Functionalization of pillararenes

Graduate School of Natural Science and Technology, Kanazawa University, Kakuma-machi, Kanazawa 920-1192, Japan Email ogoshi se.kanazawa-u.ac.jp 

We have introduced a nomenclature system for identifying the functionalizing position in pillar[n]arenes. The numbers and letters refer to the upper/lower rims and unit positions of the pillar[n]arenes, respectively. Mono-, di-, tetra-, penta- and per-functionalized pillar[5]arenes have been obtained to date. In the case of pillar[6]arenes, mono-, di- and per-functionalization have been achieved. Mono- and per-functionalized pillar[ ]arenes have only one constitutional isomer. Therefore, synthesis of mono- and per-functionalized pillar[ ]arenes is relatively easy. However, multi-functionalized pillar[n]arenes have many conformers. For example, di-functionalized pillar[5]arenes have five possible conformers (A1/B2, All Cl, A1/A2, Al/Bl and A1/C2). The number of possible conformers increased as the number of the functional groups increased the tri-functionalized pillar[5]arene has 10 constitutional isomers (Al/Bl/Cl, Al/Bl/Dl, A1/B1/C2, A1/B1/D2, A1/A2/B1, A1/A2/B2, A1/A2/C1, A1/A2/C2, A1/B2/C1 and A1/B2/D1). As the number of rings increased, the number of conformers also increased di-functionalized pillar[6]arenes have seven isomers (Al/Bl, Al/Cl, Al/Dl, A1/A2, A1/B2, A1/C2 and A1/D2), while di-functionalized pillar[5]arenes have five. Isolation of a particular constitutional isomer from a mixture is quite difficult, therefore position-selective functionalization based on the unique 

2 Per-functionalization of Pillar[n]arenes by Pre-formed Pillar[n]arenes 

Pillar[5]arene with 10 carboxylate anions (3.13) can be used as nano-valves (details in Chapter 10) and Xe-based biosensors. Pillar[5]arenes with 10 ethylene oxide (3.18) and tri(ethylene oxide) moieties (3.19) were also synthesized by etherification. Pillar[5]arene with 10 tri(ethylene oxide) moieties 3.19 was completely soluble in aqueous media, while pillar[5]arene with 10 ethylene oxide moieties 3.18 was insoluble. The introduction of longer water-soluble parts is required to increase the water solubility. 

Interestingly, pillar[5]arene with 10 tri(ethylene oxide) moieties 3.19 was soluble in aqueous media at 25 °C, but not soluble with heating (45 °C), indicating that 3.19 showed a lower critical solution temperature (LCST). This is because pillar[5]arene with 10 tri(ethylene oxide) is an amphiphilic molecule the core of pillar[5]arene consists of water-insoluble hydrophobic benzene and methylene bridge moieties and the 10 tri(ethylene oxide) arms are water-soluble amphiphilic moieties. 

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In this chapter, we have tried our best to summarize different examples of the pillararene-based supramolecular polymers shown in the published journals so far. It is worth mentioning that many of these examples have been beyond the simple construction of supramolecular polymers. These outstanding researches are focusing on stimuli-responsive polymers, supramolecular gels, molecular devices, and the modification of polymeric materials. However, more functions and applications of pillararene-based supramolecular polymers need to be explored in the future. On the other hand, most supramolecular polymers discussed in this chapter are based on pillar[5]arenes, which is attributed to the facile synthesis of pillar[5]arenes when compared with other pillar[n]arenes. Therefore, the research on supramolecular polymers based on other pillar[n]arenes, especially pil-lar[6]arenes, will also be a challenge. 

Figure 7.1 The equilibrium of the self-assembly of mono-functionalized pillararenes.

Thus, the formation of a cylinder-like supramolecular dimer is the key step in the construction of some micellar and vesicular assemblies. Several methods can be employed to make building blocks having cylinder-shaped structures. For example, cylinder-like supramolecular dimers can be initially formed from mono-functionalized pillararenes through supramolecular interactions such as ti-ti stacking. Functional groups can be used to modify pillararenes, and the pillararene derivatives obtained may form daisy-chain type dimers through the complexation. However, such dimers prefer to form further supramolecular polymers rather than spherical assemblies at certain concentrations. Therefore, it should be noted that the functional groups... 

Numerous examples of artificial channels have been reported since the pioneering work of Tabushi using tetrachained cyclodextrin as a model. Of these examples, the macrocycles, such as crown ethers, calixarenes, cyclodextrins, and cucurbiturils, play an important role. The macrocycles provide not only the platforms for construction of whole channels but also the functional sites necessary to achieve transport selectivity and efficiency. From this viewpoint, pillararenes, new macrocycles with unique structural features, may also act as a platform for building such channels, which will give the channels new functions. This hypothesis has been systematically explored by us. 

Although pillararenes have been developed for several years, the chemistry of this family is beginning to blossom, as witnessed by the heightened interest in the field for the last several years. In particular, the efficient synthesis of the pillararene backbones and their subsequent direct functionalization have removed one of the major obstacles to pillararene chemistry and henceforth they will be available for exhaustive synthetic manipulations to pursue biomedical applications, including transmembrane transport and drug delivery, as we have demonstrated in this chapter. 

The LCST of the polymer is lower than that of typical PNIPAM due to the hydro-phobic nature of the dibenzofuran end functionality. However, the addition of CB[8] and M2V increased the hydrophilicity of the polymer due to the dibenzofuran unit by CB[8], increasing the LCST. Also, pillararenes are known as hosts that incorporate hydrophobic guests in water. Ogoshi et al. demonstrated the LCST behaviors of pil-lar[5]arenes 14 modified with oligoethylene oxide groups and LCST control by the addition of viologen derivatives (Figure 5.16). ... 

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