Ring-chain tautomerism of 2-mercaptobenzoylhydrazones of aromatic aldehydes

It has been shown by 1H NMR spectroscopy that 2-mercaptobenzoylhydrazones of aromatic aldehydes 2-HSC6H4CONHN=CHC6H4X (X = 4-NO2, 3-NO2, 4-Br, H, 4-Me, 4-MeO, 4-Me2N) exist in DMSO-d6 solution as tautomeric mixtures of linear and cyclic benzo-1,3,4-thiadiazepine forms. The linear hydrazone form is represented by (E,Z′)-conformational isomers, differing in the disposition relative to the amide C–N bond. It was shown that the logarithm of the tautomeric equilibrium constant K T correlates with the σ-constants of the substituents in the aromatic nucleus.