Calixarenes ammonium complexation

Calixarene derivatives involving crown ethers like 233 exhibit complexation ofalkali or ammonium cations typical of crown ethers that is especially effective in the partial cone conformation [26]. By a proper functionalization selective synthetic receptors for specific hosts have been developed. For instance, two 2,4-diaminotriazine groups at diametrical positions of the upper rim of a... 

The chiral cavitands 3.109 have been developed by combining the amino acid residue L-alanine with macrocyclic cavitands (calixarenes - Section 3.14).51 These ammonium ion receptors are able to complex a range of amino acids and their methylester hydrochloride salts, all of which contain an -NH3+ functionality capable of interaction with the carboxylate residues of the host. In general amino acids are bound only very weakly in aqueous solution, while association constants with the chiral methyl esters range from 620 M-1 for L-tryptophan methylester to 110 M-1 for L-alanine methylester. The methylester of glycine is not bound at all. Receptors related to 3.109 with variable four peptide loops arrayed around a central calixarene core have been used to bind to the surfaces of proteins. The... 

Calixarenes are formed by condensation of a p-substituted phenol with formaldehyde [8]. These macrocycles are conformationally quite flexible but, by introducing suitable substituents in the aromatic subunits, the so-called cone conformation, in which all aromatic subunits point into the same direction, can be stabilized. This conformation is usually best suited to complex guest molecules because it has a well defined hydrophobic cavity. An inclusion of cations such as ammonium ions or quaternary ammonium ions into this cavity can be demonstrated, for example, by the characteristic upfield shifts of guest signals in the NMR, an effect that is a consequence of the close proximity of the corresponding protons to the surfaces of the aromatic receptor subunits in the complex. 

The corresponding dependence of cation complex stability on the anion differs profoundly from that of most other cation receptors such as cyclophanes or calix-arenes [16]. For these cation complex stability decreases on changing the anion from picrate through iodide to tosylate, a dependence that has been attributed to ion-pair aggregation in non-polar solvents. Because the interaction of quaternary ammonium ions with tosylate or iodide in chloroform is considerably stronger than with picrate, cation complexes in the presence of the latter anion are usually more stable. Only when iodide or tosylate cooperatively contributes to cation binding, as in 3 or in some recently described calixarene derivatives [17], is reversal of this order observed. 

The tube-like receptor 39 was shown to bind two imidazolidin-2-one molecules inside the calixarene subunits or, more interestingly, ion triplets comprising an anion, which binds to the central urea moieties, and two ammonium ions, which are incorporated into the calixarene rings [86]. Particularly stable complexes are formed with ammonium sulfate salts even in relative polar media (CD3OD/ CDCI3 3 1), but chloride ions can also induce the formation of such complexes... 

Scheme 12.19 Extended calixarene hosts which form complexes with ammonium ions...

J. M. Lehn, R. Meric, J.-P. Vigneron, M. Cesario, J. Guilhem, C. Pascard, Z. Asfari, J. Vicens, Binding of acetylcholine and other quaternary ammonium cations by sulfonated calixarenes. Crystal structure of a [choUn-tetrasulfonated calix[4]arene] complex, Supramol. Chem., 1995, 5, 97-103. 

The capacity of the calixarenes to exhibit cation selection or to recognize the chirality o/amines and amino acids, as well as their ability to form complexes with quaternary ammonium, led to a number of potential and actual applications. 

Harrowfield, J.M. Richmond. W.R. Sobolev. A. Inclusion of quaternary ammonium compounds by calixarenes. J. Inch Phenoin. Mol. Recognit. Chem. 1994. 19. 257. Lippmann, T. Wilde. H. Pink. M. Schaefer. A. Hesse. M. Mann. G. Host-guest complexes between resorcinol-derived cal ix [4] arenas and alkyl ammonium ions. Angew. Chem. 1993, 105, 1258-1260. ... 

The chemical behavior of clays is also mimicked by the title complexes in two respects the presence of cation-exhange capabilities and the reduced water content in the ammonium salt [20]. Currently, other similarities between clays and the water-soluble calixarenes are being probed. In particular, the size- and polarity-selective cavity of the calix[4]arene sulfonates is finding utility in the separation of organic substances, cations, anions, and neutral molecules, from aqueous feed streams. 

The calix[5]arene 43, synthesized by Parisi et aL, for example, presents a marked shape and size selectivity even in ISEs for n-alkyl over iso-, sec-, and t rt-alkylammonium salts. It was proven that linear fz-alkyl ammonium salts form endo-complexes with the NH3+ head group interacting with the phenolic oxygen atoms and the alkyl residue with the calixarene cavity, while other branched alkyl ammonium salts generally form -complexes (see review for specific reference). 

The versatility of the threading of calixarene wheels with dialkylammonium and bis-ammonium threads prompted us to investigate the synthesis of more complex architectures such as an handcuff rotaxane [27]. As previously described, the threading of double-calix[6]arene 20 with bis-ammonium axle led to the specific formation of the handcuff pseudorotaxane (7 ,7 )-17 c20, as predict by the endo-dX ay rule (Scheme 30.3). In order to permanently trap the handcuff architecture, this pseudorotaxane was then stoppered by reaction with 4-tritylphenyl isocyanate (Scheme 30.12) to give the first example of predefined orientational handcuff [2]rotaxane (r,T)-26 in 28 % yield [27]. 

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