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/k(o)) = rho[sigma-degrees + r+(sigma+-sigma-degrees)+ r-(sigma--sigma-degrees)], 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 pi 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 deviation suggests the existence of a reaction route where the two phenyl groups become coplanar in the transition state. Ab initio calculations on selected N-phenylformaldimines and N-benzylideneanilines were performed to characterize the actual relation between both reaction possibilities as alternative and parallel routes, respectively. On the basis of the experimental data, the rate constants for the two inversion isomerizations were estimated by assuming parallel reactions for three cases.