Cinchonidine platinum surface

In catalysis, adsorbed CO may retard some reactions such as olefin hydrogenation, fuel cell conversion, and enantioselective hydrogenation. For instance, Lercher and coworkers observed the deactivation of Pt/Si02 in the liquid-phase hydrogenation of crotonaldehyde, and ascribed this deactivation to the decomposition of crotonaldehyde on platinum surface to adsorbed CO [138]. Blaser and coworkers found that the addition of a small amount of formic acid decreases the rate of liquid-phase hydrogenation of ethyl pyruvate on cinchonidine-modified Pt/Al203 catalyst, which they explained as the decomposition of formic acid on the catalyst to adsorbed CO. Interestingly, the addition of acetic acid does not decrease the reaction rate, but whether acetic acid decomposes on the catalyst as formic acid does was not mentioned [139]. 

We then designed model studies by adsorbing cinchonidine from CCU solution onto a polycrystalline platinum disk, and then rinsing the platinum surface with a solvent. The fate of the adsorbed cinchonidine was monitored by reflection-absorption infrared spectroscopy (RAIRS) that probes the adsorbed cinchonidine on the surface. By trying 54 different solvents, we are able to identify two broad trends (Figure 17) [66]. For the first trend, the cinchonidine initially adsorbed at the CCR-Pt interface is not easily removed by the second solvent such as cyclohexane, n-pentane, n-hexane, carbon tetrachloride, carbon disulfide, toluene, benzene, ethyl ether, chlorobenzene, and formamide. For the second trend, the initially established adsorption-desorption equilibrium at the CCR-Pt interface is obviously perturbed by flushing the system with another solvent such as dichloromethane, ethyl acetate, methanol, ethanol, and acetic acid. These trends can already explain the above-mentioned observations made by catalysis researchers, in the sense that the perturbation of initially established adsorption-desorption equilibrium is related to the nature of the solvent. 

Figure 17. The effect of cyclohexane (A) and dichloromethane (B) solvents on the desorption of cinchonidine (abbreviated as CD) from platinum [66], In both cases, a clean platinum surface was first exposed to a cinchonidine solution in CC14 to allow for the adsorption of cinchonidine, and the platinum disk was then exposed to either cyclohexane or dichloromethane. In the case of cyclohexane, a total rinsing with 180 mL in several sequential flushings did not lead to significant change of the infrared spectra. On the other hand, with dichloromethane (B), one flush was sufficient to remove most of the adsorbate. [Reproduced by permission of the American Chemical Society from Ma, Z. Zaera, F. J. Phys. Chem. B 2005,109, 406-414.]...

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. 

Cinchonidine, being a bulky molecule, reduces the accessible active platinum surface as it adsorbs and should causes some deactivation with respect to racemic hydrogenation. The decrease in formation rate of the main product after the maximum can be a result of poisoning by adsorbed spectator species, which inhibit enantiodifferentiating substrate-modifier interaction. Adsorbed cinchonidine in parallel mode (active form) provides an enantioselective site (Figure 7.8) and when the reactant is adsorbed in the vicinity, interaction between reactant and modifier leads to such orientation that hydrogenation towards the main product (e.g. B or 1-R enantiomer) is preferred. However, when the tilted form (Figure 7.8) of... 

The simplest cinchona alkaloids, cinchonidine and its 10,11-dihydro-derivative, have been shown by D-tracer studies and by NEXAFS and ATR-IR spectroscopy to adsorb by interaction of the quinoline moiety with the platinum surface. Mechanistic studies have established that a site exists adjacent to the open-3 conformation of adsorbed cinchonidine at which pyruvate ester can undergo selective enantioface adsorption. Hydrogenation of the preferred enantioface results in preferential formation of one enantiomer of the product, methyl lactate, MeC H(OH)COOMe. Pt modified by cinchonidine provides R-lactate preferentially, whereas the near enantiomer cinchonine provides 5-lactate in excess. Values of the enantiomeric excess of 75% can be obtained without optimisation, and of 98% under special conditions. In solution, conditions that achieve enantioselectivity normally promote the reaction rate by a factor of 2 to 100 depending on conditions. ... 

Cinchonidine in its lowest energy state is L-shaped and it can approach a Pt (100) or Pt (111) surface in a configuration that would permit adsorption by the quinoline moiety without conformational disturbance (22]. The model proposed by Wells involves the flat adsorption on the platinum surface in a non-close packed array, thus leaving exposed shaped ensembles of platinum atoms. 

Scheme 5.25. The calculated structures of complexes formed between protonated cinchonidine and pyruvate adsorbed on a platinum surface (according to Schwalm et al. ).

Figure 4. Side and top views of the energetically most favorable complexes formed between protonated cinchonidine and methyl pyruvate which would yield (R)-methyl lactate (left) and (S)-methyl lactate (right), respectively, upon hydrogenation. The complexes have been accomodated on a space filling model of platinum (111) surface in order to illustrate the space requirements of the adsorbed complexes. For the sake of clarity, in the side views the carbon atoms of the reactant are marked with a white square and the oxygen atoms with an o. Data taken from ref. [41].

The coordination of ligands at the surface of metal nanoparticles has to influence the reactivity of these particles. However, only a few examples of asymmetric heterogeneous catalysis have been reported, the most popular ones using a platinum cinchonidine system [65,66]. In order to demonstrate the directing effect of asymmetric ligands, we have studied their coordination on ruthenium, palladium, and platinum nanoparticles and the influence of their presence on selected catalytic transformations. 

CD cinchonidine S substrate 3,4-HD 3,4-hexanedione premixing technique Tr 20 °C pm 50 bar amount of catalysts used was calculated to have equal amount of surface platinum (Pts) in each reaction. 

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],... 

Scheme 1 a The [2 + 2] cycloaddition product of prochiral trans 2-butene with Si dimers of the Si(100) surface leads to chiral adsorbate complexes, b Hydrogenation of prochiral a-keto esters over platinum is a heterogeneously catalyzed reaction leading to chiral alcohols. Using cinchonidin as chiral modifier makes this surface reaction enantioselective. In a similar fashion, TA-modified nickel is a highly enantioselective catalyst for /3-keto ester hydrogenation... 

Another group of cinchona alkaloids lacks the 6 -mclhoxy group. Cinchonine (7) and its diastereomer cinchonidine (5) are commercially available and have been used as catalysts in the addition of zinc alkyls to aldehydes (Section D. 1.3.1.4.). Cinchonidine and dihydrocin-chonidine (6) were used to modify the surface of platinum catalysts used in the enantioselective reduction of z-oxo esters to a-hydroxy esters (see Section D.2.3.1. for such applications). Dihydrocinchonidine may conveniently be obtained by catalytic reduction of the double bond of cinchonidine, e.g., with nickel and hydrogen7. Cinchonidine also acts as a catalyst in the enantioselective formation of C-S and C-Se bonds by the addition of thiols and selenols to activated alkenes, such as 1-nitroalkenes (Sections D.5. and D.6.). Another application is the enantioselective protonation of kelenes (SectionD.2.I.). 

---