Amination of Phenylketene Revisit. Substituent Effect on Reactivity

Abstract

Abstract The asymmetric stretching frequencies of the ketene group of the m,p-substituted phenylketenes were found to be correlated with σ. 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 σ°-correlations with a plateau at a high reactivity region were observed for meta-substituents and p-chloro. The substituent effects including para-π-acceptors were described in terms of the Yukawa–Tsuno equation modified by a quadratic term as an approximation. The resultant ρ 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 ρ 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 ρ value and large r− value attributes to early transition state and larger ρ value with smaller r− value for 2,2,2-trifluoroethylamine attributes to late transition state.