Remarkable catalytic effect of H+ in Michael-type additions of anilines to 3-butyn-2-one

Second-order rate constants (kN) for the Michael-type reaction of 3-butyn-2-one (1) with a series of anilines in H2O have been determined spectrophotometrically. The kN values are dependent on the free aniline fraction (FN). The plot of log kN vs pKa of the conjugate acid of the anilines is linear for the reactions run at FN=1.00. However, the Brønsted-type plot for the reactions performed at FN=0.50 is nonlinear, suggesting a change in the reaction mechanism as the basicity of anilines changes. The analysis of the kinetic results has revealed that the reaction of anilines proceeds through the protonated form of the substrate 1 as well as the unprotonated. The protonated form of 1 is of the order of 109 times more reactive than the unprotonated form toward anilines. The contribution of the reaction with the protonated species to the kN is suggested to be responsible for the nonlinear Brønsted-type plot obtained for the reactions run at FN=0.50.