Rotaxane stoppers

Figure 4.23 A cyclodextmn-hased rotaxane stoppered with cobalt(III) complexes ...

The synthesis of [3]- (figuratively shown as 7) and a [5]rotaxane (8) with one central and two terminal porphyrins in the open configuration has been reported <96AG(E)906> also a rotaxane with two Ru(terpy>2 stoppers has appeared <96CC1915>. A pseudorotaxane comprised of a macroring of 2,9-diphenyl-1,10-phenanthroline unit and a molecular string... 

Rotaxane Dendrimers Bearing Dendron Stoppers Type I-A.117... 

Fig.1. Rotaxane dendrimers from crown ether type macrocycle ring, bipyridinimn derivatives rod component and Frechet-type dendron stoppers...

Fig. 2. Rotaxane dendrimers from isophthalamide [2]rotaxane-core and Frechet-type den-dron stoppers...

Fig. 3. Rotaxane dendrimers incorporating cucurbituril threaded on l,6-di(bipyridmium)hex-ane with Fr chet dendrons as stoppers...

The axial coordination of metalloporphyrins to a pyridyl ligand was successfully exploited by two groups to produce porphyrin-stoppered rotaxanes. Sanders (48) assembled a rotaxane by simply mixing the constituent parts. Zn(II), Ru(II)CO, and Rh(II)Cl porphyrins were used as stoppers. Branda (49) reported the stoppering of a pseudorotaxane by adding two equivalents of a Ru(II)CO porphyrin that coordinated to... 

In a recent report [141] Stoddart et al. reported a new class of rotaxanes with dendritic stoppers by using a so-called threading approach (Fig. 25). Alkylation of bipyridinium based units with Frechet s third tier branched aryl ethereal dendron, in the presence of BPP34C10 afforded 58 as one of the products. Variable temperature H-NMR spectroscopy in different NMR solvents helped determine the novel shuttling process of BPP34C10 from one bipyridinium unit to the other in 58. The dendritic framework of 58 assists in its solubility in a wide range of solvents. 

Fig. 25. Rotaxanes with dendritic stoppers synthesized by Stoddart et al.

Pseudorotaxanes are precursors of both rotaxanes and catenanes they consist of a guest molecule threaded through a macrocyclic host. Stoppering both ends of the threaded molecule gives a rotaxane, cycliza-tion of the thread gives a catenane. Pseudorotaxane formation may occur by spontaneous self-assembly, or may be template-controlled. Anion size can be of paramount importance for such templates - Cl- is effective, Br, I- less good, and PFe ineffective when the recognition motif demands a small template (454). 

Concept Rotaxane-like entities are created by the slippage protocol in which the cavities of macrocyclic polyethers dilate", at elevated temperatures, so as to allow their passage over the relatively bulky stoppers of chemical dumbbells (see Cartoon below). Similarly,... 

Reaction (Scheme 2) of salt 5-2PF6 with an excess of 6, in the presence of the macrocydic polyether BPP34C10, provided1181 [BPP34C10-7] [PF6]4 via a threading-followed-by-stoppering approach.1231 This rotaxane incorporates a free 4,4-bipyridinium recognition site within its dumbbell-shaped compo-... 

The term rotamne derives (G. Schill, Histoiy , In Catenaries, Rotaxanes, and Knots, Academic, New York, 1971, pp. 1-4) from the Latin words rota, meaning wheel, and axis, meaning axle. In these molecular compounds, several wheel and axle components are constrained to be bound to one another mechanically, Le., without the aid of any valence forces, since the axle(s) is/are endowed with bulky stopper groups that prevent the extrusion of the wheel(s). 

Kinetically stable superarchitectures can be assembled by relying upon both noncovalent bonds and mechanical coercion. Thus, at elevated temperatures, rotaxane-like complexes (a) are generated when macrocycles slip over the stoppers of chemical dumbbells, while hemicarceplexes (b) are created when guests squeeze through the portals of hemicarcerands. 

With this separation of the hydrogen bond-acceptor template and the sites for the stopper attachment, the steric effects due to shielding by the wheel are avoided making the synthesis of the rotaxane 48 more efficient (in some of the systems the yields of rotaxane formation increase from 5 to 30% when the functionalised centre-piece is added). 

The templated syntheses of amide-based rotaxanes discussed until now have made use of the threading-followed-by-capping method. However there are also examples in which the clipping approach has been employed. Leigh, for example, has used a five-component clipping method to prepare [2]rotaxanes. Isophthalamide and peptide-based threads were shown to template the formation of benzylic amide macrocycles about them in non-polar solvents [69, 70]. When the peptide-based threads (49) contain bulky stoppers at their ends, the [2]rotaxanes (50) can be prepared in high yields (see Scheme 24) [71]. 

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