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/Al2O3 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)-CDH6} 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)-CDH6, 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 CDH6 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. [Copyright &y& Elsevier]