N-H⋯O assisted structural changes induced on ketoenamine systems

The tautomeric equilibrium between the ketoenamine and enolimine forms for a series of arylimines (1a-1c) derived from salicylaldehyde and enaminones (2a-2f) derived from benzoylacetone and substituted anilines has been analyzed in solution by 1H NMR and UV-vis spectroscopy, while IR and X-ray diffraction analyses were used in the solid state. The X-ray diffraction analysis of 1a, 2a-2c and 2e-2f showed that the most stable tautomer is the ketoenamine and all enaminones present intramolecular NH⋯O hydrogen bonds forming a six member ring in which the keto form is favored. In addition, the VT-NMR analysis for 2b in solvents of different polarities evidenced the behavior of the tautomeric equilibrium, showing that in DMF and acetone at low temperature, the enolimine tautomer is more stable.