Amination of Phenylketene Revisit. Substituent Effect on Reactivity

The asymmetric stretching frequencies of the ketene group of the m,p-substituted phenylketenes were found to be correlated with a. The substituent effects for the second-order rate constants of phenylketenes with various amines were not correlated linearly with any single substituent constants. The curved sigma degrees-correlations with a plateau at a high reactivity region were observed for meta-substituents and p-chloro. The substituent effects including para-pi-acceptors were described in terms of the Yukawa-Tsuno equation modified by a quadratic term as an approximation. The resultant rho value decreases significantly from 2.92 for 2,2,2-trifluoroethylamine to 0.83 for benzylamine while an r(-) value increases from 0.50 to 1.10. The opposite change of rho and r(-) with amines was interpreted by the dual electronic interactions between substituents and positive/negative charges at the zwitterionic transition structure. According to this interpretation, benzylamine with small rho value and large r(-) value attributes to early transition state and larger rho value with smaller r(-) value for 2,2,2-trifluoroethylamine attributes to late transition state.