Calix-4-arenes

Two other types of host for arenediazonium salts were found by Shinkai et al., the calix[ ]arenes, 11.10 (1987 a, 1987 b) and 11.11 (1988). The hexasulfonated calix[6]arenes 11.10 suppress dediazoniation of substituted benzenediazonium ions in aqueous solution much more efficiently than 18-crown-6. The complexation of calix[ ]arenes 11.11 (n = 4, 6, and 8) with 4 -dimethylaminoazobenzene-4-diazonium ions (11.12) was measured, and was found to be weaker than that of 18-crown-6. It may be that the large difference in behavior between these two types of complexation reagents 11.10 and 11.11 is due to the significantly different diazonium ions used as guests for the two types. Electronically the azobenzenediazonium ion (11.12) is... 

The coordination chemistry of tertiary phosphine-functionalized calix[4]arenes have been described.279 Treatment of a bis(diphenylphosphino) or bis(dimethylphosphino) derivative of calix[4]arene with [PtCl2(COD)] leads to the formation of the corresponding dichloroplatinum(II) complex. The related diplatinum(II) species has also been reported with the tetrafunctionalized calix[4]arene.280 The mononuclear derivative is susceptible to oligomerization if the two free phosphine ligands are not oxidized or complexed to another metal center such as gold(I).279 The platinum(II) coordination chemistry of a mono-281 and diphosphite282 derived calix[ ]arene (n = 4 and 6, respectively) has also been described. 

Since the first examples of lower and upper rim glycocalixarenes were obtained in 1994 by Marra el al.,106 employing the Mitsunobu reaction or copper(II)-catalyzed glycosylation, the development of efficient synthetic methodologies has allowed the emergence of several examples of ()-, N-, or C-glycosyl calix arenes, and these have recently been reviewed (101-106, Fig. 8).107,10X... 

A. Dondoni, M. Kleban, and A. Marra, The assembly of Carbon-linked calix-arene-carbohydrate structures (C-calixsugars) by multiple Wittig olefination, Tetrahedron Lett., 38 (1997) 7801-7804. 

Water-soluble calix[ ]arenes (62 n = 4, 6, and 8) containing trimethylammonio-methyl groups act as efficient inverse phase-transfer catalysts in the nucleophilic substitution reactions of alkyl and arylalkyl halides with nucleophiles in water.126 (Inverse phase-transfer catalysts facilitate reactions between two immiscible reactants via the transport of an organic substrate into an aqueous solution of a second substrate, in which reactions take place.)... 

A study48 of the direct upper-rim alkylation of calix[ ]arenes has shown that, with n = 8, reaction with isopropyl chloride in 1,2-dichloroethane with AICI3 gives isopropylation, whereas when n = 4 hydroxyisopropylation is observed. With n = 6 there is a mixture of products, indicating overall an increase in phenolic behaviour as n increases from 4 to 8. 

Calixarene crown-6 compounds, which are neutral extractants like crown ethers, are able to coextract technetium with cesium. Tests carried out with several calix-arene-crown ethers (MC7, MC8, MC14, BC2, BC5, BC8, and BC10) show that the extraction of technetium, present in the aqueous phase at a concentration 10 5 M, is enhanced as the cesium concentration in the aqueous phase increases from 10 5 to 10-2 M. As expected, an increase of nitrate concentration prevents pertechnetate extraction in competition with nitrate anion. The extraction of technetium is only appreciable when the nitric acid does not exceed 1 M. Distribution ratios DCs (close to 8) are comparable for the various calixarenes. However, a decrease of extraction is observed for naphtho derivatives.88 89... 

The nature of the diluent has an important role on the degradation rate of calix-arene (see Table 8.7). In dodecane, the loss of calixarene was very high, compared with measurements in the aromatic NPOE diluent. As already mentioned with other ligands (like TBP), aromatic diluents had a protective effect, explained by a lower ionization potential. However, serious radiolytic damage (e.g., a considerable rise in viscosity) has been observed with NPOE alone (68). Therefore, authors, such as Lamouroux, have suggested the use of a mixture NPOE-dodecane (72). 

The most stable are the macrocyclic extractants, with radiolytic degradation yields lower than 1 molecule/100 eV (7), and especially the calix-arenes G(-calixarene) <0.1 (72) ... 

Calix[ ]arenes are a family of macrocycles prepared by condensation reactions between n /v/ra-substituted phenols and n formaldehyde molecules under either base or acid catalysis. Different sizes of the macrocycles can be obtained (n = 4-20) (Stewart and Gutsche, 1999) depending on the exact experimental conditions, which were mastered in the 1960 s (Gutsche, 1998), but the most common receptors are those with n =4,6,8 (macrocycles with an odd number of phenol units are more difficult to synthesize). We use here the simplified nomenclature in which the number of phenolic units is indicated between square brackets and para substituents are listed first.4 Calixarenes, which can be easily derivatized both on the para positions of the phenolic units and on the hydroxyl groups, have been primarily developed for catalytic processes and as biomimics, but it was soon realized that they can also easily encapsulate metal ions and the first complexes with d-transition metal ions were isolated in the mid-1980 s (Olmstead et al., 1985). Jack Harrowfield characterized the first lanthanide complex with a calixarene in 1987, a bimetallic europium complex with p-terf-butylcalix[8]arene (Furphy etal., 1987). 

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. 

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). 

Although nowadays the term calixarene tends to be used for all [l ]-meta-cyclophanes, this chapter will deal with [l ]-metacyclophanes bearing phenolic OH groups in the intraannular (endo, e.g. calix[ ]arenes) or extraannular (exo, e.g. resorc[4]arenes) position. 

The introduction of nitro groups at the upper rim of calix[4]arenes is a quick and useful method for the preparation of functionalized calix[4]arenes. This reaction is quite general and can be conveniently carried out on a wide variety of calix[4]arenes substituted at the lower rim. When the starting calix-arene is partially substituted at the lower rim, the substitution takes place at the more reactive phenol rings. 

Calix[ ]arenes are a family of macrocycles prepared by condensation reactions between para-substituted phenols and formaldehyde (see Fig. 4.8). In a way, they can be thought... 

During the last fifteen years, a great deal of effort have been devoted to the synthesis of suitable calix[ ]arene receptors for lanthanide ions, mainly with the purpose of developing... 

The calix[ ]arene ( indicates the number of aromatic units in the metacyclophane) building block provides a three-dimensional skeleton that is needed to fix different donor ligands in the optimal spatial positions for binding one ion selectively [94,95]. Calix[n]arenes are easily accessible from the base-eatalyzed condensation of p-tert-butylphenol and formaldehyde [96]. Gutsche proposed to define the two faces of a calix[4]arene molecule as the lower (phenolic groups) and the upper rim. The calix[4]arene with free hydroxyl groups is conformationally flexible and the... 

SCHEME 1. One-pot condensation of t-butylphenol 1 to form calix[ ]arenes 2. The p-unsubstituted calixarenes (see Section V.A.) wiU be characterized by 2h... 

SCHEME 3. Synthesis of calix[ ]arenes by fragment condensation k + m ... 

Calixarenes have attracted the attention of a number of groups for constructing a wide variety of receptors. Functionalized higher calix[ ]arenes ( = 6 and 8) are also being increasingly synthesized. 

Combined NMR-spectroscopy and molecular mechanics studies on the stable structures of calix[ ]arenes, T. Haradaand S. Shinkai, J. Chem. Soc., Perkin Trans. 2,1995, 2231. 

A new area of polymer science termed nano-macromolecular chemistry [Eirich, 1993] also has relevance to future polymer blend technology and application. Langmuir-Blodget techniques allow for the formation of films of one molecule thickness. Utilizing polymerizable molecules for these films, a polymer molecule or network can yield a film with the thickness of several nanometers. Alternating layers comprised of different polymers could be prepared to yield specific optical or electrical properties. Polymerization of calix-arenes to yield molecular sieving membranes for gas separation has been discussed by Conner et al. [1993]. 

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