Calixarenes, the third host compounds following crown ethers and cyclodextrins, have a prominent host-guest recognition ability. Extensive modifications to their structure have been carried out with the aim of discovering new functions. Thiacalixarenes are modified calixarenes in which the methylene subunits are replaced with sulfur atoms. Ever since Miyano et al.1) reported a facile synthesis of thiacalixarene (1) by base-catalyzed condensation of p-substituted phenol and elemental sulfur, various studies using thiacalixarenes have been undertaken.
For instance, by exploiting thiacalixarene-cation recognition, the liquid-liquid extraction of metal ions has been reported.2) 1 forms complexes with soft metal ions, whereas sulfonyl-calixarene 2 complexes hard metal ions, and sulfinylcalixarene 3 complexes both soft and hard metal ions. In particular, 3 is an unique host compound in which either oxygen or sulfur can act as the ligand, depending on the metal ion's soft or hard nature.
The hydrophobic recognition ability of thia-calixarenes is utilized for the removal of halogenated organic compounds in water. There is a high demand for them in waste water and drinking water treatments. To an aqueous solution containing halogenated organic compounds is added sulfonated thiacalixarene 4 to form the host-guest inclusion complex. The resulting mixture is passed through a column packed with a weak anion exchange resin. Consequently, 4 is trapped in the weak anion exchange resin, and having complexed the halogenated organic compounds, clean water free of halogenated organic compounds is eluted.3)
Thiacalixarene 5, a variant of 1 whose hydroxyl groups are modified with chiral amides, has been developed. Optical resolution of racemic amino acids, alcohols and amino derivatives has been reported by coating capillary columns with 5 for use as a chiral stationary phase.4) The separation factor of enantiomers is over 1.02 in most cases, which indicates a sufficient separation.
The bridging moiety of 1 consists of sulfur atoms, through which the characteristic host-guest recognition ability is exhibited. In addition, further chemical modification of the phenolic groups, aryl rings, or bridging moieties of 1 would facilitate the design and synthesis of novel host molecules with new functions. Therefore, 1 is regarded as a very attractive host compound, as well as a starting material for a wide range of possible novel thiacalixarenes.
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