Cyclobis formation

Scheme 5.6 The general principle for the synthesis of catenanes (or rotaxanes) based on cyclobis(paraquat-p-phenylene) and crown ethers containing hydroquinone units. The dipyridyl units are alternatingly stacked with two hydroquinones because of n-n interactions which direct the ring-closure toward the formation of a catenane.

Similar results have been reported for the host/guest complex formation between a tetracationic cyclophane host [=cyclobis(paraquat-p-phenylene)j and the planar het-eroarene indole. The binding constants Kg, determined in six solvents of different polarities, increase with increasing solvent polarity, being ca. 100-fold greater in water than in acetone [306]. Further examples of solvent-dependent cyclophane/guest complexation reactions can be found in reference [299],... 

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 . 

Stoddart et al. have recently used iodide-catalyzed reversible nucleophUic substitution in the thermodynamically controlled assembly of a donor-acceptor [3]catenane 20 (Figure 1.31) [37]. Exposure of cyclobis-(paraquat-4,4 -biphenylene) tetrakis-hexafluorophosphate (18) to 2 equiv. of bis-para-phenylene [34]crown-10 (19) in the presence of tetrabutylammonium iodide in acetonitrile at 80 °C led to the formation of [3]catenane 20 in 80% yield (Scheme 1.9). The Sn2 reaction... 

A classic example of the formation of a macrocycle by a neutral template is that of the versatile host compound and component of molecular machines, the so-called blue box, or cyclobis paraquat-para-phenylene. Reaction of the horseshoe precursor with dibromo-para-xylene leads to the formation of a tricationic intermediate that is capable of binding the template molecule (Scheme 3), which closes the macrocycle to form the tetracationic cyclophane. The jT-ir interactions of the charge-transfer variety (the complex of the product and template is colored, whereas the components are not) assisted by the charge on ihe product are a major driving force in the process, as revealed in X-ray structures of complexes. It should be noted that the interaction is of the jr-n type assisted by the complementary positive charge on the bipyridinium residues and r-electron-rich nature of the template. This supramolecu-lar synthon can be used for other cyclophanes, catenanes, and rotaxanes (see Self-Assembly of Macromolecular Threaded Systems, Self-Assembled Links Catenanes, and Rotaxanes—Self-Assembled Links, Self-Processes). 

The formation of pseudopolyrotaxanes incorporating cyclobis(paraquat-p-phenylene) (75) and n-electron rich polyethers has been achieved [163-165] through a reliance on a combination of [C—H O] hydrogen bonds, [Ji ... 

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