Um Ik-Hwan, Lee Ji-Youn, Lee Hai Whang, Nagano Yoshiya, Fujio Mizue, Tsuno Yuho
Department of Chemistry, Ewha Womans University, Seoul 120-750, Korea.
J Org Chem. 2005 Jun 24;70(13):4980-7. doi: 10.1021/jo050172k.
Second-order rate constants have been determined spectrophotometrically for the reactions of 4-nitrophenyl X-substituted 2-methylbenzoates (2a-e) and Y-substituted phenyl 2-methylbenzoates (3a-e) with alicyclic secondary amines in 80 mol % H(2)O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The o-methyl group in the benzoyl moiety of 2a-e retards the reaction rate but does not influence the reaction mechanism. The Hammett plots for the reactions of 2a-e are nonlinear, while the corresponding Yukawa-Tsuno plots are linear with large r values (1.06-1.70). The linear Yukawa-Tsuno plots suggest that stabilization of the ground-state through resonance interaction between the electron donating substituent X and the carbonyl group is responsible for the nonlinear Hammett plots, while the large r values imply that the ground-state resonance interaction is significant. The reactions of 2a-e resulted in smaller rho(X) values but larger r values than the corresponding reactions of 4-nitrophenyl X-substituted benzoates (1a-e). The small rho(X) value for the reactions of 2a-e (e.g., rho(X) = 0.22) is suggested to be responsible for the large r value (e.g., r = 1.70). The reactions of 3a-e with piperidine are proposed to proceed in a stepwise manner with a change in the rate-determining step on the basis of the curved Brønsted-type plot obtained. Microscopic rate constants associated with the reactions of 3a-e are also consistent with the proposed mechanism.
已在25.0±0.1℃下，通过分光光度法测定了4-硝基苯基X-取代的2-甲基苯甲酸酯（2a - e）和Y-取代的苯基2-甲基苯甲酸酯（3a - e）与脂环族仲胺在80 mol% H₂O/20 mol% DMSO中的反应二级速率常数。2a - e的苯甲酰部分中的邻甲基减缓了反应速率，但不影响反应机理。2a - e反应的哈米特图是非线性的，而相应的汤川-津野图是线性的，r值较大（1.06 - 1.70）。线性的汤川-津野图表明，给电子取代基X与羰基之间的共振相互作用使基态稳定，这是哈米特图非线性的原因，而较大的r值意味着基态共振相互作用显著。与4-硝基苯基X-取代的苯甲酸酯（1a - e）的相应反应相比，2a - e的反应导致较小的ρ(X)值但较大的r值。2a - e反应的小ρ(X)值（例如，ρ(X)=0.22）被认为是导致大r值（例如，r = 1.70）的原因。基于所获得的弯曲的布朗斯特型图，3a - e与哌啶的反应被认为是以逐步方式进行的，速率决定步骤发生了变化。与3a - e反应相关的微观速率常数也与所提出的机理一致。
