Nickel Catalysts Modified with Tartaric Acid

Nickel Catalysts Modified with Tartaric Acid 

---

Nickel Catalysts Modified with Tartaric Acid... 

Investigation of heterogeneous chiral hydrogenation catalysts started in the late fifties in Japan and has seen a renaissance in the last few years [2,4,6]. In spite of many efforts, only two classes of modified catalyst systems have been found that are of synthetic use at this time nickel catalysts modified with tartaric acid... 

Ketones carrying a sulfone substituent in the -position were subjected to enantioselective hydrogenation of the carbonyl group. With the heterogeneous Raney nickel catalyst, modified with tartaric acid and sodium bromide (see Section 2.3.1.1.), l-methylsulfonyl-2-butanone was reduced to (R)-l-methylsulfonyl-2-butanol, 1-methylsulfonyl-2-heptanone to (R)-l-methylsul-fonyl-2-heptanol, and l-methylsulfonyl-2-deeanone to (/ )-l-methylsulfonyl-2-decanol in 100% yield 67-71% ee51. 

Preparation variables for new nickel catalysts modified with tartaric acid... 

Table 4.3. Effect of different kinds of activated nickel catalysts modified with tartaric acid in the enantioselective hydrogenation of MAA (adapted...

Tai, A., Kikukawa, T., Sugimura, T., Inoue, Y., Osawa, T., and Fujii, S. (1991) Highly efficient enantio-differentiating hydrogenation over an ultrasonicated Raney nickel catalyst modified with tartaric acid, J. Chem. Soc., Chem. Comm. 795 -796. 

Nitta, Y., Sekine, F., Imanaka, T., and Teranishi, S. (1982) Effect of preparation variables on enantioselectivity of supported Nickel catalysts modified with tartaric acid, J. Catal. 74,382 -392. 

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. 

Among the various strategies [34] used for designing enantioselective heterogeneous catalysts, the modification of metal surfaces by chiral auxiliaries (modifiers) is an attractive concept. However, only two efficient and technically relevant enantioselective processes based on this principle have been reported so far the hydrogenation of functionalized p-ketoesters and 2-alkanons with nickel catalysts modified by tartaric acid [35], and the hydrogenation of a-ketoesters on platinum using cinchona alk oids [36] as chiral modifiers (scheme 1). 

Since RNi contains a large amount of aluminum and 2-hydroxy acid is a strong chelating reagent, one difference between RNi and RNiA could be ascribed to their difference in aluminum contents. Table XII (49) shows the correlation between the aluminum content and the EDA of those catalysts modified with tartaric acid. The aluminum content of RNi was decreased by pretreatment with hydroxy acid. Moreover, reduced nickel prepared from NiO (HNi-1) gives an effective modified catalyst and its pretreatment with hydroxyacid does not affect its EDA. 

A particularly successful approach to the catalytic hydrogenation of dialkyl ketones with hydrogen has been the use of the heterogeneous contact catalyst system - Raney nickel chirally modified with tartaric acid [18]. Here too, selectivity is enhanced by branching of the alkyl substituent in the alkyl methyl ketones (e.g., 85 % ee for the hydrogenation of isopropyl methyl ketone). With... 

Orito, Y., Niwa, S., and Imai, S. (1976) As3munetric hydrogenation of methyl acetoacetate using Nickel-Platinum metal-Kieselguhr catalysts modified with tartaric acid, Yuki Gosei Kagaku Kiokaishi J. Synth. Org. Chem. Jpn.) 34, 236 - 239. 

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],... 

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. 

Systematic studies on the enantioselective heterogeneous catalytic hydrogenation of carbonyl compounds were carried out by Izumi using Raney nickel modified with various chiral reagents. Hydroxy acids or amino acids were used for the modification of the nickel catalyst, and (-i-)-tartaric acid (2R,3R)... 

For 30 years, Raney nickel, modified with optically active compounds, has been used as an efficient heterogeneous catalyst for the reduction of 3-oxo esters such as methyl or ethyl 3-oxobutanoate. Many optically active compounds as modifiers and many supports other than Raney nickel have been tested 62 63. However, the best system by far is Raney nickel modified with tartaric acid and sodium bromide 64 6t This heterogeneous catalyst gives enantioselectivities close to 90% cc for a broad range of 3-oxo esters (Table 9). The alkyl substituents in the alcohol moiety, as well as in the 4-position of 3-oxobutanoate, can be varied without changing the enantioselectivity of 86-88% ee66. 

The heterogeneous catalyst Raney nickel, modified with tartaric acid and sodium bromide, was used in the hydrogenation of 4-hydroxy-2-butanone and 4-methoxy-2-butanone. (/ )- ,3-Buianediol and (/ )-4-mcthoxy-2-butanol were obtained in quantitative yield and in 70 and... 

Chernysheva et al. were the first to have obtained evidence of inhibition from the product of the reaction of hydrogenation of ethyl aceto-acetate on Raney nickel catalyst modified with (2/ ,3R)-tartaric acid into ethyl (R)-(-)-3-hydroxybutyrate. Similar results were obtained also in the enantio-... 

Figure 5.9. Effect of the amoimts of previously added methyl ( )-(-)-3-hydroxybutyrate to methyl acetoacetate before hydrogenation on Ni-silioa catalyst modified with (2R,3i )-tartaric acid ( ) (according to Nitta et al. and the hydrogenation of ethyl acetoacetate on Raney nickel catalyst modified with (2R,3R)-tartaric acid (o) (according to Chernysheva et al...

Raney Nickel catalyst modified with (2R,3R)-tartaric acid... 

Catalysts modified by tartaric acid (commonly referred to as tartrate-modified catalysts) constitute the most extensively studied class of modified catalysts (Izuma, 1983 Bartok, 1985 Sachtler, 1985 Tai and Harada, 1986). The catalyst usually consists of Raney nickel or a noble metal-containing bimetallic system, modified by tartaric acid with NaBr as a comodifier in a mildly polar aprotic solvent such as methyl propionate (Izuma, 1983 Tai and Harada, 1986). The reaction is usually carried out in the temperature range 60-100 °C at 80-120 atm. 

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],... 

Surface modification of skeletal nickel with tartaric acid produced catalysts capable of enantiose-lective hydrogenation [85-89], The modification was carried out after the formation of the skeletal nickel catalyst and involved adsorption of tartaric acid on the surface of the nickel. Reaction conditions strongly influenced the enantioselectivity of the catalyst. Both Ni° and Ni2+ have been detected on the modified surface [89]. This technique has already been expanded to other modified skeletal catalysts for example, modification with oxazaborolidine compounds for reduction of ketones to chiral alcohols [90],... 

Izumi et al. then developed another type of catalyst, Raney nickel modified by tartaric acid [ 14]. Using this, methyl acetoacetate could be hydrogenated into methyl P-hydroxybutyrate with an ee of up to 80%. Unfortunately, only some specific substrates were reduced enantioselectively. However, some interesting developments were later realized (vide infra). 

Besides several diastereoselective heterogeneous catalytic hydrogenations [1-3] only two enantioselective hydrogenation reactions are known the reduction of p-keto-esters with Raney-nickel modified by tartaric acid and of pyruvic acid esters with Pt modified by cinchona alkaloids. Garland and Blaser [4] described the reduction of pyruvic acid ester as a "ligand-accelerated" reaction with the adsorption of the modifier new active sites are generated on the catalyst surface. On these new centers the selective reaction is faster and the increased reaction rate is accompanied by greater enantioselectivities. 

Instead of the conventionally used modification by pre-immersion of the catalyst in a solution of modifier, Osawa et al. used an in situ modification during the enantioselective hydrogenation of MAA. Fine nickel powder modified with (2R,3R)-tartaric acid was used and sodium salt was added to the reaction media. By this method the optical yield was increased up to an ee of 79%. Improvement of this method consists in modification in situ of finely reduced Ni-powder by addition of (2R,3R)-tartaric acid and NaBr to the reaction media. In this case, an ee of 89% was obtained in the hydrogenation of MAA. The addition of small amounts of NaBr to the reaction media increased both the ee and the reaction rate, while, in contrast, the rate decreased with the addition of NaBr to the modification solution in... 

The chiral catalyst was made from Raney nickel, which was prepared by addition in small portions of 3.9 g Raney nickel alloy to 40 ml water containing9 g NaOH. The mixture was kept at 100 C for 1 h, and then washed 15 times with 40 ml water. Chirality was introduced by treatment of the Raney nickel for I h at lOO C with 178 ml water adjusted to pH 3.2 with NaOH and containing 2g (S,S)-tartaric acid and 20 g NaBr. The solution was then decanted, and the modifying procedure was twice repeated. Hydrogenation over this catalyst of acetylacctone (100 atm, 100" C) in THF containing a small amount of acetic acid gave an isolated yield of chiral pentanediol of 44% (99.6% optical purity). 

---