Insoluble polymer-supported catalyst

Insoluble polymer-supported catalysts ion exchange resins, poly-DMAP, poly-vinyl pyridine, and others... 

Despite the advantage of their easy separation, the use of conventional insoluble polymer-supported catalysts often suffered from a reduced catalytic activity and stereoselectivity, due either to diffusion problems or to a change of the preferred conformations within the chiral pocket created by the ligand around the metal center. In order to circumvent these problems, a new class of crosslinked macromolecule-namely dendronized polymers-has been developed and employed as catalyst supports. In general, two types of such solid-supported dendrimer have been reported (i) with the dendrimer as a hnker of the polymer support and (ii) with dendrons attached to the polymer support [12, 113]. 

The insoluble polymer-supported Rh complexes were the first immobilized chiral catalysts.174,175 In most cases, however, the immobilization of chiral complexes caused severe reduction of the catalytic activity. Only a few investigations of possible causes have been made. The pore size of the insoluble support and the solvent may play important roles. Polymer-bound chiral Mn(III)Salen complexes were also used for asymmetric epoxidation of unfunctionalized olefins.176,177... 

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. 

Kragl 13) pioneered the use of membranes to recycle dendritic catalysts. Initially, he used soluble polymeric catalysts in a CFMR for the enantioselective addition of Et2Zn to benzaldehyde. The ligand a,a-diphenyl-(L)-prolinol was coupled to a copolymer prepared from 2-hydroxyethyl methyl acrylate and octadecyl methyl acrylate (molecular weight 96,000 Da). The polymer was retained with a retention factor > 0.998 when a polyaramide ultrafiltration membrane (Hoechst Nadir UF PA20) was used. The enantioselectivity obtained with the polymer-supported catalyst was lower than that obtained with the monomeric ligand (80% ee vs 97% ee), but the activity of the catalyst was similar to that of the monomeric catalyst. This result is in contrast to observations with catalysts in which the ligand was coupled to an insoluble support, which led to a 20% reduction of the catalytic activity. 

The multifaceted applications of phase-transfer catalysts (PTC) in organic synthesis contributed decisively to the establishment of organic catalysts as useful preparative tools. Polymer-supported PTC was examined extensively but it was noted that the catalytic activity of the insoluble polystyrene-supported catalysts was strongly reduced in com-... 

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. 

The use of thiazolium salts enables the benzoin condensation to proceed at room temperature. It can also be performed in dipolar aptotic solvents or under phase transfer conditions. Thiazolium salts such as vitamin Bi, thiazolium salts attached to y-cyclodextrin, macrobicyclic thiazolium salts, thiazolium carboxylate, ° naphtho[2,l-d]thiazolium and benzothiazolium salts catalyze the benzoin condensation and quaternary salts of 1-methylbenzimidazole and 4-(4-chlorophenyl)-4//-1,2,4-triazole are reported to have similar catalytic activity. Alkylation of 2-hydroxyethyl-4-methyl-l,3-thiazole with benzyl chloride, methyl iodide, ethyl bromide and 2-ethoxyethyl bromide yields useful salts for catalyzing 1,4-addition of aldehydes to activated double bonds. Insoluble polymer-supported thiazolium salts are catalysts for the benzoin condensation and for Michael addition of aldehydes. Electron rich al-kenes such as bis(l,3-dialkylimidazolidin-2-ylidenes) bearing primary alkyl substituents at the nitrogen atoms or bis(thiazolin-2-ylidene) bearing benzyl groups at the nitrogen atoms are examples of a new class of catalyst for the conversion of ArCHO into ArCHOHCOAr. 

An insightful discussion on an alternahve approach is provided in Chapter 6 of this Handbook, where insoluble polymer-supported chiral catalysts (chiral palladium phosphine complexes supported on TentaGel-type amphiphilic polymer bearing PEG chains) are used for heterogeneous asymmetric processes in water. 

Based on this concept, Seebach et al. developed the first example of TADDOL-cored dendrimers (Figure 4.41) immobilized in a PS matrix [116]. The resultant internally dendrimer-functionalized polymer beads were loaded with Ti(OiPr)4, leading to a new class of supported Ti-TADDOLate catalysts for the enantioselective addition of diethylzinc to benzaldehyde. Compared to the conventional insoluble polymer-supported Ti-TADDOLate catalysts, these heterogeneous dendrimer catalysts gave much higher catalytic activities, with turnover rates close to those of the soluble analogues. The polymer-supported dendrimer TADDOLs were recovered by simple phase separation and reused for at least 20 runs, with similar catalytic efficiency. 

Very recently, Portnoy et al. described the design and synthesis of insoluble polymer-supported dendrimers bearing proline end groups for asymmetric aldol reachons [123]. The zeroth- to third-generahon supported dendrimer catalysts were prepared by ahaching (2S,4R)-4-hydroxyproline onto the insoluble PS-bound... 

Scheme 8.4 Insoluble polymer-supported phase-transfer catalysts.

Several enantioselective reductions that use polymer-supported chiral catalysts have been reported. A maj or advantage of performing enantioselective reactions with polymer-supported catalysts is that their use allows both the recycHng of the catalysts and the easy separation of the low molecular weight chiral products. One of the most attractive methods to carry out asymmetric synthesis is the continuous flow system by using an insoluble, polymeric catalyst. 

Another option is the use of both solid-supported versions of the catalyst and the co-oxidant. The Toy group in Hong-Kong has developed a multipolymer system for the TEMPO-catalyzed alcohol oxidation in which both the pre-catalyst and the co-oxidant are attached to a polymer. As it was clearly impossible to use two insoluble supported reagents, the idea was to use an insoluble polymer in conjunction with a soluble one. An insoluble polymer-supported diacetoxyiodosobenzene (PSDIB), an analog of 10, and a soluble polymer-supported TEMPO were used,... 

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