Um Ik-Hwan, Hwang So-Jeong, Yoon Sora, Jeon Sang-Eun, Bae Sun-Kun
Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea.
J Org Chem. 2008 Oct 3;73(19):7671-7. doi: 10.1021/jo801539w. Epub 2008 Sep 4.
A kinetic study is reported for aminolysis of O-Y-substituted phenyl thionobenzoates (1a-f) and O-4-nitrophenyl X-substituted thionobenzoates (2a-f) in 80 mol % H2O/20 mol % DMSO at 25.0 +/- 0.1 degrees C. The reaction proceeds through one or two intermediates (i.e., a zwitterionic tetrahedral intermediate T(+/-) and its deprotonated form T(-)) depending on the basicity difference between the nucleophile and nucleofuge, that is, the reaction proceeds through T(+/-) when the leaving aryloxide is less basic than the attacking amine, but through T(+/-) and T(-) when the leaving group is more basic than the amine. However, the reaction mechanism is not influenced by the electronic nature of the substituent X in the nonleaving group. The Hammett plot for the reactions of 2a-f with benzylamine is consisted of two intersecting straight lines, which might be interpreted as a change in the rate-determining step (RDS). However, the Yukawa-Tsuno plot for the same reactions exhibits an excellent linear correlation, indicating that the nonlinear Hammett plot is not due to a change in the RDS but caused by stabilization of the ground-state of the substrate through resonance interaction between the electron-donating substituent X and the thionocarbonyl moiety.
本文报道了在25.0±0.1℃下，80摩尔%水/20摩尔%二甲基亚砜体系中，O-Y-取代苯基硫代苯甲酸酯（1a-f）和O-4-硝基苯基X-取代硫代苯甲酸酯（2a-f）的氨解动力学研究。根据亲核试剂和亲核离去基团之间的碱性差异，反应通过一种或两种中间体进行（即两性离子四面体中间体T(+/-)及其去质子化形式T(-)），也就是说，当离去的芳氧基碱性弱于进攻的胺时，反应通过T(+/-)进行，但当离去基团碱性强于胺时，反应通过T(+/-)和T(-)进行。然而，反应机理不受非离去基团中取代基X电子性质的影响。2a-f与苄胺反应的哈米特图由两条相交的直线组成，这可能被解释为速率决定步骤（RDS）的变化。然而，相同反应的汤川-津野图呈现出良好的线性相关性，表明非线性哈米特图不是由于RDS的变化，而是由于供电子取代基X与硫代羰基部分之间的共振相互作用使底物基态稳定所致。
