18-Crown-6

18-Crown-6 (1,4,7,10,13,16-hexaoxacyclooctadecane) is a macrocyclic polyether with six oxygen atoms in an 18-membered ring. The size of its cavity suits the potassium, ammonium and rubidium cations, and it binds the potassium cation with particular affinity.

Potassium salts in organic solvents. 18-Crown-6 binds the potassium cation inside its ring, so it dissolves potassium salts in nonpolar and dipolar aprotic solvents in which they are otherwise insoluble. The anion of the dissolved salt is only weakly paired with the complexed cation, so it reacts as a “naked” anion. In this way the 18-crown-6 complexes of potassium fluoride, potassium cyanide and potassium acetate act as stronger nucleophiles in aprotic solvents, and substitution reactions with organic substrates are accelerated.

Phase-transfer catalysis. Because it carries potassium salts into organic solvents, 18-crown-6 is used as a phase-transfer catalyst. Reported uses include alkylation reactions with potassium carbonate, the N-alkylation of heterocycles with tert-butoxide bases, the allylation of aldehydes with potassium allyltrifluoroborate to give homoallylic alcohols, and the synthesis of diaryl ethers, diaryl thioethers and diarylamines. Its complex with potassium cyanide catalyses the addition of trimethylsilyl cyanide to carbonyl compounds.

Coordination chemistry. 18-Crown-6 is also used as a complexing agent to prepare molecular complexes of metal cations, including ionic liquids based on crown-ether coordination compounds.