Topology rotaxanes

Besides their topology, rotaxanes and catenanes are also appealing systems for the construction of molecular machines because (i) the mechanical bond allows a large variety of mutual arrangements of the molecular components, while conferring stability to the system, (ii) the interlocked architecture limits the amplitude of the intercomponent motion in the three directions, (iii) the stability of a specific... 

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... 

Structurally related to these species are the triply branched compound 56+ and its rotaxanes 66+, 76+, and 86+ (Fig. 13.6)9, in which one, two, or three acceptor units are encircled by the electron donor macrocyclic compound 2. Although these rotaxanes cannot behave as degenerate molecular shuttles because of their branched topology, they are nevertheless interesting from the electrochemical viewpoint. 

Amide-linked catenanes and rotaxanes, readily accessible via nonionic template synthesis, are molecules with fascinating topology, but their final breakthrough was only achieved when they became substrates for further chemical derivatiza-tion. A potential reaction site in the catenanes and rotaxanes presented so far is the sulfonamide group, because the proton can be selectively abstracted in the presence of carbon amide groups. Subsequent alkylation offers numerous possibilities of catenane and rotaxane chemistry. 

Sulfonamide groups incorporated in rotaxanes enable the construction of new topological assemblies provided with mechanically and covalently bonded subunits. Methylation of a [2]rotaxane containing a sulfonamide unit in the axle revealed that the substitution reaction is not sterically hindered by the macrocycle. Similar to the synthesis of the pretzelane 96, the two sulfonamide groups of rotaxane 80m were bridged with 95 to form 100 in 71% yield (Figure 39) [46]. The additional covalent bond converts the former [2]rotaxane into a [l]rotaxane and reduces the mobility of the wheel along the axle. Rotaxanes 80m and 100 are his-... 

The great synthetic potential of the covalent linkage of catenanes and rotaxanes via sulfonamide groups in such a way enables the chemist to construct larger topologically interesting assemblies and to aim at nanoscale architectures. 

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. 

Numerous examples of catenanes 1, rotaxanes 2, and trefoil knots 3 (Scheme 1) have been previously reported in the literature and are still attracting considerable attention (see Chapters 4 and 6-8) [1-5]. These aesthetically appealing molecules have in common that the topological bonds occurring in catenanes 1 and trefoil knots 2 and the mechanical bonds connecting the component parts of rotaxanes 3 are defined at a molecular scale without ambiguity [1, 2, 4]. 

Molecular catanes, rotaxanes and knots a journey through the world of molecular topology / E. Wassermann. .. Ed. by J.-P. Sauvage and C. Dietrich-Buchecker. - Weinheim New York Chichester Brisbane Singapore Toronto Wiley-VCH, 1999 ISBN 3-527-29572-0... 

This is particularly true, since rotaxanes and catenanes have entered a stage in their development, in which they have undergone the transition from beautiful structures with ever-new topologies to those which possess function. Many so-called molecular devices have been synthesized and studied with respect to a controlled motion of the two components relative... 

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). 

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],... 

Sauvage, J.-P., Dietrich-Buchecker, C. (eds.), Molecular Catenaries, Rotaxanes and Knots, A Journey Through the World of Molecular Topology, Wiley-VCH, Weinheim, 1999. 

Dodziuk, H., Nowinski, K.S. Topological isomerism should rotaxanes, endohedral fullerene complexes and in-out isomers of perhydrogenated fullerenes be considered as such Tetrahedron 54 (1998), 2917-2930. 

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