Tartaric acid, modifier

Stereochemical Studies of the Enantio-differentiating Hydrogenation of Various Prochiral Ketones over Tartaric Acid-Modified Nickel Catalyst... 

A common theme is the existence of modified (enantioselective) sites and unmodified (racemic) sites. For the case of the tartaric acid modified Ni, it is postulated that the tartaric acid is adsorbed on the surface and stereodirects (through hydrogen bonding) adsorption of the incoming 3-ketoesters.18 19 Support for this comes from an isotope effect from deuterium labeling.23 Increased enantioselectivities resulting from co-modification with NaBr is believed to result from poisoning the racemic sites.24 A similar technique in... 

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)... 

Tartaric Acid-Modified Me/Support Hydrogenation Catalysts and Related Systems.502... 

TARTARIC ACID-MODIFIED ME/SUPPORT HYDROGENATION CATALYSTS... 

The chapter Chiral Modification of Catalytic Surfaces [84] in Design of Heterogeneous Catalysts New Approaches based on Synthesis, Characterization and Modelling summarizes the fundamental research related to the chiral hydrogenation of a-ketoesters on cinchona-modified platinum catalysts and that of [3-ketoesters on tartaric acid-modified nickel catalysts. Emphasis is placed on the adsorption of chiral modifiers as well as on the interaction of the modifier and the organic reactant on catalytic surfaces. 

Reagents (i) (SS)-T artaric acid modified RaNi, H2, (ii) (RR)-Tartaric acid modified RaNi, H2... 

Historically, it took many years to achieve the almost perfect stereocontrol of an enantioselechve hydrogenahon reaction using a heterogeneous catalyst, with a resultant 97-99% enanhomeric excess (ee) of the product One of the oldest such examples is that of tartaric acid-modified platinum black for the hydrogenahon of an oxime to give a chiral amine of <20% ee [2]. Nonetheless, many reviews in this field have provided a clear history of the enanhoselechve catalysis from silk-palladium-a palladium metal supported on a chiral silk fiber to hydrogenate a... 

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

Scheme 10.7 Selected compounds applicable for the tartaric acid-modified Raney nickel, and the product ee-values at 60 and lOO C. (a) The results in parentheses were obtained in a 1 1 mixture of pivalic acid and THF (b) The result in brackets was obtained in the presence of 2 equiv. of 1-methyl-1 -cyclohexanecarboxylic acid.

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.

A heterogeneous tartaric acid modified nickel catalyst was used in the synthesis of methyl (S,.S )-3-hydroxy-2-methylbutanoate. necessary for the preparation of the sex-attractant of the pine sawfly. A syntanti ratio of 75 25 was obtained69. 

Although this approach is the only example in which catalytic activity and stereochemical control are not separated, no synthetic application can yet be foreseen because the concentration of such chiral sites in practically useful polycrystalline metal catalysts is very low and surface restructuring is likely to occur under reaction conditions. A similar conclusion can be drawn about the importance of other chiral metal structures, such as a screw dislocation or a chiral surface produced by asymmetric corrosion. Interestingly, asymmetric leaching of Ni in the presence of tartaric acid has already been proposed as an explanation for the enantio-differentiation by the tartaric acid-modified Ni catalyst [5]. 

Table 4.4. Effects of inorganic salts, added to the (2R,3R)-tartaric acid modifying solution, on enantioselectivity in the hydrogenation of MAA on MRNi (mainly according to Harada ).

According to the model of active surfaces of modified catalysts two kinds of centers, selective and non-selective, exist It may be possible to improve enantioselectivity of hydrogenation if non-modified centers producing racemic MHB can be blocked with a poison, while modified centers are being protected with tartaric acid modifier and therefore are not poisoned. The effect of poisoning depends on the strength of adsorption of... 

In method M-P the addition of thiophene after modifieation inereased the ee value from 6.8 to 11% at 10% eonversion. Thus the inhibition effect of the poison in the P-M process appears to be more effective than in the M-P process. This suggests that in the latter case all of the active centers are already occupied by the tartaric acid modifier molecules, which is more easily displaced Irom the less active chiral sites than from the racemic sites. 

In general, supported metal catalysts are less effective than Raney metal modified catalysts, and the enantioselectivities of supported catalysts are near to those of the Raney catalysts only in the cases when large amounts of metal are found on the surface of the support This can be explained, at least in the cases of bulk metal catalysts, as a consequence of an increase in crystallite sizes and diffusion of the tartaric acid modifier into the pores during modification of the catalyst (Sachtler " ). Detailed consideration of this problem is in discussed in Chapter 5. 

Figure 4.17. Effect of the pH of the (21 ,3i )-tartaric acid modifying solution on the optical yield of (i )-(-)-EHB on Cu-Pd (97.3 2.7) catalyst (according to Vedenyapin et al. ).

Nakagawa, S., Sugimura, T., and Tai, A. (1998) A substrate specific chiral modifier activation on enantio-differentiating hydrogenation over tartaric acid-modified Raney niekel, Chem. Lett. 1257 - 1258. 

Nitta, Y., Kawabe, M., and Imanaka, T. (1987) Enantioselectivity of tartaric acid modified Ni-Al203 catalysts, Appl. Catal. 30,141-149. 

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