Topological chirality molecular knots

Some racemates (Figure 3.23) are more efficiently resolved on the bonded-type CSP than the coated-type CSP by using chloroform as a component of the eluent. On the bonded-type CSP of 24n, topologically interesting catenanes and molecular knots are successfully resolved using a hexane-chloroform-2-propanol mixture.185 The first direct HPLC resolution of the smallest chiral... 

We shall now apply the techniques that we described above to prove the topological chirality of some molecular knots and links. Note that if we succeed in proving that a molecular graph is topologically chiral then it will follow that the molecule that it represents is chemically chiral, since any molecular motion corresponds to a rigid or flexible deformation of the molecular graph. In particular, it is not chemically possible for one molecular bond to pass through another molecular bond. 

We can use this same approach to prove that other molecular knots and links are topologically chiral. For example, consider the molecular link illustrated in Figure 18. This catenane was synthesized by Nierengarten et al. [12]. For this molecule the set T(G) consists of many unlinks together with many copies of the (4,2)-torus link, illustrated as L in Figure 12. However we saw earlier that this unoriented link is topologically chiral. Therefore, the molecular (4,2)-torus link is topologically chiral as well. 

The method that we have described enables us to prove the topological chirality of most topologically chiral molecular knots and links. 

We shall now see how to apply the theorem to the molecular trefoil knot, which was illustrated in Figure 17. We can create a molecular cell complex G by replacing each isolated benzene ring by a cell and each chain of three fused rings by a single cell. We prove by contradiction that our molecular cell complex is topologically chiral. Suppose that it is topologically achiral. Then there is a defor-... 

In addition, chiral dendrimers (see Section 4.2) can be resolved with the aid of HPLC into their enantiomers, if the silica gel material used as stationary phase has optically active substances bound to its surface [9]. Since the chiral stationary phase (CSP) [10] undergoes different intensities of interaction with the enantiomeric dendrimers, they are retained to different degrees, and in the ideal case two completely separated (baseline separated) peaks are obtained. This separation technique was successfully applied inter alia to racemic mixtures of planar-chiral dendro[2.2]paracyclophanes, cycloenantiomeric dendro[2] rotaxanes, topologically chiral dendro[2]catenanes [11] as well as topologically chiral, dendritically substituted molecular knots (knotanes) [12] (Section 4.2.3). 

Lukin, O., Muller, W.M., Muller, U., Kaufmann, A., Schmidt, C., Leszczynski, J., Vogtle, F. Covalent chemistry and conformational dynamics of topologically chiral amide-based molecular knots, Chem. Eur. J. 9 (2003), 3507-3517. 

There are also chiral topological supramolecular structures in molecular knots, rotaxanes, catenanes, and Borromean rings with metal complexes.73,74... 

Trefoil knots are the classic examples of topologically chiral structures. The rational synthesis of molecular trefoil knots, suggested as long ago as 1953,81 was finally achieved in 1989, by Christiane Dietrich-Buchecker and Jean-Pierre Sauvage82 (Figure 18) the enantiomers of this knot were subsequently resolved.83... 

To our knowledge, topologically chiral molecules have not yet been resolved into enantiomers. However, we may anticipate that their energy barrier to racemization will be extremely high, compared to Euclidean chiral molecules. Therefore they are expected to be useful in enantioselective interactions or reactions. For example, it has been shown that tetrahedral copper(I) bis-2,9-diphenyl-l,10-phenanthroline complexes (which form the catenate subunits) are good reductants in the excited state [97] therefore the chiral Cu(I) catenates could be used for enantioselective electron-transfer reactions. Alternatively, the resolution of topologically chiral molecules would allow to answer fundamental questions, such as what are the chiroptical properties of molecular trefoil knots ... 

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