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 σp with the tautomeric equilibrium constants KT

Abstract

AbstractThe 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 R1 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 σp (‐0.17 to +0.78) with the tautomer ratios of A to B or the tautomeric equilibrium constants KT. However, the presence of the ester group R2 in compounds 1a‐e induced the exclusive existence of the tautomer A regardless of the nature of the p‐substituents R1. 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.