Crown ether separation

As outlined in (1) at the beginning of Section 2, a solution containing a metal salt (MX) and a crown ether (Cr) may contain three different anionic species, namely, the free anion X-, the tight ion pair M+.X and the crown ether-separated ion pair M+.Cr.X. All of these species may react with a substrate SY with different rate constants. As the reaction proceeds, any leaving group Y may affect the rate of the reaction by competing with M+. X-for crown ether as shown in (11). It is evident from (1) and (11) that the effect... 

The behaviour of the dibenzo-18-crown-6 derivative is similar, yet the highest attainable rate is only one eighth of that observed with dicyclohexyl-18-crown-6, which points to an important difference in reactivity between crown ether-separated ion pairs. Compared to tetrabutylammonium phenoxide, the dicyclohexyl-18-crown-6/K+ phenoxide was 2.6 times less reactive. The addition of 0.05 M dicyclohexyl-18-crown-6 to dioxan resulted in the alkylation rate constant becoming the same as that observed in pure tetraglyme. 

The effect of crown ethers on the alkylation of sodium diethyl n-butylmalonate by 1-bromobutane has been studied by Zaug et al. (1972). The absence of a common-ion rate depression in dimethylformamide (DMF) pointed to an ion pair being the kinetically active species. The addition of dicyclohexyl-18-crown-6 (a mixture of [20] and [21]) accelerates the alkylation in both benzene and tetrahydrofuran (THF) (Table 24). The rates reach a plateau, indicating that at a crown-ether concentration of 0.5 M the ion pair is fully converted to the crown ether-separated ion pair which is slightly less reactive than the uncomplexed ion pair in DMF. The rate constant in pure dimethoxyethane (DME) is equal to that observed in THF or benzene... 

Conversion of tight ion pairs into crown ether-separated ion pairs leads in many cases to increased basicity. For example, Dietrich and Lehn (1973) have shown that a homogeneous solution of sodium t-amyloxide in benzene is unable to deprotonate triphenylmethane, whereas the reaction occurs rapidly in the presence of [2.2.2]-cryptand [37]. In THF or diethyl ether, alkali metal enolates do not react with triphenyl- or diphenylmethane (Pierre et al.,... 

It is clear from Table 44 that the E2/SN2 ratios observed for reactions of crown ether-separated KOt-Bu ion pairs will greatly depend on the type of substrate and solvent. Di Biase and Gokel (1978) have recently reported many examples of the use of this reagent either as a nucleophile [for example, in its reaction with benzyl chloride and in the reaction with isatoic anhydride (34)] or as a strong base [for example, in the basic oxidation of fluorene to 2-carboxybiphenyl (35)]. 

Mn nmr spectroscopy the authors concluded that the crown ether-separated ion pair prevails in solution. At 25°C, the complex reacts slowly (half life 48 h) with benzene. The complex reacts readily with olefins to give the potassium salts of the corresponding acids in high yields. The authors proposed a mechanism, in which the first step is a [2 + 4] electrocyclic addition as... 

Crown and lariat ethers formed the basis of several synthetic channel model systems. These involved multiple crown ethers separated by fixed spacers and macrocycles having flexible side arms. Polymers and peptides were also used as backbones to organize multiple crowns that inserted into phospholipid bilayers. 

Also, in this case, a rate plateau is reached when the sodium enolate is totally complexed by the ligand (w4 molar equivalents of 5). Under these conditions, the crown-ether-separated ion pair is only 3.8 in THE and 2.3 times in benzene less reaetive than the kinetically active species in DMF. Onee again, these results highlight the remarkable ability of crown ethers to specifically solvate cations activating counterions. 

Agrawal, Y.K., Shrivastava, P. Menon, S.K. (2000) Solvent extraction, separation of uranium(VI) wifli crown ethers. Separation and Purification Technology, 200 (2-3), 177-183. 

Among various types of chiral stationary phases, the host-guest type of chiral crown ether is able to separate most amino acids completely (58). 

Appllca.tlons. The first widely appHcable Ic separation of enantiomeric metallocene compounds was demonstrated on P-CD bonded-phase columns. Thirteen enantiomeric derivatives of ferrocene, mthenocene, and osmocene were resolved (7). Retention data for several of these compounds are listed in Table 2, and Figure 2a shows the Ic separation of three metallocene enantiomeric pairs. P-Cyclodextrin bonded phases were used to resolve several racemic and diastereomeric 2,2-binaphthyldiyl crown ethers (9). These compounds do not contain a chiral carbon but stiU exist as enantiomers because of the staggered position of adjacent naphthyl rings, and a high degree of chiral recognition was attained for most of these compounds (9). 

Another group of macrocyclic ligands that have been extensively studied are the cycHc polyethers, such as dibenzo-[18]-crown-6 (5), in which the donor atoms are ether oxygen functions separated by two or three carbon atoms. The name crown ethers has been proposed (2) for this class of compounds because of the resemblance of their molecular models to a crown. Sandwich stmctures are also known in which the metal atom is coordinated with the oxygen atoms of two crown molecules. 

So many different crown ether systems have been prepared over the recent decade that it sometimes seems that any of them could be placed in a miscellaneous category. On the other hand, each has its interesting features and probably merits a separate section for adequate discussion. Because both of these criteria cannot be met simultaneously, we have placed a number of compounds in this section which are fully deserving of detailed discussion, but not enough examples are yet available to group them separately. 

Cyclic low molecular weight compounds. Chiral separations using chiral crown ethers immobilized on silica or porous polymer resins were first reported in the... 

Enantioresolution in capillary electrophoresis (CE) is typically achieved with the help of chiral additives dissolved in the background electrolyte. A number of low as well as high molecular weight compounds such as proteins, antibiotics, crown ethers, and cyclodextrins have already been tested and optimized. Since the mechanism of retention and resolution remains ambiguous, the selection of an additive best suited for the specific separation relies on the one-at-a-time testing of each individual compound, a tedious process at best. Obviously, the use of a mixed library of chiral additives combined with an efficient deconvolution strategy has the potential to accelerate this selection. 

Inspired by the separation ability of cyclic selectors such as cyclodextrins and crown ethers, Malouk s group studied the synthesis of chiral cyclophanes and their intercalation by cation exchange into a lamellar solid acid, a-zirconium phosphate aiming at the preparation of separation media based on solid inorganic-organic conjugates for simple single-plate batch enantioseparations [77-80]. 

Armstrong and Jin [15] reported the separation of several hydrophobic isomers (including (l-ferrocenylethyl)thiophenol, 1 -benzylnornicotine, mephenytoin and disopyramide) by cyclodextrins as chiral selectors. A wide variety of crown ethers have been synthesized for application in enantioselective liquid membrane separation, such as binaphthyl-, biphenanthryl-, helicene-, tetrahydrofuran and cyclohex-anediol-based crown ethers [16-20]. Brice and Pirkle [7] give a comprehensive overview of the characteristics and performance of the various crown ethers used as chiral selectors in liquid membrane separation. 

Table 6. Maximum molar ratios of transported alkali metal ions to crown ether carrier for several separation techniques...

Heumann, K. G. Isotopic Separation in Systems with Crown Ethers and Cryptands. 127, 77-132 (1985). 

Crown ethers have been used to separate isotopes of cations, for example Ca from Ca. For a review, see Heumann, K.G. Top. Curr. Chem., 1985, 127,11. 

Another material based on the crown ether extractant 4,4 (5 )-bis(t-butyl-cyclohexano)-18 crown-6, marketed under the name Sr-Spec, is useful for separations involving divalent cations including Pb, Ba, and Ra (Horwitz et al. 1991). For Ra analysis by TIMS, Ra-Ba separations are required because the presence of Ba drastically decreases the ionization efficiency of fg Ra samples from 10% to <1%. This material has been widely used for separations of Ra from Ba (e.g., Chabaux et al. 1994 Lundstrom et al. 1998 Rihs et al. 2000 Joannon and Pin 2001 Pietruszka et al. 2002) and is a complement or alternative to cation exchange separations for EDTA complexes of these elements (Volpe et al. 1991 Cohen and O Nions 1991). Sr-Spec material would also be useful for °Pb analysis, since Pb has a greater distribution coefficient than Sr with this extractant. 

Horwitz EP, Dietz ML, Fisher DE (1991) Separation and preconcentiation of strontinm from biological, environmental, and nuclear waste samples by extraction chromatography nsing a crown ether. Anal Chem 63 522-525... 

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