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 spectroscopy. 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 ...