Tetra- calix arene

As illustrated in Figure 5.6 for a tetra-tolylurea (as an example), the two calix[4]-arenes forming the dimer are turned by 45° around their common fourfold axis. The tolyl residues attached to the urea groups point in opposite directions. Based on M D-simulations, the distance between adjacent tolyl methyl groups in the same calix-arene is slightly larger (13.4 A) than the distance to the adjacent methyl groups in the other calixarene (12.3 A). 

Calestani et al. [8] prepared dietylamide of the tetra-carboxymethyl ether of p-tert-h W.y calix[4]arene and reported alkali metal picrate extraction constants of 1.9... 

Recently, Deligoz and Yilmaz [24,25] described the preparation of two polymeric calix[4]arene tetra esters (Scheme 4) and their Na -complexation. Based on phase-transfer experiments with these compounds using alkali picrates in water-dichloromethane, they confirmed that polymers are as Na selective as monomers. 

Deligoz and Yilmaz [52] reported that the selective liquid-liquid extraction of various alkali and transition metal cations from the aqueous phase to the organic phase as carried out by using p-tert-h iy calix[4]arene (1), p-tert-b x. y calix[6]arene (2), tetra-ethyl-p-tm-butylcalix[4]arene-tetra-acetate (3), tetra-methyl-p-/< /-/-butyl calix[4]arene-tetraketone (4), calix[n]arenes ( = 4 and 6) bearing oxime groups on the lower rim (5 and 6) and a polymeric calix(4]arene (8). It was found that compounds 5 and 6 showed selectivity towards Ag, Hg, Hg, Cu, and Cr and the order of the ex-tractability was Hg > Hg > Ag > Cu > Cr. The polymeric calix[4]arene (8) was selective for Ag, Hg, and Hg , unlike its monomeric analog. 

The azo coupling reaction of the calix[4]arene 12.32 shows an unexpected auto-catalytic effect. If the molar ratio of the diazonium ion (X=N02) to the calix[4]arene is 4 1, the yield of the tetra(arylazo)calix[4]arene 12.33 is 99%, but with ratios 3 1, 2 1, and 1 1 the tetra(arylazo) compound is also a major product (70%, 45%, and 22% respectively). If ratios 3 1, 2 1, and 1 1 are used the yields of the tris- and bis(arylazo) products are in the range of only 1.2-3.4%, and the mono(arylazo) compound is formed with a yield of 5.3-6.0%. Using the ratio 4 1 the bis- and mono(arylazo) products are not found at all, and the tris(arylazo) com-... 

Typically, the cumulative electron-withdrawing effect of metallating more than half of the arenes of the macrocycle is required in order to observe anion inclusion. Thus, we have focused on tetrametallated derivatives of calix[4]arene and di- or trimetalated derivatives of the related orr/io-cyclophane, cyclotriveratrylene (CTV). The X-ray crystal structure of the BF salt of the water soluble bear trap 51 demonstrates the suitability of the narrow calix[4]arene cavity for small tetra-... 

Wide-rim tetra(CMPO) calix[4]arenes present good extraction abilities and also rather good selectivities at high acidity. But, the stability toward hydrolysis is not very good. N-methylated compounds exhibit low extraction efficiency, but higher stability toward hydrolysis when they are in contact with strong nitric acid (3 M). 

Tetra(CMPO) cavitands present nearly the same extraction properties as wide-rim tetra CMPO calix[4]arenes, but a slightly lower selectivity. 

Several classes of synthesized calixarenes bearing several moieties (ether, ester, and amide derivatives), were tested for the extraction of strontium picrates (from aqueous solutions into dichloromethane).128 Only a few of them show appreciable extraction levels. The p-i-butyl calix[6]arene hexa(di-/V-ethyl)amide (CA4) shows a very high extraction level of alkaline earth cations with respect to alkali metal cations. Moreover, dealkylation of the calix[6]arene hcxa(di-/V-cthyl)amidc (CA5) decreases the extraction of both sodium and strontium. As this decrease is much more important for sodium than for strontium, the Sr/Na selectivity, which increases from 3.12 to 9.4, is better than that achieved for DC18 derivative under the same conditions (8.7). These results were confirmed by extraction of strontium (5 x 10 4 M) from 1 M HN03 solutions, where it was found that p-t-butyl calix[4]arene tetra(di-N-ethyl) amide (CA2) (10 2 M in NPOE) extracts only sodium (DNa = 12.3, DSl < 0.001). 

The X-ray structure of the L-Sr(Picrate)2 (L = p-tert-butyl-calix[4]arene-tetra(diethylamide)) is reported, as well as MD simulations on the L M2+ complexes in vacuo, in water, and in acetonitrile solutions for alkaline earth cations with a comparison of converging and diverging conformers.130 In the simulated and solid-state structures of the L M2+ complex, the ligand wraps around the complexed cations M2+ (more than it does with alkaline cations), which are completely encapsulated within the polar pseudo-cavity of L, without coordination to its counterion in the crystal or to solvent molecules in solution. In contrast to alkali cation complexes, which display conformational flexibility in solution, computations show that the alkaline earth cation complexes are of the converging type in water and in acetonitrile. Subtle structural changes from Mg2+ to Ba2+ are observed in the gas phase and in solution. Based on FBP calculations, a binding sequence of alkaline earth cations was determined Mg2+ displays the weakest affinity for L, while Ca2+ and Sr2+ are the most stable complexes, which is in agreement with the experiment. 

FETS for the selective detection of Ca2+ and Na+. The receptor molecules are a modified bisamide (4) and a tetra-O-alkylated calix[4]arene (5). 

Synthesis of the closely related acyclic (19) and macrocyclic (20) polyradicals has recently been reported (Figure 5.1).1231 The -conjugated carbanions (e.g., the calix[4]-arene-based tetraanion and the related calix[3]arene-based trianion) were synthesized and studied.1241 Oxidation of these tetra- and tri-anions gave the corresponding tetra- and tri-radicals, respectively. It has been shown in closely related systems that it is not the shape or overall geometric symmetry of the molecules, but rather it is the juxtaposition of the carbenic centers within the jt-cross-conjugated structure, that is most important in determining the spin multiplicity of the alternant hydrocarbon molecule.1251... 

Tetra-urea Calix[4]arenes - From Dimeric Capsules to Novel Catenaries and Rotaxanes... 

Hydrogen-bonded dimers of tetra-urea calix[4]arenes offer a unique, unprecedented preorganization [28], Before we describe how this can be used for the rational construction of molecules with novel topologies, it is reasonable to discuss the properties of such dimeric capsules and the conditions under which they exist. 

The isocyanate can be replaced by the corresponding activated urethane (as a milder reagent), and this activation can also involve the p-amino functions of the calix[4]arene (step e) and their subsequent reaction with a suitable amine (step f). Multigram quantities of simple tetra-urea calix[4]arenes are easily available in this way. 

Scheme 5.8 Synthesis of tetra-urea calix[4]arenes. (a) Alkylating reagents, NaH, DMF, rt (b) HN03/H0Ac, CH2CI2, rt (c) H2, Raney-Ni, toluene, rt (d) R-NCO, rt (e) p-nitrophenyl chloro-formate (f) R-NH2, rt.

Scheme 5.9 Synthesis of tetra-urea calix[4]arenes bearing two different urea residues in the sequence ABAB (a) and AABB (b), using the protection of amino functions by trityl- or Boc-groups. (Y = C5Hln).

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