Department of Chemistry and Nano Science, Ewha Womans University, Seoul 120-750, Korea.
J Org Chem. 2013 Sep 6;78(17):8689-95. doi: 10.1021/jo401415f. Epub 2013 Aug 21.
This paper comprises results of our investigation of the α-effect phenomenon for the reaction of O-p-nitrophenyl thionobenzoate (PNPTB) with butane-2,3-dione monoximate (Ox(-), α-nucleophile) and p-chlorophenoxide (p-ClPhO(-), normal-nucleophile) in DMSO-H2O mixtures of varying compositions at 15.0 °C, 25.0 °C, and 35.0 °C. The reactivity of Ox(-) and p-ClPhO(-) increases significantly as the DMSO content in the medium increases, although the effects of medium on reactivity are not the same for the reactions with Ox(-) and p-ClPhO(-). Ox(-) exhibits the α-effect in all solvent compositions and temperatures. The α-effect increases up to 50 mol % DMSO and then decreases thereafter, resulting in a bell-shaped α-effect profile. Dissection of the activation parameters (i.e., ΔH(‡) and TΔS(‡)) has revealed that the bell-shaped α-effect behavior is due to entropy of activation differences rather than enthalpy terms, although the enthalpy term controls almost entirely the solvent dependence of the reaction rate. Differences in the transition-state (TS) structures for the reactions with Ox(-) (a six-membered cyclic TS) and p-ClPhO(-) (an acyclic TS) are consistent with the entropy-dependent α-effect behavior.
本文介绍了我们在 15.0°C、25.0°C 和 35.0°C 下，在 DMSO-H2O 混合溶剂中，对邻硝基苯硫代苯甲酸酯（PNPTB）与丁烷-2,3-二酮单肟（Ox(-)，α-亲核试剂）和对氯苯氧负离子（p-ClPhO(-)，正常亲核试剂）反应的 α-效应现象的研究结果。Ox(-)和 p-ClPhO(-)的反应性随着介质中 DMSO 含量的增加而显著提高，尽管介质对反应性的影响对于 Ox(-)和 p-ClPhO(-)的反应并不相同。Ox(-)在所有溶剂组成和温度下均表现出 α-效应。α-效应在 DMSO 含量达到 50 mol%时达到最大值，随后降低，呈现出钟形的 α-效应曲线。活化参数（即 ΔH(‡)和 TΔS(‡)）的剖析表明，钟形的 α-效应行为是由于活化熵的差异，而不是焓项，尽管焓项几乎完全控制着反应速率对溶剂的依赖性。Ox(-)（一个六元环状过渡态）和 p-ClPhO(-)（一个非环状过渡态）反应的过渡态（TS）结构的差异与熵依赖的 α-效应行为一致。
