Cinchona palladium-modified

P-ketoesters and their analogues. As tartaric acid is tightly adsorbed onto the catalyst surface and is stable under the hydrogenation condihons, the modified catalyst can be used for the recover/reuse process, which is different from the cinchona alkaloid-modified plahnum and palladium systems. 

The present study aimed at revealing the mode of enantiodifferentiation in the asymmetric hydrogenation of ethyl pyruvate and isophorone over platinum and palladium catalysts. The effect of adding the modifier after an initial phase of racemic hydrogenation and the combined use of different vinca and cinchona type modifiers on enantioselectivity and activity were studied. A mechanistic rationale is proposed to account for the experimental observations. 

Szollosi, G., Kun, 1., Bartok, M. (2001) Heterogeneous Asymmetric Reactions. Part 24. Heterogeneous Catalytic Enantioselective Hydrogenation of the C=N Group Over Cinchona Alkaloids Modified Palladium Catalyst,... 

Salen palladium(II) complexes have been reported to be effective homogeneous, or when immobilized, to be active heterogeneous catalysts for the hydrogenation of alkenes [1,2]. It is also known that platinum(O) catalysts modified with cinchona alkaloids catalyze the enantioselective hydrogenation of a-ketoesters to the corresponding a-hydroxyesters[3]. The platinum(O) catalyst is attached to AI2O3 [4] or to zeolites [5,6]. 

Platinum and palladium dominate the enantioselective catalysis scene as far as alkaloid modifiers are concerned. Of the other platinum group metals, Ir follows Pt in cinchona-modified pyruvate ester hydrogenation,132 the fact that Rh can hydrogenolyse alkaloids adsorbed at its surface133 makes it unlikely to function successfully and Ru tends to be too easily oxidised by adventitious oxygen in these reactions. 

Alternatively, the surface of a metal can be modified with an enantiomerically pure additive. For example, in 1956 palladium metal/silk fibroin was used for the hydrogenation of alkenes with moderate enantioselectivity. However, most success with modified metal siufaces has been achieved in the hydrogenation of ketones. The reduction of 3-keto esters with a Raney nickel/tartaric acid/sodium bromide catalyst provides good enantioselectivities. For example, P-keto ester (3.108) affords the P-hydroxy ester (3.109). Platinum metal modified with cinchona alkaloids has been successfully used with a-keto esters. The reaction yields product with up to 90% ee in the reduction of a-keto ester (3.110). ... 

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