Hydrazinolysis of aryl cinnamates and related esters: the alpha-effect arises from stabilization of five-membered cyclic transition state

Abstract

A kinetic study is reported for nucleophilic substitution reactions of Y-substituted-phenyl cinnamates (1a-1h) with a series of primary amines including hydrazine in H2O containing 20 mol % DMSO at 25.0 degrees C. The Bronsted-type plot for the reaction of 2,4-dinitrophenyl cinnamate (1a) is linear with beta(nuc) = 0.57 except hydrazine, which exhibits positive deviation from the linear correlation (i.e., the alpha-effect). The Bronsted-type plots for the reactions of 1a-1h with hydrazine and glycylglycine (glygly) are also linear with beta(1g) = -0.71 and -0.87, respectively, when 1a is excluded from the linear correlation. Thus, the reactions have been concluded to proceed through a concerted mechanism on the basis of the linear Bronsted-type plots and magnitudes of the beta(nuc) and beta(1g) values. The alpha-effect shown by hydrazine is dependent on electronic nature of the substituent Y in the leaving group, e.g., it increases as the substituent Y becomes a weaker electron-withdrawing group (or as basicity of the leaving aryloxide increases), indicating that the alpha-effect is not due to destabilization of the ground state but mainly due to stabilization of the transition state. A five-membered cyclic TS structure, which could increase nucleofugality of the leaving aryloxide through H-bonding interaction, has been proposed to account for the leaving-group dependent alpha-effect found in this study. The theories suggested previously to rationalize the alpha-effect found for the related systems are also discussed.