimines to ketoamines as well as slow hydrolysis were noted. The tendency to tautomeric interconversion was significant in the case of 2-(3-pyridylmethyliminomethyl)phenol, whereas in the case of other Schiff bases it was very low. The corresponding tautomeric constants were estimated in a variety of solvents on the basis of UV spectral data. The protonation constants of all compounds were determined in methanol/water mixtures by the spectrophotometric method and discussed with respect to their structural characteristics and solvent properties.