Synthesis, Characterization, and Conformational Study of Acylhydrazones of α,β-Unsaturated Aldehydes

ABSTRACT New acylhydrazone derivatives 1–6 have been synthesized by condensation of tert‐butylphenoxyhydrazide and cinnamaldehyde A or β‐chloro‐α,β‐unsaturated aldehydes B–F. They were characterized by IR, (1H, 13C, 19F) nuclear magnetic resonance (NMR), and high‐resolution mass spectroscopy. The NMR data show the existence of the cis/trans‐amide conformers due to N–C(O) bond rotation in addition to the E/Z isomers around the C=C bond of some of the starting aldehydes. The solvent polarity effects on the ratios of the cis/trans‐amide rotamers have also been investigated. Importantly, rotational barriers around the N–C(O) bond for all compounds 1–6 (62.9–68.8 kJ mol–1) were calculated using the coalescence‐temperature method according to the Eyring equation. The results are discussed and compared with those previously reported for related acylhydrazones of aryl adehydes and acetone.