Pyruvate, asymmetric hydrogenation

Knoop and Martius102a) reported the asymmetric hydrogenation of the Schiff base obtained from (S)-arginine and pyruvic acid as early as 1939. 

The equipment depicted in Fig. 17 also allows monitoring of species adsorbed on a solid catalyst. For this application, the ZnSe IRE is coated with a layer of the catalyst before assembly of the cell and the start of the reaction. This approach was chosen for investigation, for example, of the interaction of the reactant with the catalyst during the asymmetric hydrogenation of ethyl pyruvate catalyzed by cinc-honidine (CD)-modified Pt/Al2O3 in the presence of supercritical ethane (79). 

The temperature effect could be explained by the chelation mechanism. - The Schiff s base composed of a-keto acid and optically active amine interacted with the catalyst to form a substrate-catalyst complex (14) at lower temperature at higher temperatures, the population of the unchelated structure (15) would increase as shown in Scheme 6. Asymmetric hydrogenation involving () )-a-phenylglycinate and ethyl pyruvate has also been studied. ... 

Orino, Y. Imai, S. Niwa, S. Asymmetric hydrogenation of methyl pyruvate using Pt/C catalyst modified with cinchonidine. Nippon Kagaku Kaishi (J. Chem. Soc. Jpn.) 1979, 1118-1120. 

Baiker and colleagues have extensively studied the heterogeneously catalyzed asymmetric hydrogenation of ethyl pyruvate, giving ethyl D-lactate, in the presence of compressed gases [Eq. (20)] (98,99,108,109) ... 

The present study aimed at revealing the mode of enantiodifferentiation in the asymmetric hydrogenation of ethyl pyruvate and isophorone over platinum and palladium catalysts. The effect of adding the modifier after an initial phase of racemic hydrogenation and the combined use of different vinca and cinchona type modifiers on enantioselectivity and activity were studied. A mechanistic rationale is proposed to account for the experimental observations. 

This paper deals with the asymmetric hydrogenation of ethyl pyruvate to ethyl lactate showing a high enantiomeric excess in favour of the R-enantiomer over (-)cinchonidine modified Pt/carrier catalysts. Due to their regular structures, zeolites in particular have been used as carrier materials. 

Enantiomeric excess and catalytic activity of the asymmetric hydrogenation of ethyl pyruvate over (-)cinchonidine modified Pt/carrier catalysts depend significantly on the specific Pt surface area This is due to the morphology of the Pt particles and to surface chemical Pt/support interaction. Thus, reaction pathway control is possible by varying these parameters. 

In contrast to asymmetric hydrogenation, examples of stereoselective reduction of functionalized ketones are rare. Scheme 43 illustrates the highly enantioselec-tive reduction of methyl benzoylformate in 2-propanol containing KOH using a catalyst prepared in situ from [RhCl(CgHjo)]2 and (S,S)-3 [101]. With the same catalyst, methyl pyruvate is reduced in 5% optical yield. 

Heterogeneous asymmetric hydrogenation of ethyl pyruvate in SCC2HS using Pt/ alumina modified with cinchonidine as the catalyst has, in comparison to the reaction in toluene, the benefit of a shorter reaction time [46] (Scheme 13.6). 

The asymmetric hydrogenation site A is accessible to methyl pyruvate provided adsorption is in the configuration required to give R-(+)-lactate on hydrogenation. The spatial relationship of adsorbed methyl pyruvate to molecule x of cinchonidine is such as to permit the hydrogen bonding interaction that is proposed to occur when the reactant is converted to its half-hydrogenated state. 

Cinchonidine molecule Y directly prevents methyl pyruvate adsorbing in a form that would give S-(-)- lactate as product and molecules Z1 and Z2 form the remainder of the boundary of the asymmetric hydrogenation site. Such boundaiy molecules are obviously not required at the edges of the Pt crystallites (site... 

The asymmetric hydrogenation of prochiral ketones is often an important step in the industrial synthesis of fine and pharmaceutical products. Several noble metal nanoparticles have been investigated for asymmetric catalysis of prochiral substrates but platinum colloids have been the most widely studied and relevant enantiomeric excesses have been reported (>95%). Nevertheless, the enantioselec-tive hydrogenation of ethyl pyruvate catalyzed by PVP-stabilized rhodium nanocluster modified by cinchonidine and quinine was reported by Li and coworkers (Scheme 11.7) [68]. 

Table 5.1. Asymmetric hydrogenation of methyl pyruvate (MePy) to (iS)-(+)-methyl lactate (MeLa) on 5% Pt-C and 5% Pt-Alumina catalysts modified with supplemental addition of different alkaloids in different solvents (according to Orito et al. ).

Torok, B., Felfoldi, K., Szakonyi, G., Molnar, A., Bartok, M. (1998) Ultrasonics in chiral metal catalysis. Effect of presonication on the asymmetric hydrogenation of ethyl pyruvate over Pt catalysts. Chemical Industries Series Catalysis of Organic Reactions, Herkes, F.E., ed.) (Dekker) 75, 129-140... 

Wang, J., Sun, Y., LeBlond, C., Landau, D.G., Blackmond, D.G. (1996) Asymmetric hydrogenation of ethyl pyruvate relationship between conversion and enantioselectivity, J. Catal. 161, 752-758. 

Bartok, M., Balazsik, K., Szollosi, G., Bartok, M. (2001) Solvent and support effects in the case of acetic acid and alumina Oxonium cations in asymmetric hydrogenation of ethyl pyruvate over dihydrocinchonidine modified platinum, Catal. Commun. 2,269-272. 

Very promising are supercritical fluids as solvents for asymmetric hydrogenations. Thus, ethyl pyruvate can be hydrogenated into ethyl lactate in supercritical ethane with a 3.5 times increased reaction rate and with the same ee as in the common solvent toluene (see Chapter 5.5 of this book). 

The new mannitol-based diphosphinite ligands 233 (R = Ph, cyclopentyl, cyclohexyl) have been prepared, and used in rhodium complexes for the asymmetric hydrogenation of prochiral ketones. The cases where R = cyclohexyl gave highest enantioselectivity, up to 86% for the reduction of methyl pyruvate to give methyl R-lactate. °... 

Asymmetric hydrogenation using an H-Cube reactor has also been investigated. Bartok reported the enantioselective hydrogenation of a-ketoesters by using a 5% Pt/ AI2O3 cartridge [81,82]. In the presence of cinconidine and cinconine as chiral sources, ethyl pyruvate was reduced into (R)- and (S)-lactate in about 90% ee (Scheme 7.17). 

Baiker and coworkers (189,190) have investigated the asymmetrical hydrogenation of ethyl pyruvate in SCF solvents using a heterogeneous 5 wt % Pt/alumina catalyst modified with cinchonidine to promote the asymmetric in-... 

Figure 6 Enantiomeric excess for asymmetrical hydrogenation of ethyl pyruvate in liquid ethanol and supercritical ethane. (Reprinted with permission from Ref. 189. Copyright 1995 Baltzer Science Publishers.)... 

Table 5 Asymmetrical Hydrogenation of Ethyl Pyruvate over 5% Pt/Alumina Catalyst in Supercritical and Conventional Solvents (190)... 

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