Cinchona Alkaloid-Modified Platinum Catalysis

One notable property of the Orito reaction is that two chiral modifiers, CD and CN, are diastereomers of each other. The configurations of the functional groups which are expected to create the molecular recognition are antipodes, and only the 

Important information regarding the stereocontrol mechanism was reported by the same group. For the interachon model of linear a-ketoesters with CD, a ther- 

Murzin et al. reported detailed studies of the hydrogenation of l-phenyl-1,2-propanedione (acetyl benzoyl), which has two carbonyl groups to give four possible stereoisomers by double hydrogenation [32]. A detailed theoretical analysis has also been reported for this substrate [33]. 

---

Felfoldi, K., Balazsik, K., Bartok, M. (2003) Heterogeneous asymmetric catalysis. Part 32. High enantioselectivities in the hydrogenation of activated ketones on cinchona alkaloid modified Platinum-alumina catalysts, J. Mo/. Catal. A. Chem. 202, 163-170. 

Enantioselective hydrogenation of a-ketoesters on cinchona alkaloid-modified Pt/Al203 is an interesting system in heterogeneous catalysis [143-146], The key feature is that on cinchonidine-modified platinum, ethyl pyruvate is selectively hydrogenated to R-ethyl lactate, whereas on einchonine-modified platinum, S-ethyl pyruvate is the dominant product (Figure 16) [143]. 

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],... 

In addition to the enantioselective effect, cinchona alkaloids also produce a rate acceleration, i.e. this is an example of ligand accelerated catalysis [14]. The model of a non-closepacked ordered array of cinchonidine molecules adsorbed on platinum, proposed by Wells and co-workers, was abandoned in their later study [15]. Augustine [16] deduced from the behaviour of this system at low modifier concentrations that the chiral sites are formed at the edge and comer platinum atoms, which involve the adsorbed cinchonidine and a metal adatom. The different authors agreed that the quinoline ring of the modifier is responsible for the adsorption on platinum, the quinuclidine part, through the nitrogen atom, interacts with... 

Platinum and palladium dominate the enantioselective catalysis scene as far as alkaloid modifiers are concerned. Of the other platinum group metals, Ir follows Pt in cinchona-modified pyruvate ester hydrogenation,132 the fact that Rh can hydrogenolyse alkaloids adsorbed at its surface133 makes it unlikely to function successfully and Ru tends to be too easily oxidised by adventitious oxygen in these reactions. 

Several studies have been reported on the mechanism of chiral catalysis by modified solid catalysts. Both modifier and substrate structures play important roles. Refer to reviews by Fish and Ollis (1978), Izuma (1983), Sachtler (1985), Tai and Harada (1986), and Blaser et al. (1988) for details of various postulated mechanisms, but one particular conclusion is significant. For the cinchona-modified platinum catalyst, the presence of nitrogen is considered essential, and the configuration at Cg of the alkaloid determines which enantiomer of the product is formed. 

---