Rotaxanes topological chirality

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). 

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. 

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... 

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. 

Schalley, C.A., Beizai, K. and Voegtle, F. (2001) On the way to rotaxane-based molecular motors studies in molecular mobility and topological chirality, Acc. Chem. Res. 34, 465-476. 

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],... 

Scheme 6.5.4. A topologically chiral rotaxane. Arrows indicate the directionality imposed by the sulfonamide group in the wheel.

C.A. Schalley, K. Beizai, F. Vogtle, On the Way to Rotaxane-Based Molecular Motors Studies in Molecular Mobility and Topological Chirality , Acc. Chem. Res., 34, 465 (2001)... 

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]. 

Molecular mobility and topological chirality of rotaxanes 01ACR465. Rotaxanes. From supramolecular self-organization to molecular devices 00MI21. 

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... 

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

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