Polymer-bound catalysts, used

The use of such an oxazaborolidine system in a continuously operated membrane reactor was demonstrated by Kragl et /. 58] Various oxazaborolidine catalysts were prepared with polystyrene-based soluble supports. The catalysts were tested in a deadend setup (paragraph 4.2.1) for the reduction of ketones. These experiments showed higher ee s than batch experiments in which the ketone was added in one portion. The ee s vary from 84% for the reduction of propiophenone to up to >99% for the reduction of L-tetralone. The catalyst showed only a slight deactivation under the reaction conditions. The TTON could be increased from 10 for the monomeric system to 560 for the polymer-bound catalyst. 

Scheme 4.37 The Pauson-Khand reaction using polymer-bound catalyst (60).

Soluble polymer-bound catalysts for epoxidation reactions have also been explored, with a complete study into the nature of the polymeric backbone performed by Janda [70]. Chiral (salen)-Mn complexes were appended to MeO-PEG, NCPS, Jan-daJeF and Merrifield resin via a glutarate spacer. It was found that for the Jacobsen epoxidation of ds-/ -mefhylstyrene, the enantioselectivities for each polymer-supported catalyst were comparable (86-90%) to commercially available Jacobsen catalyst (88%). Both soluble polymer-supported catalysts could be used twice before a decline in yield and enantioselectivity was observed. However, neither soluble polymer support proved as suitable as the insoluble JandaJel-supported (salen)-Mn complex for the epoxidation because residual impurities during precipitation and leaching of Mn from the complex, resulted in lowered yields. 

The affinity of the polymer-bound catalyst for water and for organic solvent also depends upon the structure of the polymer backbone. Polystyrene is nonpolar and attracts good organic solvents, but without ionic, polyether, or other polar sites, it is completely inactive for catalysis of nucleophilic reactions. The polar sites are necessary to attract reactive anions. If the polymer is hydrophilic, as a dextran, its surface must be made less polar by functionalization with lipophilic groups to permit catalytic activity for most nucleophilic displacement reactions. The % RS and the chemical nature of the polymer backbone affect the hydrophilic/lipophilic balance. The polymer must be able to attract both the reactive anion and the organic substrate into its matrix to catalyze reactions between the two mutually insoluble species. Most polymer-supported phase transfer catalysts are used under conditions where both intrinsic reactivity and intraparticle diffusion affect the observed rates of reaction. The structural variables in the catalyst which control the hydrophilic/lipophilic balance affect both activity and diffusion, and it is often not possible to distinguish clearly between these rate limiting phenomena by variation of active site structure, polymer backbone structure, or % RS. 

The modified polymer 70a with the larger concave pyridine was then tested in the base catalyzed addition of ethanol to diphenylketene (59a) and proved to be catalytically active [17]. To compare the polymer bound concave pyridine 70a to the corresponding free concave pyridine 3k (MeO-substituted in 4-position of the pyridine ring), the same quantities of pyridine units were used, in solution (3k) or in suspension (70a) respectively. The polymer bound catalyst 70a catalyzed 2-3 times slower than the analogous 4-methoxy-substituted... 

Hydroformylation of 1-hexene was studied at 80 °C under 30 bar of CO/H2 (1 1) with the in-situ generated catalyst 7. The reaction was monitored via the CO/H2-uptake which was measured with mass flow meters. The polymer-bound catalysts possessed moderate activities in methanol (200-400 TO/h), and the activity was approx. 4-times higher when non polymer-bound TPPMS was used as the ligand. The activity of the catalyst 7 decreased in recycling experiments (entries 2a-2b in Table 2), which can presumably be attributed to a partial oxidation of the phosphine ligands. Moreover, the activity of the complex was not significantly affected by the change of P Rh... 

In reactions with polymer-bound catalysts, a mass-transfer limitation often results in slowing down the rate of the reaction. To avoid this disadvantage, homogenous organic-soluble polymers have been utilized as catalyst supports. Oxazaborolidine 5, supported on linear polystyrene, was used as a soluble immobilized catalyst for the hydroboration of aromatic ketones in THF to afford chiral alcohols with an ee of up to 99% [40]. The catalyst was separated from the products with a nanofiltration membrane and then was used repeatedly. The total turnover number of the catalyst reached as high as 560. An intramolecularly cross-linked polymer molecule (microgel) was also applicable as a soluble support [41]. 

Heterogeneous catalysts have also been reported to effect the arylation of secondary amines using aryl bromides. Buchmeister reported the preparation of a polymer-bound catalyst, which effects the arylation reaction at elevated temperatures. No attempts to recycle the catalyst were reported, however [62]. Djakovitch and co-workers reported the use of Pd particles immobilized on metal oxides or Pd-loaded zeolites as a catalyst [63]. The yields and selectivities for the reaction were diminished compared to homogeneous systems previously described. 

BQC is derived from quinine, which is a member of the cinchona family of alkaloids. Ammonium salts derived from quinidine, a diastereomer of (1) at the hydroxyl substituent, have been used less frequently in catalysis than BQC. Quini-dinium salts often give rise to products with enantioselectivity opposite to that from (1). Other related compounds, such as those derived from cinchonine and cinchonidine (which lack the methoxy substituent on the quinoline nucleus), have found application in organic synthesis. The cinchona alkaloids, as well as salt derivatives in which the benzyl group bears various substituents, have also been studied. Results from polymer-bound catalysts have not been promising. ... 

Continuous homogeneous catalysis is achieved by membrane filtration, which separates the polymeric catalyst from low molecular weight solvent and products. Hydrogenation of 1-pentene with the soluble pofymer-attached Wilkinson catalyst affords n-pentane in quantitative yield A variety of other catalysts have been attached to functionalized polystyrenes Besides linear polystyrenes, poly(ethylene glycol)s, polyvinylpyrrolidinones and poly(vinyl chloride)s have been used for the liquid-phase catalysis. Instead of membrane filtration for separating the polymer-bound catalyst, selective precipitation has been found to be very effective. In all... 

S. Erase, F. Lauterwasser, R. E. Ziegert, Recent advances in asymmetric C-C and C-heteroatom bond forming reactions using polymer-bound catalysts, Adv. Synth. Gated. 345 (2003) 869. 

Polymer bound catalyst systems have become highly sophisticated. For supports, commercial resins have in cases given way to custom tailored polymers designed to optimize a supported catalyst s performance. This progression from simple to complex systems is illustrated by advances in supported catalysts used for the asymmetric hydrogenation of enamides. 

Catalysis with water-soluble polymer-bound catalysts in a single homogeneous aqueous phase, the subject to this section, can be of interest for the conversion of water-soluble organic substrates. With a view to applications, the use of water as a nonhazardous, environmentally benign solvent can be advantageous. 

An important consideration in catalyst, reagent or substrate recovery is measuring and verifying how effective such recovery actually is. While we have modeled such recovery using dye-modified polymers, analyses of catalysts typically requires additional analytical work. For example, ICP analysis for residual metal can be used as a quantitative and sensitive assay. Such assays are however more problematic with non-metallic catalysts. In this paper, we show that bifunctional polymers where both a catalyst and a colorimetric label are included in the same polyacrylamide polymer provide a simple way to monitor separability and catalyst recovery for non-metallic polymer-bound catalysts. 

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