Polyethylene supported phase-transfer catalysts

Several microwave-assisted protocols for soluble polymer-supported syntheses have been described. Among the first examples of so-called liquid-phase synthesis were aqueous Suzuki couplings. Schotten and coworkers presented the use of polyethylene glycol (PEG)-bound aryl halides and sulfonates in these palladium-catalyzed cross-couplings [70]. The authors demonstrated that no additional phase-transfer catalyst (PTC) is needed when the PEG-bound electrophiles are coupled with appropriate aryl boronic acids. The polymer-bound substrates were coupled with 1.2 equivalents of the boronic acids in water under short-term microwave irradiation in sealed vessels in a domestic microwave oven (Scheme 7.62). Work-up involved precipitation of the polymer-bound biaryl from a suitable organic solvent with diethyl ether. Water and insoluble impurities need to be removed prior to precipitation in order to achieve high recoveries of the products. 

Sauvagnat, B., Lamaty F., Lazaro, R. and Martinez, J., Polyethylene glycol (PEG) as polymeric support and phase-transfer catalyst in the soluble polymer liquid phase synthesis of ct-amino esters, Tetrahedron Lett., 1998, 39,821. 

Benaglia M, Annunziata R, Cinquini M, Cozzi F, Tocco G (2000) A polyethylene glycol)-supported quaternary ammonium salt an efficient, recoverable, and recyclable phase-transfer catalyst. Qrg Lett 2 1737-1739... 

Xia, M., Wang, Y.-G. A novel microwave-activated Sonogashira coupling reaction and cleavage using polyethylene glycol as phase-transfer catalyst and polymer support. J. Chem. Res., Synop. 2002,173-175. 

Cross-linked supports have also been made by copolymerization of polyethylene glycol terminated with methacrylate groups with cross-linking agents such as trimethylolpropane trimethacrylate.166 This type of resin has also been used as phase-transfer catalysts.167... 

Xia, M. and Wang, Y.G. (2001) Polyethylene glycol as support and phase transfer catalyst in aqueous palladium-catalyzed liquid-phase S3mthesis. Chin. Chem. Lett., 12, 941-2. 

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. 

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). 

Given the structural diversity of the ligands that can be attached to polyethylene oligomers, it is not surprising that there is a similar diversity in the sorts of catalysts that have been supported on these materials. Selected examples of catalysts prepared using polyethylene ligands are shown in structures 14-26 in Fig. 4 [32-34,38-40,45-49]. While most of these catalysts contain transition metals, non-transition metal catalysts like poly-ethyldibutyltin chloride 14 or phase-transfer onium catalysts like 24 have also been prepared. 

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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