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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 βacyl = −2.34 to −2.92, ρ(Y) = 2.71–3.39, βnuc = 0.74–0.83, and |βlg| = 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. [ABSTRACT FROM AUTHOR]

Reactions of 4‐nitrophenyl 2‐furoates (1a–e) with R2NH/R2NH2+ in 20 mol% DMSO(aq) have been studied. The reactions produced aminolysis products and exhibited second‐order kinetics. The Brönsted plots were linear with βnuc values of 0.75–0.89, which remained nearly the same for all 5‐furyl substituents. The rate data showed excellent correlations on the Yukawa‐Tsuno plots with ρ(x) = 0.72–1.1, and r = 0.55–0.95, 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 contribution. The results have been interpreted with the addition–elimination mechanism in which the second step is the rds. By comparing with the data for ArC(O)OC6H4‐4‐NO2 (Ar = Ph, thienyl), the effect of the aryl group on the acyl transfer reaction was assessed. [ABSTRACT FROM AUTHOR]

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 Brönsted plots showed downward curves with pKa0 = 9.5, β1 = 0.22–0.34, and β2 = 0.85–0.92. The k1 values increased with a stronger electron‐withdrawing 5‐thienyl substituent and a stronger nucleophile, whereas the k2/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 ρ = 1.28–2.16 and r = 0.20–0.60. The ρ 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. [ABSTRACT FROM AUTHOR]

Ketene-forming elimination from C4H3(O)CH2C(O)OC6H3-2-X-4-NO2 (1) promoted by R2NH in MeCN has been studied. The reactions produced elimination products and exhibited second-order kinetics with Brönsted β = 0.51, and |βlg| = 0.47-0.53, indicating that the reaction proceeds by the E2 mechanism via an E2-central transition state. Comparison of β, |βlg|, ∆H6≠, and ∆S6≠ values for R2NH-promoted eliminations from ArCH2C(O)OC6H3-2-X-4-NO2 reveals that the transition-state structures for Ar = furyl and thienyl are similar and more symmetrical than that for Ar = Ph. This outcome has been attributed to the greater double bond stabilizing ability of the former than that of the latter. [ABSTRACT FROM AUTHOR]

Elimination reactions of ( E)-2,4,6-( NO2 ) 3C6H2CH NOC(O) C6H4X ( 3) promoted by R2NH/ R2NH2 + in 70 mol% MeCN (aq) have been studied. The reactions produced elimination products and exhibited second-order kinetics. The β and |βlg| values remained nearly the same for all leaving groups and bases. The results can be described by the negligible pxy interaction coefficient, pxy = ∂β/∂p Klg = ∂βlg/∂p KBH ≈ 0, which provides a strong support for the (E1cb)irr mechanism. For eliminations from ( E)-ArCHNOC(O)C6H4X ( 1, 3) and ( E)-2,4,6-(NO2)3C6H2CHNOAr′ ( 2, 3), the change of the β-aryl group (Ar) from 2,4-dinitrophenyl ( 1) to 2,4,6-trinitrophenyl ( 3) increased the rate by 270-fold without appreciable change in the transition state structure. On the other hand, the leaving group (OAr′) variation from benzoate ( 3) to 4-nitrophenoxy ( 2) induced a change in reaction mechanism from (E1cb)irr to E2. These results have been attributed to the cyclic transition state for the nitrile-forming eliminations involving the benzoate leaving group. [ABSTRACT FROM AUTHOR]