Mechanism of thermal Z/E isomerization of substituted N-benzylideneanilines. 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, and the results could be described satisfactorily by log (k/ko) = ρ[σ∘ + r+(σ+ − σ∘) + r−(σ−−σ∘)], except for the 4-(dimethylamino) 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 π 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. When a dimethylamino group exists in the 4-position of the aniline ring, the rate constants observed were larger than that expected from the above equation. This de...