Nickel enantioselective hydrogenation

When we first contemplated thermochemical products available from Glu, a search of the literature revealed no studies expressly directed at hydrogenation to a specific product. Indeed, the major role that Glu plays in hydrogenation reactions is to act as an enantioselectivity enhancer (17,18). Glu (or a number of other optically active amino acids) is added to solutions containing Raney nickel, supported nickel, palladium, or ruthenium catalysts and forms stereoselective complexes on the catalyst surface, leading to enantioselective hydrogenation of keto-groups to optically active alcohols. Under the reaction conditions used, no hydrogenation of Glu takes place. 

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],... 

Recently, it has been shown that ultrasonic agitation during hydrogenation reactions over skeletal nickel can slow catalyst deactivation [122-124], Furthermore, ultrasonic waves can also significantly increase the reaction rate and selectivity of these reactions [123,124], Cavitations form in the liquid reaction medium because of the ultrasonic agitation, and subsequently collapse with intense localized temperature and pressure. It is these extreme conditions that affect the chemical reactions. Various reactions have been tested over skeletal catalysts, including xylose to xylitol, citral to citronellal and citronellol, cinnamaldehyde to benzenepropanol, and the enantioselective hydrogenation of 1-phenyl-1,2-propanedione. Ultrasound supported catalysis has been known for some time and is not peculiar to skeletal catalysts [125] however, research with skeletal catalysts is relatively recent and an active area. 

One of the important conclusions of the early attempts was that it is fruitful to place the functionality near an optically active support. Already in 1958, Isoda and coworkers reported for the first time the enantioselective hydrogenation with a Raney nickel catalyst modified with optically pure amino acids. Optical yields reported at that time were from low (2.5%) to moderate (36%) values (for references see [12]). Subsequently, in 1963, Izumi and coworkers [100] initiated an extended study of the modified Raney nickel system with TA. As a result of their initial researches, this system was the first heterogeneous chiral catalyst to give high enantioselectivities in the hydrogenation of / -ketoesters (95%) [101,102],... 

It is clear that the influence of surface geometry upon catalytic activity is extremely complex and many more studies are required before any definitive relationship between catalytic activity and metal particle size can be established. Such studies will require to take cognisance of such factors as the perturbation of surface structure due to the formation of carbidic residues, as noted by Boudart [289] and by Thomson and Webb [95], and by the modification of catalytic properties on adsorption, as noted by Izumi et al. [296—298] and by Groenewegen and Sachtler [299] in studies of the modification of nickel catalysts for enantioselective hydrogenation. Possible effects of the support, as will be discussed in Sect. 6.3, must also be taken into account. 

As a rule, synthetic chemists will consider only those new reactions and catalysts for preparative purposes where the enantioselectivity reaches a certain degree (e.g. >80%) and where both the catalyst and the technology are readily available. For heterogeneous catalysts this is not always the case because the relevant catalyst parameters are often unknown. It is therefore of interest that two types of modified Nickel catalysts are now commercially available a Raney nickel/tartrate/NaBr from Degussa [64] and a nickel powder/tartrate/NaBr from Heraeus [65, 66]. It was also demonstrated that commercial Pt catalysts are suitable for the enantioselective hydrogenation of a-ketoesters [30, 31]. With some catalytic experience, both systems are quite easy to handle and give reproducible results. 

The second, effective heterogeneous enantioselective catalytic system is nickel modified with tartaric acid and sodium bromide. This system is most effective for the hydrogenation of P keto esters giving chiral P hydroxy esters, 41, with ee s as high as 95% (Eqn. 14.29). 0,72,84,85 n also been used for the enantioselective hydrogenation of p diketones (Eqn. 14.30) and methyl ketones. ... 

Keane MA, Webb G (1992) The enantioselective hydrogenation of methyl acetoacetate over supported nickel catalysts I. The modification procedure. 1 Catal 136 1 Keane MA (1997) Interaction of optically active tartaric acid with a nickel-sUica catalyst role of both the modification and reaction in determining enantioselectivity. Langmuir 13 41... 

Scheme 10.6 Enantioselective hydrogenation of MAA over tartaric acid-modified nickel catalyst.

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. 

MA Keane. The role of catalyst activation in the enantioselective hydrogenation of methyl acetoacetate over silica-supported nickel catalysts. Can. J. Chem. 72 372-381, 1994. 

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. 

Table 4.1. Enantioselective hydrogenation of the C=0 bond in diethyl 2-oxoglutarate to diethyl 2-hydroxyglutarate, of the C=N bond in diethyl 2-oximmoglutarate to diethyl glutamate, and of the C=C bonds in 2-acetamidocmnamic acid to phenylalanine on Raney nickel catalysts, modified with optically active ketones, alcohols and amino acids (according to mainly Isoda et al. ).

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... 

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

Table 4.7. Rate of formation of (R)-(-)-MHB (mmol h g ) and ee values in the enantioselective hydrogenation of methyl acetoacetate on deposited nickel-kieselguhr catalysts, promoted with 1% noble metals and modified with (2R,3R)-tartaric acid (according to summarized data of Orito et al. ).

Yokozeki, M., Shimokoshi, K., and nMiyazaki, E. (1985) Enantioselective hydrogenation of methyl acetoacetate over a modified nickel surface MNDO calculation of stability for intermolecular interactions, J. Phys. Chem. 89,2397 -2400. 

Vedenyapin, A.A., Chankvetadze, B.G., and Klabunovskii, E.L (1984) Studies of Copper- Nickel catalysts for enantioselective hydrogenation. React. Kinet Catal. Letters 24,77-80. 

Fu L., Kung H.H., and Sachtler, W.M.H. (1987) Particle size effect on enantioselective hydrogenation of methyl acetoacetate over silica-sup-ported nickel catalyst, J. Mol Catal 42, 29-36. 

Bostelaar, L.J. (1984) Enantioselective hydrogenation on supported nickel catalysts. Thesis. Promoter Sachtler W., Leiden Univ., 157 pp. 

Fish, M.J., and Ollis, D.J. (1977) Characterization of enantioselective hydrogenation catalysts transient electrochemical oxidation of D-(+)-tartaric acid on Nickel, J. Catal. 50, 353-363. 

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. 

Jackson, S.D. (1998) New heterogeneous nickel eatalysts for enantioselective hydrogenation. Stud. Surf Sci. Catal., 118, 305-312. 

Chernysheva, V.V. (1984) Studies of effects of genesis of modified skeletal nickel and powder copper-nickel catalysts and effect of additives of organic compounds for their properties in enantioselective hydrogenation, Cand. Diss. Thesis, N.D. Zelinskii Institute of Organic Chemistry, Academy of Sciences of the USSR Moscow (supervisor Klabunovskii, E.l.). 

Figure 5.4. Dependence of optical yields on mean crystallite size of nickel in various catalysts in the enantioselective hydrogenation of methyl aceto-acetate over RNi ( ), Ni-B (A), Ni-P (V) or RNi (o) catalysts.

The mechanism of enantioselective hydrogenation on nickel catalysts modified with optically active compounds was suggested for the first time by Balandin s group They proposed that the reaction proceeded through an intermediate complex involving catalyst metal atom, (Ni), modifier molecule, (M), and substrate molecule, (S) Ni-M-S. Similar models were proposed later by Yasumori et al and Izumi and Tai... 

This chapter summarizes some of the most characteristic results obtained with the use of mainly homogeneous metal complex eatalysts either in the industry or in processes recommended for practical use. These are large seale processes of asymmetric synthesis of the herbicide metolachlor, synthesis of optically pure menthol with the use of chiral iridium and rhodium phosphine complexes, consideration of the synthesis of ethyl 2-hydroxybutyrate as a monomer for the preparation of biodegradable polyesters with use of heterogeneous ehiral modified nickel catalyst, the manufacturing of (fJ)-pantolactone by means of a possible eata-IjTic systems for enantioselective hydrogenation of ketopantolactone, and catalytic systems for the preparation of other pharmaceuticals. 

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