Geometrical isomers of the Hydrazones from 2-Formyl- or 2-Acetyl-furans and -thiophenes and 2-Benzothiazolylhydrazine

Abstract

The reaction of 2-formyl-, 2-formyl-5-methyl-, 2-acetylfurans, 2-formyl- and 2-acetylthiophenes with 2-benzothiazolylhydrazine in refluxing ethanol gave the corresponding hydrazones [1] (X=O, R1=R2=H), [2](X=O, R1=H, R2=CH3), [3] (X=O, R1=CH3, R2=H), [4] (X=S, R1=R2=H) and [5] (X=S, R1=CH3, R2=H). From each hydrazone mixture, the E- and Z-isomers were separated by fractional recrystallization and column chromatography. The steric configurations of the E- and Z-isomers were determined by NMR spectra and their spectral characters were investigated. Moreover, the photoisomerizations between the E- and Z-isomers were studied. In the NMR spectra, the protons of the 3- and 4-positions of furan rings in [1E]~[3E] were shielded more than those of [1Z]~[3Z] and the NH protons in [1E],[2E] and [4E] were deshielded more than those of the corresponding Z-isomers. But no difference was observed in the NH protons between [3E], [5E] and [3Z], [5Z]. The E- and Z-isomers were isomerized to each other under the irradiation by tungsten lamp at 25°C in a few organic solvents. The rate constants k1 for the isomerization of showed the solvent dependence; that is : but k1 of did not show remarkable solvent dependence. On the other hand, the solvent dependence of the rate constants k-1 for the [E]→[Z] isomerizations were opposite to the above tendency. © 1974, The Chemical Society of Japan. All rights reserved.