Mechanism of thermal Z/E isomerization of substituted N-benzylidenanilines. Nature of the activated complex with an sp-hybridized nitrogen atom

In order to study the mechanism of thermal geometrical isomerization involving a sp2-hybridized nitrogen atom, kinetic effects of substituent, solvent, and pressure were studied in substituted N-benzylideneanilines. The effect of the substituent on the aniline moiety was almost independent of the electronic nature of the benzylidene group. The r values were more than twice as large as r+, suggesting strongly that the aniline ring is in conjugation not with the carbon-nitrogen u bond but with the nitrogen lone pair in the transition state. The lower activation enthalpies and fairly large negative activation entropies observed in N-(4-X-benzylidene)-4-nitroanilines also support this view.