Polymer-supported cinchona catalyst

In 2005, Lectka and coworkers also reported a-cMorination of acid halide by using polymer-supported cinchona catalyst via a column-based flush and flow system (Scheme 6.37) [66]. To a column of quinine-bound Wang resin 126 were added 120 and 123, then the eluent (THF) was flowed by flushing to afford the corresponding a-chloroesters 125 up to 94% ee. 

Catalytic asymmetric alkylations of 28 have also been carried out with polymer-bound glycine substrates [43], or in the presence of polymer-supported cinchona alkaloid-derived ammonium salts as immobilized chiral phase-transfer catalysts [44], both of which feature their practical advantages especially for large-scale synthesis. 

Lohray and coworkers reported the first application of silica gel-supported Cinchona alkaloids in AD in 1996 [67], A 3,6-DHQ2-pyridazine derivative was linked to a silica gel support with an attachment point at one of the quinudidine moieties (Fig. 4, catalyst 13). The alkaloidic ligand was expected to bind to the silica surface resulting in better availability of the active site compared to polymer-... 

A few years ago Cahard reported a series of studies on the use of immobilized cinchona alkaloid derivatives in asymmetric reactions with phase-transfer catalysts [17[. Two types of polymer-supported ammonium salts of cinchona alkaloids (types A and B in Scheme 8.4) were prepared from PS, and their activity was evaluated. The enantioselectivity was found to depend heavily on the alkaloid immobilized, with the type B catalysts usually giving better results than the type A catalysts. By performing the reaction in toluene at -50 °C in the presence of an excess of solid cesium hydroxide and 0.1 mol equiv of catalyst 10, benzylation of the tert-butyl glycinate-derived benzophenone imine afforded the expected (S)-product in 67% yield with 94% ee, a value very close to that observed with the nonsupported catalyst. (Scheme 8.4, Equation b) Unfortunately-and again, inexplicably-the pseudoenantiomer of 10 proved to be much less stereoselective, affording the R)-product in only 23% ee. No mention of catalyst recycling was reported [18]. 

A member of the new ligand class for the asymmetric dihydroxylation is the bis(dihydroquinidine) ether of l,4-dihydroxy-9,10-anthraquinone. Cinchona alkaloid ligands bound to soluble polymer supports" are effective catalysts for asymmetric dihydroxylation. 

Despite the many studies reported, only two combinations of catalytic and modifying functions have emerged as commercial possibilities, tartrate-modified catalysts and cinchona-modified catalysts. The substrates that have been most studied are the a, -ketoesters, and the polymer support most frequently used is preformed styrene-DVB(2%) resin. Asymmetric ligand monomers that contain asymmetric ligand sites have also been synthesized. 

In addition to the recyclable solid-supported Cinchona-derived PTC catalysts, described in Section 16.2.1.3, polymer-bound glycine substrates... 

In addition, in 2(X)4 Mamoka and co-workers [72] synthesized a recyclable fluorous chiral phase-transfer catalyst which was successfully applied for the catalytic asymmetric alkylation of a glycine-imine derivative followed by extractive recovery of the chiral phase-transfer catalyst using fluorous solvent. Later, in 2010 Itsuno and co-workers [73] published a new type of polymer-supported quarternary ammonium catalysts based on either cinchona alkaloids or Maruoka s-type catalyst bound via ionic bonds to the polymeric sulfonates. 

Other functionalized supports that are able to serve in the asymmetric dihydroxylation of alkenes were reported by the groups of Sharpless (catalyst 25) [88], Sal-vadori (catalyst 26) [89-91] and Cmdden (catalyst 27) (Scheme 4.13) [92]. Commonly, the oxidations were carried out using K3Fe(CN)g as secondary oxidant in acetone/water or tert-butyl alcohol/water as solvents. For reasons of comparison, the dihydroxylation of trons-stilbene is depicted in Scheme 4.13. The polymeric catalysts could be reused but had to be regenerated after each experiment by treatment with small amounts of osmium tetroxide. A systematic study on the role of the polymeric support and the influence of the alkoxy or aryloxy group in the C-9 position of the immobilized cinchona alkaloids was conducted by Salvadori and coworkers [89-91]. Co-polymerization of a dihydroquinidine phthalazine derivative with hydroxyethylmethacrylate and ethylene glycol dimethacrylate afforded a functionalized polymer (26) with better swelling properties in polar solvents and hence improved performance in the dihydroxylation process [90]. 

Although Cinchona alkaloids are easily separated from products by acid-base extractions and recycled, immobilisation of the catalyst on polymers was investigated by Oda. In parallel with new catalyst synthesis, their immobilisation to various solid supports was also studied. Immobilised catalysts are easily isolated by filtration and reused several times, although their initial enantioselectivity is slightly lower compared with homogeneous catalysis. 

In these cases, the polymer was used as an asymmetric support to induce the formation of optically pure product (cf. Worster et al., 1979). Few reports of the use of polymer-bound asymmetric reagents seem to exist in the literature. In this application, the reagent is used either to promote the asymmetric coupling of two groups or to add a group to a compound in an asymmetric manner. By far the largest number of applications have been those in which the polymer-bound asymmetric centers act as catalysts. Asymmetric catalysts, based on either amino acids or cinchona alkaloids, have been used to catalyze the Michael reaction in an... 

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