Lipophile Kronenverbindungen — selektive Extraktionsmittel für Metallionen; Syntheseprinzipien

Lipophilic crown ethers are useful for metal ion extraction: they are soluble in organic phases and insoluble in aqueous ones. The lipophilicity can be enhanced by long-chain hydrocarbon residues, by expanding the carbon content in the crown skeleton and by other structural variations. The complexation behaviour and, therefore, the selectivity of crowns in extraction processes can be altered by changing the crown diameter, the kind and the number of hetero atoms within the crown ring or at positions outside the cycle. Furthermore, for the electro-neutrality of the extracted species it is necessary to transfer also an anion into the organic phase. This problem can be solved with crown ether acids or with other hydrophobic inorganic or organic anions. In this review with 276 references many synthetic routes and examples of such lipophilic crown ethers are described. Special attention is given to the building blocks for the crown ethers, which are important in the extraction area. So the survey should open a view for similar syntheses of new metal extraction reagents with coronand structure.