Thiacalixarenes. Novel host compounds with high versatility

Heteroatom-bridged calixarenes have been confined into the unexplored frontier of the vast realm of the calixarene chemistry because of their synthetic difficulty. The circumstances have been changed by our facile one-step synthesis of thiacalix[4]arene, in which four methylene bridges of calix[4]arene are replaced by four sulfides, by heating a mixture of p-t-butylphenol and elemental sulfur in the presence of a base. The availability of thiacalix[4]arene in substantial amounts allowed us to engage in a project for modifications and functional developments of this cyclic entity by taking advantage of unique properties of the bridging sulfide. For example, the ready oxidizability of the bridging sulfides provided sulfinylcalix [4]arenes and sulfonylcalix[4]arenes. By stereocontrolled oxidation, all four kinds of stereo isomers of sulfinylcalix[4]arenes, imposed by configurations of four SO groups, could be obtained. Oxidation of one or two adjacent sulfides to sulfoxides in O-alkylated thiacalix[4]arenes gave novel chiral molecules. Chelation-assisted nucleophilic aromatic substitution reaction was applied to sulfinyl- and sulfonylcalix[4]-arene to lead to aminocalix[4]arenes consisting of four aniline and four bridging groups S, SO, or SO₂. In terms of functions, thiacalix[4]arene is revealed to have higher inclusion ability toward halomethanes than calix[4]arene, which was applied to the removal method of them from aqueous solutions. Solvent extraction study showed that thiacalix[4]arene has high binding ability toward soft metal ions, owing to the coordination ability of the bridging sulfur. The metal ion selectivity was conveniently controlled by the oxidation state of the bridging moiety. Chiral stationary phases for gas-chromatographic separation of enantiomers were prepared by introducing chiral substituents into thiacalix[4]arene to show higher enantio-selectivity than methylene-bridged counterparts did. Thus, we have revealed versatility of thiacalixarenes, which is not attainable by conventional calixarenes.