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

M. ROCHA; D. M. GIL; G. A. ECHEVERRÍA; O. E. PIRO; J. L. JIOS ; S. E. ULIC

Journal of Organic Chemistry

American Chemical Societyservice@acs.org

Año: 2019 vol. 84 p. 11042 - 11053

The strange tautomeric equilibrium behavior exhibited by anew o-hydroxyphenyl diazepine derivative when the compound is analyzedboth in solution and solid state opens the structural study of the enol-imino−keto-enamine forms and the influence of the intermolecular interactions intheir equilibrium. The expected enol-imino form, in which the enol is part of aphenyl system and a strong O−H···N intramolecular hydrogen bond isestablished, results the most stable tautomer in gas phase (theoreticalcalculations) and was detected by NMR spectroscopy when the compoundwas dissolved in aprotic solvents. Nevertheless, the keto-enamine form ,inwhich the keto group integrates a cyclohexadienone moiety and thearomaticity of the phenol is lost, was the only tautomer in the crystal latticeaccording to single-crystal X-ray diffraction, vibrational spectroscopy, anddiffuse reflectance results. The last form was also found as the main tautomerin UV−vis and NMR spectroscopy when a protic solvent was employed.