Kinetic studies of the mechanism of transimination

The transimination reaction between N-p-chlorobenzylideneaniline and 2,2,2-trifluoroethylamine (or other primary amines more basic than aniline) is subject to catalysis by a second molecule of the amine, by carboxyanions and by tertiary amines. Plots of second order rate constants against catalyst concentration are curved, indicating a change in rate limiting step from proton transfer to some other step. The intermediate formed in this sequence is thought to be a geminal-diamine. A Bronsted plot for general base catalysis of the reaction with 2,2,2- trifluoroethylamine is curved with limiting slopes of 1.0, for catalysts of low pK_a, and 0.0, for catalysts of high pK_a. The break occurred at the estimated pK_a for the 2,2,2-trifluoroethylamine moiety in the geminal-diamine intermediate.

The results are consistent with a mechanism involving nucleophilic attack of an amine on protonated N-p-chlorobenzylideneaniline forming a geminal-diamine intermediate protonated on the nitrogen of the attacking amine. Proton transfer to the aniline nitrogen occurs either by a water mediated proton switch or by general acid-base catalysis. Protonated aniline is expelled from the intermediate forming a new imine. A reorganization step in the proton transfer is rate limiting at low concentrations of weakly basic catalysts. Diffusion of the catalyst from the intermediate, protonated on the aniline nitrogen, is rate limiting with strongly basic catalysts. Structure reactivity correlations confirmed the assignment of rate limiting steps. Collapse of the intermediate to products is probably rate limiting at high catalyst concentration.