Synthesis and conformational studies of proton-ionizable polyethers

Proton-ionizable lariat ethers are crown ethers (macrocyclic polyethers) which possess a pendent addic group (carboxylic add, sulfonic add, phosphonic acid). Compared with neutral crown ethers, the protonionizable lariat ethers have an important advantage that transfer of a metal ion into an organic medium in a separation process does not require concomitant transport of an aqueous phase anion. This factor is of immense importance for potential practical applications in which hard, hydrophilic aqueous phase anions, such as chloride, nitrate and sulfate, would be involved.

Attachment of an alkyl group to the central carbon of the three-carbon bridge on the polyether ring in sym-dibenzo-16-crown-5- xyacetic acid gives rise to significant enhancement in the metal extraction selectivity. To explore the causative factors for this phenomenon, the synthesis of several series of proton-ionizable polyethers with structural variations which include the identity of the proton-ionizable group, the crown ether cavity size, and the length of the linkage which joins the proton-ionizable group to the polyether ring was undertaken. Synthetic strategies for the preparation of these proton-ionizable polyethers are described.

Pre-organization of the binding site by intramolecular hydrogen bonding of the carboxylic acid function for sym-(R)dibenzo-16-crown-5- oxyacetic adds, where R is an alkyl, alkenyl, alk5myl or aryl group, in homogeneous solution has been studied by ^H NMR spectroscopy, which utilized 2-D techniques as well as solvent-dependent and temperature dependent measurements. Results of these conformational studies are supported by X-ray crystal structure determinations, as well as association constant measurements.