Organocatalysts polymers

Polymer-supported organocatalysts have been used for cycloaddition of ketene, 127, to chloral, 128 [141]. Use of homo-acrylate polymers of cinchona alkaloids led to formation of the desired /Mactone (S)-130 with enantioselectivity up to... 

Researchers at Degussa AG focused on an alternative means towards commercial application of the Julia-Colonna epoxidation [41]. Successful development was based on design of a continuous process in a chemzyme membrane reactor (CMR reactor). In this the epoxide and unconverted chalcone and oxidation reagent pass through the membrane whereas the polymer-enlarged organocatalyst is retained in the reactor by means of a nanofiltration membrane. The equipment used for this type of continuous epoxidation reaction is shown in Scheme 14.5 [41]. The chemzyme membrane reactor is based on the same continuous process concept as the efficient enzyme membrane reactor, which is already used for enzymatic a-amino acid resolution on an industrial scale at a production level of hundreds of tons per year [42]. 

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 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]. 

The potential for the use of cinchona alkaloids as organocatalysts has long been recognized [22]. As the alkaloid skeleton offers different sites for polymer attach-... 

Since the bulk properties of PLA are highly dependent on the stereoregularity or the tacticity of the polymer, i the development of catalysts for rac-lactide (or meso-lactide) polymerization has been focused on achieving stereoselectivity. 2 2 " Numerous organocatalysts such as A-heterocyclic carbenes (NHCs) and phosphine-based compounds " have been investigated for the controlled ROP of lactide. 

The application of secondary amines as organocatalysts in different processes makes use of the inherent nucleophilicity of these compounds, but in this case they function as catalysts rather than reagents, and are thus covered in Chapter 4 of this book. Polymer-bound tertiary amines have so far only found use as depro-tecting agents for phosphorus-borane complexes (see above) [9]. They are widely used as scavengers and linkers, however. 

Polymer-Supported Organocatalysts Based on Amino Acids 285... 

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