Calixarene calix arene

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. 

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... 

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. 

Fig. 4 Exemplary cases of enforced cavity hosts and their supramolecular compounds (guests symbolized in broken rings) (a) Cleft-type (b) tweezer-type (c) spherand (d) cyclophane (monocyclic) (e) cyclophane ibicyclic) (f) cavitand (g) carcerand (h) calixarene (calix[4]arene) (i) resorcarene (j) cryptophane (k) cyclodextrin (P-cyclodextrin) and (1) cucurbituril.

The cavity of these calixarenes is usually too small to accommodate fullerenes, and is confirmed by solution, unless they have extended cavity walls as in calix[4]-naphthalenes. Nevertheless, some calix[4]arenes were shown to form stable crystalline complexes with C60 with the fullerene eno- to the calixarene cavity, and they include p-Ph-calix[4]arene, which has a toluene molecule in the cavity, the overall structure dominated by fullerene-fullerene and eso-calixarene fullerene interactions p-Br-calix[4]arene propyl ether, the structure showing very close interfullerene contacts in a columnar structure (Fig. 2d), which most likely results in opposing induced dipoles from the unidirectionally aligned calix-arenes " and /7-I-calix[4]arene benzyl ether, where the fullerenes are ordered without appreciable interfullerene interactions. There is also a calix[4]resorcinarene, R = CH2CH2Ph, 3, which has a molecular capsule derived from head-to-head hydrogen bonding of two resorcinar-enes and propan-2-01 molecules, with the fullerenes also... 

Functional calixarenes can be used as plurifunctional ATRP initiators for the synthesis of star polymers. Show the structures of 2-bromopropionate and 2-bromoisobutyrate esters of 4-ten-butyl calix[ ] arene used as tetrafunctional ( = 4), hexafunctional (h = 6), and octafunctional (n = 8) initiators for ATRP. 

Fig. 24.2 Different cationic calixarene derivatives upper rim (trimethyl-ammonium)methyl calix [4]arene 1 in cone conformation, 13-alternate calix[4]arene 2, flexible calix[n]arenes 3 and 4 upper rimpara-guanidinium calix[ ]arenes 5-8 [13, 14]...

In our discussion, many calixarens have been mentioned in number of chemical reactions as catalyst, however out of these, quaternary ammonium-terminated calixarenes calix[4]arene 4 and calix[6]arene 2 catalysts were used in one-pot Mannich reaction of benzaldehyde, acetophenone and aniline in aqueous media to produce p-aminocarbonyl compounds (see Fig. 27.7) [19]. These calix[n]arene catalysts emphasized not only the high catalytic activity but also have an outstanding reusability with high yield, at low loading amoxmt. Advantages of that were, short reaction times, high yields, and easy work-up that make them great candidate for Mannich-type reactions (see Table 27.5). 

Some of calix[ ]arene sulfonic acids were also developed as Bronsted acid catalysts for Mannich-type reactions [23]. Because of their water-solubility, the reaction can be performed in water with obvious advantages from an environmentally point of view. As depicted in Fig. 27.8, two water soluble calixarene bearing a sulphonic acid group are suitably employed in the Mannich reaction in water as a catalyst. 

Silva and collaborators described calixarenes (CAT-30) as efficient catalysts in the Biginelli reaction [41]. They found that the presence of a sulfonyl group on these calixarenes improved their catalytic properties, and the calix-arene CAT-30 showed better results for the preparation of Biginelli adducts than other catalysts (Table 6). They also showed that macrocyclic monomers, in amounts equivalent to CAT-30, resulted in lower efficiency, justifying the use of calixarenes as supramolecular catalysts in these reactions. Aromatic aldehydes were good substrates (yields >52%), while nonaromatic aldehydes were poor substrates (yields >34%). CAT-30 was reused in up to five cycles without any loss of efficiency. 

Recently, Kennedy and co-workers [37] have reported the synthesis of well-defined star polymers having well-defined arms of PIB emanating fi-om a calix-arene derivative initiator. Calixarene derivative initiators in conjunction with... 

Kimura and coworkers [17], Diamond [18], and Damien et al. [19] have described that the polymeric calix-[4]arenes have been used as ionophores in ion selective electrodes for Na (based on calixarene esters and amides) and for Na and Cs (based on p-alkylcalixarene acetates). The electrodes are stated to function as poten-tiometric sensors as well, having good selectivity for primary ion, virtually no response to divalent cations, and being stable over a wide pH range. 

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