Enantioselective heterogeneous catalysis immobilization

H. U. Blaser, B. Pugin, M. Studer, Enantioselective heterogeneous catalysis academic and industrial challenges, in D. E. de Vos, I. F. J. Vankelecom, P. A. Jacobs (Eds.), Chiral Catalyst Immobilization and Recycling, Wiley-VCFI, Weinheim, 2000, p. 1. 

A. HECKELand D. Seebach, Preparation and characterization of TAD DO Ls immobilized on hydrophobic controlled-pore-glass silica gel and their use in enantioselective heterogeneous catalysis, Chem - Eur.J. 2002, 8, 559-572. 

Blaser H-U, Pugin B, Studer M (2000) Enantioselective heterogeneous catalysis academic and industrial challenges. In De Vos DE.Vankelecom IFJ, Jacobs PA (eds) Chiral catalyst immobilization and recycling. Wiley-VCH,Weinheim,p 1... 

The immobilized catalyst provided good enantioselectivity and activity in the heterogeneous catalysis of the oxidative kinetic resolution of secondary alcohols and can be recovered and recycled for four times without obvious loss of enantioselectivity and activity. Oxidative kinetic resolutions of meso-diols, hydroxyl esters, and primary alcohols were also studied using this catalytic system. 

Heterogenization of homogeneous metal complex catalysts represents one way to improve the total turnover number for expensive or toxic catalysts. Two case studies in catalyst immobilization are presented here. Immobilization of Pd(II) SCS and PCP pincer complexes for use in Heck coupling reactions does not lead to stable, recyclable catalysts, as all catalysis is shown to be associated with leached palladium species. In contrast, when immobilizing Co(II) salen complexes for kinetic resolutions of epoxides, immobilization can lead to enhanced catalytic properties, including improved reaction rates while still obtaining excellent enantioselectivity and catalyst recyclability. 

The heterogeneous nature of the catalysis was confirmed by the inactivity of the supernatant THF solution of the catalyst composed of L41, for the epoxidation of chalcone under the same experimental conditions, and the less than 0.4 ppm lanthanum leaching in consecuhve runs as determined by inductively coupled plasma (ICP) spectroscopy. The catalyst with L4b could be recovered by simple filtration under argon, and reused for at least six cycles in the epoxidation of chalcone without any significant loss of enantioselectivity. Furthermore, the substrate general adaptability of the immobilized catalyst was demonstrated for the enantioselective catalytic epoxidation of a variety of a,P-unsaturated ketones (7), affording the corresponding a,P-epoxy ketones (8) in excellent yields (91-99%) and enantioselechvities (84-97% ee) (Scheme 9.7). 

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