Tartaric Acid-Modified Nickel Catalysis

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. 

Results identified for the nickel catalysis before 1999 are summarized as follows 

The hydrogenation of MAA is usually carried out in the presence of acetic acid (ca 0.1%, v/v). A careful kinetic analysis indicated that acetic acid specifically accelerates the enantioselective hydrogenation interacting with tartaric acid, but decelerates the nonenantioselective hydrogenation [71]. Kukula and Cerveny also reported the kinetic analysis of this hydrogenation system [72]. 

Although the contribution of the nonenantioselective site to the total hydrogenation is difficult to predict, it is firmly believed that there must indeed be some degree of contribution. By comparison with the best substrate that gives the product of 98.6% ee, lower ee-values with other P-ketoesters were analyzed [77]. 

In a simplified model, the total product ee-value is expressed as  

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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 . 

Scheme 10.8 Quantitative analysis of the product ee-values obtained by the hydrogenation of P-ketoesters over the tartaric acid-NaBr-modified Raney nickel. Factor-/ indicates the intrinsic enantioselective ability of the tartaric acid-modified sites, and E and N indicate the contribution of the enantioselective catalysis sites and nonenantioselective hydrogenation sites, respectively.

The enantioselective hydrogenation of prochiral substances bearing an activated group, such as an ester, an acid or an amide, is often an important step in the industrial synthesis of fine and pharmaceutical products. In addition to the hydrogenation of /5-ketoesters into optically pure products with Raney nickel modified by tartaric acid [117], the asymmetric reduction of a-ketoesters on heterogeneous platinum catalysts modified by cinchona alkaloids (cinchonidine and cinchonine) was reported for the first time by Orito and coworkers [118-121]. Asymmetric catalysis on solid surfaces remains a very important research area for a better mechanistic understanding of the interaction between the substrate, the modifier and the catalyst [122-125], although excellent results in terms of enantiomeric excesses (up to 97%) have been obtained in the reduction of ethyl pyruvate under optimum reaction conditions with these Pt/cinchona systems [126-128],... 

The transformations that use asymmetric heterogeneous catalysis will be highlighted P-keto esters and diketone reductions by Raney nickel catalyst modified with R,R-tartaric acid and NaBr. a-Keto acid reductions with cinchona modified Pt catalysts are discussed in Chapter 18. 

Until recently, access to optically active 3-hydroxyalkanoates by enantioselective transition metal catalysis was based primarily on the heterogeneous hydrogenation of 3-oxo esters by Raney nickel modified with tartaric acid and sodium bromide (see Section 2.3.1.1.). As far as homogeneous catalysis is concerned, the best optical induction (71 % ee) in the hydrogena-... 

Bennett, A., Cristie, S., Keane, M.A., Peacock, R.D., and Webb, G. (1991) Enantioselective hydrogenation of methyl acetoacetate over nickel catalysts modified with tartaric acid, Catalysis Today, 10, 363 -370. 

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