Chiral modification of rh and pt surfaces: Effect of rotational flexibility of cinchona-type modifiers on their adsorption behavior

The adsorption behavior of cinchona-type chiral modifiers (beta-isocinchonine, cinchonidine, and cinchonine) on model rhodium and platinum catalysts has been studied using attenuated total reflection infrared (ATR-IR) spectroscopy and density functional theory (DFT). The ATR-IR studies performed under conditions closely resembling those encountered during enantioselective hydrogenation as well as in the absence of hydrogen revealed significant differences in the adsorption mode of the modifiers, depending on the rotational flexibility of the modifier, the platinum group metal, and the presence of hydrogen. The more rigid beta-isocinchonine showed preferential tilted adsorption on both metals, independent of the presence or absence of hydrogen. Cinchonidine and cinchonine were adsorbed on Pt predominantly with the quinoline ring nearly parallel to the surface, irrespective of the presence or absence of hydrogen, whereas on Rh in the presence of hydrogen only tilted species were observed. The latter behavior is attributed to the fast hydrogenation of the aromatic anchoring group (quinoline) on Rh that interferes with the adsorption process. DFT calculations confirmed that the energy difference between tilted and adsorbed species decreases in the case of the beta-isocinchonine. This combined spectroscopic and theoretical investigation indicates a greater bias toward the tilting of the anchoring group in the case of beta-iCN as compared to the natural occurring alkaloids cinchonidine and cinchonine. Such behavior explains the greater stability toward quinoline ring saturation observed for beta-isocinchonine on rhodium-supported catalysts during enantioselective hydrogenation. This study thus helps define the delicate equilibrium between formation of chiral surface sites and their stability in strongly reducing conditions in terms of the structure and consequent adsorption behavior of the modifier molecule, as a function of the metal catalyst of choice.