Prins L J, De Jong F, Timmerman P, Reinhoudt D N
Laboratory of Supramolecular Chemistry and Technology, MESA Research Institute, University of Twente, Enschede, The Netherlands.
Nature. 2000 Nov 9;408(6809):181-4. doi: 10.1038/35041530.
Chiral molecules have asymmetric arrangements of atoms, forming structures that are non-superposable mirror images of each other. Specific mirror images ('enantiomers') may be obtained either from enantiomerically pure precursor compounds, through enantioselective synthesis, or by resolution of so-called racemic mixtures of opposite enantiomers, provided that racemization (the spontaneous interconversion of enantiomers) is sufficiently slow. Non-covalent assemblies can similarly adopt chiral supramolecular structures, and if they are held together by relatively strong interactions, such as metal coordination, methods analogous to those used to obtain chiral molecules yield enantiomerically pure non-covalent products. But the resolution of assemblies formed through weak interactions, such as hydrogen-bonding, remains challenging, reflecting their lower stability and significantly higher susceptibility to racemization. Here we report the design of supramolecular structures from achiral calix[4]arene dimelamines and cyanurates, which form multiple cooperative hydrogen bonds that together provide sufficient stability to allow the isolation of enantiomerically pure assemblies. Our design strategy is based on a non-covalent 'chiral memory' concept, whereby we first use chiral barbiturates to induce the supramolecular chirality in a hydrogen-bonded assembly, and then substitute them by achiral cyanurates. The stability of the resultant chiral assemblies in benzene, a non-polar solvent not competing for hydrogen bonds, is manifested by a half-life to racemization of more than four days at room temperature.
手性分子具有不对称的原子排列，形成彼此不可叠合的镜像结构。特定的镜像（“对映体”）可以从对映体纯的前体化合物中获得，通过对映选择性合成，或者通过拆分所谓的对映体相反的外消旋混合物来获得，前提是外消旋化（对映体的自发相互转化）足够缓慢。非共价组装体同样可以采用手性超分子结构，如果它们通过相对较强的相互作用（如金属配位）结合在一起，那么类似于用于获得手性分子的方法可以得到对映体纯的非共价产物。但是，通过弱相互作用（如氢键）形成的组装体的拆分仍然具有挑战性，这反映了它们较低的稳定性以及对外消旋化的显著更高的敏感性。在这里，我们报告了由非手性杯[4]芳烃二三聚氰胺和氰尿酸盐设计的超分子结构，它们形成多个协同氢键，共同提供足够的稳定性，以允许分离对映体纯的组装体。我们的设计策略基于非共价“手性记忆”概念，即我们首先使用手性巴比妥酸盐在氢键组装体中诱导超分子手性，然后用非手性氰尿酸盐取代它们。所得手性组装体在苯（一种不竞争氢键的非极性溶剂）中的稳定性表现为在室温下外消旋化的半衰期超过四天。
