Calix-cryptands

Calix-crowns, calix-cryptands and extended networks.361... 

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>. 

The earliest recognised examples of synthetic supramolecular structures were the complexes formed from crown ethers and metal cations [19]. Since then numerous macrocycles have been synthesised. Representative examples are the cryptands [20], These differ from crown ethers in that the former contains a tridimensional cavity while the latter are characterised by a hole. Similarly, calix[4]arenes are compounds with a cup -like structure that through lower rim functionalisation gives rise to a hydrophilic and a hydrophobic cavity, thus allowing the reception of ionic species in the former and neutral species in the latter. Most of the above mentioned macrocycles are known for their capability to serve as cation receptors. 

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... 

A remarkable high yield [1 -1- l]-macrocyclization of a 1,3,5-rr/smethylated calix[6]arene with fris(2-formylphenyl)phosphine gave (91%) a frw-imine intermediate, which was reduced to afford the desired Csv-symmetrical PN3-calix[6]cryptand 36 the ability of the cavity to host ammonium guests was demonstrated by NMR studies <04JOC6886>. The related non P-... 

Pyrrole-containing receptors include porphyrins, extended porphyrins and calix[n]pyrroles. A variety of calixpyrroles have been used as sensors and in anion-separation technologies. An interesting example is the cryptand-like calixpyrrole (2.69) that binds fluoride via six of the nine hydrogen atoms, forming a one-to-one host-guest complex in dichloromethane and THF mixed solvent. The actual stoichometry is rather complicated as each of the pairs of clefts can bind fluoride ions, forming one-to-one, one-to-two or one-to-three stoichiometries. 

Figure 7.8. Ligands useful for ISEs with liquid membranes. a dycyclohexyl-18-crown-6. b Cryptand-222. c Monensin. d ETH 295. e a-cydodextrin. f calix[4]arene...

Separate from these cryptand-type systems, the attachment of functional groups on the weso-positions has led to the development of deep cavity systems, capsules, strapped calix[4]pyrroles, and calix[4]pyrroles with pendant arms. 

Synthetic developments in the area of Supramolecular Chemistry are currently leading to a massive production of new macrocycles. The driving force for this continuous growth is the search for selective hosts to target a particular neutral or ionic species. There is no doubt that the impact produced by the discovery of macrocyclic ligands such as the crown ethers [1] and the cryptands [2] resulted from their cation complexation and this prompted us to consider the thermodynamic characterisation of these systems (mainly cryptands) which has been extensively reported [3-5]. Calixarenes, an important class of macrocyclic compounds, are products of the base-catalysed condensation reaction of p-substituted phenols and formaldehyde [6, 7]. These compounds are characterised by their low solubilities in most solvents, although until recently [8], no quantitative data has been reported. Functionalisation of the lower or upper rim of parent calix[n]arenes has... 

The strapped calix[2] furan[4]pyrrole 63 with a cryptand-like structure showed moderate binding ability for fluoride. It was suggested that 63 can possibly bind CsF as an ion pair in the cavity [44]. 

Sessler and co-workers also prepared cryptand-like calix[4]pyrrole 83 [147], which has three identical binding cavities and could bind solvents in the cavities in the solid state. Huoride was found to be bound to six of the nine pyrrolic subunits, presumably in a 1 1 fashion, while chloride was found to interact with two 83 molecules in solution and the was estimated to be 3.08 x 10 in dichlo-romethane-d2. For nitrate, a 1 2 stoichiometry was also observed, and the K i and K 2 values were determined to be 1,740 and 420 respectively, in dichloro-methane-d2. Kohnke et al. reported cryptand 84 showed a preferred binding for over Cr, NO3, HSO4, CH3C00, and H2PO4 (as tetrabutylammonium salt) [148] and complexation was kinelically slow in CD2CI2. 

Scheme 12.85 Synthesis of PN3-calix[6]cryptands 241-242 and copper complexes 243...

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