Fine Tuning of the Cavity Size in Calixarene-Like Cyclophanes: A Complete Series of Homooxacalix[4]arene Ligands for Quaternary Ammonium Ions
✍ Scribed by Bernardo Masci; Marco Finelli; Maurizio Varrone
- Publisher
- John Wiley and Sons
- Year
- 1998
- Tongue
- English
- Weight
- 333 KB
- Volume
- 4
- Category
- Article
- ISSN
- 0947-6539
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✦ Synopsis
Only a few oxygenated homologues of calixarenes have been reported to date, although a large number of structures can be envisaged. These can give rise to a practically new and interesting class of cyclophanes and deserve a systematic investigation. The synthesis is reported of the whole set of p-tert-butylhomooxacalix[4]arene compounds with methylated phenol functions and with various combinations and permutations of CH 2 and CH 2 OCH 2 units in the connections between the aromatic units. The homooxacalix[4]arenes obtained constitute a set of cyclophanes (5 compounds besides the typical calix[4]arene) in which the size of the potential cavity and the conformational mobility, as investigated by NMR tech-niques, smoothly increase as the number of the CH 2 OCH 2 groups is increased. Monooxa-and dioxa-homologues are found to exist in solution in only one of the possible main conformations; cone conformations appear to be disfavored in general. In spite of the lack of preorganization, the prepared compounds appear to be suitable hosts for organic cations. When tetramethylammonium picrate and N-methylpyridinium iodide are considered as possible guests in (CDCl 2 ) 2 solvent, while no evidence of association is obtained in the case of the reference calix[4]arene compound and weak binding is observed with the monooxa analogue, fairly strong complexes (À DG8 values of up to 3.8 kcal mol À1 ) are formed with dioxato tetraoxa-compounds. These are among the most effective neutral ligands for tetramethylammonium salts, particularly when simple monocyclic structures are considered. Several features of the complexation are discussed in the light of the controlled change in the host structure, which proves to be a powerful tool in the evaluation of the effects of steric and charge complementarity on binding.