Kinetic studies for amination of ketenimines: change of rate-determining step by electron-withdrawing N-substituents through electronic effects

Kinetic studies for the amination of ketenimines 1a-d and 2a-e with n-BuNH2 were carried out by means of UV spectrometry. Hammett equation was applied to the second-order rate constant (k1) for the amination of 2a-e and the Hammett plot demonstrated a linear free-energy relationship with a reaction constant (ρ1) of 2.87, indicating that the second-order rate constant (k1) corresponds to the first step of rate-determining CN addition. In contrast, Hammett equation was applied to Kk2 for the amination of 1a-d and the Hammett plot was a convex curve with ρD=ρ1=7.08 and ρA=ρK+ρ2=0.98, indicating change of the rate-determining step. The electron-withdrawing para-substituents on the N-phenyl group of ketenimines significantly stabilize the first transition state of CN addition, resulting in change of the rate-determining step to the second step of tautomerization. The N-substituent electronic effect has much more significant influence on the amination of ketenimines than the substituent electronic effect at Cβ.