School of Chemistry, University of Southampton, Southampton, UK.
School of Chemistry, University of Birmingham, Birmingham, UK.
Nat Chem. 2023 Jun;15(6):781-786. doi: 10.1038/s41557-023-01194-1. Epub 2023 May 11.
Catenanes-molecules comprising two interlocking rings held together like links in a chain-are topologically non-trivial: a catenane is a topological isomer of its separated rings, but the rings cannot be disconnected without bond scission. Catenanes can exist as topological enantiomers if both rings have directionality conferred by a defined atom sequence, but this has led to the assumption that the stereochemistry of chiral catenanes composed of oriented rings is inherently topological in nature. Here we show that this assumption is incorrect by synthesizing an example that contains the same fundamental stereogenic unit but whose stereochemistry is Euclidean. One ring in this chiral catenane is oriented by the geometry of an exocyclic double rather than determined by atom sequence within the ring. Isomerization of the exocyclic double bond results in racemization of the catenane, confirming that the stereochemistry is not topological in nature. Thus, we can unite the stereochemistry of catenanes with that of their topologically trivial cousins, the rotaxanes, enabling a more unified approach to their discussion.
轮烷——由两个相互扣合的环组成的分子，就像链中的链接一样——在拓扑学上是不平凡的：轮烷是其分离环的拓扑异构体，但如果不发生键的断裂，环就无法断开。如果两个环都由定义的原子序列赋予方向性，那么轮烷可以作为拓扑对映异构体存在，但这导致了一个假设，即由定向环组成的手性轮烷的立体化学本质上是拓扑的。在这里，我们通过合成一个含有相同基本手性单元但立体化学是欧几里得的例子证明了这个假设是不正确的。这个手性轮烷中的一个环是由环外双键的几何形状定向的，而不是由环内的原子序列决定的。环外双键的异构化导致轮烷的外消旋化，证实了其立体化学本质上不是拓扑的。因此，我们可以将轮烷的立体化学与拓扑平凡的近亲索烃结合起来，为它们的讨论提供一个更统一的方法。
