Unprecedented Chiral Molecular Recognition in a C3-Symmetric Environment
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- 作者：Sung-Gon Kim ; Kyung-Hyun Kim ; Junyang Jung ; Seung Koo Shin ; and Kyo Han Ahn
- 刊名：Journal of the American Chemical Society
- 出版年：2002
- 出版时间：January 30, 2002
- 年：2002
- 卷：124
- 期：4
- 页码：591 - 596
- 全文大小：123K
- 年卷期：v.124,no.4(January 30, 2002)
- ISSN：1520-5126
文摘
The enantiomeric recognition of 
-chiral primary ammonium ions has been studied with benzene-based tripodal tris(oxazoline) receptors. Contrary to the literature and our expectation, a good level ofchiral discrimination is observed with one of the tripodal receptors, which provides a C3-symmetric chiralenvironment on guest binding. The chiral discrimination has been found to be general in the case of -arylsubstituted guests, suggesting 
- interactions as an important factor. This result raises a question withrespect to the origin of the chiral discrimination since little steric or electronic difference is expected betweenthe diastereomeric inclusion complexes. Binding studies by NMR titration and isothermal titration calorimetryshow that the chiral discrimination results from the different thermodynamic stabilities between thediastereomeric complexes and that the host-guest complex formation is driven by favorable enthalpychanges with a minor negative contribution by entropy changes. The X-ray crystal structures for both ofthe diastereomeric inclusion complexes are resolved, which unambiguously show the binding mode andprovide clues on the origin of the chiral discrimination. Bond angle analyses indicate that the minor complexexperiences a larger steric strain, which is discernible when it is viewed from "three-body" interactionsbetween the host and the guest. The guest and oxazoline phenyl rings are well stacked, indicating interplayof the - interactions. The - interactions are believed to stabilize host-guest complexes, therebyendowing the highly flexible receptors with a substantial enantio-discrimination.
-chiral primary ammonium ions has been studied with benzene-based tripodal tris(oxazoline) receptors. Contrary to the literature and our expectation, a good level ofchiral discrimination is observed with one of the tripodal receptors, which provides a C3-symmetric chiralenvironment on guest binding. The chiral discrimination has been found to be general in the case of -arylsubstituted guests, suggesting - interactions as an important factor. This result raises a question withrespect to the origin of the chiral discrimination since little steric or electronic difference is expected betweenthe diastereomeric inclusion complexes. Binding studies by NMR titration and isothermal titration calorimetryshow that the chiral discrimination results from the different thermodynamic stabilities between thediastereomeric complexes and that the host-guest complex formation is driven by favorable enthalpychanges with a minor negative contribution by entropy changes. The X-ray crystal structures for both ofthe diastereomeric inclusion complexes are resolved, which unambiguously show the binding mode andprovide clues on the origin of the chiral discrimination. Bond angle analyses indicate that the minor complexexperiences a larger steric strain, which is discernible when it is viewed from "three-body" interactionsbetween the host and the guest. The guest and oxazoline phenyl rings are well stacked, indicating interplayof the - interactions. The - interactions are believed to stabilize host-guest complexes, therebyendowing the highly flexible receptors with a substantial enantio-discrimination.