Soluble Polymer Supported Catalysts

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. 

Membrane filtration using a polyaramide membrane [56] showed a retention of more than 99.8%. Application of this catalyst in a continuously operated membrane reactor showed conversion for more than 150 h. The ee dropped from 80% in the beginning (non-bonded analogue 97%) to about 20% after 150 h. The average ee for the first 80 h was 50%. 

The reduction of acetophenone was carried out at r.t. giving 86% yield with an ee of 97%. This is similar to the ee obtained with unbound analogues. A limited study was conducted on the retention of the catalyst by nanofiltration. It was found that the compound could be retained in the membrane reactor but no specific details were given about these measurements. 

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. 

Kragl and Wandrey made a comparison for the asymmetric reduction of acetophenone between oxazaborolidine and alcohol dehydrogenase.[59] The oxazaborolidine catalyst was bound to a soluble polystyrene [58] and used borane as the hydrogen donor. The carbonyl reductase was combined with formate dehydrogenase to recycle the cofactor NADH which acts as the hydrogen donor. Both systems were run for a number of residence times in a continuously operated membrane reactor and were directly comparable. With the chemical system, a space-time yield of 1400 g L 1 d 1 and an ee of 94% were reached whereas for the enzymatic system the space-time yield was 88 g L 1 d 1 with an ee of 99%. The catalyst half-life times were 

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Soluble polymers that have been used in hquid-phase methodologies are listed in Fig. 5.1 [3, 7, 8, 34, 35]. Polyethylene glycol and non-cross-linked polystyrene are some of the most often used polymeric carriers for organic synthesis and have found frequent use in the preparation of soluble polymer-supported catalysts and reagents consequently, a brief discussion of these polymers is warranted. 

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. 

For the use of soluble polymers in synthesis, see Liquid-Phase Chemistry Recent Advances in Soluble Polymer-Supported Catalysts, Reagents, and Synthesis, P. Wentworth, Jr., K. D. Janda, Chem. Commun. 1999, 1917-1924 and references cited therein. 

The soluble polymer-supported catalysts have also been used for asymmetrically catalyzed reactions Following a procedure for the preparation of insoluble polymeric chiral catalysts a soluble linear polystyrene-supported chiral rhodium catalyst has been prepared. This catalyst displays high enantiomeric selectivity compared to the low molecular weight catalyst. Thus, hydroformylation of styrene using this catalyst produces aldehydes in high yields. The branched chiral hy drotropaldehy de is formed in 95% selectivity. 

Wentworth, P, Janda, K D, Liquid phase chemistry recent advances in soluble polymer-supported catalysts, reagents and synthesis, Chem. Commun., 1917-1924, 1999. 

Soluble polymer-supported catalysts ligands attached to polyethylene glycol (PEG) supports... 

Some chiral salen (N,N-ethyl( n(i)is(salicylirnine) ligands were attached as the end group of PEG. The soluble polymer-supported catalyst 18, with a glutarate spacer between the ligand and PEG, performed well in toluene and provided 82% ee in the asymmetric ethylation of benzaldehyde (Scheme 3.4) [14]. 

Chiral secondary amines such as nonracemic imidazolidin-4-ones have been found to be effective asymmetric organocatalysts in the Diels-Alder cyclization of cyclopentadiene and a,p-unsaturated aldehydes [60]. A tyrosine-derived imidazoli-din-4-one was immobilized on PEG to provide a soluble, polymer-supported catalyst 110. In the presence of 110, Diels-Alder cycloaddition of acrolein 112 to 1,3-cyclohexadiene 111 proceeded smoothly to afford the corresponding cycloadduct 113 with high endo selectivity and enantioselectivity up to 92% ee (Scheme 3.31) [61]. 

The soluble polymer-supported catalysts 11 and 12 (Scheme 8.5) were prepared by attaching two different MeO-PEGsooo/spacer fragments to the N-anthracenyl-methyl salts of nor-quinine and cinchonidine, respectively [19], The behavior of the obtained catalysts, however, fell short of expectations. Whilst with 11 enantioselectivities lower than 12% ee were always obtained, 12 showed good catalytic activity in promoting the benzylation reaction (solid CsOH, DCM, -78 to 23 "C,... 

T. Nishikubo, et al., Soluble polymer-supported catalysts containing pendant quaternary onium salt residues for regioselective addition reaction of epoxy compound with active ester. Macromolecules 1994, 27(25), 7240-7247. 

Water-soluble polymers prepared from a hydrophilic polyester have been shown to be highly effective water-soluble polymer-supported catalysts for aqueous biphasic hydrogenations [169]. The necessary amphiphilic polyester 134 with a BINAP ligand in the main polymer chain was prepared according to Eq. 74 using terephthaloyl chloride, dihydroxy-PEG4000) a diaminated BI-... 

Figure 6. Thermomorphic system where the catalysis is carried out homogeneously at 70 °C in a monophasic system but where the separation is carried out at room temperature in a biphasic system with the soluble polymer-supported catalyst (e,g, 5 or 6) exclusively dissolved in the aqueous ethanol phase at 20...

With the soluble polymer-supported catalyst 57 (Scheme 27), the reduetion of acetophenone was performed in a continuous operated membrane reactor equipped with a nanofiltration membrane [54], An enhancement of total TON from 10 to 560 (equivalent to 0.18 mol % catalyst for an average ee of 91% and an almost quantitative conversion) was observed. Excellent space-time yields of up to 1.4 kg per day were reached... 

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