Cryptands polymer supported

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... 

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... 

As with polymer-supported onium ions the degree of cross-linking of the polymer support is likely to affect mainly intraparticle diffusion in reactions with polymer-supported crown ethers or cryptands. The activity of catalyst 37 decreased by a factor of about 3 as % CL with divinylbenzene changed from 1 % to 4.5 % 146). 

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). 

Polymer-supported onium ions are relatively unstable under severe conditions, especially concentrated alkali154). Polymer-supported crown ethers and cryptands are stable under such conditions. In practice, they could be reused without loss of catalytic activity for the alkylation of ketones under basic conditions, whereas the activity of polymer-supported ammonium ion 7 decreased by a factor of 3 after two recycles of the catalyst147). 

Crown ethers and cryptands, either alone or fixed on a polymer support [2.89], have been used in many processes, including selective extraction of metal ions, solubilization, isotope separation [2.90], decorporation of radioactive or toxic metals [2.17, 2.49], and cation-selective analytical methods [2.89, 2.91, 2.92] (see also Sect. 8.2.2 and 8.4.5). A number of patents have been granted for such applications. 

In a cryptate complex, the cation is enclosed wholly or partially in a hydrophobic sheath, so that not only are salts of this complexed cation soluble in nonpolar organic solvents but also extractable from aqueous solutions into organic solvents immiscible with water (144). Specific cryptands may be used to selectively complex metals from crude materials or wastes, particularly if they are immobilized on a polymer support (101, 114, 145). 

Polymer-supported crown ethers, cryptands, and polyethylene glycols in organic synthesis 87MI43. 

Covalent attachment of ligands to polymer supports retains their complexing properties156 and widens their applications. For instance, immobilized crown poly-thers and cryptands used as phase-transfer catalysts can be recycled55. Chiral ligands have been used for a chromatographical separation of D- and L-amino acids75. ... 

Other areas of interest include stabilization of noncommon oxidation states, solvent extraction of cations, transfer of cations through membranes, isotopic separation, detoxification of harmful and radioactive metals, metal recovery, metal trace analysis, ion chromatography on polymer-supported cryptands, and chromo- and fluoro-ionophores. More organic-chemistry-orientated applications can be mentioned, including enhancement of metal salt solubility in organic solvents, anion activation, phase-transfer catalysis, and anionic polymerization. Many of these applications are covered in other articles in this encyclopedia as well as in the literature. 

The rather exciting and stimulating "phase transfer" synthetic procedure based on the use of polymer supported active functional moieties, such as onium groups, crown ethers, cryptands and poly-glymes, becomes even more naive when the active sites are bound to chiral matrices, whose prevalent chirality can be either intrinsi-... 

A number of polymer systems containing macrocyclic ligands, such as the crown ethers, have been prepared. This subject has been reviewed a number of times (Blasius and Janzen, 1982 Gramain, 1982 Sahni and Reedijk, 1984 Smid, 1976, 1981a, 1981b Smid et al., 1979 Takagi and Nakamura, 1986 Yokota, 1989). These reviews report on crown ethers or cryptands attached to solid supports or as part of a polymer system. There have been no separate reviews of polymers containing only aza- or peraza-crown macrocycles. 

The majority of research on catalysis by anion-exchange resins is devoted to the use of resins in interfacial catalysis reactions [38-43]. In this connection, quaternary ammonium or phosphonium, onium salts have been commonly obtained [44]. Crown-ethers, cryptands and linear polyesters supported on polymers also catalyze similar reactions dealing with the interfacial transfer of reagents. In the simplest case, the mechanism of interfacial catalysis represents a substitution reaction of Sf 2 type typical for the interaction of the nucleophile, T , present in the aqueous solution with the alkyl halogenide, RX, in the organic phase ... 

Applications to Phase-transfer Methods.—Dehmlow has published a review on advances in phase-transfer catalysis (PTC) which discusses the introduction of crown ethers into this area. The full details are now available of a study of alkyl-substituted azamacrobicyclic polyethers (78a) as PT catalysts. When the alkyl chains are C14—C20, such molecules are very efficient catalysts in both liquid-liquid and solid-liquid phase-transfer modes, which contrasts with the lower catalytic ability of the less organophilic unsubstituted cryptand (78b). Crown ethers immobilized on polymeric supports have been demonstrated to possess increased PTC activity in 5n reactions, up to that of the non-immobilized systems, when the connection to the polymer involves long spacer chains [e.g. (79)]. 

Phase-transfer catalysts, such as the classic onium salts, crown ethers, and cryptands, have been immobilized on insoluble polymer matrices with various degrees of cross-linking. Their activity remains reasonably high if the catalytic centre is sufficiently far from the polymer backbone or if the resin is very porous. However, with phosphonium salts immobilized on silica gel die length of the hydrophobic chain between the active centre and the matrix and the solvent determine the adsorption capacity of the polar support, which then controls the rate of reaction. ... 

The use of phase-transfer catalysts bound to polymeric supports has been reported. The catalytic functional groups anchored to the polymer were (i) quaternary ammonium salts (Fig. 13-la,b,c), (ii) phosphonium salts (Fig. 13-ld), (iii) Crown ethers (Fig. 13-le), and (iv) cryptands (Fig. 13-If). Chloromethylated, 2-4% cross-linked polystyrene and silica gel were used as the support polymers, and the catalyst groups were anchored either by the reaction with the corresponding amine or phosphine or by absorption. Spacer-arms were used for linking the crown ether and cryptand (Cinouini et al., 1976 Cinquini et al., 1975 Molinari et al., 1977 Tundo, 1977, 1978). 

Quaternary ammonium and phosphonium ions bound to insoluble polystyrene present an even more complicated mechanistic problem. Polystyrene beads lacking onium ions (or crown ethers, cryptands, or other polar functional groups) have no catalytic activity. The onium ions are distributed throughout the polymer matrix in most catalysts. The reactive anion must be transferred from the aqueous phase to the polymer, where it exists as the counter ion in an anion exchange resin, and the organic reactant must be transferred from the external organic phase into the polymer to meet the anion. In principle, catalysis could occur only at the surface of the polymer beads, but kinetic evidence supports catalysis within the beads for most nucleophilic displacement reactions and for alkylation of phenylacetonitrile. 

Quaternary ammonium (3) and phosphonium ions (61), crown ethers such as (62), cryptands such as (63) and poly(ethylene glycol) ethers (64) bound to PS are catalysts for reactions of water insoluble organic compounds with organic insoluble inorganic salts. " Silica gel, alumina, polystyrene-polypropylene composite fibers, nylon capsule membranes, and polyethylene (Mn 1000-3000) have also been used as supports. The reactions are called phase-transfer-catalyzed because one or both of the reactants are transported from the normal liquid or solid phase into a polymer phase, where the reaction proceeds. 

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