CaUx arene

A study" of the direct upper-rim alkylation of caUx[ ]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 phenohc behaviour as n increases from 4 to 8. 

It would be too long and too tedious to list all the facilitated ion-transfer reactions that have been reported over the years. From aUcali-metal ions to transition metal ions, most cations have been studied with different classes of ionophores ranging from the crown family with N, O, or S electron-donating atoms to caUx-arenes, not to mention all the commercial ionophores developed for ion-selective electrode applications or for solvent extraction. In the case of anions, the number of voltammetric studies reported has been much smaller [155-163], although the field of supramolecular chemistry for anion recognition is developing fast as recently reviewed [164]. 

Fischer S, P D J Groothenuis, L C Groenen, W P van Hoorn, F C J M van Geggel, D N Reinhoudt and M Karplus 1995. Pathways for Conformational Interconversion of CaUx[4]arenes. Journal of the American Chemical Society 117 1611-1620. 

SCHEME 10. Cone-to-cone ring inversion of caUx[4]arenes... 

TABLE 3. H NMR signals in conformational isomers (atropisomers) of caUx[4]arene tetraethers la with Y = CH2R (R is a residue which shows no coupling with the adjacent CH2 group) and R = C(CH3>... 

SCHEME 12. Rearrangement of 1,3-into 1,2-diester derivatives under conditions where the monoanion is formed. No rearrangement takes place when the dianion is formed by an excess of a strong base . The caUx[4]arene skeleton is symbolized by a circle... 

FIGURE 6. Selected examples of 1,4-bridged caUx[6]arenes... 

SCHEME 16. o-Fries rearrangement of caUx[4]arene carbamates... 

Hydroxymethyl-4-benzoylresordnol (2,4-dihydroxy-3-(hydroxymethyl)benzophenone) gave a caUx[5]arene M. Tabatabai, W. Vogt, V. Bdhmer, G. Ferguson and E. F. Paulus, Supramol. Chem., 4, 147 (1994). 

Caffeic add conjugates 913 Cahxarenes—see also AminocaUxarenes, Calix[3]arenes, CaUx[4]arenes, Calix[5]arenes, CaUx[6]arenes, Calix[8]arenes, HomocaUxarenes,... 

NO2 absorb in this region, addition of as little as l equiv NO2 to the solution of 36 in chlorinated solvent results in appearance of the CT band. Its absorbance grows upon addition of larger quantities of NO2 and reaches saturation when lO equiv NO2 is added (74). Accordingly, cahx[4]arenes can detect NO2 even at micromolar-ppm concentrations. Interestingly, wider calix[5]-, calix[6]-, and caUx[8]arenes do not form encapsulation complexes with NO+. 

Based on these findings, calixarene materials have been synthesized for entrapment of NOx gases. Specifically, calix-silica gel (39) (73) and caUx[4]-arene-based periodic mesoporous silica (40) (75) were prepared (Fig. 16). In the NO2 entrapment experiments, a stream of the gas was passed through columns... 

The role of the cavity size of calixarenes is heavily predictable. For example, there is no significant hole-selectivity of hosts II-1-II-3 (Scheme 12.6) in complexes both with the tetraalkylammonium ion B-7 and the iminium ion C-3 (Table 12.1). Surprisingly, the flexible methylether II-l, which exhibits interconversion between different conformers (cone-partial cone, 1,3 alternate), binds B-7 whilst the fixed cone with propyl groups at the narrower rim does not. Also, the diffiaent cavity sizes of the caUx[4]arene II-4 and calix[5]arene n-5 fixed in their cone-conformation exhibit no selectivity with the iminiiim... 

The binding of C-3 by a simple caUx[4]arene host in nonpolar media can be improved by a factor of 25 by the formation of host clusters on the surfaces of gold nanoparticles. Applications of host-guest complexes are mainly directed towards the development of sensors. For example, acetylcholine detection at micromolar concentration is based on inclusion of a fluorophore containing the pyridinium unit (tranj-4[4-(dimethylamino)... 

For coordinating and sensing luminescent lanthanide ions, a luminescent ruthenium bipyridyl complex has been covalently linked to one, two or six lower rim acid-amide modified caUx[4]arene moieties in receptors 155-157. All these complexes coordinated with lanthanide ions, Nd , Eu and Tb, with the formation of adducts of variable stoichiometries. The adduct formation affected the Ru luminescence, which was strongly quenched by Nd ion, increased by the Tb ion and moderately quenched or increased by Eu ion (2004IC3965). 

Figure 1 H NMR spectra in CD2CI2 of caUx[6]arenes 1 and 2 at 293 K (a and c) and 223 K (b and d). Six sets of signals are observed at low temperature in the case of 2. (Reproduced from Ref. 5. American Chemical Society, 1992.)...

Figure 22 Structure of the caUx[4] arene-mono- and bis-crown-6 carriers, R=Ci2H25. (Reproduced from Ref. 83. Old-enbourg Wissenschaftsverlag, 2002.)...

To render the designation independent from the orientation of the caUx, Bohmer has suggested the alternative terms narrow rim and wide rim, which are also used in literature. In calix[4]arenes, adjacent nuclei have been named viciruil or proximal (or 1,2) and the opposite ones are indicated as distal or diametral (1,3). 

Figure 8 A supramolecular complex between caUx[6]arene and two Ceo molecules.

Figure 12 Encapsulation and isolation of cyclobutadiene in caUx[4]arene-based carcerand.

The de Mendoza group synthesized caUx[4]arene 34 (Figure 13). This supramolecular host is a highly structured amphiphile it presents four cationic guanidinium... 

Considering a molecular model of the cyclic tetramers, Gutsche introduced the term calix[4]arenes [1,2] ( calix — vase, bowl, cup), a term that emphasizes the cup-like structure of these macrocycles, which have all four aromatic rings oriented in one direction (a so-called cone conformation). The word arene indicates the presence of aromatic rings in the macrocyclic ensembles. Later the term was extended to include large macrocyclic compounds ([l ]metacyclophanes) and macrocycles, in which the methylene bridge is replaced by a heteroatom. The size of the cycle is indicated by a number placed in square brackets between the roots caUx and arene nature and position... 

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