Immobilized crown ethers

Crown-ether network polymers have also been prepared by adding to a slurry of NaH in THF, an equimolar amount of the appropriate hydroxy-containing crown-ethers followed by the addition of chloromethylated cross-linked polystyrene. Binding of Na, K, Cs picrates and of sodium tetraphenyl-borate to the immobilized crown-ether is achieved under simple conditions [74]. 

In comparison, employing the reaction strategy depicted in Scheme 48 whereby the trifluoroacetic acid (TFA) salt of tyramine 175 was N-protected by the immobilized crown ether [step (a)], O-acetylated with acetic anhydride 37 in the presence of an organic base 14 [step (c)], and then simultaneously deprotected and the crown ether regenerated... 

Fig. 7.19 Representative chromatograms forthe resolution of (a) gemifloxacin (methanesulfonate form) and the (b) O-ethyloxime analogue of gemifloxacin on an immobilized crown ether-type CSP. Column (250 x 4.6 mm i.d.) mobile phase 80% acetonitrile in H2O4-H2SO4...

Polymeric crown ether resins are mechanically unstable and are, therefore, operated with low flow rates between 0.05 and 0.1 mL/min resulting in long analysis times. Typical for all polymeric crown ether phases is the relatively low chromatographic efficiency, which does not meet today s requirements. However, this problem can be overcome by immobilizing crown ether polymers on the surface of a solid support. Modified and non-modified silica are predominantly used as support materials. Igawa et al., for instance, coated silica particles with the above-mentioned polyamide crown ether resin and obtained significantly better separations then with the rmcoated resin. 

In a recent development, Ito and Yamaguchi [45] plasma-grafted poly(N-isopropylacrylamide) (pNIPA) chains partially substituted with Ba +-selective crown ether side chains into the micropores ofa polyethylene membrane. At 40 °C and in the absence of Ba +, the pNIPA chains are relatively collapsed onto the pore walls, and the membrane is relatively permeable to water. When Ba enters the pores and binds to the immobilized crown ethers, the polymer becomes a polyelectrolyte and swells, substantially reducing hydraulic permeability. On the other hand, fixed charge present on the swollen polymer excludes further entry of Ba + because of buildup ofa Donnan potential. 

Most studies with the immobilized crown ethers and other ligands have been concerned with determining their effectiveness in separating ionic or neutral solutes and with their role as phase transfer catalysts in modifying reaction rates, yields or distribution of products. However, quantitative information on the actual binding of ionic solutes as a function of the many... 

From an economic standpoint, it is important to immobilize crown ethers on the surface of porous support to make full use of the costly crown ether. The scheme of our method can be shown as follows. 

The first examples of immobilized crown ethers are those with oxygen-donor atoms, such as dibenzo-18-crown-6 with polyamic acids. The linear 18-crown-6/polyamic acid (Fig. 1) was found as possessing a great binding ability with alkali and alkaline earth metal cations in aqueous solutions [6]. 

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

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... 

Immobilization of phase-transfer catalysts on polymeric matrices avoids the problem of separating and recycling the catalysts. In this case the chemical stability of the immobilized catalyst becomes very important quaternary salts often decompose under drastic reaction conditions whereas polydentate ligands are always stable. However, the difficult synthesis of cryptands, despite their high catalytic efficiency, can hardly justify their use. Synthesis of crown-ethers is much easier, but catalytic efficiences are often too low. 

Pillared polyethers and polyimines have been also described, which present sorbent and ion exchange properties [7, 56]. Immobilization of crown ethers and diaza crown ethers has been carried out with the bisphosphonate pillars strategy by using the co-precipitation synthesis [44, 58]. Exchange re-... 

Desired chemical functionality may also be introduced into SA monolayers via in situ cleavage of unsaturated surfactants. Crown-ether-complexed potassium permanganate was shown to break the carbon-carbon double bonds in the substrate-immobilized, unsaturated surfactants and to convert them to carboxyl groups. Slow lateral propagation of this bond breaking resulted in the formation of shorter monolayers with exposed terminal moieties (Fig. 22) [195]. 

In line with the scope of this article only developments in polymer immobilized metal phthalocyan-ines, Schiff s bases, or crown ethers are highlighted barring a few cases of unanchored metal complexes displaying novel features. 

Chiral crown ethers based on IB-crown-6 I Fig. 4> can form inclusion complexes with ammonium ions and proionated primary amines. Immobilization of these chiral crown ethers on a chromatographic support provides a chiral stationary phase which can resolve most primary amino acids, amines and amino alcohols. However, the stereogenic center must be in fairly close proximity in the primary aininc lor successful chiral separalion. Significantly, ihe chiral crown ether phase is unique in that ii is one of the few liquid chromatographic chiral stationary phases that does not require the presence of an aromatic ring to achieve chiral separations. 

The CSPs based on chiral crown ethers were prepared by immobilizing them on some suitable solid supports. Blasius et al. [33-35] synthesized a variety of achiral crown ethers based on ion exchangers by condensation, substitution, and polymerization reactions and were used in achiral liquid chromatography. Later, crown ethers were adsorbed on silica gel and were used to separate cations and anions [36-39]. Shinbo et al. [40] adsorbed hydrophobic CCE on silica gel and the developed CSP was used for the chiral resolution of amino acids. Kimura et al. [41-43] immobilized poly- and bis-CCEs on silica gel. Later, Iwachido et al. [44] allowed benzo-15-crown-5, benzo-18-crown-6 and benzo-21-crown-7 CCEs to react on silica gel. Of course, these types of CCE-based phases were used in liquid chromatography, but the column efficiency was very poor due to the limited choice of mobile phases. Therefore, an improvement in immobilization was realized and new methods of immobilization were developed. In this direction, CCEs were immobilized to silica gel by covalent bonds. 

The prevailing extraction-chromatographic method for 90Sr separation entails the use of Sr-Resin.116"119 This material is prepared by impregnating a porous polymer with a 1-octanol solution of the crown ether 4,4 (5 )-bis(t-butylcyclohexano)-18-crown-6 (DtBuCH18C6). This material will extract and retain Sr from 2-8 M HN03 solutions, while most of the matrix constituents are not retained and are removed with a column wash. The separated Sr is released by elution with water or weak nitric acid. The extraction equilibrium is shown in Equation 9.2, where the bar above a species indicates that it is immobilized on the resin.4... 

Packed capillary columns with chirally selective stationary phases (e.g., flr acid glycoprotein), as well as wall-immobilized, CD-based stationary phases, have been successfully used in CE chromatographic separations. Also, macro-cylic crown ethers, forming sterically selective complexes with the guest molecule, have been used for the resolution of optically active amines. 

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