Polymer-supported chiral organocatalyst

Since the discovery of proline-catalyzed enantioselective aldol reactions, an extensive research program to explore chiral secondary amine catalysts has been pursued. Several polymer-supported chiral amines have been synthesized for aldol, Mannich, and related reactions. Polystyrene is a popular solid phase for use in place of silica gel in the proline-based organocatalysis. In contrast, silica gel displays a slightly acidic character and has a hydrogen-bond donor or acceptor, which may change the catalytic activity and chiral space of the organocatalyst. Flow enantioselective aldol [158-161], Mannich [162], Michael [163], and related reactions... 

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]. 

In summary, development of functional polymers that can act as efficient organocatalysts is at the origin of the application of modified polymers as reagents and catalysts in chemistry. To date, the most successful accomplishments have been carried out in the area of basic and acid catalysts. Not surprisingly, this area includes some of the most important industrial applications for supported reagents and catalysts. In general, the development of non-chiral polymer-supported organocatalysts is a field of research that involves both industrial and academic research. In many cases, it has been possible to demonstrate how the use of the... 

In addition, the asymmetric reduction of A7-aryl ketimines with tri-chlorosilane could be achieved on polymer-supported organocatalysts by Kocovsky et alP Indeed, 7V-methylvaline-derived formamide anchored to a polymeric support, used at a catalyst loading of 15 mol %, allowed good enantioselectivities of up to 82% ee combined with good yields to be obtained for the formed chiral amines (Scheme 8.3). This novel methodology simplified the recovery of the catalyst, which could be reused at least five times without any loss of the activity. The best results were obtained with the catalysts directly attached to the polymer or via a suitable spacer. A strong influence of the solvents on the catalytic performance was observed with chloroform giving the... 

In general, two strategies can be applied for the construction of chiral dendrimer catalysts (i) chiral metal complex (or organocatalyst) may be incorporated into the core of the dendrimer (Figure 4.1a) or (ii) multiple chiral metal complexes (or organocatalysis) may be located at the periphery of the dendrimer (Figure 4.1b). Recently, hybrids of dendrimer and crosslinked polymer as supports have been developed (Figure 4.1c) [12]. 

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