O-Phenylcinchonidine (PhOCD) is known to efficiently induce inversion of enantioselectivity with respect to cinchonidine (CD) in the enantioselective hydrogenation of various activated ketones on Pt/Al₂O₃. To understand the origin of the switch of enantioselective properties of the catalyst, the adsorption of PhOCD has been studied by in situ ATR-IR spectroscopy, in the presence of organic solvent and dissolved hydrogen, i.e., under conditions used for catalytic hydrogenation. The adsorption structures and energies of the anchoring group of CD and PhOCD were calculated on a Pt 38 cluster, using relativistically corrected density functional theory (DFT). Both approaches indicate that both modifiers are adsorbed via the quinoline ring and that the spatial arrangement of the quinuclidine skeleton is critical for the chiral recognition. New molecular level information on the conformation of CD relative to PhOCD adsorbed on a surface is extracted from the ATR spectra and supported by DFT calculations. The result is a clearer picture of the role played by the phenyl group in defining the chiral space created by the modifiers on Pt. Moreover, when CD was added to a pre-equilibrated adsorbed layer of PhOCD, a chiral adsorbed layer was formed with CD as the dominant modifier, indicating that CD adsorbs more strongly than PhOCD. Conversely, when PhOCD was added to preadsorbed CD, no significant substitution occurred. The process leading to nonlinear effects in heterogeneous asymmetric catalysis has been characterized by in situ spectroscopy, and new insight into a heterogeneous catalytic R−S switch system is provided.

The enantioselective hydrogenation of ring-substituted acetophenones that possess no functional group in the α-position to the keto group represents the latest extension of the application range of the Pt–cinchona system. The influence of the type of solvent, pressure, temperature, and modifier/substrate/Pt molar ratios was investigated in the hydrogenation of 3,5-di(trifluoromethyl)acetophenone. Modification of a 5 wt% Pt/Al₂O₃ catalyst by cinchonidine (CD) afforded the corresponding (S)-1-phenylethanol (69.5% ee). Working in strongly polar solvents, addition of trifluoroacetic acid in a weakly polar solvent, and replacing CD by its ether derivatives resulted in the inversion of enantioselectivity. Addition of CD or any of its derivatives always led to a lower reaction rate, contrary to the generally observed rate acceleration in the hydrogenation of α-functionalized activated ketones over the same catalyst system. Another fundamental difference to the hydrogenation of α-functionalized activated ketones is that both the quinuclidine N and the OH functions of CD influence the stereochemical outcome of the reaction, as clarified by using O- and N-substituted derivatives of CD. Ab initio calculations confirmed these remarkable mechanistic differences. Inversion of enantioselectivity in the presence of strongly polar and acidic solvents is attributed to special interactions with the OH function of CD, and to the formation of a CD–acid ion pair, respectively. A possible explanation for the moderate ee's in the hydrogenation of ring-substituted acetophenones is that a reaction pathway without involvement of the OH function of CD is also feasible. This competing pathway is even faster and provides low ee to the opposite enantiomer.

The adsorption of several ketones interesting for the enantioselective hydrogenation on cinchona-modified platinum has been modeled using relativistically corrected density functional theory. Two metal clusters, containing 19 and 31 Pt atoms, respectively, have been used to model a Pt(111) surface. The two adsorption modes η¹ and η² have been described, and their importance for the mechanism of hydrogenation has been pointed out. The effect of an ester group in α position and of α-fluorination of a ketone on its adsorption has been studied, and an explanation for the reactivity enhancement due to the ketone substitution has been proposed.

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.

The origin of the rate acceleration in enantioselective hydrogenation of α-functionalised ketones over cinchona alkaloid modified platinum has been studied using a combined experimental and theoretical approach, and the rate acceleration is traced to a lowering of the energy of the carbonyl π orbitals in the diastereomeric complex formed between reactant and modifier.

The relation between the electronic structure of α-substituted ketones and their reactivity in the racemic and enantioselective platinum-catalyzed hydrogenation has been investigated using a combined theoretical and experimental approach. A correlation between the keto carbonyl orbital energy and the hydrogenation rate has been found, which rationalizes the effect of the substituent on the rate of hydrogenation. The uncovered relationship between the keto carbonyl orbital energy and the hydrogenation rate provides a rational explanation for the often observed rate acceleration that occurs when cinchona-modified platinum is used as a enantioselective hydrogenation catalyst. The previously suggested model for enantiodiscrimination based on the different stability of the diastereomeric complexes formed between the reactant and the cinchona modifier is discussed in the light of the new kinetic findings.

Vibrational circular dichroism (VCD) spectra of the chiral modifiers cinchonidine, an alkaloid, and (R)-2-(pyrrolidin-1-yl)-1-(1-naphthyl)ethanol (PNE) were measured and simulated. For cinchonidine independent information from NMR investigations on the distribution of conformers was used to simulate VCD spectra from calculated spectra of the individual conformers. Agreement with experiment is reasonably good. For the structurally similar synthetic modifier PNE VCD spectra show that an open conformer predominates in solution. The difference between the most stable conformers of cinchonidine and PNE in solution is the intramolecular hydrogen bond found in the latter, which forms due to the enhanced flexibility of the pyrrolidinyl moiety in PNE as compared to the quinuclidine moiety in cinchonidine. The similar enantiodifferentiating power of cinchonidine and PNE as chiral modifiers in the heterogeneous enantioselective hydrogenation of ethyl pyruvate indicates that the rigidity of this part of the molecule is not a prerequisite for enantioselection. It is furthermore shown that binding of a non-chiral carboxylic acid to the alkaloid induces VCD in vibrations associated with the acid. Observation of this induced VCD allows probing of the chiral binding site.

The previously proposed model for reactant–modifier interaction in the enantioselective hydrogenation of activated carbonyl compounds over platinum chirally modified by cinchona alkaloids has been extended to platinum modified by synthetic pyrrolidinyl–naphthyl–ethanol modifiers. As in the case of cinchonidine, the most used modifier, the model predicts enantiomeric excess in nearly quantitative agreement with experiment. Excellent agreement is achieved despite the fact that structural assumptions had to be made and the platinum surface was not explicitly taken into account. The one-to-one interaction between modifier and reactant was calculated at the ab initio level. A comparison of the results for different modifiers leads to the conclusion that steric repulsion caused by the anchoring group plays an important role in the enantiodifferentiating interaction. The favoured formation of the (R)-product is traced to the fact that the pro-(S) complex leading to the (S)-product upon hydrogenation is more destabilised due to repulsive interaction than the pro-(R) complex. The model calculations are a useful tool for designing effective modifiers and for gaining insight into the mechanism of enantiodifferentiation.