Catenanes topological chirality

Out of many exciting recently obtained topologically distinct structures combined rotaxanes 41,400 [23], pretzel-molecule 35 [24a] and bis(pretzelane) 401 [24b], a topologically chiral [2]catenane [25] and an interestiig catenated... 

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. 

Figure 29. Synthesis of macrocycle 65 and the corresponding, topologically chiral sulfonamide catenane 79.

Figure 47. The topological chirality of catenane 79 results from the different amide-sulfonamide succession in the macrocycles.

In the topologically chiral catenane 79 the succession of amide and sulfonamide units defines sequence information for each macrocycle (Figure 47). Consequently one wheel gives an orientation to space and the other defines the configuration [60 b], Remarkably, the enantioseparation performed by Okamoto and Vogtle et al. by HPLC on chiral column materials resulted in baseline separation with a very large separation factor a = 6.95 [61, 62],... 

Amide-linked catenanes and rotaxanes can play a major role in the study of rare forms of chirality, e.g. topological chirality and cycloenantiomerism [60]. Resolution of enantiomeric catenanes, rotaxanes, and pretzelanes has been achieved by HPLC on chiral column materials, but further work must be performed to determine absolute configurations and to realize new chiral skeletons composed of achiral building blocks. Topological asymmetric synthesis still belongs to dreams of the future yet should be kept in mind. 

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). 

In addition, sulfonamides bear more acidic, compared to carboxamides, protons that can be selectively A-alkylatcd or V-acylatcd. Thus, alkylation of sulfonamide catenane and rotaxane with Frechet-dendrons of 2nd generation led to the first representatives of the dendrocatenane 15 and dendrorotaxane 16 [46], respectively. These compounds were of especial interest, since they allowed for the first time to study chiral induction of topologically chiral cores on appended dendrons and to compare to the analogous centrochiral dendrimers for which phenomenon of the crypto-optical activity was postulated [47],... 

Yamamoto, C., Okamoto, Y., Schmidt. T., Jager, R., Vogtle, F. Enantiomeric resolution of cycloenantiomeric rotaxane, topologically chiral catenane, and pretzel-shaped molecules observation of pronounced circular dichroism, J. Am. Chem. Soc. 119 (1997), 10547-10548. 

In contrast to bis[2]catenanes 12, which are topologically chiral, bis[3]catenanes 13 and cyclic [8]catenanes 14 prepared via metathesis and subsequent hydrogenation are usually not chiral. For identical loops, their symmetry (without and with the directionality of the hydrogen-bonded belt) is C2v/ C2 and DAd/ Ss, respectively. This is reflected by their NMR-spectra (for an example, see Figure 5.17). 

Fig. 20. Two topologically chiral catenanes 71 and 72 with oriented rings m and n are the numbers of atoms in-between the substituents of the constitutive macrocycles. Note that these catenanes lose their chirality when m = n...

Keywords Catenanes Copper complexes Interlaced design Knots Kuratowski s graphs Mobius strip Topological chirality Topology... 

Finally, it needs to be noted that while a simple [2]-catenane can be made topologically chiral by orientation of the rings - see 83, a four-crossing [2]-catenane such as 77 is automatically chiral without the need for such orientation. 

Recently Vogtle and co-workers synthesized the first catenanes (Structure 53) and rotaxanes containing sulfonamide units. The sulfonamide catenanes 53a, 53b have a topologically chiral structure [67]. 

Scheme 2 Two enantiomers of a topologically chiral sulfonamide [2]catenane and their CD spectra...

Let us assume that a rotaxane is formed from an axle with two different stoppers and a wheel that bears a directionality defined by the sequence of atoms in the macrocycle. Both of these subunits are achiral, both are identical to their mirror images. Nevertheless, the corresponding rotaxane exists in two enantiomeric forms, although no chiral center is present in the molecule (Fig. 4). It is the molecular topology that makes these species chiral, and consequently, this phenomenon was coined "topological chirality.. It also exists for catenanes, knots, and other mechanically bound molecules. Shown in Fig. 4 is a detailed structure of such a rotaxane. The wheel contains one sulfonamide group that provides the directionality of... 

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