Substituent effects on the tautomer ratios between the hydra‐zone imine and diazenyl enamine forms in 3‐(arylhydra‐zono)methyl‐2‐oxo‐1,2‐dihydroquinoxalines. Correlation of the hammett constants σ p with the tautomeric equilibrium constants K T

The 3‐(arylhydrazono)methyl‐2‐oxo‐1,2‐dihydroquinoxalines 1a‐e and 2a‐i showed tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in dimethyl sulfoxide media. The sub‐stituent effects on the tautomer ratios of A to B in compounds 1a‐e and 2a‐i were studied by the nmr spec‐troscopy. The electron‐donating or electron‐withdrawing p ‐substituents R 1 in compounds 2a‐i represented a tendency to increase the ratios of the tautomer A or the tautomer B , respectively, exhibiting the linear correlation of the Hammett constants σ p (‐0.17 to +0.78) with the tautomer ratios of A to B or the tautomeric equilibrium constants K T . However, the presence of the ester group R 2 in compounds 1a‐e induced the exclusive existence of the tautomer A regardless of the nature of the p ‐substituents R 1 . In the tautomeric thermodynamic study, the elevating temperature increased the ratios of the hydrazone imine tautomer A in compounds 2a‐i . The tautomeric thermodynamic parameters ΔG°, ΔH° and ΔS° were derived from the van't Hoff plots for compounds 2a , b , h , i , wherein the entropy term dominated the free‐energy difference between the A and B tautomers.