Phase transfer catalysts, chiral polymeric

Also useful for synthetic apphcation are heterogeneous metallic catalysts, modified with chiral auxiliaries and finally chiral soluble organic bases or acids. Less easy to apply are chiral polymeric and gel-type materials, phase-transfer catalysts or immobilized complexes. 

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. 

The enantioselective ester syntheses from acid salts, chlorides and anhydrides with racemic alkyl halides, catalysed by optically active polyaminesalmost certainly proceed via in situ formation of chiral ammonium salts, and therefore fall within the scope of phase transfer catalysts. Though the optical yields obtained are low, the work is important because it explores the use of polyamine species with a potential chirality derived from the polymerization of optically active oxazolines, and as such is again a novel approach. 

Very recently, the Siva group successfully reported a chiral multisite phase-transfer catalytic Michael addition by the use of 2,4,6-(triscincho-niummethyl)phenyl-l,3,5-triazines as new polymeric chiral quaternary ammonium PTC catalysts. Catalyst 12c showed a higher catalytic efficiency than the corresponding monomeric catalyst 8q in terms of chemical yield and enantioselectivity due to the trimeric reaction sites of 12c, which can promote efficient ion-pair interactions between the a-carbon of the diethyl-malonate and trimeric catalysts due to steric factors (Scheme 16.22). ... 

The activity of the polymeric salts as catalysts in phase transfer reactions was tested in alkylation and dichlorocarbenation reactions carried out on chiral or prochiral substrates under polyphase conditions in the presence of concentrated aqueous sodium hydroxide. They were tested also in oxidation reaction of racemic alcohols under acidic conditions. ... 

There have been several examples of multi-phase systems in asymmetric syntheses, notably a polymeric chiral rhodium hydrogenation catalyst for selectively producing i -amino-acids, and the use of phase-transfer catalysis in asymmetric carbon-carbon bond formation. The incorporation of optically active cinchona alkaloids, e.g. quinine (29), into a polymer by virtue of their vinyl group has been described, with preliminary results on application to asymmetric syntheses. At the other extreme of structural complexity, enzymes themselves have been incorporated into polymers to some advantage. ... 

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