Thiacalixarenes. Novel host compounds with high versatility

Heteroatom-bridged calixarenes have been confined into the unexplored front ier of the vast realm of the calixarene chemistry because of their syntheti c difficulty. The circumstances have been changed by our facile one-step sy nthesis of thiacalix[4]arene, in which four methylene bridges of calix[4]ar ene 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 thiacal ix[4]arene in substantial amounts allowed us to engage in a project for mod ifications and functional developments of this cyclic entity by taking adva ntage 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 consi sting of four aniline and four bridging groups S, SO, or SO2. In terms of f unctions, thiacalix[4]arene is revealed to have higher inclusion ability to ward halomethanes than calix[4]arene, which was applied to the removal meth od of them from aqueous solutions. Solvent extraction study showed that thi acalix[4] arene has high binding ability toward soft metal ions, owing to t he coordination ability of the bridging sulfur. The metal ion selectivity w as conveniently controlled by the oxidation state of the bridging moiety. C hiral stationary phases for gas-chromatographic separation of enantiomers w ere prepared by introducing chiral substituents into thiacalix[4]arene to s how higher enantio-selectivity than methylene-bridged counterparts did. Thu s, we have revealed versatility of thia-calixarenes, which is not attainabl e by conventional calixarenes.