Crown Ethers of Low Symmetry. Spiro Crown Ethers and 16-Crown-5 Derivatives

Article abstract of DOI:10.1021/jo00167a007

Spiro crown ethers 3a-c, spiro bis(crown ethers) 5a-e, and the related 16-crown-5 derivatives 6a-d were synthesized and their cation binding abilities were evaluated by study of the extraction of aqueous alkali picrates.Crown ethers carrying 13-crown-4 and 16-crown-5 skeletons showed significant changes in cation selectivity as compared with the corresponding 12-crown-4 and 15-crown-5.Spiro-13-crown-4 3a and spiro-bis<4.4> 5a showed extremely low extractabilities for all cations examined, while the 16-crown-5-derivatives, including spiro-bis<4.5> 5b and spiro-bis<5.5> 5c, showed anomalously high Na⁺ selectivity.In a quantitative study of extraction equilibrium constants (K_ex), 16-crown-5 was again found to have much higher selectivity for Na⁺ than 15-crown-5.This result is attributed to the less symmetrical spatial arrangement of donor oxygen atoms in 16-crown-5; the symmetry-extractability relationship is discussed on the basis of the size-fit concept.