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 C{double bond, long}N addition. In contrast, Hammett equation was applied to Kk₂ for the amination of 1a-d and the Hammett plot was a convex curve with ρ_D=ρ₁=7.08 and ρ_A=ρ_K+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 C{double bond, long}N 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 C_β.