Cinchona platinum-modified

The molecular modelling approach, taking into account the pyruvate—cinchona alkaloid interaction and the steric constraints imposed by the adsorption on the platinum surface, leads to a reasonable explanation for the enantio-differentiation of this system. Although the prediction of the complex formed between the methyl pyruvate and the cinchona modifiers have been made for an ideal case (solvent effects and a quantum description of the interaction with the platinum surface atoms were not considered), this approach proved to be very helpful in the search of new modifiers. The search strategy, which included a systematic reduction of the cinchona alkaloid structure to the essential functional parts and validation of the steric constraints imposed to the interaction complex between modifier and methyl pyruvate by means of molecular modelling, indicated that simple chiral aminoalcohols should be promising substitutes for cinchona alkaloid modifiers. Using the Sharpless symmetric dihydroxylation as a key step, a series of enantiomerically pure 2-hydroxy-2-aryl-ethylamines... 

In order to evaluate the catalytic characteristics of colloidal platinum, a comparison of the efficiency of Pt nanoparticles in the quasi-homogeneous reaction shown in Equation 3.7, with that of supported colloids of the same charge and of a conventional heterogeneous platinum catalyst was performed. The quasi-homogeneous colloidal system surpassed the conventional catalyst in turnover frequency by a factor of 3 [157], Enantioselectivity of the reaction (Equation 3.7) in the presence of polyvinyl-pyrrolidone as stabilizer has been studied by Bradley et al. [158,159], who observed that the presence of HC1 in as-prepared cinchona alkaloids modified Pt sols had a marked effect on the rate and reproducibility [158], Removal of HC1 by dialysis improved the performance of the catalysts in both rate and reproducibility. These purified colloidal catalysts can serve as reliable... 

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]. 

Heterogeneous enantioselective hydrogenation over Cinchona alkaloid modified platinum 04ACR909. 

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. 

In the Review, Blaser describes his own Group s work with cinchona alkaloid modified platinum catalysts in the enantioselective hydrogenation of [Pg.8]

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. 

Vayner, G., Houk, K.N., Sim, Y.K (2004) Origins of enantioselectivity in reduction of ketones on cinchona alkaloid modified platinum, J. Amer. Chem. Soc. 126, 199-203. 

Among the various strategies [34] used for designing enantioselective heterogeneous catalysts, the modification of metal surfaces by chiral auxiliaries (modifiers) is an attractive concept. However, only two efficient and technically relevant enantioselective processes based on this principle have been reported so far the hydrogenation of functionalized p-ketoesters and 2-alkanons with nickel catalysts modified by tartaric acid [35], and the hydrogenation of a-ketoesters on platinum using cinchona alk oids [36] as chiral modifiers (scheme 1). 

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 fact, there are only two heterogeneous catalysts that reliably give high enantioselectivities (e.s. s) (90% e.e. or above). These are Raney nickel (or Ni/Si02) system modified with tartaric acid (TA) or alanine for hydrogenation of /(-kctocstcrs [12-30], and platinum-on-charcoal or platinum-on-alumina modified with cinchona alkaloids for the hydrogenation of a-ketoesters [31-73],... 

Cinchona-modified platinum catalysts received special interest mainly because of to the results obtained for the hydrogenation of methyl pyruvate (MP) or ethyl pyruvate. E.e. s up to 80% or even higher (98% for ethyl pyruvate) made this system one of the most interesting ones from the application point of view. At present, 5% Pt/alumina with low dispersion (metal particles > 2 nm) and a rather large pore volume constitutes one of the best catalysts commercially available. 

A striking feature of the template model is the restriction of the role of the modifier to that of a template, which does not take into account direct binding interactions of the reactant with the modifier. Furthermore, there exists no experimental evidences for the formation of ordered arrays of cinchona molecules on a platinum surface. In 1995, Margitfalvi and Hegedus [235] criticized this model showing that the model is too idealistic and oversimplified. 

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. 

The modification of platinum-group metals by adsorbed chiral organic modifiers has emerged as an efficient method to make catalytic metal surfaces chiral. The method is used to prepare highly efficient catalysts for enantioselective hydrogenation of reactants with activated C = O and C = C groups. The adsorption mode of the chiral modifier is crucial for proper chiral modification of the active metal surfaces. The most efficient chiral modifiers known today are cinchona alkaloids, particularly CD, which yields more than 90% enantiomeric excess in the hydrogenation of various reactants. 

For the Pt/cinchona catalysts only preliminary adsorption studies have been reported [30]. From the fact that in situ modification is possible and that under preparative conditions a constant optical yield is observed we conclude that in this case there is a dynamic equilibrium between cinchona molecules in solution and adsorbed modifier. This is supported by an interesting experiment by Margitfalvi [63] When cinchonine is added to the reaction solution of ethyl pyruvate and a catalyst pre-modified with cinchonidine, the enantiomeric excess changes within a few minutes from (R)- to (S)-methyl lactate, suggesting that the cinchonidine has been replaced on the platinum surface by the excess cinchonine. 

Systematic investigation of the Pt - cinchona alkaloid system, discovered by Orito et al. in 19797, has led to a better understanding of the a-ketoester-modifier-Pt interaction. It has been therefore possible to increase the enantioselectivities of the original reaction and, more importantly, to broaden the application range of cinchona-modified platinum. 

Finally, metal colloids can adsorb chiral molecules such surface-modified particles can catalyze hydrogenations in high optical yields. An example is platinum colloids treated with cinchona alkaloids.18... 

Platinum catalysts modified with members of the cinchona alkaloids are effective enantioselective catEilysts for the hydrogenation of a keto esters giving the chiral a hydroxy esters with ee s as high as 95%. >72 of the cinchona alkaloids, cinchonidine (30) and cinchonine (31) appear to be more effective than quinine (32) and quinidine (33). With cinchonidine the (R) lactate, 34, is... 

Blaser HU, Jalett HP, Muller M, Studer M (1997) Enantioselective hydrogenation of a-Ketoesters using cinchona modified platinum catalysts and related systems. Catal Today 37 441... 

As mentioned in Section 2.1, the use of cinchona alkaloids as modifiers of the classical heterogeneous hydrogenation catalyst platinum on a support was pioneered... 

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