Molecular Desidn of Crown Ethers. 4. Syntheses and Selective Cation Binding of 16-Crown-5 and 19-Crown-6 Lariats

Article abstract of DOI:10.1021/jo00388a016

A number of new lariat 16-crown-5 and 19-crown-6 ethers posesing a variety of single or double side arm(s) were synthesized, and their cation-binding abilities were evaluated by solvent extraction technique.In general, the extractabilities of the 16-crown-5 lariats for most mono- and divalent cations increased gradually with extending oxyethylene side arm.However, sodium and silver ions, which are best size fitted to the crown cavity, showed peac extractability/selectivity at a specific length of the donating side arm.The complexation stoichiometry is 1 : 1 forall cations employed as confirmed by measurement of the extraction equilibrium constants for the single-armed 16-crown-5 (3p).The tetrasubstituted pivot carbon is shown to be a reguirement for lariat 16-crown-5 to effect extra side-arm ligation, although the second side arm of lariat ether 3e seems ineffective in extractability enhancement with reference to the single-armed analogue 3p.By contrast, the corresponding 19-crown-5 lariat with potential donor side arms at an appropiate position did not show any enhancement in extractability for most cations, which may be attributed to its flexible framework.