Rotaxane entities

Rotaxane entities bearing other molecular architectures have also been synthe-sised. For example, the clipping mechanism has been employed to clip the linked reagent 20 around two dumb-bell-shaped components of type 21, giving rise to the bis-[2]-rotaxane 22 shown in Figure 4.13. 

Rotaxanes are the compounds consisting of noncovalent entities called rotor and axle [77], Figure 21 illustrates them schematically. Initially, attempts were made to prepare them by statistical methods, so that the yields were necessarily very low [78-80], Recently, methods have been proposed for their more efficient synthesis, with renewed interest in their unique structure and properties. Section 4.1 summarizes some of the typical results obtained. 

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,... 

When rotaxanes and catenanes contain redox-active units, electrochemical techniques are a very powerful means of characterization. They provide a fingerprint of these systems giving fundamental information on (i) the spatial organization of the redox sites within the molecular and the supramolecular structure, (ii) the entity of the interactions between such sites, and (iii) the kinetic and thermodynamic stabilities of the reduced/oxidized and charge-separated species. 

Figure 35. Schematic representation depicting the self organization of rotaxane-like entities using the slipping approach in the melt (high concentrations of wheel and axle). R Rotax-ane where the hydrogen bonds between wheel and axle have not yet been formed. R Rot-axane with hydrogen bonds between wheel and axle. Entropy loss upon formation of the mechanical bond is not considered in the graph.

Although emphasis has been given so far to the self-assembly of positively charged rotaxanes, it needs to be noted that a number of uncharged rotaxanes, in which both the linear and cyclic components are neutral entities, have also been synthesised by a number of workers. Typically, these systems incorporate amide linkages and. 

Not only poly-pseudo-rotaxanes but also polyrotaxanes should be considered trivial topological entanglements, extending the concepts developed for the related molecular entities. By means of ideal continuous deformations we could separate the components of any finite portion of a polyrotaxane. How-... 

Mechanical bond Used to describe a situation where the separation of two or more molecular entities in a supermolecule is prevented by repulsive intermolecular interactions, in contrast to the usual case of dissociation inhibited by attractive intermolecular interactions. Examples include catenanes that are topologically constrained, and rotaxanes and carceplexes where dissociation is exceedingly slow. Dissociation of a mechanical bond usually involves the rupture of covalent bonds. The term constrictive binding has been used to describe the kinetic barrier to formation of carceplexes and hemicarceplexes as a measure of the magnitude of the repulsive intermolecular interactions involved in the dissociation of such complexes. 

The introduction of Tt-stacking interactions in supramolecular assemblies was driven mainly by the tremendous work of Stoddart and co-workers who took advantage of the presence of electron-poor bipyridinium dications and electron-rich aromatic polyethers to template assemblies by planar aromatic interactions leading to a library of supermolecules such as rotaxanes °and catenanes. Another pioneering effort was carried out by Sanders et al. who used neutral aromatic electron-poor and -rich entities instead of charged species (Scheme 17.5). 

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