Pillar arene

Hu X-B, Chen Z, Chen L, Zhang L, Hou J-L, Li Z-T. Pillar[ ]arenes (n = 8-10) with two cavities synthesis, structures and complexing properties. Chem Commun 2012 48 10999-1001. 

In this chapter, we present the investigations of the major structural and physical properties of macrocyclic compounds in historical order. First, we compare the chemical and X-ray ciystal structures and the physical properties of the four key classes of compounds, cyclodextrins, crown ethers, calbc[n]arenes and cucurbit[n]urils, which have been dominant in the field of supramolecular chemistry. Other well-known and useful host molecules, and recently discovered compounds with great potential in the field of macrocyclic chemistry, are also described. Second, we discuss the desired properties of macrocyclic compounds, and the advantages of the novel macrocyclic compounds pillar[ ]arenes compared with the four key classes of compounds. 

Table 1.1 presents the advantages and disadvantages of typical macro-cyclic compounds in addition to pillar[ ]arenes. 

The first pillar[5]arene (2.1), which consists of five dimethoxybenzene units, was discovered by chance. The discovery of this molecule was an important starting point for pillar[ ]arene chemistry. From 2006, we investigated the reaction of l,4-dimetho>ybenzene with paraformaldehyde to synthesize new phenolic resins. 

Synthesis of pillar[ ]arene homologs ( = 5-10) from 1,4-diethoxyben-zene by Hou et air... 

Therefore, chloroform does not act as a template for particular pillar[n]ar-ene homologs, and pillar[6]arene is obtained under kinetic control. Yields of the pillar[ ]arene homologs under kinetic control were much lower than those obtained under thermodynamic control. Therefore, we investigated several reaction solvents that fit into the pillar[6]arene cavity to act as a template solvent for the synthesis of pillar[6]arenes under thermodynamic control. 

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... 

Per-functionalized pillar[n]arenes have only one isomer, therefore their synthesis is relatively easy. One of the reliable routes for per-funetionalized pillar[ ]arenes is by using pre-formed reactive pillar[ ]arenes. Per-hydro ylated pillar[n]arenes having phenolic moieties at both rims are one of the useful key compounds for the synthesis of various functionalized pillar[ ]arene derivatives because phenolic moieties are highly reaetive. Removing the methoxy groups on the rim of pillar[5]arene (3.1) with Bfirg afforded a completely de-protected pillar[5]arene with 10 OH moieties (3.6) in quantitative yield (Scheme 3.1). ... 

The synthetic protocol for per-functionalized pillar[5]arene derivatives from per-hydroxylated pillar[5]arene can be applied to the synthesis of larger per-functionalized pillar[ ]arene derivatives. A per-hydro3tylated pillar[6]ar-ene with 12 phenolic moieties (3.7) was synthesized by Huang and Hou et al. (Scheme 3.3). 3.7 can be synthesized by de-protection of ethoty moieties (3.2) with BBts, which is the same method as used for the synthesis of per-hydrotylated pillar[5]arene. 

Depending on the variety of the pillar[n]arene, in many cases the phenolic units of pillar[M]arenes can rotate and form various conformers in solution, therefore pillar[ ]arenes are classified as flexible macrocyclic compounds/ This chapter deals with the conformational mobility of pillar[ ]arenes in solution and in the crystal state. The substituents on the rims, cavity size and number of rings of pillar[ ]arenes dramatically influence the conformational mobility and the most stable structures of pillar[ ]arenes in solution and in the solid state. 

The rotation of the units enables many conformers of calix[4]arene cone, partial cone, 1,2-alternate and 1,3-alternate. In contrast, pillar[ ]arenes have highly symmetrical structures when compared with calfac[n]arenes, therefore the rotation mode for the phenolic units of pillar[ ]arenes is only the o)ygen-through-the-annulus rotation around the methylene bridges as the axis. The rotation of the phenolic units around 180° results in a change in the positions of the substituents. The positional changes of the substituents give rise to the planar chirality of the pillar[ ]arene. 

In this chapter, we have described the conformations and planar chirality of pillar[ ]arenes. In many cases, 1,4-dialkoxybenzene units in pillar[n]arenes... 

Second, we describe crystal state-assembled structures of pillar[5]arenes, pillar[6]arenes, and larger pillar[n]arene homologs (n = 8, 9, 10). Furthermore, the bulk materials formed using pillar[ ]arenes are discussed. 

The assembled structures of these large pillar[ ]arene homologs are shown in the right-hand column of Figure 6.21. Pillar[9]- and pillar[10]arene assembled to form one-dimensional channels, but the packing mode of pillar[8]arene exhibited slipped stacking of pillar[8]arene molecules. 

Macrocycles play an important role in the development of supramolecular chemistry. Based on the traditional macrocycles such as crown ethers, calixarenes, cyclodextrins, and cucurbiturils, a great many supramolecular polymers as well as their applications have been reported. Because pillar-arenes have only had 6 years of development (from 2008), the research on pillararene-based supramolecular polymers is at a preliminary stage. However, all of the efforts made and aehievements that have been reached suggest that pillararenes as well as pillararene-based supramoleeular polymers will have a bright future. 

Scheme 8.6 The first pillar[ ]arene-based [2]catenane 8.45.

Second, more interesting and advanced superstructures based on pillar-arenes and their inclusion complexes can be designed and synthesized for various functions. For example, new pillararenes with larger cavity sizes have been reported recently, and the host-guest complexes based on these pillararenes should be explored in detail. Novel assembled architectures may be achieved by using these new host-guest complexes. ... 

With regard to carbojylated pillar[ ]arenes, Park, in collaboration with Kim et al., reported a facile one-pot hydrothermal method for the synthesis of gold nanoparticles stabilized by carbojg lated pillar[5]arene that were highly stable against pH variations. The HAuCU (0.01%) was refluxed in water (10 mL) for 10 min, followed by the addition of CP[5]A solution (1%, 0.3 mL H2O). After refluxing the solution mixture for 1 h, spherical gold nanoparticles were obtained. 

The starting material for assembly of asar[ ]arene macrocycles is tetramethoxybenzene (12.31), which can be prepared from commercial dihydrojybenzoquinone (12.33) in abundant quantities. We subjected (Scheme 12.11) a mixture of tetramethojybenzene and paraformaldehyde to Friedel-Crafts alleviation conditions at 80 °C in chlorinated solvents, with BF3 OEt2 as the Lewis acid. Influenced by the prior work on the synthesis of pillar[n]arenes - where pillar[5]arene is formed primarily under similar reaction conditions - we were expecting this reaction mixture to form asar[5]ar-ene as the major product. To our surprise, we found, however, that the reaction mixture produced only asar[6]arene and not even a trace of asar[5]arene. It is most likely that the increased steric demand imposed on the macrocyclic framework by the two additional methojyl groups not present in pillar[ ]arenes is responsible for this striking difference in reactivity between the asar[ ]arene and pillar[ ]arene families of macrocycles. Soxhlet extraction of the crude reaction mixture with acetonitrile as the solvent was then used as a scalable method of purification to access pure asar[6]arene (12.32a) in bulk quantities. 

Asar[ ]arene rings with more than six phenylene units n = 6-15) can be accessed when conducting the synthesis of asar[n]arenes at lower temperatures ( 36°C). Over time, this reaction mixture starts slowly to become enriched in asar[6]arene (12.32a). This finding suggests that, as in the synthesis of pillar[ ]arenes, formation of the asar[n]arene macrocycles... 

In going from pillar[ ]arenes to asar[ ]arenes, two hydro g l groups are substituted onto each of the phenylene units in the host. Analogous pil-lar[n]arene derivatives can be prepared using different heteroatoms substituted at these positions to produce hosts with unique properties. Amino groups directly substituted on the pillar[n]arenes have the potential... 

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