Cinchonidine adsorption

Cinchonidine adsorption on Pt/a-alumina depressed the rate by a factor of 1.3 -... 

In this Discussion the assumption is made that the adsorption of cinchonidine and cinchonine on Pd is similar to that on Pt [ 10]... 

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. 

The next question is, what physicochemical parameters may influence the adsorption-desorption equilibrium We suspected that the difference with different solvents may be due to the fact that the solubilities of cinchonidine in different solvents are different, so we tested the solubilities of cinchonidine in 54 solvents, and found that if the initially established adsorption-desorption equilibrium is perturbed, that is beeause the solubility of einehonidine in that flushing solvent is relatively big (e.g., 12 g/L in diehloromethane). On the other hand, the adsorption-desorption equilibrium is not perturbed by cyclohexane, because the solubility of cinchonidine in cyclohexane is quite small (0.46 g/L). By plotting the measured cinchonidine solubility versus solvent polarity reported in the literature, nice volcano-like correlations ean be identified (Figure 18) [66]. This example shows that some empirical observations in enantioselective hydrogenation may be traeed baek to basie physieoehemieal properties sueh as the solubility of cinchonidine and the polarity 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. 

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... 

Adsorption measurements with different supports or catalysts were carried out by using a mixed solution of cinchonidine and substrate 1 (4 mmol/1 for each) in solvent B. After stirring at 298 K for 1 h, the amount of each compound adsorbed was determined from the decrease in the concentration of the solution, The concentrations were monitored by HPLC. The mean crystallite sizes of Pd in the reduced catalysts were obtained from XRD line broadening. The total surface areas (Sbet) and the areas of Pd metal surface (Spa) were determined using the nitrogen adsorption at 77 K and by the CO chemisorption at 323 K, respectively. 

Pd modified by cinchona, vinca, or ephedra alkaloids is a moderately efficient catalyst but Pd is still the catalyst of choice for the enantioselective hydrogenation of olefins with a functional group in the a position [8,20]. Modification of Pd with cinchonidine is as simple as for Pt, but Pd requires a considerably lower substrate/ modifier ratio than Pt, probably because of weaker adsorption and/or partial degradation (hydrogenation) of the modifier during reaction. Another drawback is that the reactions are not accelerated but decelerated by the chiral modifier (by a factor of up to 140 [21]). This phenomenon can rationalize the moderate performance of chirally modified Pd. 

Figure 7.8.Different forms of adsorption of cinchonidine of the surface (D. Ferri, T. Btlrgi, A. Baiker, In situ ATR-IR study of the adsorption of cinchonidine on Pd/Al203 differences and similarities with adsorption on Pt/Al2C>3, Journal of Catalysis, 210 (2002) 210).

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