Raney-Ni/tartaric acid

In the most effective, chirally modified catalytic systems, Pt/cinchonidine and Raney-Ni/tartaric acid, the enantioselectivity was also sensitive to the method of catalyst preparation and on support properties (5, 6). 

The best studied systems are the Raney Ni/tartaric acid/NaBr combination, for the hydrogenation of / -functionalized ketones, and the Pt- and Pd-on-support/cinchona alkaloid systems for the enantioselective hydrogenation of a-functionalized ketones. 

A cathode containing a Raney-Ni-Tartaric acid catalyst coated with CdS particles proved to be rather effective. Irradiation of this electrode by a xenon-lamp during reduction of methyl acetoacetate in EtOH solution leads to methyl 3-hydroxybutyrate with an ee of 67%... 

The Ni/tartaric acid/NaBr catalyst system has been extensively studied. A variety of ketone substrates have been reduced with Ni/tartaric acid/NaBr catalysts with variable enantioselectivities, but the highest (>85% ee) are obtained for the reductions of P-keto esters and P-diketones (Schemes 12.60 and 12.61).5 Asymmetric reduction of diketones results in the formation of mesa and chiral diols. The highest meso chiral diol ratio of 2 98 and enantioselectivities of 98% ee are obtained with modified Raney nickel catalysts treated by sonication.5... 

TA-NaBr-MRNi was prepared by the reported method [3]. RNi (W-1 type) was prepared from 1.9 g of Raney nickel alloy (Kawaken Fine Chemical Co., Ni/Al = 42/58). To wash out the excess base and aluminum salts, a sufficient amount of deionized water was used with ultrasonic irradiation. The modifying solution was prepared by dissolving of (R,R)-tartaric acid (1 g) and NaBr (6 g to 10 g) in 100 ml of water and adjusting the pH to 3.2 with IN NaOH aqueous solution. RNi was heated in the modifying solution at 100 C for 1 hour, washed with water (50 ml), methanol (50 ml, twice), and THF (10 ml). The TA-NaBr-MRNi obtained by this method was immediately used for the hydrogenation. 

Enantioselective hydrogenation of yS-keto sulfones in the presence of (S,S)-tartaric acid modified Raney Ni gives the S alcohols in up to 71% ee (Fig. 32.23)... 

In fact, there are only two heterogeneous catalysts that reliably give high enantioselectivities (e.s. s) (90% e.e. or above). These are Raney nickel (or Ni/Si02) system modified with tartaric acid (TA) or alanine for hydrogenation of /(-kctocstcrs [12-30], and platinum-on-charcoal or platinum-on-alumina modified with cinchona alkaloids for the hydrogenation of a-ketoesters [31-73],... 

X = COjNa Il2, cat Raney Ni-NaBr-tartaric acid JOC 52 1139 (1987) (chiral)... 

A Raney Ni catalyst modified by tartaric acid and NaBr is fairly effective for enantioselective hydrogenation of a series of (3-keto esters (Scheme 1.41) [203a,214,215]. The enantio-discrimi-nation ability of the catalyst is highly dependent on the preparation conditions such as pH (3—4), temperature (100°C), and concentration of the modifier (1%). Addition of NaBr as a second modifier is also crucial. Ultrasonic irradiation of the catalyst leads to even better activity and enantioselectivity up to 98% ee [214d-f. The Ni catalyst is considered to consist of a stable, selective and weak, nonselective surface area, while the latter is selectively removed by ultrasonication. 

We then focused on the synthesis of lipid A analogs which contain 3-hydroxy fatty acids. For this purpose, sufficient amount of (R)-3-hydroxytetradecanoic acid ( ), which is the commonest hydroxy acid in Salmonella lipid A, was first prepared by means of an asymmetric reduction of the corresponding keto ester, i. ., methyl 3-oxotetradecanoate (j ) (7). Catalytic hydrogenation of 21 in the presence of Raney Ni modified with (R, R)-tartaric acid foaBr (8) afforded the crude (R)-ester in 85% enantiomeric excess. After saponification, the resultant acid was purified through its dicyclohexylammonium salt to give the optically and chemically pure (R)-acid In a yield of 61% from... 

The noteworthy advantages of this method are that the pure acid can be prepared, on a hundred gram scale, and that even the unnatural enantiomer, (S)-acid, can be obtained simply by use of (S,S)-tartaric acid for the modification of the Raney-Ni catalyst (7). 

Later, the optical yields over the modified Raney Ni were greatly improved by hydrogenating the substrate in the presence of sodium bromide.210 Thus, the best optical yield of 88.6% ee was obtained in the hydrogenation of methyl acetoacetate over Raney Ni modified 3 times with tartaric acid in the presence of sodium bromide with addition of a small amount of acetic acid (eq. 5.48). 

Hydroxy-2-butanone and its methyl ether were also hydrogenated in 69 and 68% ee, respectively, using the same modified nickel catalyst.211 Acetylacetone was first hydrogenated to (R)-4-hydroxy-2-pentanone in 87% ee over the Raney Ni modified by (R,R)-tartaric acid. Then the (R)-hydroxy ketone was hydrogenated to a mixture of... 

Simple 2-alkanones, which usually afford optically active alcohols only in low optical yields with modified Raney Ni, have been converted to active alcohols in 72-85% ee by hydrogenation over the Raney Ni modified with optically active tartaric acid and sodium bromide in the presence of an excess amount of pivalic acid (eq. 

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