Geometrical Isomers of the Hydrazones from 2-Formyl, 2-Acetyl- and 2-Benzoylpyridine and 2-Hydrazinobenzothiazole

Abstract

The spectral characters and the isomerizations of the Z- and the E-isomers of the hydrazones prepared from 2-formyl-, 2-acetyl- and 2-benzoylpyridine and 2-hydrazinobenzothiazole were discussed. In the NMR spectra of the Z-isomers, on account of the six membered intramolecular hydrogen bonding, protones of NH and 6-position of pyridine rings were deshielded in comparison with the E-isomers. In the IR spectra of the Z-isomers, the vC=N absorptions were appeared at the lower wave number. Moreover, it was found that the λmax in the UV spectra of the Z-isomers were in the longer wave regions. The isomerizations between the Z– and the E-isomer of 2-acetylpyridine 2-benzothiazolyl-hydrazones were investigated in refluxed xylene. On this condition it was found that the Z/E ratio for the equilibrium Z-isomer ⇄ E-isomer approached to about 3:7 by weight. The E-isomer of 2-acetylpyridine 2-benzothiazolylhydrazone was easily isomerized to the Z-isomer by the tungsten lamp at 25°C in some organic solvents in concentration of 2 × 10″6 mol/l. The rate of isomerization depended on solvents, and the order of the rate was as follows: EtOH> CH3CN>cyclo-C6H12. © 1973, The Chemical Society of Japan. All rights reserved.