Rotaxane host-guest

Keywords. Rotaxane dendrimers. Host-guest interaction, Recognition, Self-assembly, Supra-molecular chemistry... 

Vogtle has developed this approach further and employed a series of anionic templates to prepare rotaxanes (instead of the neutral template in the above reaction) [65-67]. In this approach a phenolate, thiophenolate or sulfonamide anion is non-covalently bound to the tetralactam macrocycle (46) forming a host-guest complex via hydrogen bonding (see Scheme 21). 

Loeb has reported a series of pseudorotaxanes [84,85] and rotaxanes [86,87] where C-H- 0 hydrogen bonding interactions (together with N+- -O attractive forces) play an important contribution in templating the formation of the interlocked species. In particular, the formation of a pseudorotaxane was observed when equimolar amounts of [pyCH2CH2py]2+ and the crown ether 20 were mixed. The structural characterization of the resulting host-guest complex... 

Analysis of the available literature data on host-guest complexes based on cyanine and styryl dyes with CDs shows that rotaxane formation in general... 

Like the currently popular area, called nanoscience , the field of supramolecular chemistry has rather hazy boundaries. Indeed, both areas now share much common ground in terms of the types of systems that are considered. From the beginning, electrochemistry, which provides a powerful complement to spectroscopic techniques, has played an important role in characterizing such systems and this very useful book goes considerably beyond the volume on this same topic by Kaifer and Gomez-Kaifer that was published about 10 years ago. Some of the classic supramolecular chemistry topics such as rotaxanes, catenanes, host-guest interactions, dendrimers, and self-assembled monolayers remain, but now with important extensions into the realms of fullerenes, carbon nanotubes, and biomolecules, like DNA. 

Cucurbit [6] uril and increasingly its higher homologues have been used extensively in a variety of host-guest chemistry and nanostructure assemblies. Of particular appeal are rotaxanes and molecular necklaces, mechanically interlocked assemblies of molecules based on CB [6]. Efficient synthesis of ID, 2D and 3D polyrotaxanes and molecular necklaces (cucurbituril beads linked by a macrocyclic molecule string ) has been achieved by a combination of self-assembly and coordination chemistry. We discuss rotaxanes and molecular necklaces in Section 10.7, and cucurbil-based systems are summarised in a recent review.23... 

Figure 10.53 Synthesis of catenanes and rotaxanes via host-guest chemistry.

Fortunately, more efficient methods for the complexation of macrocyclic hosts with acyclic guest molecules have become available with the advent of supramo-lecular chemistry, resulting in higher yields in rotaxane and catenane synthesis. In the following sections, the preparation of different types of interlocked molecules, with the use of host-guest recognition, is discussed. It should be noted that these template-directed methods differ significantly from the above-mentioned stochastic approach [28]. 

The supermolecules that have been synthesized include large metallocyclic rings, helices, host-guest complexes, and interlocked structures such as catenanes, rotaxanes, and knots. 

The successful syntheses of the above host-guest assemblies pointed the way to the formation of molecular threads - namely, compounds formed by threading a cyclic species, such as the cyclobis(paraquat-/ -phenylene) tetracation, on to a separate extended linear component the product is a pseudorotaxane. Pseudorotaxanes are a derivative category of the rotaxanes in which one of the assembled components is a longer (often oligomeric) linear fragment. They are thus similar to rotaxanes, with the exception that the linear component is not terminated by bulky stoppers . 

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

A particularly interesting family of host-guest systems [1] is that in which a linear guest is threaded through the cavity of a macrocyclic host (Figure 1). A compound which exhibits such a supramolecular structure is usually called pseudorotaxane [2], with reference to the name rotaxane (see Volume III, Part 2, Chapter 7) used to indicate the species in which the ring is prevented to dethread by the presence of bulky stoppers at the ends of the linear guest. 

Cyclodextrin is a well-known non-aromatic host molecule, which serves not only in host-guest chemistry but also in supramolecular chemistry as a component of the topochemical molecular architecture along with rotaxane and polyrotaxanes. 

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