1H‐NMR study on the tautomer ratios between the hydrazone imine and diazenylenamine forms in 3‐(arylhydrazono)‐methyl‐2‐oxo‐1,2‐dihydroquinoxalines

Abstract The o‐substituted 3‐(arylhydrazono)methyl‐2‐oxo‐1,2‐dihydroquinoxalines 1a‐c and 2a, c were synthesized to investigate the tautomeric behavior between the hydrazone imine A and diazenylenamine B forms in a series of mixed dimethyl sulfoxide/trifluoroacetic acid media. The chemical shifts of the hydrazone NH, N4‐H, hydrazone CH, and diazenyl CH protons for o‐, m‐, and p‐substituted 3‐(arylhydrazono)methyl‐2‐oxo‐1,2‐dihydroquinoxalines 1 and 2 synthesized so far are summarized in Tables 3 and 4, respectively, which are found to be useful for the specification of the proton signals due to the hydrazone imine form A (hydrazone NH, hydrazone CH) and diazenylenamine form B (N4‐H, diazenyl CH).