Calixarene Capsules

Bottom Pictorial representation of (a) the achiral, S8 symmetric homodimer 16-16 and of (b) the two enantiomeric forms of a calixarene capsule heterodimer. The head-to-tail directionality of the urea groups (arrow) defines the chirality. 

Two similar systems of trigonal antiprismatic geometry are represented by dimeric calixarene capsules that are also based on only five building units [141,142], This is illustrated for capsule 28 [141], where two ligands 27, each with three meta-pyridyl substituents, are connected via coordination to three palladium(II) ions. Due to the meta-substitution at the coordinating pyridyl rings, the resulting capsule 28 is helically twisted (Fig. 19). The... 

The efforts to synthesize host molecules with larger, noncollapsing cavities for specific guest molecules resulted in the production of several families of hosts, including the cavitands, hernicarcerands. carcerands, cryptophanes, and calixarene capsules, etc. These molecules have well-defined portals for entry and exit of the guest molecules and the possibility of functionalization of the cavity interior to control the orientation and fluxionality of the guest, and the binding forces for complexation. While the cavitands provide no steric hindrance to complexation and decomplexation processes... 

Rebek. J.. Jr. Host-guest chemistry of calixarene capsules. Chem. Commun. 2000. (8). 637-643. 

All force-distance experiments were conducted in mesitylene (1,3,5-trimethylbenzene) solution. Mesitylene is a good solvent for calixarenes and has a high boiling point, which is desirable for single-molecule experiments. The dimeric calixarene capsules have a cavity size suitable for the inclusion of solvent molecules such as mesitylene. Because hydrogen bridges inside the capsules can be influenced by the dielectric permittivity of the guest molecules, it is important to test whether impurities of the solvent may affect the experimental result. Toluene is probably the most likely contamination. Control experiments were conducted with 5 vol% toluene as an... 

Limitations on the application of hydrogen bonded capsular assemblies come from the poor stability of these assemblies in polar media compared to those held by covalent bonds. In an attempt to overcome these Umitations, Bohmer described the synthesis of a mechanically locked chiral bis-catenane based on calixarene capsules [55]. Inspired by this pioneering work, our group produced a mechanically locked hybrid tetraurea calix[4]arene-calix[4]pyrrole capsule 30 (Fig. 32.14) [56]. This catenated capsule displayed reversible encapsulation (i.e. reversible assembly of the urea belt). The two hemispheres of the capsule never dissociate completely due to the bis-catenated topology. 

Reversible encapsulation of snitable guests by the fluorine-containing bis-calixarene capsule... 

Assembly of the negatively charged calix[4]arene 162 and its analogs 163 and 164 (Scheme 3.46) with 165 and the caging properties of supramolecular bis-calixarene capsules 377-379 have been studied in [47] using ESI-MS, H NMR, and ITC meth-... 

Recently, it has been found that the outcome of some cycloadditions can be altered remarkably when performed inside the cavity of cyclodextrins (288), self-assembled molecular capsules (289), or coordination cages (290). This fact intrigued us greatly and stimulated our interest in the Diels-Alder reactivity of the calixarene-like [M2(L19)(L )]+ complexes bearing unsaturated carboxylate coligands L (215). 

Figure 3. Capsules based on other hydrogen-bonding atoms omitted) Tetraurea calixarene dimer 16-16,...

Figure 9. Top Two tetraurea calixarene monomers connected by a rigid spacer at their bottom rim display diverging hydrogen-bonding sites ideally suited for polymerization. The polymer (53) bears capsules of ca. 1.6 nm x 2.2 nm dimensions like beads on a string. Bottom Photomicrographs of typical Schlieren textures of 53 in chloroform (top row, left) and p-difluorobenzene (top row, middle) as viewed...

C. A. Schalley, R, K. Castellano, M. S. Brody, D. M. Rudkevich, G. Siuzdak, J. Rebek, Jr, Investigating Molecular Recognition by Mass Spectrometry Characterization of Calixarene-Based Self-Assembling Capsule Hosts with Charged Guests , J. Am. Chern Soc, 1999, 121, 4568-4579. 

Hydrogen bonded capsule consisting of calixarene dimers will be briefly presented in Section 8.3.2. 

Fig. 19 Formation of the dimeric coordination capsule 28 based on the calixarene ligand 27. The helical twist results from the orientation of the meta-substituted pyridyl units upon coordination to palladium(II) ions (represented by grey spheres). Adapted with permission from [141]. Copyright 2001 American Chemical Society...

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