Hydrogen bonding calixarene dimers

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

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

Multi-component, solvent-bridged systems are usually formed between two identical units resulting in a bridged dimer [112-114]. The normal approach for these systems has been the same as that for simple multi-component capsules, namely the use of concave molecules such as calixarenes with hydrogen bonding... 

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). 

Complementarity between different molecules has also been harnessed in the pursuit of capsular complexes. Schrader and Kraft have pioneered the design of complementary systems with three- and fourfold symmetry based on 1,3,5-tris(aminomethyl)benzene derivatives [18] and calix[4]arenes [19], respectively. This approach has also been taken by Reinhoudt whose vast output of calixarene chemistry includes work on calix[4]arene dimers held together by ionic interactions [20]. Other examples show how the presence of encapsulated guests templates the formation of self-complementary capsules [21], how two host molecules may be held together by metal coordination [22-24] or where hydrogen bonding by a second entity holds the assembly together [25,26],... 

Rebek and co-workers have studied the self-assembly of molecules containing substituted ureas.65 Dimerization of two calixarenes (where Bn is benzyl and Ar is 4-fluo-rophenyl ) (7.8) by hydrogen bonding produces a host that encapsulates benzene, fluorobenzene, p-difluorobenzene, and pyrazine, but not toluene. 

Although the mechanism of the base-induced formation of calixarenes has been studied in some detail, the reaction pathways remain uncertain. The most intuitively reasonable proposal is that the immediate precursor of any particular calixarene, regardless of size, is the linear oligomer carrying the requisite number of aryl residues. Another proposal, however, postulates that calix[8]arenes, for example, arise from intermolecularly hydrogen-bonded dimers (hemicalixarenes) formed from a pair of crescent-shaped, intramolecularly hydrogen-bonded linear tetramers. Calix[4]arenes, formed under considerably more strenuous conditions, have been postulated to be the result not of direct cyclization of the linear tetramer but of reversion of the calix[8]arene. The cyclic octamer is viewed as the product of kinetic control, and the cyclic tetramer is viewed as the product of thermodynamic control. The particular efficacy of KOH and RbOH for the formation of calix[6]arenes suggests that the hexamer is the product of template control. 

Figure 23. X-ray crystal structure of the calix[4]arene-calix[4]pyrrole pseudo-dimer 18. Two pyrrole-NH groups form hydrogen bonds to the phenolic oxygen atoms at the lower rim of the calixarene. Crystal structure originally published in Gale, P. A. Sessler,). L. Lynch, V. Sansom, P. I, Tetrahedron Lett. 1996, 37, 7881. Diagram produced using data from the Cambridge Crystallographic Database.

Zadmard et al. carried on the dimer concept by introducing amino acid or peptide bridges in order to increase hydrogen bonding to the nucleic acid bases [21]. In another approach calixarene derivatives were equipped with acridine moieties, and high affinities were detected between acridine-functionalized calix[4]arenes and CT-DNA by fluorescence titration [22]. 

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