Polymer-bound cryptands

There are also many uses for nonenzymatic polymeric catalysts. For instance, polymer-bound crown ethers, cryptates, and channel compounds behave as polymeric phase-transfer catalysts. The catalytic activity is based on selective complex formation. An example is the use of polystyrene-attached oxygen heterocycles [18]-crown-6 or a cryptand[222] to catalyze replacements of bromine in n-octyl bromide by an iodine or by a cyanide groups... 

A number of other cryptand-bound polymers have been synthesized using similar procedures to those discussed previously for immobilization of crown molecules. Apart from their use in phase transfer catalysis, such polymers have been studied extensively as chromatography reagents for the separation of a range of metal-ion types (Blasius Janzen, 1982) in a number of instances quite useful separations have been achieved. 

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... 

Resin-bound PTC catalysts include polymer-NRs, -PPhs, -SR2, -crown ethers, and -cryptands, etc. 

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). 

Typical crown and cryptand catalysts were bound as active groups to a polymer matrix of the polystyrene type, crosslinked with 20% or 4% p-divinylbenzene. 

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. 

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-... 

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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