Reactions of 2,4-Dinitrophenyl 5-substituted-2-thiophenecarboxylates with R2NH/R2NH2+ in 20 Mol % DMSO(aq). Effects of 5-Thienyl Substituent and Leaving Group on the Reaction Mechanism

Abstract

Reactions of 2,4-dinitrophenyl 2-thiophenecarboxylate (2a-d) with R2NH/R2NH2+ in 20 mol % DMSO(aq) have been studied. The reactions are overall second order, first order to the substrates, and first order to the nucleophiles. The Bronsted plots showed downward curves with pK(a)(0) = 9.5, beta(1) = 0.22-0.34, and beta(2) = 0.85-0.92. The k(1) values increased with a stronger electron-withdrawing 5-thienyl substituent and a stronger nucleophile, whereas the k(2)/k(-1) values remained nearly the same for all 5-thienyl substituents. The influence of 5-thienyl substituent on the reaction rates showed excellent correlations on the Yukawa-Tsuno plots with rho = 1.28-2.16 and r = 0.20-0.60. The rho value increased and r value decreased with a stronger nucleophile, indicating an increase in the electron density at the carbonyl carbon and a decrease in the resonance demand. From these results, a stepwise mechanism with a change in the rate-determining step has been proposed.