Kinetic Study on Aminolysis of Y-Substituted-Phenyl Picolinates: Effect of H-Bonding Interaction on Reactivity and Transition-State Structure
作者:Min-Young Kim、Tae-Ah Kang、Jung Hwan Yoon、Ik-Hwan Um
DOI:10.5012/bkcs.2014.35.8.2410
日期:2014.8.20
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 % $H_2O$/20 mol % DMSO at $25.0\pm}0.1^\circ}C$. 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 Br$\o}$nsted-type plot for the aminolysis of 4-nitrophenyl picolinate (7a) is linear with $\beta}_nuc}=0.78$, which is typical for reactions proceeding through a stepwise mechanism with expulsion of the leaving group being the rate-determining step. The Br$\o}$nsted-type plots for the piperidinolysis of 7a-7h and 1a-1f are also linear with $\beta}_lg}=-1.04$ and -1.39, respectively, indicating that the more reactive 7a-7h are less selective than the less reactive 1a-1f to the leaving-group 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.
报告了在 80 mol % $H_2O$/20 mol % DMSO,$25.0\pm}0.1^\circ}C$条件下 Y-取代苯基吡啶甲酸盐(7a-7h)与一系列环状仲胺的亲核取代反应的动力学研究。将动力学结果与之前报告的 Y-取代苯基苯甲酸盐(1a-1f)相应反应的动力学结果进行比较,发现 7a-7h 的反应活性明显高于 1a-1f。4-nitrophenyl picolinate (7a) 氨解反应的 Br$\o}$nsted-type plot 是线性的,$\beta}_nuc}=0.78$,这是典型的分步机理反应,离去基团的排出是决定反应速率的步骤。7a-7h 和 1a-1f 的哌啶分解反应的 Br$\o}$nsted 型曲线图也是线性的,分别为 $\beta}_lg}=-1.04$ 和 -1.39,这表明反应活性较高的 7a-7h 对离去基团碱性的选择性低于反应活性较低的 1a-1f。有人可能会认为,7a-7h 的反应性增强是由于吡啶基中电负性 N 原子产生的感应效应,而反应性较强的底物的选择性降低则符合反应性-选择性原理。然而,中间体的性质(例如,7a-7h 反应中通过分子内 H 键相互作用产生稳定的环状中间体,而 1a-1f 反应在结构上不可能产生稳定的环状中间体)也是导致反应活性增强而选择性降低的原因。