Adsorption Of Cinchonidine Research Articles

Asymmetric Hydrogenation of α-Amino Ester Probed by FTIR Spectroscopy

Asymmetric hydrogenation reaction of dehydro-α-amino acid (i.e., α-amino ester) over cinchonidine (CD) modified Pd catalyst has been studied by an array of in situ infrared spectroscopic methods, including transmission, diffuse reflectance (DR), and attenuated total reflectance (ATR). Transmission FTIR spectra probed the hydrogenation reaction process, revealed OH–O and NH–N hydrogen bonding interactions between the adsorbed CD and during the reaction. DR and ATR spectra of the hydrogenation reaction under different conditions, which are consistent with but slightly different from the transmission spectra, evidenced the successful hydrogenation of the compound. The incorporation of DR and microfluidics flow-through design allowed us to investigate the adsorption of CD on the Pd surface efficiently. The results revealed that the N-bonded CD on Pd surface in a tilted configuration had increased abundance on the Pd surface with high coverage. These valuable insights provided an image of the reaction pathway ...

An In Situ Spectroscopic Study of Prochiral Reactant–Chiral Modifier Interactions on Palladium Catalyst: Case of Alkenoic Acid and Cinchonidine in Various Solvents

In situ attenuated total reflection infrared (ATR-IR) spectroscopy has been used to study the adsorption of a model α,β-unsaturated carboxylic acid (2-methyl-2-pentenoic acid) and chiral modifier (cinchonidine), as well as their intermolecular interactions, on Pd/Al2O3 in typical polar (methanol) and nonpolar (dichloromethane) solvents. It has been found that the solvent has essentially no effect on cinchonidine adsorption. The carboxylic acid tends to adsorb on surface through bridging bidentate carboxylate in MeOH instead of monomer or dimer species, which is prevalent in the case of CH2Cl2. Moreover, an acid–cinchonidine complex prefers to form in a substrate to modifier ratio of 1:1, regardless of whether it is in the bulk solution or adsorbed on the surface. Thus, direct spectroscopic observation of an important intermediate in this catalytic system has been made.

Asymmetric Hydrogenation of Ethyl Pyruvate on Chirally Modified Pt Catalysts Supported on Al2O3@FDU-14 Mesoporous Composites

A series of Al 2 O 3 @FDU-14 composites with different alumina loadings were prepared using a solid state grinding method. The 4% Pt catalysts supported on Al 2 O 3 @FDU-14 were prepared by impregnation using H 2 PtCl 6 . X-ray diffraction, transmission electron microscopy, and nitrogen adsorption characterization of the Pt/Al 2 O 3 @FDU-14 catalysts indicated that the mesoporous features of the FDU-14 mesopolymer were retained. The Pt/Al 2 O 3 @FDU-14 catalysts were used in the asymmetric hydrogenation of ethyl pyruvate after they were chirally modified with cinchonidine. For Al 2 O 3 loading from 5% to 15%, Al 2 O 3 was uniformly dispersed inside the mesoporous channels of FDU-14 host. Ethyl pyruvate conversion and their ee values increased with increasing alumina loading, with a highest ee value of 80%. It was suggested that the alumina coating inside the mesoporous channels of FDU-14 decreased the surface electronic density of the FDU-14 mesopolymer so that cinchonidine can be easily adsorbed on the surface of Pt/Al 2 O 3 @FDU-14 catalysts. As compared with chirally modified Pt/FDU-14 catalysts, chirally modified Pt/Al 2 O 3 @FDU-14 catalysts gave higher ee values. 以碳包铁纳米晶 (Fe@C) 为载体, 采用浸渍法制备了一种磁可分离的 Pd/Fe@C 催化剂, 并运用 X 射线荧光光谱、透射电镜、X 射线衍射和 X 射线光电子能谱对催化剂进行了表征. 结果表明, 纳米 Pd 颗粒的粒径分布在 4∼10 nm, 平均粒径约为 7 nm, Pd 物种以 Pd 0 为主, 其 Pd 3 d 5/2 结合能为 335.6 eV. 将该催化剂应用于苯甲醇选择性氧化反应, 考察了催化剂用量、溶剂、温度对反应的影响以及催化剂的循环使用性能. 结果表明, 催化剂可使苯甲醇高选择性生成苯甲醛, 在催化剂用量 (摩尔分数) 为 0.5%、乙腈为溶剂、80 o C 常压 O 2 条件下, 苯甲醇的氧化反应效果最佳 (苯甲醇转化率为 96%, 苯甲醛选择性为 95%). 催化剂易于磁分离和回收, 循环使用 4 次仍保持较佳的催化性能. Cinchonidine modified Pt/Al2O3@FDU-14 gave a higher ee value than a Pt/FDU-14 catalyst for the chiral hydrogenation of ethyl pyruvate, which was due to cinchonidine adsorption on the alumina coating.

Substrate-controlled adsorption of cinchonidine during enantioselective hydrogenation on platinum

Abstract It is commonly accepted that the origin of enantioselection on chirally modified metals is the control of the adsorption and reactivity of the substrate by the chiral environment of the modifier. Here, we provide the first experimental evidence to a mutual process, namely, that the substrate controls the adsorption and reactivity of cinchonidine (CD) on the metal surface. Our approach is to follow the competing hydrogenation of the quinoline ring, the anchoring moiety of CD, in the presence or absence of an activated ketone substrate. On Pt/Al 2 O 3 in the weakly interacting solvent toluene, CD (and 10,11-dihydro-CD) favors a C(4′) – pro( S ) adsorption geometry and saturation of the heteroaromatic ring gives 1′,2′,3′,4′( S ),10,11-hexahydro-CD {( S )-CDH 6 } in excess. Addition of methyl benzoylformate, ketopantolactone, or ethyl pyruvate inverts the dominant conformation of CD to C(4′) – pro( R ) as indicated by the major product ( R )-CDH 6 , and even the rate is higher by about 30% (“inverse ligand acceleration”). Acetic acid that interacts strongly with CD exerts a similar effect on quinoline hydrogenation. In contrast, the product α-hydroxyester interacts weakly with CD, decelerates the hydrogenation of the quinoline ring and the de of CDH 6 depends on the chirality of the α-hydroxyester. These unexpected observations provide a fundamentally new insight into the complexity of the surface conformation of CD and the origin of high enantioselectivity on cinchona–modified Pt.

Surface Processes Occurring on Rh/Alumina during Chiral Modification by Cinchonidine: An ATR-IR Spectroscopy Study

Cinchona alkaloids are frequently used for chiral modification of supported noble metal catalysts employed in heterogeneous enantioselective hydrogenation. In order to gain molecular insight into the surface processes occurring at the metal/liquid interface, cinchonidine (CD) adsorption on vapor-deposited Rh/Al2O3 films has been studied in the presence of solvent and hydrogen by means of attenuated total reflection infrared (ATR-IR) spectroscopy. The spectrum of CD adsorbed on Rh exhibited two dominant signals at 1593 and 1511 cm(-1), which are characteristic of a surface species having a quinoline ring tilted with respect to the metal. Interestingly, no adsorbed modifier in the flat geometry (quinoline parallel to the metal plane) was observed. During desorption, these signals vanished, and a new prominent signal appeared at 1601 cm(-1) which belongs to a species with the quinoline ring hydrogenated on the heteroaromatic side. Concentration-dependent experiments and the reversibility of the observed phenomenon indicate that CD was readily hydrogenated to 1',2',3',4',10,11-hexahydrocinchonidine (CDH(6)) on Rh. The ATR-IR spectra also reveal that the flat species was indeed immediately hydrogenated when CD was provided from solution, and the only visible adsorbed species was the tilted species, which displaced the hydrogenation product from the metal surface. In the absence of dissolved CD, during desorption, the tilted species was converted to the flat species and rapidly hydrogenated. The hydrogenation product was stable on the metal surface only in the absence of CD. Therefore, the adsorption strength of the different species is as follows: flat >> tilted > CDH(6). Evidence for the formation of the flat species and its role as an intermediate to the hydrogenation product is given by an experiment in which CD was adsorbed in the absence of dissolved hydrogen after surface cleaning. The adsorption and hydrogenation of CD on Rh deviate significantly from that observed earlier on Pt and Pd under similar conditions, where the flat species could be observed even in the presence of hydrogen. This difference is attributed to the weaker interaction and lower hydrogenation rate occurring on Pt and Pd.

Towards the engineering of enantioselective properties of supported platinum catalysts

Understanding the sub-molecular structure of conformationally complex adsorbed molecules is still a difficult task for experimentalists interested in the structure of modified surfaces, therefore first principles calculations can be a fundamental tool for the investigation of structural details otherwise impossible to identify. Density functional theory (DFT) calculations of the adsorption of cinchonidine (CD) and O-phenyl-cinchonidine (OPhCD) have been performed, using large metal clusters to simulate the metal surface and a zero order regular approximation (ZORA) Hamiltonian to account for relativistic effects due to the heavy nuclei involved. The local geometry of chiral surface sites formed by CD was investigated in detail and discussed in relation to the reaction of enantioselective hydrogenation of activated ketones on cinchona modified platinum. Also, the relevant conformations of OPhCD were investigated and the resulting structure of the chiral sites is discussed and compared to those obtained for the parent alkaloid CD. The adsorption behavior on platinum of a series of substituted anisoles was also investigated, in order to evince the effect that phenyl substitution might have on the relative proportion of surface conformers in substituted OPhCD ethers, that have been shown to possess interesting enantioswitching properties when used as surface modifiers in the enantioselective hydrogenation of activated ketones on cinchona alkaloid modified platinum. A correlation between conformational distribution of the modifiers and the selectivity of the catalyst is proposed.

Time-Lapse STM Studies of Diastereomeric Cinchona Alkaloids on Platinum Metals

The adsorption of cinchonidine (CD) and cinchonine (CN) on Pt(111) and Pd(111) single crystals has been investigated by means of scanning tunneling microscopy (STM) in an ultrahigh vacuum system. In time-lapse series the mobilities of different adsorption species have been determined on a single molecule basis and with varying hydrogen background pressures in the system. The diastereomeric cinchona alkaloids, CD and CN, which are widely used as chiral modifiers of platinum group metals in catalytic enantioselective hydrogenation, showed similar adsorption modes and diffusion behavior on Pt(111), except that the flatly adsorbed CN molecules which were free (not in a dimer/cluster) were significantly more mobile than their CD analogues. CD adsorbed on Pd(111) showed similar adsorption modes as observed on Pt(111) but at considerably higher mobility of the flatly absorbed species already in the low-pressure region. The observed adsorption behaviors are discussed in the context of independent ATR-IR measurements and theoretical calculations. Special emphasis is put on the nonlinear effect observed in hydrogenation reactions with CD/CN mixtures. Our observations corroborate that this effect is mainly a consequence of the different adsorption strengths of CD and CN on Pt.

Cinchonidine Adsorption on Gold and Gold-Containing Bimetallic Platinum Metal Surfaces: An Attenuated Total Reflection Infrared and Density Functional Theory Study

Adsorption of cinchonidine on monometallic Au and bimetallic Pt-Au and Pd-Au thin model films prepared by physical vapor deposition has been investigated with attenuated total reflection infrared (ATR-IR) spectroscopy. On Au the alkaloid forms an adsorbed layer that shows higher stability against desorption than the corresponding adsorption on Pt. In this adsorption layer the intermolecular interactions dominate over metal-adsorbate interactions as indicated by the absence of the spectroscopic features attributed to strongly flat adsorbed species. This behavior is further supported by Density Functional Theory (DFT) calculations indicating that flat and tilted orientations of the quinoline ring have comparable adsorption energy on Au but lower (7-10 kcal/mol) compared to adsorption on Pt (ca. 40 kcal/mol). As a consequence, the creation of a metal surface with isolated chiral sites is prevented by formation of an adsorbed structure formed by intermolecularly bound cinchonidine molecules on Au. While the binding to Pt is due to the formation of sigma-bonds to surface atoms, such aggregates are bound to Au mainly by van der Waals forces. Given this different nature of bonding of cinchonidine to Au and Pt, addition of Au to Pt and Pd films could be used to probe the changes of fractional coverage of the different adsorbed species of cinchonidine on the platinum metals. It is demonstrated that the lowering of the domain size of the platinum group metal by Au can simulate the effect of particle size on the distribution of the surface conformations of the alkaloid on a metal surface.

Adsorption and surface mobility of cinchonidine on Pt(111) studied by STM

The adsorption of cinchonidine (CD) on Pt(111) has been investigated by means of STM in the presence and absence of hydrogen. The time-resolved studies revealed that the mobility of adsorbed CD molecules depends on the hydrogen pressure. At low hydrogen pressure the CD molecules are rather immobile, whereas considerable mobility is observed at higher hydrogen pressure. Based on the different adsorption geometries and the surface mobility three different species could be distinguished and their dynamic behaviour was investigated on a single molecule level.

In situ Characterization of the Adsorption of Cinchona Chiral Modifiers on Platinum Surfaces

AbstractFor Abstract see ChemInform Abstract in Full Text.

Adsorption of cinchonidine on platinum: a DFT insight in the mechanism of enantioselective hydrogenation of activated ketones

The adsorption of cinchonidine on platinum has been calculated with relativistically corrected density-functional theory, by first studying the interaction of the 1( S)-(4-quinolinyl)ethanol with a platinum cluster of 31 metal atoms, and by successive addition and separate optimization of the quinuclidine moiety. The conformations of the alkaloid on the surface were analyzed and their possible interactions with a surface chemisorbed methylpyruvate and acetophenone are discussed. A chiral space that is able to selectively accommodate surface enantiomers and to promote their rapid hydrogenation in a ligand-accelerated fashion has been determined. The role of the O-alkylation of the alkaloid in the modulation of enantioselectivity has been rationalized within the new interaction model.

Continuous Enantioselective Hydrogenation of Ethylbenzoylformate over Pt/Al2O3Catalyst: Bed Dilution Effects and Cinchonidine Adsorption Study

Depending on the reaction medium (toluene or acetic acid) striking differences were observed in the development of enantiomeric excess with time-on-stream in continuous enantioselective hydrogenation of ethylbenzoylformate on cinchonidine-modified Pt/Al2O3, which correlated well with the amount of adsorbed cinchonidine on alumina and silica from acetic acid and toluene solutions. Maximally 93% enantiometric excess (ee) of (R)-ethyl mandelate was obtained.

In situ characterization of the adsorption of cinchona chiral modifiers on platinum surfaces

Graphic The adsorption of cinchona chiral modifiers on platinum surfaces has been characterized with surface-sensitive techniques. The uptakes of quinoline and lepidine, models for the aromatic moiety of the cinchona, were first studied on Pt(1 1 1) single crystal surfaces and under ultrahigh vacuum by reflection–absorption infrared spectroscopy (RAIRS). Adsorption at low (∼100 K) temperatures is molecular and without any preference for a distinct molecular geometry. Temperature programmed desorption (TPD) experiments indicated extensive dehydrogenation starting at about 250 K, and infrared data obtained at room temperature corroborated the conversion of some of the aromatic CH bonds to a more aliphatic environment. An in situ RAIRS characterization of the adsorption of cinchonidine from carbon tetrachloride solutions onto a polycrystalline platinum substrate proved that in hydrogen-treated solvents the decomposition of the adsorbate is inhibited, so that the cinchona retains its molecular character on the surface at room temperature. Three adsorption regimes were identified depending on the concentration of cinchonidine in solution, namely, no observable adsorption at low concentrations, bonding via a flat-lying quinoline moiety in the intermediate concentration range, and a tilted adsorption geometry near saturation. The optimum activity and enantioselectivity reported previously in catalytic processes for intermediate concentrations can then be associated with the orientation of the aromatic ring parallel to the surface plane. In a separate study, it was determined that hydrogen plays a unique role in cinchonidine adsorption, initially conditioning the surface for the uptake (presumably by reduction and removal of surface contaminants), and later hydrogenating the quinoline fragment of the modifier and facilitating its desorption. Finally, different solvents were shown to behave differently in these systems. Specifically, adsorbed cinchonidine remains on the platinum surface after flushing with non-polar solutions such as cyclohexane, but can be easily removed by more polar solvents such as dichloromethane. This behavior correlates well with both the solubility of cinchonidine and the performance of the cinchona/platinum catalyst as a function of the nature of the solvent.

Surface Raman characterization of cinchonidine-modified platinum in ethanol: effects of liquid-phase concentration and co-adsorbed hydrogen

Surface-enhanced Raman spectroscopy has been utilized to probe the adsorption of the chiral modifier cinchonidine on platinum in ethanol at 25 °C. The modifier is found to be strongly and irreversibly adsorbed through the quinoline portion of cinchonidine by π-bonding with the Pt surface. The degree of tilt of the quinoline group with respect to the surface increases as cinchonidine concentration increases. Addition of hydrogen appears to result in hydrogenation of the vinyl group on cinchonidine to form 10,11-dihydrocinchonidine, resulting in a decrease in the tilt of the aromatic ring. Surface-enhanced Raman spectroscopy has been utilized to probe the adsorption of the chiral modifier cinchonidine on polycrystalline platinum. Surfaces were prepared by electrodeposition of ultrathin platinum films onto roughened gold, which provided stable and intense SERS activity for performing these studies. The vibrational properties of adsorbed cinchonidine on platinum in ethanol solutions at 25 °C have been probed in situ. Based on the appearance and trends in the strong ring breathing mode at 1357 cm −1 , the modifier is strongly and irreversibly adsorbed through the quinoline portion of cinchonidine by π-bonding with the Pt surface. Furthermore, analysis of both in-plane and out-of-plane vibrations suggests that the aromatic group of cinchonidine is tilted with respect to the surface. The degree of tilt appears to increase as concentration increases over the range of cinchonidine liquid-phase concentrations examined here (0.03–1.2 mM). The presence of an H-abstracted α-quinolyl species is also tentatively suggested by the appearance of a downshifted aromatic CC stretching band not associated with adsorbed cinchonidine. These findings are largely consistent with what has been observed previously for cinchonidine adsorption on Pt from different solvents using in situ infrared spectroscopy. Addition of hydrogen into the system results in enhanced Raman scattering from adsorbed cinchonidine. Comparisons with SERS measurements of 10,11-dihydrocinchonidine adsorption in ethanol suggest that the H 2 -induced changes likely result from hydrogenation of the vinyl group on cinchonidine. The data suggest a more flat orientation of this species, resulting from increased interaction of the aromatic ring structure with the surface. The results are consistent with kinetic studies of cinchonidine hydrogenation, which have implied much stronger adsorption of 10,11-dihydrocinchonidine as compared with cinchonidine.

Vibrational Band Assignments for the Chiral Modifier Cinchonidine: Implications for Surface Studies

Vibrational assignments for the chiral molecule cinchonidine using a combination of experimental vibrational spectroscopic measurements and ab initio computational methods are reported. Several bands in both the IR and Raman spectra are identified as useful in providing information regarding the mode of adsorption of cinchonidine on metal surfaces, a system of great relevance to enantioselective catalysis. In particular, vibrational modes associated with the quinoline group can be used to diagnose the degree of tilt of the molecule with respect to the plane of the substrate. The use of this information for the study of cinchonidine adsorption on polycrystalline platinum with both reflection−absorption infrared spectroscopy (RAIRS) and surface-enhanced Raman spectroscopy (SERS) is discussed. Adsorption geometries for cinchonidine on platinum at low and high concentrations are reported.

The influence of dissolved gases on the adsorption of cinchonidine from solution onto Pt surfaces: an in situ infrared study

The influence of different dissolved gases on the adsorption of cinchonidine from CCl 4 solutions onto polycrystalline platinum surfaces has been studied by in situ reflection–absorption infrared spectroscopy (RAIRS). It was observed that Ar, N 2, O 2, air, or CO 2 neither enhances the adsorption of cinchonidine nor damages cinchonidine adlayers once they have formed on the surface. On the other hand, H 2 plays a unique role, initially facilitating the uptake of cinchonidine, but later removing some of the resulting adsorbed cinchonidine from the platinum surface. It was also found that CO is a strong inhibitor, significantly retarding the adsorption of cinchonidine. Adsorbed CO can be removed by H 2 dissolved in the cinchonidine solution, a process that also boosts the adsorption of cinchonidine. O 2 can remove the surface adsorbed CO as well, but without facilitating cinchonidine adsorption. The cleanliness of Pt surfaces at different stages of the experiment was probed by CO adsorption. Possible correlations between the RAIRS results from the present study and reported data from catalytic studies are discussed.

Asymmetric hydrogenation on platinum: nonlinear effect of coadsorbed cinchona alkaloids on enantiodifferentiation

Prominent nonlinear effects in enantioselectivity were observed with a transient technique when ethyl pyruvate was hydrogenated over Pt/Al2O3 in the presence of two cinchona alkaloids, which alone afford the opposite enantiomers of ethyl lactate in excess. The changes in reaction rate and ee, detected after injection of the second alkaloid, varied strongly with type and amount of the alkaloid, and with the order of their addition to the reaction mixture. For example, under ambient conditions in acetic acid cinchonidine (CD) afforded 90% ee to (R)-ethyl lactate and addition of equimolar amount of quinidine (QD) reduced the ee to (R)-ethyl lactate only to 88%, though QD alone provided 94% ee to (S)-lactate in a slightly faster reaction. The stronger adsorption of CD on Pt in the presence of hydrogen and acetic acid was proved by UV–vis spectroscopy. The different adsorption strengths result in an enrichment of CD on the Pt surface and also in a crucial difference in the dominant adsorption geometries. CD is assumed to adsorb preferentially via the quinoline rings laying approximately parallel to the Pt surface. In this position it can interact with ethyl pyruvate during hydrogen uptake and control the enantioselectivity. The weaker adsorbing QD adopts mainly a position with the quinoline plane being tilted relative to the Pt surface and these species are not involved in the enantioselective reaction. Competing hydrogenation of the alkaloid, and steric and electronic interactions among the adsorbed species, can also influence the alkaloid efficiency and the product distribution. Hydrogenation of the quinoline rings at low alkaloid concentration resulted in unprecedented swings in the enantiomeric excess.

In Situ ATR–IR Study of the Adsorption of Cinchonidine on Pd/Al 2O 3: Differences and Similarities with Adsorption on Pt/Al 2O 3

Pd/Al 2O 3 model catalysts have been prepared by physical vapour deposition and characterised by means of XPS, STM, and in situ ATR–IR spectroscopy. Morphological changes in the Pd film induced by dissolved hydrogen leads to enhanced infrared absorption and could be followed with both STM measurements and IR spectroscopy. Adsorption of CO, pyridine, quinoline, 2-methylquinoline, and the chiral auxiliary cinchonidine has been studied in situ at 283 K in CH 2Cl 2 solvent. Two different species have been observed for cinchonidine on Pd. One is oriented with the quinoline moiety nearly parallel to the Pd surface, likely through the π-system, whereas in the second the σ-bonding through the N lone pair prevails and induces a tilting of the ring with respect to Pd. No indication of the presence of α-quinolyl species has been found, in contrast to adsorption on Pt/Al 2O 3 catalysts. Compared to adsorption on Pt, cinchonidine is more weakly bound on Pd under hydrogenation conditions. Also, the relative stability of the π- and N lone pair-bonded species is different for the two metals, with the π-bonded species being relatively more stable on Pt. Similarities and differences found in the adsorption of the chiral modifier on the two metals are discussed and traced mainly to the different d-orbital diffuseness of Pd and Pt.

Adsorption of cinchonidine on platinum: an electrochemical study

The adsorption of cinchonidine (CD) on polycrystalline platinum surfaces in 0.5 mol dm −3 H 2SO 4 has been investigated. It has been found that adsorption is irreversible, desorption takes place at relatively high potentials simultaneously with the oxidation of CD molecules. According to our results CD covers only a part of the Pt surface and maximal coverage is influenced by the surface structure. Some assumptions for the site requirement of an adsorbed CD molecule have also been made.

An in situ attenuated total reflection infrared study of a chiral catalytic solid-liquid interface: cinchonidine adsorption on pt.

An in situ attenuated total reflection study of the chiral solid-liquid interface created by cinchonidine adsorption on a Pt/Al(2)O(3) model catalyst is presented. Experiments were performed in the presence of dissolved hydrogen, that is under conditions used for the heterogeneous enantioselective hydrogenation of alpha-functionalized ketones. Cinchonidine adsorbs via the quinoline moiety. The adsorption mode is coverage dependent and several species coexist on the surface. At low concentration (10(-6)M) a predominantly flat adsorption mode prevails. At increasing coverage two different tilted species, alpha-H abstracted and N lone pair bonded cinchonidine, are observed. The latter is only weakly bound and in a fast dynamic equilibrium with dissolved cinchonidine. At high concentration (10(-4)-10(-3) M) all three species coexist on the Pt surface. A slow transition from an adsorbate layer with a high fraction of alpha-H abstracted cinchonidine to one with a high fraction of N lone pair bonded cinchonidine is observed with the cinchonidine concentration being the driving force for the process. The reverse transition in the absence of dissolved cinchonidine is fast. Cinchonidine competes with solvent decomposition products for adsorption sites on the Pt, which may contribute to the observed solvent dependence of the heterogeneous enantioselective hydrogenation of ketones by cinchonidine-modified Pt.