Cryptates polyether

Sodium or potassium ions can also participate in the phase-transfer process when they are converted to lipophilic cations by complexation or by strong specific solvation. A variety of neutral organic compounds are able to form reasonably stable complexes with K+ or Na + and can act as catalysts in typical phase-transfer processes. Such compounds include monocyclic polyethers, or crown ethers (1), and bicyclic aminopolyethers (cryptates) (2). They can solubilize inorganic salts in nonpolar solvents and are particularly recommended for reactions of naked anions. Applications of these compounds have been studied.12,21-31... 

Although acyclic polyethers, e.g., polyethylene glycols, form less stable solvates than the cyclic counterparts, they are also able to act as catalysts in biphasic systems. Typical structures of open-chain equivalents of crown ethers and cryptates are glymes32-34 (3) polyethylenamines35,36 (4) poly-podes37,38 (5) lariat ethers39,40 (6) and octopus41 (7). 

It seems likely that the future will see major developments in the use of metal complexes as bacteriocidal, viricidal, immunosuppressive, anti-arthritic and biocidal agents. One class of complexing agents that appears to have particular promise are the macrocyclic antibiotics, macrocyclic polyethers, polyamines and cryptates . In this group the ligand wraps around a metal ion to form a lipid-soluble complex... 

The use of cyclic (crown) polyethers and cryptates to enhance metal solubilities also opens up many interesting possibilities for studying expanded-metal compounds in the low-dielectric-constant solvents... 

For all systems, the lighter isotope Li is enriched in the organic phase compared with Li. Obviously, this also proved true for the Chinese work where the obtained e-value of 30 x 10" is comparable with the results of Jepson and Cairns. On the whole, a significant dependence of the isotopic separation on the polyether as well as on the anion of the lithium salt was found. This can be understood by means of the different interaction of the cryptated cation with the anion in chloroform. According to Jepson and Cairns, the enrichment of Li in the organic phase is attributed to a more stable Li polyether complex compared with the Li compound. 

Crown ethers or cryptates 493) are cyclic polyethers that selectively solvate alkali-metal ions, although depending on the size of the hole, other ions can be extracted by these reagents. Some recent work on extraction using these crown ethers has been reported 494-496) and more results can soon be expected from the use of crown ketones 408, 409). 

Stability and selectivity. The pH-metric titration method is usually used to determine the stability constants of cryptates (47). For alkali and alkaline earth metal cations, high stability constants are generally observed. As with the monocyclic polyethers, the most stable complex results when the ionic radius of the metal cation best matches the radius of the cavity formed by the cryptand on complexation. Because the cryptand host cavity is three-dimensional and spheroidal in shape, it is well adapted for a ball-like guest metal cation. Hence they have more pronounced recognition receptor... 

Cryptate effect. Cabbiness and Margenum (51) observed increased stability for the complexes of monocyclic ligands over those with an open chain analogue of similar composition and termed this extra stability the macrocyc-lic effect . For example the monocyclic tetramine cyclam-Cu complex is much more stable (by a factor of 10" ) than the open-chain tetramine Cu " complex. The same effect is found with the macrocyclic polyethers (17) (see Figure 2.16). 

The macrocyclic polyethers were prepared by Pedersen a decade ago and shown to complex a variety of cationic substrates [39]. Among these substrates are alkali metal cations, alkaline earth cations and ammonium ions [40]. It has been shown that hindered esters could be saponified by KOH in toluene solution in the presence of dicyclo-hexyl-18-crown-6 [41] or cryptate [42]. Potassium hydroxide is both solubilized in toluene and activated by the crown ether [43]. Sam and Simmons, however, first clearly demonstrated the potential of crowns as phase transfer reagents by solubilizing potassium permanganate in benzene solution (see Eq. 1.15) and using the solubilized reagent in a variety of oxidation processes [44]. 

Another type of phase-transfer catalysts is synthetic macro-cyclic polyethers, so-called cro m ethers, and poly(ethylene glycol) derivatives. Since the discovery of crown ethers and their complex-ing capabilities toward metal and ammonium ions in 1967 by Pedersen, crown ethers and modified compounds such as cryptates > have been attracting ever increasing interest among scientists having different applications in their mind. Complexes formed between these compounds and cations correspond to Q" " shown in Fig. 1 and possess phase-thransfer capacity for anions. The nature of cations is known to greatly influence the complexation with a given crown either or cryptate. ... 

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