Synthesis and selectivity for lithium of lipophilic 14-crown-4 derivatives bearing bulky substituents or an additional binding site in the side arm

Abstract

Two types of 6,6-disubstituted 14-crown-4 (1,4,8,11-tetraoxacyclotetradecane) derivatives have been synthesized in the hope of improving on the selectivity of the parent macrocycle for Li⁺, especially against Na⁺ and K⁺. Selectivity enhancement was expected to result from incorporation of bulky substituents which suppress the formation of stable sandwich-type complexes with Na⁺ and K⁺, and from the attachment of an additional binding site possessing affinity for Li⁺. Studies of the electrochemical selectivity of membranes containing the crown ethers showed that selectivities for Li⁺ with respect to Na⁺ and K⁺ are augmented in 6,6-dibenzyl- and 6-diethylcarbamoylmethyl-6-dodecyl-14-crown-4, (6) and (18), as compared with the 6-dodecyl-6-methyl derivative (2).