Evidence for a Catalytic Six-Membered Cyclic Transition State in Aminolysis of 4-Nitrophenyl 3,5-Dinitrobenzoate in Acetonitrile: Comparative Bronsted-Type Plot, Entropy of Activation, and Deuterium Kinetic Isotope Effects

TLDR: A kinetic study for reactions of 4-nitrophenyl 3,5-dinitrobenzoate with a series of cyclic secondary amines in acetonitrile is reported, and plots of the pseudo-first-order rate constant (kobsd) vs [amines] curve upward, while those of kobsd /[amine] vs [amine] exhibit excellent linear correlations with positive intercepts, indicating that the reaction proceeds through both uncatalyzed and catalyzed

Abstract: A kinetic study for reactions of 4-nitrophenyl 3,5-dinitrobenzoate (1a) with a series of cyclic secondary amines in acetonitrile is reported. Plots of the pseudo-first-order rate constant (kobsd) vs [amine] curve upward, while those of kobsd /[amine] vs [amine] exhibit excellent linear correlations with positive intercepts, indicating that the reaction proceeds through both uncatalyzed and catalyzed routes. Bronsted-type plots for uncatalyzed and catalyzed reactions are linear with βnuc = 1.03 and 0.69, respectively. The ΔH⧧ and ΔS⧧ values measured for the catalytic reaction with morpholine are −0.80 kcal/mol and −61.7 cal/(mol K), respectively. The negative ΔH⧧ with a large negative ΔS⧧ suggests that the reaction proceeds through a highly ordered transition state (i.e., a six-membered cyclic transition state, which includes a second amine molecule that accepts a proton from the aminium moiety of the zwitterionic tetrahedral intermediate and simultaneously donates a proton to the aryloxyl oxygen of the nu...