Mechanistic studies of amination of ketenimines: change of rate-determining step by N-substituents through electronic effects

Vinylidenediamine intermediate was not found in amination reactions of N-i-propyl-p-substituted-phenylketenimines with n-BuNH2, but it was found in amination reactions of N-p-substituted-phenylphenylketenimines with n-BuNH2 by low-temperature 1H NMR spectrometer, indicating that the rate-determining step is changed from the first step (Cdouble bond; length as m-dashN addition) to the second step (tautomerization) when N-substituent of the ketenimines is changed from i-propyl group to p-substituted-phenyl group. Amination reactions of ketenimines and in a solvent with ε=35.9 were designed to explore electronic effects of N-substituents on the amination reactions by means of ab initio calculations. Computation results at level of MP2/6-31+G*//HF/6-31+G* (Onsager model) show that significant electronic stabilization of the first transition state involving Cdouble bond; length as m-dashN addition by N-phenyl group is the major factor causing the change of the rate-determining step for the amination reactions.