Photoisomerisation of (E)- to (Z)-N,N-dimethylhydrazones and thermal reversion

Abstract

The E-isomers of aliphatic aldehyde N,N-dimethylhydrazones undergo competing E to Z isomerisation and photolysis to nitriles under UV (254 nm) irradiation. The Z–E isomer mixture which results thermally reverts to the more stable E-isomer and under short irradiation times the photoisomerisation/thermal reversion cycle can be reversible. Trimethylacetaldehyde N,N-dimethylhydrazone undergoes similar reactions (ruling out the intervention of other types of hydrazone isomerisation) and the rates of the thermal Z→E reactions for RCHN–NMe₂ are in the order R = Bu^t > Et > Me. The results are considered in terms of conformations of the hydrazones and the mechanism of E–Z isomerisation.