A kinetic study is reported for SNAr reactions of 1-(Y-substituted-phenoxy)-2,4-dinitrobenzenes (1a-1h) with amines in MeCN. The plots of pseudo-first-order rate constant versus amine concentration curve upward, indicating that the reactions are catalyzed by a second amine molecule. The Brønsted-type plots for the reaction of 1-(4-nitrophenyl)-2,4-dinitrobenzene (1a) with secondary amines are linear with βnuc = 1.10 and 0.85 for the uncatalyzed and catalyzed reactions, respectively, while the Yukawa-Tsuno plots for the reactions of 1a-1h with piperidine result in excellent linear correlations with ρY = 1.85 and r = 0.27 for the uncatalyzed reaction and ρY = 0.73 and r = 0.23 for the catalyzed reaction. The catalytic effect decreases with increasing amine basicity or electron-withdrawing ability of the substituent Y in the leaving group. Activation parameters calculated from the rate constants measured at five different temperatures for the catalyzed reaction of 1a with piperidine are ΔH(‡) = 0.38 kcal/mol and ΔS(‡) = -55.4 cal/(mol K). The catalyzed reaction from a Meisenheimer complex (MC(±)) is proposed to proceed through a concerted mechanism with a cyclic transition-state rather than via a stepwise pathway with an anionic intermediate, MC(-). Deuterium kinetic isotope effects provide further insight into the nature of the concerted transition state.

A kinetic study is reported for the S N Ar reaction of 1-Y-substituted-phenoxy-2,4-dinitrobenzenes (1a-1h) with OH -in 80 mol % H 2 O/20 mol % DMSO at 25.0 ± 0.1 o C. The second-order rate constant (k OH −) increases as the substituent Y in the leaving group changes from an electron-donating group (EDG) to an electronwithdrawing group (EWG). The Brønsted-type plot for the reactions of 1a-1h is linear with β lg = -0.16, indicating that the reactivity of substrates 1a-1h is little affected by the leaving-group basicity. A linear Brønsted-type plot with β lg = -0.3 ± 0.1 is typical for reactions reported previously to proceed through a stepwise mechanism in which formation of a Meisenheimer complex is the rate-determining step (RDS). The Hammett plot correlated with σ Y o constants results in a much better correlation than that correlated with σ Y − constants, implyng that no negative charge is developing on the O atom of the leaving group (or expulsion of the leaving group is not advanced at all in the TS). This excludes a possibility that the S N Ar reaction of 1a-1h with OH -proceeds through a concerted mechanism or via a stepwise pathway with expulsion of the leaving group being the RDS. Thus, the current reactions have been concluded to proceed through a stepwise mechanism in which expulsion of the leaving group occurs rapidly after the RDS.

A kinetic study is reported on nucleophilic substitution reactions of Y-substituted-phenyl picolinates (7a-7h) with a series of cyclic secondary amines in 80 mol % H 2 O/20 mol % DMSO at 25.0 ± 0.1 o C. Comparison of the kinetic results with those reported previously for the corresponding reactions of Y-substituted-phenyl benzoates (1a-1f) reveals that 7a-7h are significantly more reactive than 1a-1f. The Brønsted-type plot for the aminolysis of 4-nitrophenyl picolinate (7a) is linear with β nuc = 0.78, which is typical for reactions proceeding through a stepwise mechanism with expulsion of the leaving group being the rate-determining step. The Brønsted-type plots for the piperidinolysis of 7a-7h and 1a-1f are also linear with β lg = -1.04 and -1.39, respectively, indicating that the more reactive 7a-7h are less selective than the less reactive 1a-1f to the leavinggroup basicity. One might suggest that the enhanced reactivity of 7a-7h is due to the inductive effect exerted by the electronegative N atom in the picolinyl moiety, while the decreased selectivity of the more reactive substrates is in accord with the reactivity-selectivity principle. However, the nature of intermediate (e.g., a stabilized cyclic intermediate through the intramolecular H-bonding interaction for the reactions of 7a-7h, which is structurally not possible for the reactions of 1a-1f) is also responsible for the enhanced reactivity with a decreased selectivity.