Kim Sung-Gon, Kim Kyung-Hyun, Jung Junyang, Shin Seung Koo, Ahn Kyo Han
Department of Chemistry, Center for Integrated Molecular Systems, Division of Molecular and Life Science, Pohang University of Science and Technology, San 31 Hyoja-dong, Pohang 790-784, Republic of Korea.
J Am Chem Soc. 2002 Jan 30;124(4):591-6. doi: 10.1021/ja0119696.
The enantiomeric recognition of alpha-chiral primary ammonium ions has been studied with benzene-based tripodal tris(oxazoline) receptors. Contrary to the literature and our expectation, a good level of chiral discrimination is observed with one of the tripodal receptors, which provides a C3-symmetric chiral environment on guest binding. The chiral discrimination has been found to be general in the case of alpha-aryl substituted guests, suggesting pi-pi interactions as an important factor. This result raises a question with respect to the origin of the chiral discrimination since little steric or electronic difference is expected between the diastereomeric inclusion complexes. Binding studies by NMR titration and isothermal titration calorimetry show that the chiral discrimination results from the different thermodynamic stabilities between the diastereomeric complexes and that the host-guest complex formation is driven by favorable enthalpy changes with a minor negative contribution by entropy changes. The X-ray crystal structures for both of the diastereomeric inclusion complexes are resolved, which unambiguously show the binding mode and provide clues on the origin of the chiral discrimination. Bond angle analyses indicate that the minor complex experiences a larger steric strain, which is discernible when it is viewed from "three-body" interactions between the host and the guest. The guest and oxazoline phenyl rings are well stacked, indicating interplay of the pi-pi interactions. The pi-pi interactions are believed to stabilize host-guest complexes, thereby endowing the highly flexible receptors with a substantial enantio-discrimination.
已使用基于苯的三脚架式三（恶唑啉）受体研究了α-手性伯铵离子的对映体识别。与文献及我们的预期相反，在其中一种三脚架式受体中观察到了良好的手性识别水平，该受体在客体结合时提供了一个C3对称的手性环境。已发现对于α-芳基取代的客体，手性识别具有普遍性，这表明π-π相互作用是一个重要因素。由于非对映体包合物之间预计几乎没有空间或电子差异，这一结果就手性识别的起源提出了一个问题。通过核磁共振滴定和等温滴定量热法进行的结合研究表明，手性识别源于非对映体复合物之间不同的热力学稳定性，并且主客体复合物的形成是由有利的焓变驱动的，熵变有较小的负贡献。两种非对映体包合物的X射线晶体结构都得到了解析，这明确显示了结合模式并为手性识别的起源提供了线索。键角分析表明，次要复合物经历了更大的空间应变，从主体与客体之间的“三体”相互作用来看这是可辨别的。客体和恶唑啉苯环堆积良好，表明存在π-π相互作用。据信π-π相互作用稳定了主客体复合物，从而赋予高度灵活的受体显著的对映体识别能力。
