Rotaxanes, containing cyclodextrin

For examples of cyclodextrin-containing rotaxanes, see (a) Stoddart, J.F. Angew. 

The formation of pseudorotaxane complexes in aqueous solutions has also been exploited [10] to template the synthesis of cyclodextrin-containing rotaxanes. In the example illustrated in Figure 3-8, the diamine 29 is mixed [26, 27] with a methylated a-cyclodextrin derivative - either 30 or 31 - in aqueous solution. In both instances, a pseudorotaxane complex self-assembles spontaneously. The subsequent covalent attachment of bulky stoppers is achieved by reacting these complexes with sodium 2,4,6-trinitroben-zenesulfonate 32. The resulting [2]rotaxanes 33 and 34 can be isolated in yields of 42 and... 

Figure 30. The threading approach to the cyclodextrin-containing [2]rotaxanes 96 and 97.

Figure 3-8 Template-directed synthesis of cyclodextrin-containing [2]rotaxanes 33 and 34.

An interesting example of a system whose components become mechanically bound upon surface immobilization is rotaxane trans-21 (Fig. 13.24), consisting of a ferrocene-functionalized /3-cyclodextrin (fi-CD) macrocycle threaded on a molecule containing a photoisomerizable azobenzene unity and a long alkyl chain.33 A monolayer of trans-21 was self-assembled on a gold electrode. Therefore, the ring... 

Yamaguchi, I., Osakada, K., Yamamoto, T., Polyrotaxane containing a blocking group in every structural unit of the polymer chain. Direct synthesis of poly(alkylenebenzimidazole) rotaxane from Ru complex-catalyzed reaction of 1,12-dodecanediol and 3,3-diaminobenzidine in the presence of cyclodextrin. J. Am. Chem. Soc. 1996, 118, 1811-1812. 

Any artificial rotaxane-based mimic of the natural ATP synthase motor must bear a chiral element that helps to define the direction of rotation. Chirality can, of course, be implemented in such molecules by adding chiral groups as the stoppers or the wheel. Rotaxanes with cyclodextrins as the wheels have been described [11], and rotaxanes with glucose-containing stoppers are known [14]. Rotaxanes with elements of planar chirality have also been realized [15]. 

The first example of a photoresponsive [2]rotaxane, published in 1997 by Nakashima and co-workers, is one of those cases [61]. Molecular shuttle E/Z-224+ consists of an a-cyclodextrin macrocycle, and a tetracationic thread containing an azobiphenoxy moiety, very closely related to azobenzene, and two bipyridinium stations. The well-known E-Z isomerizations of azobenzenes and the ability of cyclodextrins to bind lipophylic compounds in water are exploited in this system to achieve shuttling. When the azobiphenoxy station is in its trans form, E-224+, the cyclodextrin encapsulates it preferentially over the more hydrophilic bipyridinium station (Scheme 12). 

A. Harada, J. Li, M. Kamachi, The Molecular Necklace A Rotaxane Containing Many Threaded a-Cyclodextrin , Natme, 356, 325 (1992)... 

When supramolecular polymers are treated with bulky stopper groups, they may form poly[2]rotaxane daisy chains [32,60-68]. Cyclic tri[2]rotaxanes (daisy chain necklace) containing cyclodextrins have been prepared from the mixture of 6-(4-aminocinnamoyl)-Q -CD and 2,4,6-trinitrobenzene sulfonic acid sodium salt [50,59] in an agueous solution (Fig. 21). If the molecule changes its conformation (or co-conformation), the ring may expand or shrink by external conditions (temperature, solvents, photochemically, elec-trochemically). These compounds are important because the cycle can be used as a chemical valve as seen in ion channels in biological membranes. 

Since a very large number of rotaxanes and poly rotaxanes that contain cyclodextrins and cyclodextrin derivatives are known, discussion of only a representative selection of such compounds is presented in this section. 

Since these studies, many other rotaxanes incorporating cyclodextrins or their substituted derivatives have been reported, including other metal-containing systems. The latter include a series of a-cyclodextrin rotaxanes formed by the reaction of labile [Fe(CN)50H2J ions with pre-threaded l,l"-(a,(o-alkanediyl)-bis(4,4 -bipyridinium) dicationic moieties (incorporating methylene chains between 8- and 12-members long). " ... 

A photochemically induced ring movement has been obtained in a [2]rotaxane based on Cu+ [45], as described in detail in Volume III, Part 2, Chapter 8. Shuttling of a a-cyclodextrin torus along an azobiphenoxy-containing dumbbell has also been obtained by exploiting the trans/cis photoisomerization of the azobiphenoxy unit... 

Rotaxanes are considered to be one typical prototype of molecular devices machines, because they have a rooter and an axle in the molecule. a-Cyclodextrin was first used as a rotor of rotaxane by Ogino et al. in 1978. ° They used metal complexes as stoppers. Since then some rotaxanes containing cyclodextrins have been reported (Fig. 1.20). Most of them used metal complexes as stopper groups. However, the metal coordination bonds are so weak that cyclodextrin rings may escape from the bond. All the rotaxanes reported are ionic so as to solubilize the rotaxanes in water, except one example in which all the components are non-ionic and soluble in organic solvents." In this case there are some inter-... 

Fig. 1.32 Daisy chain necklace, cyclic tri[2]rotaxane containing a-cyclodextrin.

In more recent times, many structures based on substituted tetrahedra such as phosphonate RPO3 or phosphinate R2PO2 have been explored. This structural world is now merging with that of phosphate-containing calixarenes, cavitands, cyclodextrins, rotaxanes and other large organic ring compounds. 

---