The use of calixarenes in metal-based catalysis.

The condensation of formaldehyde with p-alkylphenols under alkaline conditions, a reaction dating back to the days of Baeyer and Zinke, affords initially linear polyphenols, which, once a certain temperature is achieved, yield cyclic oligomeric phenolic compounds (Scheme 1). One-step, multigram synthetic procedures have now been developed, which by varying either the temperature or amount of base used in the preparation, readily afford the tetrameric, hexameric, and octameric phenolic ring systems, bridged by methylene (-CH2-) spacers. Ring systems with up to 20 phenolic residues are now known, although methods of preparation for these higher members, and for those with an odd number of phenolic residues, are usually low yielding. The smaller members of the family adopt a cone-shaped structure, hence the name calix[n]arene coined by Gutsche, from the Greek “calix” meaning vase, where n denotes the number of phenolic residues. Such a bowl shape results in the formation of a hydrophobic, electron-rich cavity, which is well-suited to the formation of inclusion complexes, particularly cations.5 The synthetic methodology has also been adapted to allow for the incorporation of bridging groups other than methylene, such as dimethyleneoxa (-CH2OCH2-), thia (-S-), and aza [-CH2N(R)CH2-] bridged calixarenes,6 and these additional donors can provide extra binding sites.