Complexing agents crown polyethers

There were not many new developments in the use of crown ethers for fluorination during the last decade, since the basicity of the fluoride anion is also enhanced along with the nucleophilicity. What is more, cheaper substitutes such as open-chain polyethers frequently led to similar results. However, there are still some reactions which are difficult to accomplish without these complexing agents and the synthesis of /-butyl fluoroformates... 

Due to their pronounced selectivity in metal ion ccmplexation (6), crown ethers (macrocyclic polyethers) and related macrocyclic multidentate ligands are attractive mobile carriers for metal ion transport across liquid membranes. As summarized in recent reviews of macrocycle-facil itated transport of ions in liquid membrane systems (7,8), most studies have been conducted with macrocyclic carriers which do not possess ionizable groups. For such carriers, metal ions can only be transported down their concentration gradients unless some type of auxiliary complexing agent is present in the receiving aqueous phase. 

Lipophilic cation-complexing agents are used as electrically neutral carriers. They selectively extract hydrophilic cations from aqueous solution into a hydrophobic membrane phase and transport them across the membrane. The first such carrier was the antibiotic valynomycin [59, 60] later, a group of synthetic macrocylic polyethers ( crown compounds) became very popular [61, 62]. 

Potassium permanganate/crown polyether Crown polyethers as complexing agents Oxidations with potassium permanganate in benzene... 

KMnO solubilized in benzene by a crown polyether as complexing agent as well as a complex prepared from N-chlorosuccinimide and di-... 

It is known that the solubility of metal salts in nonpolar media is drastically increased if small amounts of crown ethers (cyclic polyethers) are used as complexation agents. Such a concept has been demonstrated in various areas of chemistry. For example, they are used as phase-transfer catalysts in organic synthesis. Moreover, Cheng27 and Schue28 have expanded this idea in the areas of anionic polymerization. 

A fairly different approach to enhance the probability of the intramolecular pathway emerged with the discovery of crown ethers as a new class of complex-ing agents. Pedersen s pioneering experiments in this area revealed an unusual phenomenon. The 18-membered polyether 315 was formed in high yields by the condensation of two moles of catechol 316 with two moles of bis(2-chloroethyl) ether 317 (Scheme 2.115). The result was truly remarkable as, in this case, the... 

Carrier Chemistry. The use of structurally modified macrocycllc polyethers (crown ethers) as CcU rlers In bulk, emulsion, and Immobilized liquid membranes Is the subject of the chapter by Bartsch et al. (111). They discuss the use of lonlzable crown ethers for the coupled transport of alkali metal cations. The lonlzable carboxylic and phosphonlc acid groups on the macrocycles eliminate the need for an anion to accompany the catlon-macrocycle complex across the liquid membrcuie or for an auxiliary complexlng agent In the receiving phase. The influence of carrier structure on the selectivity and performance of competitive alkali metal transport across several kinds of liquid membranes Is presented. 

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