Kinetic study on aminolysis of aryl X-substituted-cinnamates in acetonitrile: differential medium effect determines reactivity and reaction mechanism

Abstract

A kinetic study on nucleophilic substitution reactions of 2,4-dinitrophenyl X-substituted-cinnamates (1a-1f) and Y-substituted-phenyl cinnamates (2a-2g) with a series of alicyclic secondary amines in MeCN at 25.0 +/- 0.1 degrees C is reported. The Bronsted-type plots for the reactions of 1a-1f are linear with beta(nuc) = 0.47 similar to 0.50, indicating that the bond formation between the amine nucleophile and the electrophilic center is advanced slightly in the transition state. The Bronsted-type plot for the reactions of 2a-2g with piperidine is also linear with beta(1g) = -0.66, which is a typical beta(1g) value for reactions reported previously to proceed through a concerted mechanism. Furthermore, the Hammett plot correlated with sigma-constants results in much better linearity than that correlated with sigma degrees constants, implying that expulsion of the leaving group is advanced in the rate-determining step (RDS). Thus, the reactions are concluded to proceed through a concerted mechanism. The Hammett plots for the reactions of 1a-1f consist of two intersecting straight lines, whereas the corresponding Yukawa-Tsuno plots exhibit excellent linear correlations with rho(X) = 0.62 similar to 0.71 and r = 0.65 similar to 0.68. Apparently, the nonlinear Hammett plots are not due to a change in the reaction mechanism (or the RDS) but are caused by stabilization of the substrate possessing an electron-donating group (EDG) in the cinnamoyl moiety through resonance interactions between the EDG and the C=O bond of the substrate. Medium effects on reactivity and reaction mechanism are also discussed.