Potassium cryptates

The rubidium and cesium complexes of [2.2.2] are isomorphous with approximate D3 symmetry and a crystallographic twofold rotation axis (41). While the rubidium cation is complexed almost without strain, the Cs+ is accommodated only by enlarging the cavity, increasing the mean C—C torsion angle to 71° (compared with 54° for the potassium cryptate). The ligand deformations required to complex Na+ and Cs + are reflected in their lower solution stability constants with respect to the K+ and Rb+ cryptates (see Section III,D). 

In the potassium cryptate with the carbon-bridgehead cryptand di-... 

ClsHa6CS2N2S4, Caesium di-n-butyldithiocarbamate, 34B, 401 Cl8H36IKN2O6, Potassium cryptate, 39B, 518 Cl8H36lN2Na06, Sodium cryptate, 39B, 519 Cl8H36N2Na20g, Disodium[2.2.2]-cryptate, 40B, 664 Cl8H4aBraCaNaOs, Calcium cryptate, 39B, 519... 

The macrocychc hexaimine stmcture of Figure 19a forms a homodinuclear cryptate with Cu(I) (122), whereas crown ether boron receptors (Fig. 19b) have been appHed for the simultaneous and selective recognition of complementary cation—anion species such as potassium and fluoride (123) or ammonium and alkoxide ions (124) to yield a heterodinuclear complex (120). 

The crown ethers and cryptates are able to complex the alkaU metals very strongly (38). AppHcations of these agents depend on the appreciable solubihty of the chelates in a wide range of solvents and the increase in activity of the co-anion in nonaqueous systems. For example, potassium hydroxide or permanganate can be solubiHzed in benzene [71 -43-2] hy dicyclohexano-[18]-crown-6 [16069-36-6]. In nonpolar solvents the anions are neither extensively solvated nor strongly paired with the complexed cation, and they behave as naked or bare anions with enhanced activity. Small amounts of the macrocycHc compounds can serve as phase-transfer agents, and they may be more effective than tetrabutylammonium ion for the purpose. The cost of these macrocycHc agents limits industrial use. 

Both of the above-mentioned catalyst types get the anions into the organic phase, but there is another factor as well. There is evidence that sodium and potassium salts of many anions, even if they could be dissolved in organic solvents, would undergo reactions very slowly (dipolar aprotic solvents are exceptions) because in these solvents the anions exist as ion pairs with Na or and are not free to attack the substrate (p. 443). Fortunately, ion pairing is usually much less with the quaternary ions and with the positive cryptate ions, so the anions in these cases are quite free to attack. Such anions are sometimes referred to as naked anions. 

The proportion of the /rans-O-alkylated product [101] increases in the order no ligand < 18-crown-6 < [2.2.2]-cryptand. This difference was attributed to the fact that the enolate anion in a crown-ether complex is still capable of interacting with the cation, which stabilizes conformation [96]. For the cryptate, however, cation-anion interactions are less likely and electrostatic repulsion will force the anion to adopt conformation [99], which is the same as that of the free anion in DMSO. This explanation was substantiated by the fact that the anion was found to have structure [96] in the solid state of the potassium acetoacetate complex of 18-crown-6 (Cambillau et al., 1978). Using 23Na NMR, Cornelis et al. (1978) have recently concluded that the active nucleophilic species is the ion pair formed between 18-crown-6 and sodium ethyl acetoacetate, in which Na+ is co-ordinated to both the anion and the ligand. 

For potassium zeolites, cryptofix 222 and cryptofix 222BB, for example, can be used. The structures together with the stability constants Ks of the complexes (cryptates) of cryptofix 222 and cryptofix 222BB with potassium are shown in... 

Table 2 Structures of Potassium Selective Cryptands Cryptofix 222 and Cryptofix 222BB and the Stability Constants of the Matching Cryptates...

For incorporation of crown ethers and cryptates into the RTV encapsulant system as sodium and potassium ion scavengers, the total ionic contaminants must first precisely be determined. Atomic absorption is used to measure these ions in commercial silicone RTVs and silicone fluids. Values of "10 ppm for sodium and potassium were obtained in the best samples. Chloride level was determined by potentiometric titration of the silicone with AgN03. A quantity of ion trap (either crown ethers or cryptates) was then added to the RTV silicone encapsulant, and its molar concentration was equal to the combined sodium and potassium contaminant levels. 

In macrobicyclic cryptate complexes where the cation is more efficiently encapsulated by the organic ligand these ion pair interactions are diminished and the reactivity of the anion is enhanced. This effect is seen in the higher dissociation constant, by a factor of 104, of Bu OK in Bu OH when K+ is complexed by [2.2.2]cryptand (12) compared to dibenzo[18]crown-6 (2). The enhanced anion reactivity is illustrated by the reaction of the hindered ester methyl mesitoate with powdered potassium hydroxide suspended in benzene. 

The Kp of cryptated potassium phenoxide is higher (about 4 times) for 2,6 dimethylphenoxide compared to 3,5 dimethylphenoxide. 

Alkylation of nonenoRzable ketones. Potassium hydride in THF reduces cyclopropyl phenyl ketone however, in the presence of cryptate [2.2.21 (1 equivalent) the ketone can be alkylated in high yield (equation I). 

As noted earlier, the similarities between H+ and alkali metal cations have led to the use of the former as a probe in biological studies, including studies with various macrocydic ligands, especially those with oxygen donor atoms. The thallium(I) cryptates behave kinetically like the potassium compounds, and the binding constants to 18-crown-6 have been measured by 205T1 NMR methods.347 Several Tl1 compounds with crown ethers (L) have been prepared in... 

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... 

Kirch and Lehn have studied selective alkali metal transport through a liquid membrane using [2.2.2], [3.2.2], [3.3.3], and [2.2.C8] (146, 150). Various cryptated alkali metal picrates were transported from an in to an out aqueous phase through a bulk liquid chloroform membrane. While carrier cation pairs which form very stable complexes display efficient extraction of the salt into the organic phase, the relative rates of cation transport were not proportional to extraction efficiency and complex stability (in contrast to antibiotic-mediated transport across a bulk liquid membrane). Thus it is [2.2.Ca] which functions as a specific potassium ion carrier, while [2.2.2] is a specific potassium ion receptor (Table VI). 

Cryptand was obtained as the corresponding sodium cryptate 20 in 27% yield. The cation-free cryptand was isolated by passing an acidic solution of the complex through cation- and anion-exchangers. However, the overall yield was not reported. When sodium carbonate was replaced by potassium carbonate, no detectable amount of cryptate K+ c [2.2.2] was observed this confirms the involvent of a template effect, although this method is rather limited to simple mononucleating cryptands. 

It should be mentioned that cation complexation by crown-type ligands can itself be solvent-dependent. For example, the dissociation rate of potassium [2.2.2]cryptate in EPD solvents increases with the donor number of the solvent [650]. Moreover, coronands themselves can interact with organic solvent molecules [651]. Such cation-solvent and ligand-solvent interactions can influence the formation of cation-ligand complexes. 

Williamson ether synthesis. Cryptate [2.2.2] is an efficient catalyst for the Williamson ether synthesis. Thus reaction of the potassium salt of cydooctanol with benzyl bromide at 20° for 5 hours in the presence of the cryptate gives the ether in 98% yield. ... 

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