Rotaxanes cyclodextrin-based

Examples of cyclodextrin-based [2]rotaxanes, which reflect the diversity of possible structures, are depicted in Figure 6.3. Aliphatic as well as aromatic compounds can complex with CDs. Different end groups can be used in the rotaxanes to prevent the dethreading of the CD, even within one rotaxane molecule as in 2 [32]. 

Figure 6.3 Examples of two cyclodextrin-based [2]rotaxanes possessing mainly hydrophobic interactions.

The first cyclodextrin-based rotaxanes were prepared by Ogino in 1981, l8a The idea was to use kinetically inert Co(III) complex fragments as stoppers. Therefore the molecular thread had to be functionalized with coordinating end groups. One of the synthetic routes is depicted in Figure 2.16a. 

Fig. 34 Example of mechanized mesoporous silica nanoparticles (MSNPs). SEM (a) and TEM (b) images show the structure and morphology of the MSNP platform [238]. (c) Structural formula of the a-cyclodextrin-based snap-top rotaxane that blocks the pores of an enzyme-cleavable mechanized MSNP. The stopper is connected to the stalk (dumbbell) by an ester or an amide bond [254]. (d) Release profile of rhodamine B from the snap-top MSNP. The addition of an esterase enzyme cleaves the ester bond, releasing the stopper, a-cyclodextrin, and cargo from the nanoparticles, which is monitored by the fluorescence intensity of rhodamine B. Controls employing an amide bond snap-top or deactivated enzyme do not release significant amounts of cargo...

While lying outside the scope of the present discussion, it is noted that a range of polymeric systems incorporating cyclodextrin-based rotaxanes, often with unusual architectures, have been synthesised. 

Finally, as mentioned earlier, a considerable number of other cyclodextrin-based rotaxane and pseudorotaxane systems have been reported but, by and large, they represent variations on a theme with respect to the individual systems discussed in this section. 

Figure 12 Threading by hydrophobic interactions. Preparation of the cyclodextrin-based [2]-rotaxanes (19n) [9, 17].

Cyclodextrin-based catenanes and rotaxanes, S. A. Nepogodiev and J. F. Stoddart,... 

Figure 16 A redox-switchable a-cyclodextrin-based [2]rotaxane R16 prepared by Click reaction.

Obtaining polyrotaxanes by the statistical method through polymerization of various monomers in the presence of macrocycles was attempted by Maciejewski who investigated the thermal radical copolymerization of vinylidene chbride, methyl methacrylate (MMA), acrylonitrile (AN), acrylamide, and thdr mixtures with ace-tonaphtylene in aqueous and dimethylformamide solutions in the presence of p-cyclodextrin or of its acylic derivative. All these attempts failed, however. Only, when copolymerization was initiated by y-radiation with an exposure dose of 5 Mrad, did some of the above-listed monomers form polymers exhibiting optical activity. However, the rotation of the initial p-cyclodextrin was 162°, while that of the obtained products only l°-6°. Such a small polarization plane rotation angle could result only from an insignificant amount of macrocycles in the synthesized compounds. Therefore, for the purpose of a more successful synthesis of cyclodextrin-based rotaxanes, a number of researchers tried to introduce elements of directionality into... 

Figure 4 The first rolaxane (4) bearing transition metal complex fragments as stoppers. It is also the first cyclodextrin-based rotaxane the a-cyclodextrin ring is threaded onto an alkyl chain ended by amino groups, which bind cobalt(III) centres coordinated to cthylcncdiaminc chelates [4a].

Polyrotaxanes have been prepared by threading multiple a-cyclodextrin units onto a bulky benzimidazole-based linear chain polymer with an aliphatic spacer where the cyclodextrin resides. The rotaxane forms as the cyclodextrin-bound precursor amine is polymerised. Compared to the polymer in the absence of the cyclodextrins, the glass transition temperature is raised by some 20 °C even though only ca. 16 % of the aliphatic spacers in the polymer are rotaxanated.28 A novel approach to polyrotaxanes involved the use of a metal ion such as Zn2+ to thread a phenanthroline-based macrocycle onto a thiophene-based complementary monomer. The resulting pseudorotaxane can then be electropolymerised and the Zn2+ ions removed to give a polyrotaxane, Scheme 14.5. The redox and conductivity properties of the polymer are very much dependent on whether a metal ion is bound or not.29... 

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]. 

---