Reactions of 2,4-dinitrophenyl 5-substituted-2-thiophenecarboxylate promoted by 4-ZC6H4O−/4-ZC6H4OH in 20 mol% DMSO(aq). Effects of leaving group and nucleophile on the acyl transfer reactions

Abstract

The kinetics of nucleophilic substitution reactions involving 2,4-dinitrophenyl 5-substituted-2-thiophenecarboxylate were studied kinetically with 4-Z-C₆H₄O⁻/4-Z-C₆H₄OH, which facilitated the reactions, in 20 mol% DMSO(aq). The reactions followed second-order kinetics and exhibited β_acyl = −2.34 to −2.92, ρ(Y) = 2.71–3.39, β_nuc = 0.74–0.83, and |β_lg| = 0.40–0.57. Based on the interpretation of the results, we have concluded that the reaction follows an addition–elimination mechanism in which the first step is the rate-determining step (rds). The transition state structures for the 4-ZC₆H₄O⁻-promoted reactions remained nearly the same with a change in the leaving group from 4-nitrophenoxide to 2,4-dinitrophenoxide. The mechanism of the 4-ZC₆H₄O⁻-promoted reaction was similar to that of R₂NH-promoted reactions, except that the former proceeded with the 1st step being the rds and the latter proceeded with a change in the rds from the second to the first step with a stronger nucleophile.