Molecular Design of Crown Ethers. 6. Substitution Effect in 16-Crown-5

Article abstract of DOI:10.1021/jo00283a019

A number of 14-, 15-, and/or 16-substituted 16-crown-5 derivatives were synthesized for systematic analysis of the effect of substitution in these crown ethers.The cation-binding ability of substituted 16-crown-5 rings was evaluated by the solvent extraction technique and found to be a critical function of the position, number, type, and stereochemistry of the substituent(s) introduced.Both 15- and 14/16-substituted 16-crown-5 exhibited gradually decreased extractabilities with increasing substitution, although the profiles of change in extractability were distinctly different between 15- and 14/16-substitution.The substitution-induced decrease of extractability is attributed to the limited access of counteranion and/or to the lack of conformational adjustments necessary to make a structure suitable for complex formation.The bridging substitutions at 14- and 16-positions dramatically enhanced the extractabilities for all cations without lowering the original relative cation selectivity.This may arise from the favorable entropic contribution of the conformational freezing by bridging substitution.