Soluble Polymers as Supports

This strategy appears to be very attractive because of the possibility of completely solubilizing the support in most of the common solvents. From a chemical perspective, that property allows one to benefit from all the solvent conditions used in classical solution chemistry. This could prove to be very advantageous, especially to obtain stereoselective glycosylation without neighboring-group assistance. Moreover, isolation and purification of the polymer is easily achieved by precipitation usually in diethyl ether or methyl-tert-butyl ether (MTBE) and recrystallisation from ethanol. One major drawback of this type of support is its tendency to solidify at low temperature, thus limiting the variety of temperature conditions. 

synthesis of lactosamine 72 was reported using the new linker system and an 

Using 5% Pd black, 1 atm H2 in ethanol at r.t. for 48 h, p-tolylmethyl derivative 74 was selectively obtained. Under stronger conditions (10% Pd/C, 50% aqueous AcOH, 3 atm H2 at 50°C) MPEG-DOX was completely cleaved to release, after acetylation, the derivative 75. This selective cleavage was reproduced using different 

Glycosylation of MPEG-DOX-OH with 76 was achieved in 95% yield. After deacetylation using DBU in methanol, glycosylation using disaccharide donor 78 was performed and resin-linked trisaccharide was obtained in 85% yield. 

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In the early 1970 s, Bayer et al. reported the first use of soluble polymers as supports for the homogeneous catalysts. [52] They used non-crosslinked linear polystyrene (Mw ca. 100 000), which was chloromethylated and converted by treatment with potassium diphenylphosphide into soluble polydiphenyl(styrylmethyl)phosphines. Soluble macromolecular metal complexes were prepared by addition of various metal precursors e.g. [Rh(PPh3)Cl] and [RhH(CO)(PPh3)3]. The first complex was used in the hydrogenation reaction of 1-pentene at 22°C and 1 atm. H2. After 24 h (50% conversion in 3 h) the reaction solution was filtered through a polyamide membrane [53] and the catalysts could be retained quantitatively in the membrane filtration cell. [54] The catalyst was recycled 5 times. Using the second complex, a hydroformylation reaction of 1-pentene was carried out. After 72 h the reaction mixture was filtered through a polyamide membrane and recycled twice. 

The subject of soluble polymers as supports in catalysis has been discussed in a number of recent reviews [ 1-5]. These other reviews each focused on a particular polymer or groups of polymers or on some aspect of catalysis (e.g., organic catalysis or asymmetric synthesis) [2, 3, 6-8]. Soluble polymers use as supports in synthesis has also been reviewed, but this topic is not covered below because in synthesis the polymer is used in a stoichiometric amount and is generally not recyclable [5,9-11]. This review takes a general approach, focusing on soluble polymers used as catalyst supports. It discusses these supports within a context of the separation strategies that could be or were used to separate or recover the soluble polymer-bound catalyst from the products. This review emphasizes examples from the last few years where soluble polymers are used but includes, for completeness, earlier examples if a particular... 

Pentene was hydroformylated to Cg aldehydes at 22 °C under 0.1 MPa H2/CO. The reaction solution was membrane-filtered and the products (77% n-hexanal and 23% methylpentanal) were analyzed by GC. The retained catalyst could be recycled twice [3], Along with Ohkubo et al, Bayer and Schurig also reported on soluble polymers as supports for asymmetric catalytic systems. The soluble polystyrene-bound analogue of DlOP (4,5-bis(diphenylphosphinomethyl)-2,2-dimethyl-l,3-dioxalane) was used for the asymmetric Rh-catalyzed hydroformylation of styrene but the ee of the predominantly obtained branched product was only 2% [Eq. (2)]. 

Bergbreiter gave a critical review mainly of work carried out in his own laboratory [4]. An overview of soluble polymers as supports for catalysts has been given by Dickerson et al. [5]. Polymer enlargement as a means for tuning catalyst properties toward hydrophilic solubility was covered by Sinou [6] and other chapters of Comils and Herrmann s books [7, 8]. 

The application of soluble polymers as supports in this field has essential advantages in comparison to the solid-ph e technique or conventional condensation in solution [28, 29, 31, 36, 40] ... 

Many different soluble polymers have been used as supports for catalyst immobilization. Since solvation of otherwise insoluble catalysts can frequently be accom-pHshed by attachment to a soluble polymer, these supports have found significant use in the immobihzation of classical solution phase catalysts. Here, we will only survey polyethylene glycol (PEG) as a soluble polymeric support for catalysis. The use of other types of soluble polymers (e.g., polyethylene, non-cross-linked polystyrene) has been reviewed elsewhere [49]. 

An approach that is intermediate between solid-phase chemistry and solution-phase chemistry is to use soluble polymers as a support for the product. PEG is a common vehicle in many pharmaceutical preparations. Depending on (he degree of polymerization. PEG can be liquid or. solid at room temperature and show varying degrees of solubility in aqueous and organic solvents. Each molecule of PEG has an OH group at either end ... 

Polymers as solids are ubiquitous in our modern society. They are some of the most common synthetic materials. Biologically derived macromolecules are also abundant. Whether it is a piece of wood, a natural fiber, or a lobster shell, nature uses solid organic macromolecular materials as key architectural material. This abundance of examples of synthetic and natural solid polymeric materials is mirrored in the prevalence with which insoluble cross-linked polymer supports are used in synthesis and catalysis [23-25]. However, while solid-phase synthesis and related catalysis chemistry most commonly employ cross-linked supports that resemble those originally used by Merrifield [26], the polymers found in nature are neither always insoluble nor always cross-linked. Indeed, soluble polymers are as common materials as their insoluble cross-linked analogs. Moreover, nature quite commonly uses soluble polymers as reagents and catalysts. Thus, it is a bit surprising that synthetic soluble polymers are so little used in chemistry as supports for reagents, substrates, and catalysts. 

Palladium species immobilized on various supports have also been applied as catalysts for Suzuki cross-coupling reactions of aryl bromides and chlorides with phenylboronic acids. Polymers, dendrimers, micro- and meso-porous materials, carbon and metal oxides have been used as carriers for Pd particles or complexes for these reactions. Polymers as supports were applied by Lee and Valiyaveettil et al. (using a particular capillary microreactor) [173] and by Bedford et al. (very efficient activation of aryl chlorides by polymer bound palladacycles) [174]. Buch-meiser et al. reported on the use of bispyrimidine-based Pd catalysts which were anchored onto a polymer support for Suzuki couplings of several aryl bromides [171]. Investigations of Corma et al. [130] and Plenio and coworkers [175] focused on the separation and reusability of Pd catalysts supported on soluble polymers. Astruc and Heuze et al. efficiently converted aryl chlorides using diphosphino Pd(II)-complexes on dendrimers [176]. 

Soluble Inorganic Polymers as Supports for Transition Metal Catalysts... 

Jesberger, M., Jaunzems, J., Jung, A., Jas, G., Schonberger, A., and Kirschning, A. (2000) Soluble versus insoluble polymers as supports for the preparation of new glycosteroids—a practicability study. Synlett 1289-1293. 

High-performance size exclusion chromatography is used for the characterization of copolymers, as well as for biopolymers (3). The packings for analyses of water-soluble polymers mainly consist of 5- to 10-/Am particles derived from deactivated silica or hydrophilic polymeric supports. For the investigation of organosoluble polymers, cross-linked polystyrene beads are still the column packing of choice. 

As with organic solvents, proteins are not soluble in most of the ionic liquids when they are used as pure solvent. As a result, the enzyme is either applied in immobilized form, coupled to a support, or as a suspension in its native form. For production processes, the majority of enzymes are used as immobilized catalysts in order to facilitate handling and to improve their operational stability [24—26]. As support, either inorganic materials such as porous glass or different organic polymers are used [27]. These heterogeneous catalyst particles are subject to internal and external... 

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. 

In addition to the insoluble polymers described above, soluble polymers, such as non-cross-linked PS and PEG have proven useful for synthetic applications. However, since synthesis on soluble supports is more difficult to automate, these polymers are not used as extensively as insoluble beads. Soluble polymers offer most of the advantages of both homogeneous-phase chemistry (lack of diffusion phenomena and easy monitoring) and solid-phase techniques (use of excess reagents and ease of isolation and purification of products). Separation of the functionalized matrix is achieved by either precipitation (solvent or heat), membrane filtration, or size-exclusion chromatography [98,99]. 

Soluble support-based synthetic approaches offer the advantages of both homogeneous solution-phase chemistry (high reactivity, ease of analysis) and solid-phase synthesis (large excess of reagents, simple product isolation and purification) [98,99]. As a representative example, PEG, one of the most widely used soluble polymers, has good solubility in most organic solvents (i.e., dichloromethane, acetonitrile, dimethylformamide, and toluene), but it... 

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