Tartrate-modified Nickel catalysts

Keywords. 3-Functionalized ketones, a-Keto acid derivatives. Cinchona modified Pt catalysts. Chiral imprints. Chiral metal surfaces. Chiral polymers. Cyanohydrin formation. Cyclic Dipeptides, Epoxidation catalysts. Heterogeneous catalysts. Hydrogenation catalysts. Modified metal oxides. Polypeptides, Tartrate-modified Nickel catalysts... 

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. 

Raney nickel, modified by free tartaric acids 2.69 (R = H) or their salts, has frequently been used as a catalyst for asymmetric hydrogenations of carbonyl compounds [578,948]. Several industrial applications have been described [578, 811, 812], Neveriess, hydrogenations of prochiral carbon-carbon double bonds in the presence of such catalysts gives disappointing results [578], The use of tartrate-modified copper catalysts in cyclopropanation of styrenes by diazoketones takes place with a modest asymmetric induction [578,936]. 

Sachtler s group (73) and Yasumori (64) studied the IR spectra of silica-supported Ni modified with amino acid and 2-hydroxy acid and the XPS of TA-MRNi. Both authors deduced almost the same model as proposed by Suetaka. Recently Sachtler s group proposed other models as shown in Fig. 22 from results obtained in enantio-differentiating hydrogenations of MAA with nickel catalysts modified with nickel and copper tartrates (74). The nickel tartrate adsorbs at the vacant coordination site of nickel in this model. 

Hoek, A., and Sachtler, W.H.M. (1979) Enantioselectivity of nickel catalysts modified with tartraic acid or nickel-tartrate complexes, J. Catal. 58, 276 - 286. 

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. 

Several groups have reported methods for the preparation of optically active 3-hydroxy-carboxylic acids from 3"keto esters by means of asymmetrical reduction. For example, Tai et al. used a nickel catalyst modified with optically active tartrate [1], Mukaiyama et... 

The most conventional investigations on the adsorption of both modifier and substrate looked for the effect of pH on the amount of adsorbed tartrate and MAA [200], The combined use of different techniques such as IR, UV, x-ray photoelectron spectroscopy (XPS), electron microscopy (EM), and electron diffraction allowed an in-depth study of adsorbed tartrate in the case of Ni catalysts [101], Using these techniques, the general consensus was that under optimized conditions a corrosive modification of the nickel surface occurs and that the tartrate molecule is chemically bonded to Ni via the two carbonyl groups. There were two suggestions as to the exact nature of the modified catalyst Sachtler [195] proposed adsorbed nickel tartrate as chiral active site, whereas Japanese [101] and Russian [201] groups preferred a direct adsorption of the tartrate on modified sites of the Ni surface. 

The temperature, concentration, and pH of the modifying solution have strong influences on the effectiveness of the resulting modified catalyst. Increasing the time of modification and the concentration of modifier at optimal temperature and pH enhances enantioselectivity of the catalyst up to an optimal value due to corrosion of the nickel surface with formation of nickel-tartrate chelates coming into the solution. Therefore, for the best enantioselective effect the amounts of TA on the surface of Ni must reach an optimal value. 

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