TAUTOMERIC BEHAVIOR OF RYLHYDRAZONO)METHYL-2-OXO-1,2-DIHYDRO-QUINOXALINES BETWEEN THE HYDRAZONE IMINE AND DIAZENYL ENAMINE FORMS IN ACIDIC MEDIA - SOLVENT AND SUBSTITUENT EFFECTS

The arylhydrazono)methyl-2-oxo-1,2-dihydroquinoxalines 1a-h and 2a-e s howed tautomeric equilibria between the hydrazone imine A and diazenyl enamine B forms in a series of mixed trifluoroacetic acid/dimethyl su lfoxide media. The substituent and solvent effects on the tautomer rat ios 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 r atios of the tautomer B gradually increased with elevation of acid con centration, and the tautomer B exclusively existed in trifluoroacetic acid media. The various acid concentrations (C v/v%, C' mol/l) giving the 1:1 tautomer ratios [C(A:B = 1:1): C'(A:B = 1:1)1 were obtained fr om all compounds (Figures 1-13), and the linear correlation of the Ham mett sigma(p) values with the log C'(A:B = 1:1) values were observed f or compounds 1a-h. The larger Hammett sigma(p) values brought about th e 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 sta bilization of tautomer B possessing the electron-withdrawing p-substit uents R(1), which weakened the basicity of the azo nitrogen atom. More over, the ester group R(2) of compounds 2a-e was found to decrease the electron density of the azo nitrogen atom, since the acid concentrati on C(A:B = 1:1) of compound 2c (R(1) = H, R(2) = COOMe, sigma(p) = 0) was 52%, whose value was larger than that of compound Ib (R(1) = CN, R (2) = H, sigma(p) = 0.66) [C(A:B = 1:1)= 42%].