Applications to Phase-transfer Methods

Applications to Phase-transfer Methods.—Dehmlow has published a review on advances in phase-transfer catalysis (PTC) which discusses the introduction of crown ethers into this area. The full details are now available of a study of alkyl-substituted azamacrobicyclic polyethers (78a) as PT catalysts. When the alkyl chains are C14—C20, such molecules are very efficient catalysts in both liquid-liquid and solid-liquid phase-transfer modes, which contrasts with the lower catalytic ability of the less organophilic unsubstituted cryptand (78b). Crown ethers immobilized on polymeric supports have been demonstrated to possess increased PTC activity in 5n reactions, up to that of the non-immobilized systems, when the connection to the polymer involves long spacer chains [e.g. (79)]. 

The potassium complex of mono-aza-18-crown-6 (80) is a self-solvating base potentially useful for performing clean E2 eliminations from alkyl halides in non-polar solvents,whilst avoiding the loss of reactivity and changes in product distribution sometimes observed in such media due to base-cation association. 

A new potential area of crown application is illustrated by the report of the separation of small amounts of Sr from large amounts of Ca by liquid-liquid extraction in the presence of dicyclohexyl-18-crown-6.  

The concept of naked anions, i.e. anions solubilized in non-solvating media by the crown complexation of their counter-cations, has been applied further this year. Naked fluoride ion has been used as a catalyst for Michael additions,such as the cyanoethylation of active methylene compounds, and also as a base to mediate the acylation (and protection) of the indole nitrogen of tryptophan in peptides. The selective cleavage of protected amino-acids from oxyacyl resins [equation (18)] is a 

Kimura, K. Iwashima, T. Ishimori, and H. Hamaguchi, Chem. Letters, 1977, 563. 

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