A Kinetic Study on Michael-type Reactions of 1-(X-Substituted PhenyI)-2-propyn-1-ones with Amines:Effect of Amine Nature on Reactivity and Mechanism

摘要

Second-order rate constants have been measured spectrophotometrically for the Michael-type reaction of 1-(X-substituted phenyl)-2-propyn-l-ones (2a-f) with amines in H2O at 25.0 ± 0.1 °C.A linear Br0nsted-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 p=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.