Formation of Rotaxanes

When diphenyhnagnesium is crystallized from a solution containing l,3-xylyl-18-crown-5, an X-ray crystal-structure determination showed the formation of rotaxane 116 (Figure 56) . Only four of the five oxygen atoms of the crown are involved in coordination to magnesium, two with a relatively short bond distance [2.204(3) and 2.222(4) A] and two with a longer bond distance [2.516(4) and 2.520(4) A]. The C(l)-Mg-C(2)... 

The amide hydrogen-bonding motif has been shown to support the clipping, threading, and slipping approaches to the formation of rotaxanes. The wide scope of the neutral template synthesis is enriched by the promising possibilities of the non-template synthesis performed in the melt. Numerous rotaxanes were obtained on a preparative scale and are available for further chemistry. 

Figure 9. a) The amide-mediated template which was hypothesized to provide access to rotaxane 22. b) A control reaction which was thought not to yield any rotaxane, but led to almost quantitative formation of rotaxane 24. 

Scheme 5.7 Pathways leading to the formation of rotaxanes or catenanes.

Figure 4.21 Formation of rotaxanes 53 and 54. Each preparation is directed by the formation of the corresponding intermediate host-guest complex shown ...

Metal-Templated Formation of Rotaxanes and Other Rack or Grid Structures... 

Fig. 3 shows the routes by which components bearing suitable recognition sites lead to the formation of rotaxanes ... 

The templates used by Stoddart [39a, b,c] in the syntheses of catenanes and rotaxanes use r- r interactions to enhance both cyclization and interweaving. The template directed cyclization (Scheme 1-9) results in rotaxane formation if the ends of the template 33 are made so bulky that it cannot escape from the macrocycle, and in catenane formation if the two ends of the template are joined to form a macrocycle interlocked with the new macrocycle. The template is an integral part of the final product [39a,d]. Cucurbituril 48 can act as both a linear template and a topological template in the formation of rotaxane, as discussed later (Scheme 1-20). 

These examples of template effects illustrate the close relationship between host-guest and rotaxane chemistry. Appropriate concave host molecules bind convex guests in such a way that they can be equipped with two stoppers that mechanically prevent dissociation of the host-guest complex. Any macrocyclic receptor may thus be suitable for the formation of rotaxanes. 

Later, gel formation was observed by Garcia et al. during the preparation of polyamides containing the benzo-18-crown-6 unit, and was ascribed to the formation of rotaxane/polyrotaxane and catenane/ polycatenane structures [156]. However, as the ring size of the benzo-18-crown-6 was considered too small to be threaded, it was thought uidikely that gel formation would occur due to catenane formation via threading of the crown ether moieties. 

The formation of catenanes by CDs is less explored than the formation of rotaxanes. Part of an extensive study of the formation of several catenanes in aqueous solution is shown in Figure 21 When the bitolyl derivative diamine chain link precursor 104, in which n is either 3... 

The use of imines as the dynamic covalent bond significantly augmented the versatility of the thermodynamic approach toward the formation of interlocked systems. Stoddart et al. reported on the synthesis of [2]rolaxanes by the clipping protocol, in which a dialdehyde was used to clip the macrocycle under thermodynamically controlled conditions (Figure 6). In this system, the driving force for the formation of rotaxane 10 is the stabilizing interaction between ammonium ions and crown ethers, which was widely exploited by Stoddart and coworkers. In the first instance, a library of macrocyclic and linear compounds was prepared on condensation of tetraethylene glycol bis(2-aminophenyl)ether 6 and 2,6-diformylpyridine 7. 

In the remaining sections of this chapter, the physical and host properties of CB[n] as they relate to their apphcations as nanoreactors will be discussed in detail, and a detailed representative survey of the specific types of reactions which have been templated or catalyzed by them will be presented. This is not intended to be a comprehensive review, as the number of publications on CB[n] both as hosts and as nanoreactors is extensive. Furthermore, the many other various uses of CB[ ] (briefly described previously) for the formation of rotaxanes and other interlocked species, self-assembled capsules and adducts, and in drug dehvery, as inhibitors of reactions, and as components of nanomachines and nanostructures, will not be covered. This chapter will, in a nutshell, explore the uses of CB[ ] as nanosize reaction flasks. 

Severin s group also prepared the first examples of boron-based rotaxanes utilizing boronic esters as stoppers. The MCR of l,2-di(4-pyridyl) ethylene 16, catechol 17, 3,5-bis(trifluoromethyl)phenylboronic acid 18, and 1,5-dinaph-tho-38-crown-lO 19 resulted in the formation of rotaxane 20 (Scheme 4.12) [14]. 

A similar active-template strategy was applied by Goldup etal. forthe formation of doubly interlocked [3]rotaxanes [93]. A photochemical thiol-yne click reaction (a variant of thiol-ene click) was used for the formation of rotaxanes, proceeding mildly at ambient temperature and humidity under an air atmosphere [94]. 

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