The α-Effect in S_NAr Reaction of Y-Substituted-Phenoxy-2,4-Dinitrobenzenes with Amines: Reaction Mechanism and Origin of the a-Effect

摘要

Second-order rate constants (k_N) have been measured spectrophotometrically for SnAt reactions of Y-substituted-phenoxy-2,4-dinitrobenzenes (la-lg) with hydrazine and glycylglycine in 80 mol % H2O/2O mol % DMSO at 25.0 ± 0.1 °C. Hydrazine is 14.6-23.4 times more reactive than glycylglycine. The magnitude of the a-effect increases linearly as the substituent Y becomes a stronger electron-withdrawing group (EWG). The Bransted-type plots for the reactions with hydrazine and glycylglycine are linear with β_(lg) = -0.21 and -0.14, respectively, which is typical for reactions reported previously to proceed through a stepwise mechanism with expulsion of the leaving group occurring after rate-determining step (RDS). The Hammett plots correlated with σ~° constants result in much better linear correlations than σ~- constants, indicating that expulsion of the leaving group is not advanced in the transition state (TS). The reaction of la-lg with hydrazine has been proposed to proceed through a five-membered cyclic intermediate (T_(III)), which is structurally not possible for the reaction with glycylglycine. Stabilization of the intermediate T_(III) through intramolecular H-bonding interaction has been suggested as an origin of the a-effect exhibited by hydrazine.