Rotaxanes with dendritic stoppers

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.

Figure 20 A rotaxane with dendritic stoppers. An example of a dendritic-linear-cyclic architectural copolymer. (Courtesy Chem. Rev. 97 1701, 1997. Copyright 1997 American Chemical Society.)...

Fig. 6.24 Synthesis of a rotaxane (schematic) with dendritic stopper (cone) and tri-tylphenol stopper (sphere). SN = nucleophilic substitution. The grey arrows indicate dethreading of the wheel (red ellipse) - to the left or the right according to choice - from...

Besides simple rotaxanes and catenanes, a great variety of more complex systems have been synthesized, including branched [n]rotaxanes [26a], [n]rotaxanes bearing dendritic stoppers [26b], catenanes composed by two, three, five, or seven interlocked macrocycles [26c,e], polyrotaxanes and polycatenanes [2-6, 8], catenanes with very special shapes [27], rotocatenanes [28], pretzelanes [29], and knots [4], Some of these compounds are shown in Figure 8. 

Polyethers, either involving aromatic groups or not, have shown wide application in the formation of this class of rotaxanes. In the case of the former type, the interaction of the electron-rich aromatic groups with the bipyridinium units is the most often used as the means to form the precursor and is demonstrated by rotaxane 95 in which dendritic stoppers are used <1996JA12012>. In the case of the latter type, the interaction of the ether oxygen atoms in the ring... 

Selected examples of rotaxanes bearing very bulky stoppers include a [2]rotaxane with fullerenes <2003AGE3158>, [2]-, [3]-, and [4]rotaxanes end-capped with dendritic moieties <1996JA12012>, as well as derivatives 142 <2000CC847>, 143 <2001NJC166>, and 144 <2003EJO3744>, which rely on axially coordinated metalloporphyrins as stoppers. 

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