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

Abstract

Second-order rate constants have been measured spectrophotometrically for the Michael-type reaction of 1-(Xsubstituted phenyl)-2-propyn-1-ones (2a-f) with amines in <TEX>$H_2O$</TEX> at 25.0 <TEX>${\pm}$</TEX> 0.1 <TEX>${^{\circ}C}$</TEX>. A linear Brønsted-type plot is obtained with <TEX>${\beta}_{nuc}$</TEX> = 0.25 <TEX>${\pm}$</TEX> 0.02, a typical <TEX>$\beta_{nuc}$</TEX> 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 ρ = 0.62 and r = 0.82. Thus, it has been proposed that the nonlinear Hammett plot is not due to a change in the ra te-determining step but due to ground-state stabilization through resonance interactions.