NMR study on the tautomeric equilibria between the hydrazone imine and diazenyl enamine forms in side chained quinoxalines: Solvent effects and temperature dependence

The reaction of 7‐chloro‐4‐ethoxycarbonylmethylene‐4,5‐dihydro‐1,2,4‐triazolo[4,3‐ a ]quinoxaline 6 with 4‐ethoxycarbonyl‐1‐methyl‐1 H ‐pyrazole‐5‐diazonium chloride or 4‐cyano‐1,3‐dimethyl‐1 H ‐pyrazole‐5‐diazonium chloride gave 7‐chloro‐4‐[α‐(4‐ethoxycarbonyl‐1‐methyl‐1 H ‐pyrazol‐5‐ylhydrazono)‐ethoxycarbonylmethyl]‐1,2,4‐triazolo[4,3‐ a ]quinoxaline 8a or 7‐chloro‐4‐[α‐(4‐cyano‐1,3‐dimethyl‐1 H ‐pyrazol‐5‐ylhydrazono)ethoxycarbonylmethyl]‐1,2,4‐triazolo[4,3‐ a ]quinoxaline 8b , respectively, while the reaction of 7‐chloro‐4‐ethoxycarbonylmethylene‐4,5‐dihydrotetrazolo[1,5‐ a ]quinoxaline 7 with 4‐ethoxycarbonyl‐1‐methyl‐1 H ‐pyrazole‐5‐diazonium chloride or 4‐cyano‐1,3‐dimethyl‐1 H ‐pyrazole‐5‐diazomum chloride provided 7‐chloro‐4‐[α‐(4‐ethoxycarbonyl‐1‐methyl‐1 H ‐pyrazol‐5‐ylhydrazono)ethoxycarbonylmethyl]tetrazolo[1,5‐ a ]quinoxaline 9a or 7‐chloro‐4‐[α‐(4‐cyano‐1,3‐dimethyl‐1 H ‐pyrazol‐5‐ylhydrazono)ethoxycarbonylmethyl]tetrazolo[1,5‐ a ]quinoxaline 9b , respectively. Compounds 8a,b and 9a,b showed the tautomeric equilibria between the hydrazone imine C and diazenyl enamine D forms in dimethyl sulfoxide and/or trifluoroacetic acid, and the effects of solvent and temperature on the tautomer ratios of C to D were studied by the nmr measurements in a series of mixed trifluoroacetic acid/dimethyl sulfoxide media (compounds 8a,b and 9a,b ) and at various temperatures (compounds 8a,b ).