New syntheses of crown and proton-ionizable lariat ethers
Hanes, Robert E.
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Macrocyclic polyethers (crown ethers) are known for their ability to bind metal ions within their central cavities. This allows crown ether compounds to be utilized in the separation and determination of metal ion species. Traditional routes for crown ether synthesis often give only low yields due to competitive cyclization and oligomentation reactions. The efficacy of cesium-assisted cyclizations for the preparation of lipophilic benzocrown ether compounds and of di-, tri-, and tetrabenzocrown ethers has been explored. In the synthesis of lipophilic benzocrown ethers by cesium-assisted cyclizations, the desired [1+1] cyclization products were found to be contaminated with corresponding [2-1-2] adducts. The influences of reactant concentrations, stirring method and rate, reactant addition method and rate, and leaving group identity were investigated to determine optimized conditions for formation of the highest possible proportion of the [l + l] adduct in the product mixture. For formation of 18-membered rings or larger, the synthesis of di-, tri-, and tetrabenzocrown ether compounds by cesium-assisted cyclizations produced only the desired [1+1] adducts in good-to-excellent yields. These results provide insight into the nature of the "cesium effect" in cylization reactions. Solid-state structures for several of the benzo group-substituted crown ether compounds and for one crown ether-metal salt complex have been determined. Alkali metal cation binding selectivity for the lipophilic benzocrown ether compounds was assessed by solvent extraction and with solvent polymeric membrane electrodes. For use in the study of ring size effects upon the efficiency and selcctivity of alkali metal cation complexation by proton-ionizablc lanat ethers (crown ethers with an attached arm bearing a teminal acidic group), a series of N-(X) sulfonyl sym-(dccyl)diben/o-13-crown-4-oxyacctamides was prepared. In this series, variation of X is used to "tunc" the acidity of the pendant acidic function.