A kinetic study on Michael-type reactions of 1-(X-substituted phenyl)-2-propyn-1-ones with amines: Effect of amine nature on reactivity and mechanism

Second-order rate constants have been measured spectrophotometrically for the Michael-type reaction of 1-(X-substituted phenyl)-2-propyn-1-ones (2a-f) with amines in H2O at 25.0 +/- 0.1 degrees C. A linear Bronsted-type plot is obtained with beta(nuc) = 0.25 +/- 0.02, a typical beta(nuc) value for reactions which proceed through a stepwise mechanism with attack of amine on the electrophilic center being the rate-determining step. Secondary alicyclic, amines are found to be more reactive than isobasic primary amines. The Hammett plot for the reactions of 2a-f with morpholine is not linear, i.e., the substrate with a strong electron-donating group (e.g., 4-MeO) exhibits a negative deviation from the Hammett plot. However, the Yukawa-Tsuno plot for the same reactions exhibits an excellent linear correlation with rho = 0.62 and r = 0.82. Thus, it has been proposed that the nonlinear Hammett plot is not due to a change in the rate-determining step but due to ground-state stabilization through resonance interactions.