Conformational analysis of hydrazones. 1H dynamic nuclear magnetic resonance and solvent effects in aryl‐ and 2‐furylaldehyde ethylaminoacetylhydrazones

Aryl- and 2-furylaldehyde ethylaminoacetylhydrazones were examined in different solvents and over a wide temperature range with 1H NMR in order to study their conformational properties. Nearly equal amounts of the E/Z isomers, relative to the CN bond, are present, even when the solvents and the substituents on the aldimino carbon produce small changes in the isomeric mixture. The activation parameters of the thermal isomerization process were measured, and the results are in the line with a lateral-shift type mechanism, also supported by theoretical calculations on a model compound. No other internal process was noted from the low-temperature spectral behaviour, and this was interpreted in terms of a highly biased equilibrium concerning the rotation around the C(O)N bond. Chemical shifts obtained in different solvents also enable the most stable arrangement of the whole molecule of these compounds to be postulated.