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

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 °C is reported. The Brønsted-type plots for the reactions of 1a–1f are linear with βnuc = 0.47∼0.50, indicating that the bond formation between the amine nucleophile and the electrophilic center is advanced slightly in the transition state. The Brønsted-type plot for the reactions of 2a–2g with piperidine is also linear with βlg = –0.66, which is a typical βlg value for reactions reported previously to proceed through a concerted mechanism. Furthermore, the Hammett plot correlated with σ– constants results in much better linearity than that correlated with σo 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 ρX = 0.62∼0.71 and r = 0.65∼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.