The Critical Role of Tilted Cinchona Surface Species for Enantioselective Hydrogenation

Abstract

On chirally-modified metal catalysts enantioselectivity is controlled by the molecular orientation of adsorbates at the chiral catalytic solid-liquid interface. To gain information on the surface coverage and to assess the enantioselecting ability of chiral modifiers depending on their molecular orientation we employed an operando spectroscopic technique using attenuated total reflection-IR and UV-Vis spectroscopies. Our study reveals that the enantiodifferentiation of the cinchona chiral modifier cinchonine varies significantly depending on its adsorption mode. In contrast to the prevailing mechanistic models, which imply that the most stable surface species is responsible for enantiodifferentiation, our results show that less stable cinchona species with a tilted aromatic ring anchor provide significantly stronger enantioselective control. As a consequence the so-called anchoring moiety of the chiral modifier exceeds the mere function of adsorptive anchoring and its molecular orientation has to be also taken into account in explaining the observed enantioselectivity of chirally-modified metals.

Molecular beam/infrared reflection-absorption spectroscopy apparatus for probing heterogeneously catalyzed reactions on functionalized and nanostructured model surfaces

“…A number of successful cases of such ligand-directed heterogeneous catalysis were reported recently, in which the surfaces were functionalized with organic ligands to control either chemo- [13][14][15][16] or enantioselectivity. 5,11,17,18 Despite the impressive progress in this field, the development of practically relevant powdered heterogeneous catalysts is frequently relying on a trial and error method. To develop a more rational approach for the synthesis of new functionalized catalytic materials, the atomistic understanding of mechanisms governing liganddirected heterogeneous catalysis must be significantly improved.…”

“…Finally, the movable sample plate can be attached to the sample holder using magnetic transfer rods [VAB, MDS 40-1200, VAB, MDS 40-700, see Fig. 3(a) (7) and (17)] containing an adaptor with rotating locking mechanism which is attached to the clamp of the movable sample plate (14). A photograph of the movable sample plate inserted into the sample holder of the manipulator in the reaction and preparation chambers is shown in Fig.…”

“…To ensure temperature measurements on the sample only, two ceramic plates are inserted in between the sample holder adaptor (10) and the thermocouple sheets(12) and(13). Finally, the movable sample plate can be attached to the sample holder using magnetic transfer rods [VAB, MDS 40-1200, VAB, MDS 40-700, see Fig.3(a)(7) and(17)] containing an adaptor with rotating locking mechanism which is attached to the clamp of the movable sample plate(14). A photograph of the movable sample plate inserted into the sample holder of the manipulator in the reaction and preparation chambers is shown in Fig.8(c) with the same labeling of the components shown in Figs.…”

Partial Hydrogenation of Unsaturated Carbonyl Compounds: Toward Ligand-Directed Heterogeneous Catalysis

“…There is a growing number of reports in the catalytic literature on heterogeneously catalyzed reactions proceeding on ligand-modified surfaces. ,,− A large body of these reports is related to enantioselective heterogeneous catalysis on chirally modified surfaces. In this type of catalysis, the surfaces are modified by adsorption of chiral adsorbates, exhibiting enantiospecific interaction with a pro-chiral reactant, which directs the catalytic process toward formation of only one enantiomeric form of the product.…”

“…(ii) The second way is adsorption of chiral modifiers, which form 1:1 complexes with the prochiral reactants. In these chiral complexes, the prochiral reactant has a specific adsorption geometry with respect to the underlying metal substrate, which allows only asymmetric chemical transformations resulting in enantiopure product. , One of the first reported examples of such type of chiral interactions is enantioselective hydrogenation of esters over chincona-modified Pt catalysts. ,, Recently, impressive progress was made by a number of groups studying the mechanisms of enantioselective processes at the atomistic level; some of these studies are summarized in recent reviews. ,,, To implement this technology into the rational design of new catalytic materials, more atomistic-level insights into the surface phenomena directing adsorbate–adsorbate interaction need to be gained in microscopic-level studies.…”