Metal heterogeneous asymmetric hydrogenation

The presence of the quinuclidine base functionality makes them effective ligands for a variety of metal-catalyzed processes (Chapters 2-4). The most representative example is the osmium-catalyzed asymmetric dihydroxylation of olefins [9]. The metal binding properties of the quinuclidine nitrogen also allow to use cinchona alkaloids as metal surface modifiers, for example, in the highly enantioselective heterogeneous asymmetric hydrogenation of a-keto esters (Chapter 2). Both... 

Chirally Modified Metal Surface for Heterogeneous Asymmetric Hydrogenation 17... 

Nickel and other transition metal catalysts, when modified with a chiral compound such as (R,R)-tartaric acid 5S), become enantioselective. All attempts to modify solid surfaces with optically active substances have so far resulted in catalysts of only low stereoselectivity. This is due to the fact that too many active centers of different structures are present on the surface of the catalysts. Consequently, in asymmetric hydrogenations the technique of homogeneous catalysis is superior to heterogeneous catalysis56). However, some carbonyl compounds have been hydrogenated in the presence of tartaric-acid-supported nickel catalysts in up to 92% optical purity55 . 

Metal complexes can be attached to membranes. For example, Ru BINAP complexes (cf. Scheme 22-VII) and a Brpnsted acid attached to a polydimethyl-siloxane membrane act as effective heterogenized catalysts for asymmetric hydrogenations.172... 

Pini, D., Petri, A., Mastantuono, A., Salvador , P. Heterogeneous enantioselective hydrogenation and dihydroxylation of carbon carbon double bond mediated by transition metal asymmetric catalysts. Chiral Reactions In Heterogeneous Catalysis, [Proceedings of the European Symposium on Chiral Reactions In Heterogeneous Catalysis], 1st, Brussels, Oct. 25-26, 1993 1995, 155-176. 

Scheme 9.10 Supramolecular metal-organic polymer 17 for the heterogeneous catalysis of asymmetric hydrogenations.

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