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 R 2 NH/R 2 NH 2 + in 20 mol % DMSO (aq) have been studied kinetically. The 2 nd order kinetics, β nuc = 0.88-0.98, and linear Hammett and Yukawa-Tsuno plots observed for these reactions indicate an addition-elimination mechanism in which the 2 nd step is rate limiting. The β 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 ρ = 0.79-1.32 and r = 0.28-0.93, respectively. The ρ 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.

Reactions of Aryl 5‐substituted‐2‐Thiophenecarboxylates Promoted by 4‐Z‐C6H4O−/4‐Z‐C6H4OH in 20 mol % DMSO(aq). Effect of Nucleophile on Acyl‐Transfer Reaction

“…Compounds 1aa′‐ea′ were available from previous study . The reaction of 1aa′ with C 6 H 5 O − /C 6 H 5 OH in 20 mol % DMSO(aq) produced 4‐nitrophenoxide as indicated by the gradual increase in the absorption at λ max = 410 nm (4‐NO 2 C 6 H 4 O − ).…”

“…Aryl 5‐substituted‐2‐thiophenecarboxylates 1 were available from previous investigations . DMSO was distilled under reduced pressure from CaH 2 .…”

“…Reactions of 1 with Z‐C 6 H 4 O − /4‐Z‐C 6 H 4 OH in 20 mol % DMSO(aq) were followed by monitoring the increase in the absorbance of the aryloxides in the range of 400–434 nm with a UV–Vis spectrophometer as described before …”

“…The stability of 1 was determined as reported earlier . The solutions of 1 in MeCN were stable for at least 2 weeks when stored in the refrigerator.…”

“…In an earlier paper, we reported the acyl‐transfer reactions of 4′‐nitrophenyl 5‐substituted‐2‐thiophenecarboxylates [5‐XC 4 H 2 (S)C(O)OC 6 H 4 ‐4‐NO 2 ( 1 )] promoted by secondary amines . The reactions proceeded via an addition–elimination mechanism in which the break‐down of the intermediate is the RDS.…”