Tautomeric behavior of 3‐(arylhydrazono)methyl‐2‐oxo‐1,2‐dihydroquinoxalines between the hydrazone imine and diazenyl enamine forms in acidic media. Solvent and substituent effects

Abstract

AbstractThe 3‐(arylhydrazono)methyl‐2‐oxo‐1,2‐dihydroquinoxalines 1a‐h and 2a‐e showed tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in a series of mixed trifluoroacetic acid/dimethyl sulfoxide 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‐h and 2a‐e by the nmr spectroscopy. In compounds 1a‐h and 2a‐e, the ratios of the tautomer B gradually increased with elevation of acid concentration, and the tautomer B exclusively existed in trifluoroacetic acid media. The various acid concentrations (C v/v%, C' mol/1) giving the 1:1 tautomer ratios [C(A:B = 1:1), C'(A:B = 1:1)] were obtained from all compounds (Figures 1–13), and the linear correlation of the Hammett σp values with the log C'(A:B = 1:1) values were observed for compounds 1a‐h. The larger Hammett σp values brought about the larger acid concentrations C(A:B = 1:1) in compounds 1a‐h and 2a‐e, indicating that the higher acid concentration was required for the stabilization of tautomer B possessing the electron‐withdrawing p‐substituents R1, which weakened the basicity of the azo nitrogen atom. Moreover, the ester group R2 of compounds 2a‐e was found to decrease the electron density of the azo nitrogen atom, since the acid concentration C(A:B = 1:1) of compound 2c (R1 = H, R2 = COOMe, σp = O) was 52%, whose value was larger than that of compound 1b (R1 = CN, R2 = H, σp = 0.66) [C(A:B = 1:1) = 42%].