Cyclodextrin rotaxanes

Fig. 1.18 The cyclodextrins (left to right) an a-cyclodextrin rotaxane [67], a fi-cyclodextiin complex with methylparaben [68] and y-cyclodextrin [69]...

In a related study, a- or P-cyclodextrin rotaxanes based on [(en)2CoNH2(CH2)2 S(CH2) jS(CH2)2NH2Co(en)2]Cl (where en = ethane-1,2-diamine) were synthe-... 

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). " ... 

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. 

Cyclodextrin Rotaxanes and Catenanes 7 Cyclodextrin Molecular Devices and 15... 

Zhao, T. and Beckham, H.W. (2003) Ditect synthesis of cyclodextrin-rotaxanated poly(ethylene glycol)s and their self-diffusion behavior in dilute solution. Macromolecules, 36,9859-9865. 

Template effects have been used in rotaxane synthesis to direct threading of the axle through the wheel. Since macrocycHc compounds such as cyclodextrins, crown ethers, cyclophanes, and cucurbiturils form stable complexes with specific guest molecules, they have been widely used in the templated synthesis of rotax-anes as ring (wheel) components. Here, we briefly discuss macrocycles used in the synthesis of rotaxane dendrimers and their important features. 

Park JS, Wilson JN, Hardcastle KI, Bunz UHF, Srinivasarao M (2006) Reduced fluorescence quenching of cyclodextrin-acetylene dye rotaxanes. J Am Chem Soc 128 7714—7715... 

Polymers are generally not discussed in this book. However, a polymer with rotaxane structures involving cyclodextrins in side chains 408 [36], pseudorotaxane superstructures [37], doubly twisted polyrotaxane [38] as well as infinite polyrotaxane network (Figure 8.2.5) [39] can be mentioned here. 

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... 

The ability of cyclodextrins to bind acyclic guests with pseudorotaxane geometries has been employed (Figure 30) by Harada et al. [46, 47] to self-assemble [2]rotaxanes after the threading procedure depicted schematically in Figure la. The diamine 92 is bound by the methylated a-cyclodextrins 93 and 94 with 1 1 stoichiometries in H20. Reaction of the terminal amine groups of the guest with... 

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

A similar template-directed approach was employed (Figure 31) by Anderson et al. [48] to self-assemble azo-dye [2]rotaxanes. The azobenzene diazonium salt 98 is bound by a-cyclodextrin 99 and also by / -cyclodextrin 100 in H20 with pseudorotaxane geometries. Reaction of the terminal diazonium functions with / -naphthol 101 affords the purple [2]rotaxanes 102 and 103 in yields of 12 and 15%, respectively. These two [2]rotaxanes were characterized by electrospray mass spectrometry which revealed peaks corresponding to the loss of all four sodium cations in both cases. By irradiating some of the protons of the dumbbellshaped components of these [2]rotaxanes, enhancement of the resonances of some of the cyclodextrin protons could be observed as a result of an intercomponent... 

Since caroviologens are rather fragile compounds, they can be protected from the environment by inclusion into polyanionic derivatives of (J-cyclodextrin in a rotaxane fashion 102 [8.156]. Also, in the design of molecular devices, it may be desirable to introduce some extent of redundancy in order to reduce the risk of device failure. This is the case in the tris-carotenoid macrobicycle 103 that represents a sort of triple-threated molecular cable whose crystal structure 104 has been determined. It forms a dinuclear Cu(i) complex 105 in which the bound ions introduce a positive charge at each of the species, a feature of potential interest for transmembrane inclusion [8.157]. 

PDMS chains have also been threaded through cyclodextrins, to form pseudo-rotaxanes.108... 

Cyclodextrin rings can be threaded by a long chain, and rotaxanes and polyrotax-anes are formed when stopper moieties are connected to the ends of the threading chain [93], Tamura and Ueno prepared a rotaxane, in which naphthalene-ap-... 

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... 

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