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

Nucleophilic substitution reactions of 5-XC₄H₂(S)C(O)OC₆H₃-2-Y-4-NO₂ (1) promoted by 4-Z-C₆H₄O-/4-Z-C₆H₄OH in 20 mol % dimethyl sulfoxide (DMSO)(aq) have been studied kinetically. The reactions exhibited second-order kinetics with β_acyl = -2.52 to -2.83, ρ(x) = 2.81-3.16, β_nuc = 0.88-0.04 and β_lg = -0.94, respectively. The results have been interpreted with an addition-elimination mechanism in which the nucleophilic attack occurs in the rate-determining step. Comparison with existing data reveals that the ratedetermining step changes from the second to the first step by the change in the nucleophile from R₂NH/R₂NH₂ ⁺ to 4-Z-C₆H₄O-/4-Z-C₆H₄OH.