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A calixarene is a macrocycle or cyclicoligomer based on a hydroxyalkylation product of a phenol and an aldehyde[1]. The word calixarene is derived from calix or chalice because this type of molecule resembles a vase and from the word arene that refers to the aromatic building block. Calixarenes have hydrophobic cavities that can hold smaller molecules or ions and belong to the class of cavitands known in Host-guest chemistry. Calixarene nomenclature is straightforward and involves counting the number of repeating units in the ring and include it in the name. A calix[4]arene has 4 units in the ring and a calix[6]arene has 6. A substituent in the meso positionRb is added to the name with a prefix C- as in C-methylcalix[6]arene.




The aromatic components are derived from phenol, resorcinol or pyrogallol, For phenol, the aldehyde most often used is simply formaldehyde, while larger aldehydes (acetaldehyde, or larger) are generally required in condensation reactions with resorcinol and pyrogallol. The chemical reaction ranks under electrophilic aromatic substitutions followed by an elimination of water and then a second aromatic substitution. The reaction is acid catalyzed or basecatalyzed. Calixarenes are difficult to produce because it is all too easy to end up with complex mixtures of linear and cyclic oligomers with different numbers of repeating units. With finely tuned starting materials and reaction conditions synthesis can also be surprisingly easy.

In 2005, researchers produced a pyrogallol[4]arene by simply mixing a solvent-free dispersion of isovaleraldehyde with pyrogallol and a catalytic amount of p-toluenesulfonic acid in a mortar and pestle[2]. Calixarenes as parent compounds are sparingly soluble and are high melting crystalline solids [3].

[2]Antesberger J, Cave GW, Ferrarelli MC, Heaven MW, Raston CL, Atwood JL (2005). "Solvent-free, direct synthesis of supramolecularnano-capsules". Chemical communications (Cambridge, England). (7): 892-4. PMID 15700072.

Structure mixing a

Calixarenes are characterized by a three-dimensional basket, cup or bucket shape. In calix[4]arenes the internal volume is around 10 cubic nanometers. Calixarenes are characterised by a wide upper rim and a narrow lower rim and a central annulus. With phenol as a starting material the 4 hydroxyl groups are intrannular on the lower rim. In a resorcin[4]arene 8 hydroxyl groups are placed extraannular on the upper ring. Calixarenes exist in different chemical conformations because rotation around the methylene bridge is not difficult. In calix[4]arene 4 up-down conformations exist: cone ( point group C2v,C4v), partial cone Cs, 1,2 alternate C2h and 1,3 alternate D2d. The 4 hydroxyl groups interact by hydrogen bonding and stabilize the cone conformation. This conformation is in dymamic equilibrium with the other conformations. Conformations can be locked in place with proper substituents replacing the hydroxyl groups which increase the rotational barrier. Alternatively placing a bulky substituent on the upper rim also locks a conformation. The calixarene based on p-tert-butyl phenol is also a cone [1].

Applications mixing a

Calixarenes are applied in enzyme mimetics, ion sensitive electrodes or sensors, selective membrames, non-linear optics [6]

and in HPLC stationary phase [7].

In addition, in nanotechnologycalixarenes are used as negative resist for high-resolution electron beam lithography[8].

A tetrathia[4]arene is found to mimic aquaporin proteins [6]. This calixarene adopts a 1,3-alternate conformation (methoxy groups populate the lower ring) and water is not contained in the basket but grabbed by two opposing tert-butyl groups on the outer rim in a pincer. The nonporous and hydrophobic crystals are soaked in water for 8 hours in which time the calixarene:water ratio nevertheless acquires the value of one.

Calixarenes are able to accelerate reactions taking place inside the concavity by a combination of local concentration effect and polar stabilization of the transition state. An extended resorcin[4]arenecavitand is found to accelerate the reaction rate of a Menshutkin reaction between quinuclidine and butylbromide by a factor of 1600 [7].

In heterocalixarenes the phenolic units are replaced by heterocycles[8], for instance by furans in calix[n]furanes and by pyridines in calix[n]pyridines. Calixarenes have been used as the macrocycle portion of a rotaxane and two calixarene molecules covalently joined together by the lower rims form carcerands.

Applications host guest interactions
Applications: Host guest interactions mixing a

Calixarenes are efficient sodiumionophores and are applied as such in chemical sensors. With the right chemistry these molecules exhibit great selectivity towards other cations.

Calixarenes are used in commercial applications as sodium selective electrodes for the measurement of sodium levels in blood.

Calixarenes also form complexes with cadmium, lead, lanthanides and actinides. [3] Calix[5]arene and the C70fullerene in p-xylene form a ball-and-socket supramolecular complex. [4] calixarenes also form exo-calix ammonium salts with aliphatic amines such as piperidine. [4]

Self assembly
Self assembly mixing a

Resorcinarenes and pyrogallolarenes self-assembly lead to larger supramolecular structures [5]. Both in the crystalline state and in solution, they are known to form hexamers that are akin to certain Archimedean solids with an internal volume of around one cubic nanometer (nanocapsules). (Isobutylpyrogallol[4]arene)6 is held together by 48 intermolecular hydrogen bonds. The remaining 24 hydrogen bonds are intramolecular. The cavity is filled by a number of solvent molecules. [5]

References mixing a

  • [1] Gutsche, C. David (1989). Calixarenes. Cambridge: Royal Society of Chemistry. ISBN 0-85186-385-X.

  • [2] Antesberger J, Cave GW, Ferrarelli MC, Heaven MW, Raston CL, Atwood JL (2005). "Solvent-free, direct synthesis of supramolecular nano-capsules". Chemical communications (Cambridge, England). (7): 892-4. PMID 15700072.

  • [3] McMahon G, O’Malley S, Nolan K and Diamond D (2003). "Important Calixarene Derivatives – their Synthesis and Applications". ArkivocPart (vii). Article

  • [4] Nachtigall FF, Lazzarotto M and Braz FNJ (2002). "Interaction of Calix[4]arene and Aliphatic Amines: A Combined NMR, Spectrophotometric and Conductimetric Investigation". Journal of the Brazilian Chemical Society13 (3). Article

  • [5] Atwood JL, Barbour LJ, Jerga A (2002). "Organization of the interior of molecular capsules by hydrogen bonding". Proceedings of the National Academy of Sciences99 (8): 4837-41. PMID 11943875.

  • [6] Thallapally PK, Lloyd GO, Atwood JL, Barbour LJ (2005). "Diffusion of water in a nonporous hydrophobic crystal". Angewandte Chemie (International ed. in English)44 (25): 3848-51. PMID 15892031.

  • [7] Purse BW, Gissot A, Rebek J Jr (2005). "A deep cavitand provides a structured environment for the menschutkin reaction". Journal of the American Chemical Society127 (32): 11222-3. PMID 16089433.

    [8] Subodh Kumar, Dharam Paul, Harjit Singh (2006). "Syntheses, structures and interactions of heterocalixarenes". Arkivoc05-1699LU: 17 - 25. PMID. Article

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