Crown Ether and Cryptand Phases

Other Materials for Anion Separations 3.4.3.1 Crown Ether and Cryptand Phases 

Crown ethers can be introduced into the ion chromatographic system via the mobile and/or the stationary phase. Sousa et al [67] used them for the first time as mobile-phase additives describing a separation of amino acid isomers separated with dinaphthal-18-crown-6. But even simple inorganic anions can be separated with this method [68]. However, the number of applicable crown ethers is limited because of their solubilities. Moreover, crown ethers are relatively expensive, so their application at higher concentrations is not justified. 

The introduction of crown ethers via the stationary phase can be carried out in three different ways  

Pioneering work in the field of adsorbing crown ethers onto chemically bonded reversed phases and PS/DVB polymers have been carried out by Kimura et al. [69]. They employed dodecyl-substituted crown ethers such as 12-crown-4, 15-crown-5, and 18-crown-6, all of which have a highly hydrophobic side chain that enables these molecules to adsorb onto the stationary phase. The capacity of the separator column can be controlled by the amount of adsorbed crown ether. Stationary phases of this type are stable over a longer period of time only when used with purely aqueous eluents. If the mobile phase contains more than 40% 

Polymeric crown ether phases have been thoroughly investigated for their applications in chromatography by Blasius et al. [70-75]. They are characterized by chemical and thermal stability, are compatible with a variety of organic solvents, and are of high capacity. Thus, Blasius et al. separated anions with pure methanol as the mobile phase. Because the stability constants of the cation crown ether complexes depend on the type of solvent, the solvent content in the mobile phase can be used as an additional parameter for optimizing the chromatographic separation. 

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With a view to producing catalysts that can easily be removed from reaction products, typical phase-transfer catalysts such as onium salts, crown ethers, and cryptands have been immobilized on polymer supports. The use of such catalysts in liquid-liquid and liquid-solid two-phase systems has been described as triphase catalysis (Regen, 1975, 1977). Cinquini et al. (1976) have compared the activities of catalysts consisting of ligands bound to chloromethylated polystyrene cross-linked with 2 or 4% divinylbenzene and having different densities of catalytic sites ([126], [127], [ 132]—[ 135]) in the... 

Both quaternary onium salts and cation complexes of lipophilic multidentate ligands (crown-ethers and cryptands) have been used as catalysts in two-phase systems in the presence of base (OH, F, etc.). However, under these conditions, the lack of chemical stability of quaternary salts and the very low complexation constants of multidentate ligands (especially crown-ethers) make all these systems barely effective in the activation of such anions. 

Polymer-supported crown ethers and cryptands were found to catalyze liquid-liquid phase transfer reactions in 1976 55). Several reports have been published on the synthesis and catalytic activity of polymer-supported multidentate macrocycles. However, few studies on mechanisms of catalysis by polymer-supported macrocycles have been carried out, and all of the experimental parameters that affect catalytic activity under triphase conditions are not known at this time. Polymer-supported macrocycle... 

Complexation constants of crown ethers and cryptands for alkali metal salts depend on the cavity sizes of the macrocycles 152,153). ln phase transfer nucleophilic reactions catalyzed by polymer-supported crown ethers and cryptands, rates may vary with the alkali cation. When a catalyst 41 with an 18-membered ring was used for Br-I exchange reactions, rates decreased with a change in salt from KI to Nal, whereas catalyst 40 bearing a 15-membered ring gave the opposite effect (Table 10)l49). A similar rate difference was observed for cyanide displacement reactions with polymer-supported cryptands in which the size of the cavity was varied 141). Polymer-supported phosphonium salt 4, as expected, gave no cation dependence of rates (Table 10). 

The versatile solubility properties of the crown ethers and cryptands are important in two of their major applications, phase transfer catalysis and anion activation. Phase transfer catalysis involves the transport of guest species from one phase to another. The two phases in question are usually two immiscible liquids (liquid-liquid phase transport). In practice, this usually means the use of a... 

The use of crown ethers and cryptands in phase transfer catalysis and anion activation are illustrated in the following case studies. 

Hosts such as crown ethers and cryptands are useful as phase transfer agents and mimics of biological membrane transporting ionophores. 

Lamb s group has adsorbed the hydrophobic crown ethers and cryptands shown in Fig 1 onto reversed phase chromatography packings for application to ion chromatography in the analysis of cations and anions. A brief introduction to ion chromatography (IC) is in order to lay the foundation for a description of this work. 

In addition to their application to liquid chromatography, macrocycles have been applied to gas chromatographic separations as well. Kartsova et al. [20] performed a systematic study of a series of crown ethers and cryptands adsorbed onto GC stationary phases. The influence on the polarity and selectivity of the stationary phases of the type and number of heteroatoms, conformational lability of the cavity, the presence of substituents, and the concentration of macrocycle was studied with respect to the separations of various classes of organic compounds. Phases containing mixtures of two macrocycles were found to be most promising. 

Thus, while pyridinium based compounds function like quats do in normal PTC, cyclodextrin compounds behave like host molecules (such as crown ethers and cryptands) and transport the entire molecule into the other phase. 

Landini, D., A. Maia, and F. Montanari, Dehydrating Effect of Concentrated Aqueous Alkaline Solutions in Aliphatic Nucleophilic Substitutions Carried Out in Aqueous-Organic Two-Phase Systems The Different Behavior of Various Phase Transfer Catalysts Quaternary Salts, Crown Ethers and Cryptands, Isr.J. Chem., 26, 263 (1985). 

The function of solid-liquid phase transfer catalysis (SLPTC) is to conduct the reaction of a solid salt and the organic reactant using a PT catalyst that is easily dissolved in the organic phase in the absence of water. These catalysts can be tertiary amines, quaternary ammonium salts, diamines, crown ethers and cryptands, among which crown ethers, act as the catalysts because of their specific molecular structures [183-186]. Starks et al. [183] indicated that 100% of the yield of product benzyl acetate was obtained at 25°C in 2 h for... 

Crown ethers and cryptands can also be used for such reactions . In this regard is should be mentioned that, in homogeneous organic phase, the complexation of LiAlH4 and NaBH4 by specific cryptands ([2.1.1] and [2.2.1] for Li and Na , respectively) inhibits or greatly slows down the reduction of ketones, due to the absence of the electrophilic activation by the inorganic cation 263 -266)... 

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