SUBSTITUENT EFFECTS ON THE TAUTOMER RATIOS BETWEEN THE HYDRA-ZONE IMINE AND DIAZENYL ENAMINE FORMS IN RYLHYDRA-ZONO)METHYL-2-OXO-1,2-DIHYDROQUINOXALINES - CORRELATION OF THE HAMMETT CONSTANTS SIGMA(P) WITH THETAUTOMERIC EQUILIBRIUM-CONSTANTS K(T)

The arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1a-e and 2a-i s howed tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in dimethyl sulfoxide media. The substituent effects on the tautomer ratios of A to B in compounds 1a-e and 2a-i were studi ed by the nmr spectroscopy. The electron-donating or electron-withdraw ing p-substituents R1 in compounds 2a-i represented a tendency to incr ease the ratios of the tautomer A or the tautomer B, respectively, exh ibiting the linear correlation of the Hammett constants sigma(p) (-0.1 7 to +0.78) with the tautomer ratios of A to B or the tautomeric equil ibrium constants K(T). However, the presence of the ester group R2 in compounds 1a-e induced the exclusive existence of the tautomer A regar dless of the nature of the p-substituents R1. In the tautomeric thermo dynamic study, the elevating temperature increased the ratios of the h ydrazone imine tautomer A in compounds 2a-i. The tautomeric thermodyna mic parameters DELTAG-degrees, DELTAH-degrees and DELTAS-degrees 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.