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

摘要

Second-order rate constants (kN) have been measured spectrophotometrically for SNAr reactions of Ysubstituted- phenoxy-2,4-dinitrobenzenes (1a-1g) with hydrazine and glycylglycine in 80 mol % H2O/20 mol % DMSO at 25.0 ± 0.1 oC. Hydrazine is 14.6-23.4 times more reactive than glycylglycine. The magnitude of the α-effect increases linearly as the substituent Y becomes a stronger electron-withdrawing group (EWG). The Brønsted-type plots for the reactions with hydrazine and glycylglycine are linear with βlg = –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 σo 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 1a-1g with hydrazine has been proposed to proceed through a five-membered cyclic intermediate (TIII), which is structurally not possible for the reaction with glycylglycine. Stabilization of the intermediate TIII through intramolecular H-bonding interaction has been suggested as an origin of the α-effect exhibited by hydrazine.