Enol-imino-keto-enamine tautomerism in a diazepine derivative: How decisive are the intermolecular interactions in the equilibrium?

Abstract

The strange tautomeric equilibrium behaviour exhibited by a new o-hydroxyphenyl diazepine derivative, when the compound is analysed both in solution and solid state, open the structural study of the enol-imino-keto-enamine forms and the influence of the intermolecular interactions in their equilibrium. The expected enol-imino form, in which the enol is part of a phenyl system and a strong O-H...N intramolecular hydrogen bond is established, results the most stable tautomer in gas phase (theoretical calculations) and was detected by NMR spectroscopy when the compound was dissolved in aprotic solvents. Nevertheless, the keto-enamino form in which the keto group integrate a cyclohexadienone moiety and the aromaticity of the phenol is loss, was the only tautomer in the crystal lattice according to single-crystal X-ray diffraction, vibrational spectroscopy, and diffuse reflectance results. The last form was also found as the main tautomer in UV-Vis and NMR spectroscopy, when a protic solvent was employed.