Polyethylene glycol solid support catalysts

Polymer-supported polyethylene glycol) analogues 50 145.156.l67-16H> anc[ 5/ 167> were effective catalysts for hydroxide, iodide, and phenoxide displacement reactions, but not for cyanide, chloride, and acetate displacementsI69). These catalysts are highly active for various solid/solid/liquid phase transfer reactions (Sect. 6). 

Substrate selectivity effects were investigated with polystyrene-supported polyethylene glycol) catalysts 51 (n = 3,61% RS) and 56 (Z = phenyl, n = 30, 8 % RS) under solid/solid/liquid conditions 180). The difference in rates of reaction of 1-bromobutane and 1-bromooctane with solid potassium phenoxide was a factor of about 3 (51 was more active). Measurements of the distribution of both bromides between... 

All peptide-catalyzed enone epoxidations described so far were performed using insoluble, statistically polymerized materials (neat or on solid supports). One can, on the other hand, envisage (i) generation of solubilized poly-amino acids by attachment to polyethylene glycols (PEG) and (ii) selective construction of amino acid oligomers by standard peptide synthesis-linked to a solid support, to a soluble PEG, or neat as a well-defined oligopeptide. Both approaches have been used. The former affords synthetically useful and soluble catalysts with the interesting feature that the materials can be kept in membrane reactors for continuously oper-... 

Systems have been developed that allow the recycling of catalysts. The first case study involved simple adsorption of proline onto silica gel [6], but the system suffered from a loss in enantioselectivity. More recently, promising results have been obtained with fluorous proline derivatives [64] used for aldol reactions the recycling of fluorous catalysts has been demonstrated using fluorous solid-liquid extraction. Solid phase-supported catalysts through covalent bonds [65] and through noncovalent interactions [66] were also used for aldol reactions. Proline and other catalysts can be recycled when ionic liquids or polyethylene glycol (PEG) were used as reaction solvents [67]. 

Various polymeric and solid supports, such as polyethylene glycol (PEG), can be used to immobilise these catalysts.[46 48] Exchanging the polymer support on the styrene moiety for charged ionic liquid tags affords complexes 38 and 39, which are retained to a significantly higher degree in the ionic liquid phase (Scheme 7.4). 

Tentagel, described above, is in wide use today in solid phase syntheses. Polyethylene glycol has also been attached to various other polymers to form support resins. For instance, Frechet and coworkers [37] used cross-linked methacrylate esters of ethylene glycol oligomers in a suspension polymerization to synthesize hydroxyl group functionalized beads. These beads swell well in a variety of polar solvents. Another example is that of Grubbs attaching a ruthenium metathesis catalyst to polyethylene [38] ... 

A number of other chemical modifications of polymers have been performed under phase transfer conditions Including the cleavage of peptides from a solid support in a Merrlfield solid phase synthesis (Ref. 65), and the hydrolysis of methyl methacrylate in the presence of catalysts such as polyethylene glycol or 18-crown-6 (Ref. 66). Hradil and Svec (Ref. 67) have very recently completed a study of the reaction of hydrolyzed copoly(glycldyl methacrylate ethylene dimethacrylate) with propane sultone in the presence of tetrabutyl ammonium hydroxide. While the reaction gave only 25% yield in the absence of catalyst, a drastic improvement to 68% conversion was observed when a phase transfer catalyst was added. 

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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