Reactions of 4-Nitrophenyl 2-Thiophenecarboxylates with R2NH/R2NH2+ in 20 mol % DMSO (aq). Effects of 5-Thienyl Substituent and Base Strength

Abstract

Reactions of 4-nitrophenyl 2-thiophenecarboxylate (1a-e) with R2NH/R2NH2+ in 20 mol % DMSO (aq) have been studied kinetically. The 2nd order kinetics, beta(nuc) = 0.88-0.98, and linear Hammett and Yukawa-Tsuno plots observed for these reactions indicate an addition-elimination mechanism in which the 2nd step is rate limiting. The beta(nuc) value increased with a stronger electron-withdrawing 5-thienyl substituent, the Hammett plots are linear except for X = MeO, and Yukawa-Tsuno plots are linear with rho = 0.79-1.32 and r = 0.28-0.93, respectively. The rho value increased and r value decreased with a stronger nucleophile, indicating an increase in the electron density at the C=O bond and a decrease in the resonance demand. These results have been interpreted with enhanced N-C bond formation in the transition state with the reactivity increase.