Anions, calixarene conformations

It is not only p-substituted phenols that have been connected by methylene groups an extended calixarene based on naphthalene units [57] has also been synthesized and its acidity constants have been measured (see Table 25 Poh and Lim, 1989, 1990). In contrast to the calix[4]arenes, the pKa of the expanded calixarene [57] did not differ much from the of the analogue [58]. This may have two explanations (i) the conformation of the expanded calixarene is different from phenol-based calix[4]arenes and a special stabilization of the anion does not occur, (ii) this stabilization occurs but it is also present in the analogue. A stabilization by the neighbouring... 

Cuest-Induced Changes in Membrane Permeability. Calixarene derivatives are also used for sensing systems other than ISEs or optodes. Recently, a systematic investigation on the control of membrane permeability by use of oriented monolayers composed of calixarene esters was carried out. The hosts used were short alkyl chain esters of calix[6]arene [28 (R = Bu )] and calix[4]arene [26 (R = Bu ), 30 both cone conformers]. The permeabilities through the intermo-lecular voids of these monolayers were evaluated by cyclic voltammetry, as described earlier for oriented membranes of nucleobase derivatives. Cationic, anionic, and neutral electroactive compounds were used as the permeability markers. The voltammetric measurements were carried out either for a monolayer... 

For Ag+, complexation via the calixarene aromatic rings has been firmly established for both the partial cone and cone conformations of the immobile tetra/j-propyl ethers by X-ray crystallography.60 In both cases, the Ag+ cation is sandwiched between two distal (opposite sides of the molecule, as opposed to proximal, meaning adjacent to one another) calixarene rings, which are nearly mutually perpendicular giving a pinched cone conformation. The exposed face of the cation is coordinated to a triflate anion in both cases. For the partial cone case, the Ag+ ion also interacts with one of the anisole oxygen atoms, which is inverted with respect to the other three at the lower rim (Figure 3.85). 

The related monodentate ligands 5.33 and 5.34 form MT4 type complexes with the relatively inert platinum(II). The complex [Pt(5.33)4]2+ adopts a 1,2-alternate conformation (by analogy with the calixarenes, Section 3.14) giving two anion-binding pockets that can bind planar anions such as nitrate and acetate in a 1 2 ratio. Acetate displays a positive allosteric effect with Kn being more than twice Ku. Tetrahedral anions are bound in a 1 1 ratio, however, and it is likely the complex can adopt a variety of conformations in solution. Related to [Pt(5.33)4]2+ is [Pt(5.34)4]2+. Like the complex with 5.33, X-ray diffraction results show that the chloride complex adopts a 1,2-alternate conformation with a... 

The possible number of inherently chiral structures and conformers further increases if the calixarene contains both different phenolic units and different bridges in the macrocyclic skeleton. For example, two chiral monoethers 88a,b are available from dihomooxacalix[4]arenes (one -CH2-0-CH2- bridge instead of -CH2-).17188b is the preferred product of the mono-O-alkylation, since the negative charge of the respective phenoxide anion is better stabilized by intramolecular hydrogen bonds due to the smaller distance between the phenoxide anion and the hydroxy groups. Tetraketone derivatives (Y = CH,-C(0)-R) in the two possible partial cone conformations, have been prepared in moderate yields. 

The present chapter will concentrate on these two types, including more or less close modifications of their structure, while cyclic oligomers derived from pyrocatechol which are [l ]orthocyclophanes and various other cyclooligomers for which meanwhile the prefix calix is used will be excluded. The main emphasis will be also on the synthesis and the chemical modification of calixarenes and some basic properties, such as their conformational behaviour. Their host properties towards cations, anions or neutral guests... 

The NMR spectra of the oxyanions generated by the action of BuLi on p-allylcalix[4]arene in DM SO suggest that the conformations of the mono- and tetra-anions are cone, while those of the di- and tri-anions are less clear " " (see ref. 1, pp. 124-126). However, X-ray crystal structures of the monosodium and dilithium salts of 4 , generated by the action of NaH or BuLi on the calixarene followed by treatment with MeOH, show both to be in the cone conformation in the solid state. "" ... 

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