Polymer supports, soluble

Seliger H, Aumann G, Polymer support synthesis, 5, Oligonucleotide synthesis on a polymer support soluble in water and pyridine, Tetrahedron Lett., 2911-2914, 1973. 

The applications reported for polymer-supported, soluble oxidation catalysts are the use of poly(vinylbenzyl)trimethylammonium chloride for the autooxidation of 2,6-di-tert-butylphenol [8], of copper polyaniline nanocomposites for the Wacker oxidation reaction [9], of cationic polymers containing cobalt(II) phthalocyanate for the autooxidation of 2-mercaptoethanol [10] and oxidation of olefins [11], of polymer-bound phthalocyanines for oxidative decomposition of polychlorophenols [12], and of a norbornene-based polymer with polymer-fixed manganese(IV) complexes for the catalytic oxidation of alkanes [13], Noncatalytic processes can also be found, such as the use of soluble polystyrene-based sulfoxide reagents for Swern oxidation [14], The reactions listed above will be described in more detail in the following paragraphs. 

Miscible Blends. Sometimes a miscible blend results when two polymers are combined. A miscible blend has only one amorphous phase because the polymers are soluble in each other. There may also be one or more crystal phases. Simple theory (26) has supported the empirical relation for the permeabihty of a miscible blend. Equation 18 expresses this relation where is the permeabihty of the miscible blend and ( ) and are the volume fractions of polymer 1 and 2. 

Polymer-supported catalysts incorporating organometaUic complexes also behave in much the same way as their soluble analogues (28). Extensive research has been done in attempts to develop supported rhodium complex catalysts for olefin hydroformylation and methanol carbonylation, but the effort has not been commercially successful. The difficulty is that the polymer-supported catalysts are not sufftciendy stable the valuable metal is continuously leached into the product stream (28). Consequendy, the soHd catalysts fail to eliminate the problems of corrosion and catalyst recovery and recycle that are characteristic of solution catalysis. 

This is an ion-exchanger like the sulfonated polymer. The siUca surface can also be functionalized with phosphine complexes when combined with rhodium, these give anchored complexes that behave like their soluble and polymer-supported analogues as catalysts for olefin hydrogenation and other reactions ... 

Styrene-based polymer supports are produced by o/w suspension polymerization of styrene and divinylbenzene. Suspension polymerization is usually carried out by using a monomer-soluble initiator such as benzoperoxide (BPO) or 2,2-azo-bis-isobutylnitrile (AIBN) at a temperature of 55-85°C (19). A relatively high initiator concentration of 1-5% (w/w) based on the monomer is used. The time required for complete monomer conversion must be determined by preliminary experiments and is usually between 5 and 20 h, depending on the initiator concentration, the temperature, and the exact composition of the monomer mixture (11-18). 

Polymer-supported amino alcohols and quaternary ammonium salts catalyze the enan-tioselective addition of dialkylzinc reagents to aldehydes (Table 31). When the quaternary ammonium salt F is used in hexane, it is in the solid state, and it catalyzes the alkylation of benzaldehyde with diethylzinc in good chemical yield and moderate enantioselectivity. On the other hand, when a mixture of dimethylformamide and hexane is used as solvent, the ammonium salt is soluble and no enantioselectivity is observed21. 

Abstract Current microwave-assisted protocols for reaction on solid-phase and soluble supports are critically reviewed. The compatibility of commercially available polymer supports with the relatively harsh conditions of microwave heating and the possibilities for reaction monitoring are discussed. Instrmnentation available for microwave-assisted solid-phase chemistry is presented. This review also summarizes the recent applications of controlled microwave heating to sohd-phase and SPOT-chemistry, as well as to synthesis on soluble polymers, fluorous phases and functional ionic liquid supports. The presented examples indicate that the combination of microwave dielectric heating with solid- or soluble-polymer supported chemistry techniques provides significant enhancements both at the level of reaction rate and ease of purification compared to conventional procedures. 

By replacing insoluble cross-linked resins with soluble polymer supports, the well-estabhshed reaction conditions of classical organic chemistry can be more readily apphed, while still fadhtating product purification. However, soluble supports suffer from the hmitation of low loading capacity. The recently introduced fluorous synthesis methodology overcomes many of the drawbacks of both the insoluble beads and the soluble polymers, but the high cost of perfluoroalkane solvents, hmitation in solvent selection, and the need for specialized reagents may hmit its apphcations. 

The N-substituted aminoacids required could be prepared by microwave-assisted reductive amination of aminoacid methyl esters with aldehydes, and although in the Westman report soluble NaBH(OAc)3 was used to perform this step, other reports have shown how this transformation can be performed in using polymer-supported borohydrides (such as polymer-supported cyanoborohydride) under microwave irradiation [90]. An additional point of diversity could be inserted by use of a palladium-catalyzed reaction if suitably substituted aldehydes had been used. Again, these transformations might eventually be accomplished using supported palladium catalysts under microwave irradiation, as reported by several groups [91-93]. 

In 2004, Weberskirch and co-workers tried a new approach by synthesising [RhBr(COD)(NHC)] 23 (NHC = l-(2 -hydroxyethyl)-3-methylimidazolidine-2-ylidene) [28]. Subsequently, attaching the unsymmetrical, monohydroxy-functionalised NHC by ester groups to an amphiphilic, water-soluble polymer support (ps)... 

Geckeler, K, E, Soluble Polymer Supports for Liquid-Phase Synthesis. VoL 121, pp. 31-80. Gehrke, S, H. Synthesis, Equilibrium Swelling, Kinetics Permeability and Applications of Environmentally Responsive Gels. Vol. 110, pp. 81 -144. de GenneSy P.-G> Flexible Polymers in Nanopores. VoL 138, pp. 91-106. 

Polymer-supported catalysts often have lower activities than the soluble catalysts because of the intraparticle diffusion resistance. In this case the immobilization of the complexes on colloidal polymers can increase the catalytic activity. Catalysts bound to polymer latexes were used in oxidation reactions, such as the Cu-catalyzed oxidation of ascorbic acid,12 the Co-catalyzed oxidation of tetralin,13 and the CoPc-catalyzed oxidation of butylphenol14 and thiols.1516 Mn(III)-porphyrin bound to colloidal anion exchange resin was... 

Fig. 2. Lysine-based dendron 1 of 2nd generation and a similar dendron 2 of 4th generation with PEG as soluble polymer support [8,9]...

Initiator (233), and a polymer-supported analog,641 are commercially available and have found widespread use in the ring-closing metathesis (RCM) and ROMP of functionalized substrates. In addition, water-soluble variants such as (234) and (235) have been synthesized using aliphatic ionic phosphines and employed in aqueous media.642-645... 

Scheme 7.61 Principles of syntheses on soluble polymer supports.

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. 

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