Ring-chain tautomerism of 2-mercaptobenzoylhydrazones of aromatic aldehydes

It has been shown by 1 H NMR spectroscopy that 2-mercaptobenzoylhydrazones of aromatic aldehydes 2-HSC 6 H 4 CONHN=CHC 6 H 4 X (X = 4-NO 2 , 3-NO 2 , 4-Br, H, 4-Me, 4-MeO, 4-Me 2 N) exist in DMSO-d 6 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.