Mono-, bis-, and tris(crown ether)s assembled around 1,3,5-triaroylbenzene scaffolds

A concise and experimentally straightforward method for assembling multiple benzo(crown ether) units around 1,3,5-triaroylbenzene scaffolds has been developed. Symmetrical tris(crown ether)s possessing three benzo(15-crown-5) or three benzo(18-crown-6) peripheral substituents have been prepared in good yield via cyclotrimerization of monomeric enaminones. Efficient cross-cyclotrimerizations have also been demonstrated through construction of unsymmetrical triaroylbenzenes functionalized with only one or two benzo(15-crown-5) moieties. The alkali cation-binding abilities of these mono- and polytopic crown ethers have been probed through picrate extraction experiments and isothermal titration calorimetry. Thermodynamic binding parameters uncovered using the latter technique reveal increasing K+/Na+ selectivity in the benzo(15-crown-5) series of compounds as a function of increasing numbers of benzo(crown) units. The data also indicate that the triaroylbenzene-derived bis- and tris-crown ethers do not engage in intramolecular chelation of cations too large to be accommodated by individual crown macrorings. Instead, cation/triaroylbenzene stoichiometries and binding profiles indicate formation of alkali metal-bridged dimers.