Multiple rotaxanes

Tetra-ureas 20 form homodimers in solvents such as CDC13 or C6D6, but they are completely converted to heterodimers upon addition of an equimolar amount of a tetra-loop compound 9. When a 5-10% excess of the anthracene derivative 21 was applied, a quantitative conversion was obtained in refluxing toluene after 72 h (as judged by NMR), and the tetra[2]rotaxane 22 was isolated in 40-50% yield. 

The rotaxane structure of 22 can be demonstrated by ESI-MS or MALDI-TOF-MS. It remains intact in TH F-d8, a solvent in which the tetra-urea dimers usually dissociate. However, an upfield shift of the NH-signals in 1 H NMR indicates that the 

Y=R = C5H11) by stoppering. Introduction of bulky stopper residues by Diels-Alder cycloaddition. 

The synthesis of fourfold [2]rotaxanes by clipping requires the efficient formation of heterodimers between a tetra-urea substituted by bulky stopper groups and an octaalk-enyl urea 6. We initially hoped that the steric crowding in homodimers of a tetra-tritylphenylurea calix[4]arene would be sufficient to shift the equilibrium toward the heterodimers in a mixture with 6. However, the distribution of the dimers was close to the statistical ratio and the desired rotaxane could be obtained in only 5% yield [59]. 

ESI-MS. Obviously heterodimers oftetra-arylureas, e.g., tolylurea (1), with 4 have a larger internal volume than homodimers 1 1 (260-270A3 vs. 190-200 A3, based on MD simulations) [61] because of differences in the hydrogen-bonding pattern [62], 

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The preorganization of functional groups attached to the urea residues could also be used for the synthesis of multiple rotaxanes. Schematically, this is shown for tetra[2] rotaxanes in Scheme 5.20. Bis[2]rotaxanes will be available analogously, using a bisloop compound 8 instead of the tetraloop compound 9. 

This approach can also be extended to the preparation of multiple rotaxanes down a single, flexible, filamentous ligand. However, as with helicates, rigid or semiflexible spacers are needed... 

Topologically novel multiple rotaxanes and catenanes based on tetraurea calix[4]arenas 06CC2941. 

In a somewhat related work, Nolte, Rowan et al. [13] described in 2003 a rotaxane complex that mimics the ability of processive enzymes to catalyze multiple rounds of reaction while the polymer substrate stays bound. The catalyst, which consists of a substrate-binding cavity incorporating a manganese] 111) porphyrin complex acting as the catalytic center, can oxidize alkenes complexed within the toroid cavity, provided a ligand has been attached to the outer face of the toroid to both activate the porphyrin complex and prevent it from being able to oxidize alkenes outside the cavity. 

To overcome the low yields encountered in statistical methods, Schill and coworkers imaginatively introduced the chemical conversion method [4, 5, 16]. As illustrated in Figure 2, this method requires very careful design (i) the cavity of a cyclic species covalendy linked to a difunctional linear species should be penetrated by the linear species, structure 14 and (ii) both the cyclic and the linear moieties must be inert to the cleavage reaction of the covalent linkage Z between them. By this means, the yield for rotaxane synthesis was increased to about 40% in last step. The disadvantages of this method are its multiple steps and time-consuming nature. 

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... 

Multiple ring-closure reactions between adjacent urea functions of a suitably functionalized tetra-urea in a heterodimer with a second tetra-urea bearing bulky groups can also create the structural elements of a rotaxane. This strategy is usually called clipping (see Scheme 5.7). 

Whereas neither I.U.P.A.C. nor nodal nomenclature bother to extend their system to what is, at present, a very small group of topologically restrained compounds, the proposed system allows for a simple extension to the set of catenanes and rotaxanes. Also, one may readily adapt this system to canonically naming endothelial compounds, such as endothelial fullerenes, etc. Moreover, extension to "chemical" knots having multiple interconnections and windings, without having to resort to the tedium employed by Schill to extend I.U.P.A.C. nomenclature to these compounds, is straightforward. 

Continuing to other combinations, one could expand upon Schill s work and note that multiple strands in a rotaxane can be of two types The first of these, which Schill included, is to have multiple rings surround a single axis, as is illustrated in Figure 6. IUPAC names this aggregation [3]-[ 1,20-diaryl-eicosane]-[cycloeicosane]-[cycloeicosane]-rotaxane in contrast to the much simpler systemic name of ... 

Dendrons have also been used as the capping moieties for rotaxane cores (Figure 8). Amabilino et al. reported the Williamson coupling of the third generation Frechet-type dendrons 6 to cap polyether macrocycles threaded around linear bipyridinium oligomers.241 When the linear component consisted of multiple viologen units, 171, shuttling of the... 

Fig. 3 In polar aprotic solvent, a salt-binding [2]rotaxane populates multiple axle/wheel orientations. Binding CP does not measurably alter the rotaxane s dynamic properties, whereas binding K " freezes out a single coconformation (molecular brake). (Reprinted with pennission from Ref. [22]. Copyright 2002, Elsevier Science.)...

Fig. 8 The three families of oligorotaxanes, exemplified by compounds derived from a heptameric DNP thread. The rotaxanes moods depend on their adherence to a selection rule forbidding CBPOT " to occupy adjacent DNP sites. Whereas Confiased rotaxanes can adopt multiple stable translational isomers and Frustrated rotaxanes cannot obey the rule at all, Happy rotaxanes express a single rule-compliant isomer...

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