Bridged calix arenes

Harrowfield et al. [37-39] have described the structures of several dimethyl sulfoxide adducts of homo bimetallic complexes of rare earth metal cations with p-/e rt-butyl calix[8]arene and i /i-ferrocene derivatives of bridged calix[4]arenes. Ludwing et al. [40] described the solvent extraction behavior of three calixarene-type cyclophanes toward trivalent lanthanides La (Ln = La, Nd, Eu, Er, and Yb). By using p-tert-huty ca-lix[6Jarene hexacarboxylic acid, the lanthanides were extracted from the aqueous phase at pH 2-3.5. The ex-tractability is Nb, Eu > La > Er > Yb. 

A new type of calixarene-capped calixpyrrole (9) has been generated (32 %) in one-step from p-fe/t-butylcalix[4]arene tetramethylketone as the template <96TL7881>. A cylindrical calix[4]-fois-cryptand, in which the central calix[4]arene possesses two 1,3-altemating diaza-tetraoxa macrocycles on each face, has been synthesized <96TL8747>. A series of substituted l,4-(2,6-pyridino)-bridged calix[6]arenes has prepared and studied <96LA1367>. 

Calix[4]arenes can be functionalized both at the phenolic OH groups (lower rim) and at the para positions of the phenol rings (upper rim).21 A special class of selectively functionalized calix[4]arenes comprises bridged calix[4]arenes in which two phenol rings are connected by a cap.22 An important subgroup of this class are the calixspherands in which a calix[4]arene is diametrically bridged with a rigid terphenyl moiety. 

Structural elements of the chiral pool have been used to synthesize the 1,3-bridged calix[4]arenes 18a and 18b. In the former case the respective calix[4]arene... 

Bridging with tri-functional reagents in 1,3,5-position at the wide, or at the narrow rim led to a more or less rigid skeleton. In such a series of 1,3,5-bridged calix[6]arenes 84 chirality is caused by the CTV moiety used as bridging element.157,158 The calix[6]cryptands 85a and 85b on the other hand are inherently chiral due to the 1,2,4-tri-substitution pattern on the narrow rim.159... 

However, non-bridged calix[6]arene derivatives may also be chiral. It was possible to prepare two diastereomeric 1,2-dibenzyl ethers from /-butyl-calix[6]arenes with a syn and anti orientation of the ether residues. The free energy barrier for their mutual interconversion was found to be about 27 kcal mol". 160 This should be high enough to separate the enantiomers of the a/ift -isomer, which has effective C2 symmetry. The same should be true for another example of this type, a 1,2-p-nitrobenzoate with an anti-configuration161 and for 1,2,4-tri-O-alkyl or acyl derivatives.162... 

Figure 16. Survey of O-O-bridged calix[8]arenes (schematic representation) with two identical intercrossing bridges and their symmetry classes.

The low-temperature H NMR spectra for calix[4]- and calix[5]arenes reveal that all four or five phenolic units are identical.225 This is compatible with a cone conformation with C4v and C5v symmetry, respectively.226,227 For calix[8]arenes the same pattern is interpreted by a / -symmetrical pleated loop conformation, with a regular up-and-down of the methylene bridges. Calix[7]arenes, however, show seven singlets for the OH protons and seven (overlapping pairs of doublets) for the methylene protons. This total lack of any symmetry element (C,) makes this conformation chiral.228... 

Oxa-thiacrown ether-esters having calix[4]arene 163 and calix[6]arene 164 were prepared by Yang et al. <2005MI42>. In both compounds, only the 1,3-bridged calix[4]- and calix[6]arenes were isolated. 

When the ring size of ethyleneoxy bridged calix[4]arene is further increased by one or two oxyethylene units the preference is changed in favor of lai er ions, e.g. 

The preorganization of the receptor molecule by introduction of an ethyleneoxy bridge imit at the upper rim was also described [131]. The cavities of these bridged calixarenes (27) were larger than the lower rim bridged calixarenes and extraction experiments were done with both solid alkali metal and ammonium picrates into chloroform. The measurements showed that these calixarenes have a lower affinity for alkali metal ions (extraction <3%) than the lower rim bridged calix[4]arenes. However, primary ammonium ions were efficiently extracted with efficiencies of 27-57%,... 

The theme of double-bridged calix[4]arenes has been further shown by the three-step conversion of 25,27-6w(5-chloro-3-oxapentyloxy)calix[4]arene to 1,3-altemate 25,27-[4-(10-cyano-9-anthrylmethyl)-1,2-phenylene-ft (5-dioxy-3-oxa-1 -pentyloxy)]calix[4]-aza-crown-5 32, which was demonstrated to be a fluorescent sensor <03JOC597>. The first Cjv-symmetrical calix[6](aza)crown 33 has been synthesized in five-steps from a l,3,5-/r/ tosylate calix[6]arene precursor <03JOC3416>. 

The structure of a di-/i-oxo bridged calix[3]arene complex has been reported whilst other complexes have been isolated with p-t-butylcalix[n]arenes n = 4,6,8). p-t-Butylcalix[4]arene forms a 2 1 complex whilst p-t-butylcalix[6]arene and p-t-butylcalix[8]arene form 1 1 complexes, but these compounds at present lack structural characterization. " ... 

Interest in phosphorus-containing calixarenes continues. Structures reported include hexa(diethoxyphosphoryloxy)calix[6]arene (8), inherently chiral 1,2-bridged calix[4]arene diphosphates, and a calixarene like C3 symmetric receptor with a phosphate function at the cavity bottom. " The purification of phosphate substituted calixarenes has been studied by chiral HPLC and by normal reverse phase HPLC. Mono(6-0-diphenoxyphosphoryl)-P-cyclodextrin (9) and mono(6-0-ethoxyhydroxyphosphoryl)-p-cyclodextrin (10) have been synthesised and show enantioselective inclusion of D and L amino acids e.g. 3.6 for D/L serine in the case of 9). ... 

Eight regioisomers are possible for doubly bridged calix[6]arenes (Figure 7) from which examples (mainly crown and benzocrown ethers) for five types have been realized (1,4 2,3-, 1,2 3,5-, 1,2 4,5- and the 1,3 2,5- and l,4 2,5-derivatives with crossed bridges). The situation may be even more complicated since two stable conformational isomers (all-up and uuuddd u = up, d = down) have been obtained from the 1,4-diallyl ether by the introduction of diethylene glycol bridges . ... 

SCHEME 24. Tandem Claisen rearrangement of doubly bridged calix[4]arenes. The intramolecular pathway is indicated by dashed arrows. BTMSU = bistrimethylsilylurea... 

The first stable arylselenenic acid, Ar-Se-OH (109) as a bridged calix [6]arene, was synthesized and characterized by an X-ray study (Scheme 17) Se chemical shifts of selenenic acids seem to be comparable to those of selenonic acids. This is confirmed by the data of the selenoperacetate 110 (same 5-value for the selenoperbenzoate) and the selenenic anhydride 111. 

Development in the research of bridged calix[6]arenes 00MI28. 

Another group of upper rim-bridged calix[4]arenes employs A,C-p-Gibbs, C. G. Gutsche, C. D. unpublished results. 

Several examples of upper rim doubly-bridged calix[4]arenes are known. In the case of A,C/B,D bridging the necessary conformation is 1,3-alternate, so the designation upper rim simply specifies that the bridges are attached to the p-carbons (i.e. exo rim) rather than the phenolic oxygens i.e. endo rim). The... 

The complexation of C q, discussed above for the solid state, has also been investigated in solution. First observed in aqueous solution with a water soluble calixarene, a complex in toluene with =110 has been demonstrated using the triply-bridged calix[6]arene 211 as the host moleeule. An even tighter complex is formed with the calix[5]arene 278 (X = I), with a assoc = 2120 M " Mn toluene. " With 278 (X = Me or H) the values fall to 1673 M and 588 M" S respectively, and the K soc values in other solvents, particularly CS2 and o-dichlorobenzene, also fall considerably. A survey of 28 calixarenes produced a similar conclusion, viz that the calix[5]arene system is particularly well suited for C q complexation. ... 

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