Alternate conformer, calix arenes

The first studies performed at Strasbourg University by the picrate extraction method developed by Pedersen reveals a high preference of calix-crowns fixed in the 1,3-alternate conformation for cesium. In contrast to its conformational isomer, di-isopropoxy-calix[4]arene-crown-6 in the cone conformation does not extract cesium (Table 4.4). 

The Cadarache group39 focused its modeling contribution on di(isopropoxy)-calix[4]arene-crown-6. It was deduced from these simulations that the 1,3-alternate conformation is much more preorganized than the cone conformation to bind a large cation in water and in vacuo. It can be explained by the fact that the isopropoxy groups, which are near to the crown basis, can prevent the complexation of small cations by shielding the complexation site constituted by the four phenoxy oxygen atoms. 

Calix[4]arene-crown-6 derivatives MC7, MC8, and MC10 in the 1,3-alternate conformation, incorporated in polyvinylchloride) membranes of CHEMFETs, exhibit high Cs+ selectivity and Nernstian behavior. The selectivity Cs+ over Na+, given by logX ° Na = 3.3, is slightly better than that observed for bis(18-crown-6) derivatives, logX ° Na = 3.0. The CHEMFETs display a sub-Nernstian response in the presence of K+ and NH4+ behavior, which can be explained, respectively, by the small difference between the stability constants of the Cs+ and K+ complexes and by the high ratio of NH4+ in favor of the membrane phase.76... 

Calixarenes are potential platforms on which specific binding arms can be grafted. The extractive properties of these molecules for metallic ions depend on the cavity size, the conformation, and the nature of the ligating groups. Different calix[4] arene-crown-6 derivatives in the 1,3-alternate conformation have been studied for Cs recovery from both basic and acidic solutions (257-262). Calixarene-based picol-inamide ligands have been proposed as candidates for separating actinides from lanthanides (263, 264). 

There is indirect evidence that in solution (see below) the 1,3-alternate conformer is in equilibrium with a cone conformer of idealized C4 symmetry (Fig. 15, a). The tetramer of Fig. 14 can be considered to be a metal analogue of a calix[4]arene [24], and this analogy includes also the propensity of this compound to coordinate metal ions. In fact, after further deprotonation of UrH- to the dianion Ur2-, the complex binds additional divalent cations to yield octanuclear species, [ (en)PtnM(Ur2, N(1),N(3),0(2), 0(4)) 4f+ with M = ds-(NH3)2Ptn, (en)Pt11, (H20)3Nin, (en)Pd11, and Cu11. The nitrate salts of these cations were isolated and the stmctures of the first... 

Figure 6 X-ray structures of calix[4]arene derivatives 1 (a), 3 (b), 4 (c), 5 (d) 7 (e) and 8 (f) adopting the cone conformation and 9 (g) adopting the 1,3-alternate conformation in the crystalline phase For the sake of clarity, solvent molecules and hydrogen atoms are not represented.

Calix[4]arene 142 in the 1,3-alternate conformation has two pairs of pyridine residues (hydrogen-bond acceptor) pointing in opposite directions. Together with... 

The structures and abbreviations used for designating the calix[4]arenes are shown in (5). These calix[4]arenes, have both a wide (upper) and a narrow (lower) rim that can be chemically modified to produce complexants that are selective for particular metal ions. In the simple calixarene framework the wide rim has hydrocarbon functionalities, and the narrow rim phenolic groups. Calixarenes are conformationally mobile, and the extreme structures for the calix[4]arenes have been termed the cone, partial cone, 1,3-alternate, and 1,2-alternate conformations (6). Because of the conical geometry of the calix[4]arene structure, the cavity size of the wide rim is larger than that of the narrow rim. 

Less is known about tetraesters of calix[4]arenes . Tetrabutanoates in partial cone, 1,2- and 1,3-alternate conformation have been obtained from 2a °, while aU four isomers are known for the tetraacetates. The tetratosylate (or p-bromophenylsulphonate) was used as protective group while modifying substituents in the p-position, and derivatives in the and 1,3-alternate conformation have been obtained in excellent yield,... 

The functionalization of two hydroxy groups in calix[4]arenes may lead to two regioi-somers (1,2 or A,B vs. 1,3 or A,C ) and for sufficiently large residues to two conformational isomers (synlanti) for each case, while three conformational isomers exist for three residues (synisyn, synlanti and antUsyn). It must be emphasized, that in such partial ethers/esters the mutual orientation of the residues Y should not be mixed with the conformation. A 5yn-diether still can assume the cone (usually the most stable conformation), the partial cone, and one of the alternate conformations ... 

Various examples are known for all possible bis-crown ether derivatives from calix[4]arenes l,2 3,4-bis-crowns in the cone (39) and 1,2-alternate (40) and 1,3 2,4-bis-crowns in the 1,3-alternate conformation (41). Figure 5 gives a survey. Especially, the latter series is weU developed, comprising examples with identical and different ether loops, including structures describable as calixcryptands (41g). Compounds 41a (n = 6) were studied as ligands for the removal of caesium from nuclear wastes in analogy to their mono-crown counterparts in the 1,3-alternate conformation (37) . The 1,2 3,4-bis-crown-3 (39a, n = 3), on the other hand, is an important building block, since its calix[4]arene skeleton is fixed in a nearly perfect C4v-symmetrical cone conformation. ... 

Finally, double calix[4]arenes should be mentioned in which two calix[4]arenes in the 1,3-alternate conformation are connected by two bridges between the narrow (F) or the wide rim (G). ... 

A rather sophisticated ditopic ligand, based on a calix[4]arene, suitable for acidity controlled metal translocation has been recently described by Shinkai [23]. The calix-derivative 10 exhibits an 1, 3-alternate conformation and possesses two distinct coordinating compartments an N04-crown-like moiety on one side, and a bis(ethoxyethoxy) chelating subunit on the opposite... 

Numerous examples of 1,3-alternate conformers of calix[4]arenes are known, but relatively few have been established by X-ray crystallography. In addition to earlier examples, more recent ones include the tetra-O-substituted 92a and 92b as a K complex) (see Figure 4.9), the doubly-bridged 92c,the bis-calixarene 93, and the calix[4]arene 40 containing four extraannular OH groups. 

The dihomooxacalix[4]arene 67 favors the cone conformation but is more conformationally mobile than its calix[4]arene analog, showing a conformational barrier of ca. 13 kcal mol The azacalix[4]arene 73, on the other hand, shows a temperature dependent NMR spectrum which the authors interpret in terms of a major 1,2-alternate conformer accompanied by a small amount of cone conformer, ", although MM3 calculations show the latter to be lOkcal mol more stable. 

Replacement of all four of the hydrogens of the OH groups of a calix[4]arene with alkyl, acyl, or aroyl groups generally results in conformational immobilization, with the result that non-interconverting cone, partial cone, 1,2-alternate, and 1,3-alternate conformers can be formed. Although approximate guidelines... 

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