Bowl-shaped arenes

In the approach of Puddephatt et al., the P-phenyl-phosphonitocavitand 2 was obtained by the reaction of phenylphosphonous chloride on re-sorc[4]arene lb (1, R=CH2CH2C6H5) in presence of pyridine as base. The reaction is stereoselective and yielded the bowl-shaped molecule 2 with the four P-phenyl groups directed outwards and the four lone pairs directed inwards ini configuration) [45-49] (Scheme 6). Molecular mechanics calculations performed on the six possible isomers of 2, showed that the iiii isomer is preferred and the orientation of one phenyl group toward the macrocyclic cavity is probable iiio isomer), but two or more phenyl groups oriented inwards are highly unlikely [48]. 

Steed, J. W., Johnson, C. P, Barnes, C. L., etal., Supramolecular chemistry of p-sulfonatocalix[5]arene - a water-soluble, bowl-shaped host with a large molecular cavity. J. Am. Chem. Soc. 1995, 117, 11426-11433. 

Recent studies have shown that the bowl-shaped C-alkyl substituted pyrogal-lol[4]arene macrocycles readily self-assemble to form a gobular hexameric cage, which is structurally robust and remains stable even in aqueous media (Fig. 20.4.38). Slow evaporation of a solution of C-heptylpyrogallol[4]arene in ethyl acetate gives crystalline [(C-heptylpyrogallol[4]arene)6(EtOAc)6... 

The synthesis of hollow architectures based on cavitands started in 1985, when Cram et al. reported on the inclusion of solvent molecules in a carcerand obtained by covalent linkage of two cavitands [30]. Since then, molecular and supramolecular capsules have been prepared from cavitands and calix[n]arenes due to two important properties the bowl-shaped form and the various functionalizations which can be introduced at the upper rim of the cavitands or the wider rim of the calix[n]arenes (Fig. 1). 

Some other capsules have been described in which both complimentary functions, carboxylic acids and nitrogen-containing aromatic rings, are respectively attached to the rims of two bowl-shaped molecules. Reinhoudt has prepared selfassembling, multi-hydrogen bonding molecular capsules in which a calix[4]arene substituted with four carboxylic functions at the upper rim interacts with a complimentary calix[4]arene with four pyridines attached to the lower rim. These capsules have been identified by H-NiMR, IR and VPO measurements but encapsulation properties have not been reported [106]. 

Bohmer has reported the ability of a bowl-shaped molecule known as a calixarene to self-assemble in the solid state to form a homodimer.35 A calix[4]arene with four urea groups attached to the upper rim self-assembled via 16 hydrogen bonds to give a cavity with an approximate volume of 200 A3 (Fig. 17). The cavity hosted a highly... 

In addition to calixarenes, Sherman has reported the ability of a resorcin[4]arene, a bowl-shaped molecule with eight hydroxyl groups at the upper rim, to form a homodimer in the solid state.39 Deprotonation of two hydroxyl groups of a re-sorcin[4]arene using l,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as a base produced a dianion that self-assembled to form an anionic capsule. The dimer hosted a molecule of pyrazine. Four FI+ DBU ions also interacted with the periphery of the dimer via four N+ H O hydrogen bonds (Fig. 19). 

We have recently shown that it is possible to extend the cavities of bowl-shaped molecules known as resorcin[4]arenes (e.g. C-methylcalix[4]resorcinarene 2), su-pramolecularly, using hydrogen bond acceptors as extender units. As a starting point, we chose readily available 2 as a platform for the assembly process. Indeed, solid-state studies had revealed the ability of 2 to adopt a bowl-like conformation... 

The calixarenes are a popular and versatile class of macrocycle formed from the condensation of a p-substituted phenol (e.g. p-tert-butylphenol) with formaldehyde. Since they contain bridged aromatic rings, they are formally members of the cyclophane family (Section 6.5). In cyclophane nomenclature they are termed substituted [l.l.l.ljmetacyclophanes . The descriptive name calixarene was coined by C. David Gutsche (Washington University, USA) because of the resemblance of the bowl-shaped conformation of the smaller calixarenes to a Greek vase called a calix crater (Figure 3.78). The number of phenolic residues is denoted by a number in square brackets. Thus the most common cyclic tetramer with p-f-butyl substituents is termed p-t-butyl-calix[4]arene (3.118). It is easy to understand why this appealing nomenclature has found wide acceptance within the field when it is compared to the Chemical Abstracts systematic name for 3.118, [19.3.1.U U l ]octacosa-l(25),3,5,7(28),9,ll,... 

Cyclotriveratrylene is a trimeric, bowl-shaped molecular skeleton, known to form solid-state inclusion compounds. A variety of cyclotriveratrylene derivatives, e.g. 96 and 97, resulted from r-complexation of the aromatic nuclei with one, two, or three Ru( / -arene) (arene = CeHe, p-MeC6H4Pr ) and Ir( -C5Me5) organometallic moieties [195-197], Some of their structures were established by X-ray diffraction. 

Calixarenes, for example, 2.120, have already been discussed as hosts for cations and both cation and neutral molecules simultaneously in Sections 2.3.6 and 2.5.5, respectively. Resorcarenes e.g. 2.121) are very closely related to calixarenes and are sometimes referred to as calixresorcarenes. They are prepared from the acid-catalysed condensation of resorcinol (1,3-dihydroxybenzene) with aldehydes, a reaction first carried out by Baeyer in 1872. This macrocycle is arguably one of the most important molecules in the field, as resorcarenes serve as the basic building blocks for many cavitands and (hemi)carcerands. Both calixarenes and resorcarenes posses a shallow bowl-shaped conformation in their most stable forms. However, [4]resorcarenes (the [4] prefix refers to the ntimber of resorcinol units that make up the molecule, just as in calix[n]arenes) tend to produce a... 

One of the earliest examples of a carcerand was prepared by joining two hemispherical bowl-shaped components together (cavitands, such as calix[4]arenes or the closely related [4]resorcarenes (Section 2.7.2). This was achieved by reacting a thiol-appended resorcarene with an analogous alkyl halide to form a thioether-bridged equatorial seam (2.123). This led to a challenging analysis as carcerand... 

Figure 22 Structures of various calix[4]arenes that typically assume bowl-shaped conformations, some of which have also been shown to form noncovalent nanotube assanbUes.

FIGURE 11.6 Bowl-shaped bridged calix[6]arene. 

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