15N, 13C and 1H NMR study of tautomerism and E/Z isomerism in 3-[(Z)-(2-phenylhydrazinylidene)methyl]quinoxalin-2(1H)-one and 3-[(E)-(2-phenylhydrazinylidene)methyl]quinoxalin-2(1H)-one

Abstract

The one- and two-dimensional 15N, 13C and 1H NMR spectra of benzenediazonium chloride coupling product with 3-methylquinoxalin-2(1H)-one were measured and analysed. It was convincingly proved that two components of the reaction product obtained by coupling reaction are 3-[(Z)-(2-phenylhydrazinylidene)methyl]quinoxalin-2(1H)-one ((Z)-1, major component) and 3-[(E)-(2-phenylhydrazinylidene)methyl]quinoxalin-2(1H)-one ((E)-1, minor component). Detailed analysis of 15N, 13C and 1H NMR spectra allowed to correct previously published incorrect data describing a tautomeric equilibrium between azo and hydrazo forms. Multinuclear and multidimensional NMR studies have been proven to be extremely effective in resolving the tautomeric forms of ‘azo’ dye systems.