Substituent and solvent effects on the tautomer ratios between the hydrazone imine and diazenyl enamine forms in p- and m-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines

The p- and m-substituted 3-(arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1a-i and 2a-d exhibited tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in a series of mixed dimethyl sulfoxide/trifluoroacetic acid media. The substituent and solvent effects on the tautomer ratios of A to B in a series of mixed media were studied for compounds 1a-i and 2a-d by the nmr spectroscopy. The linear correlation of the Hammett σp and σm values with the tautomeric equilibrium constants K_T ([A]/[B]) was found in the dimethyl sulfoxide media of compounds 1a-i and 2b-d . On the other hand, the linear correlation of the Hammett σp and σm values with the log C'(A:B = 1:1) was also observed in a series of mixed media of compounds 1a-h and 2a-c , wherein C'(A:B = 1:1) indicated the concentrations of trifluoroacetic acid (mol/l) giving 1:1 tautomer ratios in a series of mixed media. The increase in the Hammett σp or σm values decreased the K_T values in dimethyl sulfoxide media and augmented the C'(A:B = 1:1) values in a series of mixed media. The Hammett σp or σm values controlled the electron density of the side chain nitrogen atom, which influenced the C'(A:B = 1:1) values. In the K_T value temperature dependence, the higher temperature provided the larger K_T values in dimethyl sulfoxide media regardless of the Hammett σp or σm values.