Crown Ethers

structure cyclic polyethers derived from repeating —OCH 2 CH 2 — units properties form stable complexes with metal ions applications synthetic reactions involving anions Crown Ethers

18-Crown-6 negative charge concentrated in cavity inside the molecule O OO O O O

18-Crown-6 O OO O O O

O OO O O O 18-Crown-6 forms stable Lewis acid/Lewis base complex with K + K+K+

O OO O O O 18-Crown-6 K+K+

not soluble in benzene Ion-Complexing and Solubility K+F–K+F–K+F–K+F–

O OO O O O K+F–K+F–K+F–K+F– add 18-crown-6 benzene

Ion-Complexing and Solubility O OO O O O O OO O O O K+K+ 18-crown-6 complex of K + dissolves in benzene benzene F – F –

Ion-Complexing and Solubility O OO O O O + F–F–F–F– O OO O O O K+K+ F – carried into benzene to preserve electroneutrality benzene

Application to organic synthesis Complexaton of K + by 18-crown-6 "solubilizes" salt in benzene Anion of salt is in a relatively unsolvated state in benzene (sometimes referred to as a "naked anion") Unsolvated anion is very reactive Only catalytic quantities of 18-crown-6 are needed

Polyether Antibiotics

Example CH 3 (CH 2 ) 6 CH 2 Br KFKFKFKF 18-crown-6 benzene CH 3 (CH 2 ) 6 CH 2 F (92%)

Polyether Antibiotics Monensin

Monensin