An in situ attenuated total reflection infrared study of a chiral catalytic solid-liquid interface: Cinchonidine adsorption on Pt

An in situ attenuated total reflection study of the chiral solid-liquid int erface created by cinchonidine adsorption on a Pt/Al2O3 model catalyst is p resented. Experiments were performed in the presence of dissolved hydrogen, that is under conditions used for the heterogeneous enantioselective hydro genation of a-functionalized ketones. Cinchonidine adsorbs via the quinolin e moiety. The adsorption mode is coverage dependent and several species coe xist on the surface. At low concentration (10(-6)M) a predominantly flat ad sorption mode prevails. At increasing coverage two different tilted species , alpha -H abstracted and N lone pair bonded cinchonidine, are observed. Th e latter is only weakly bound and in a fast dynamic equilibrium with dissol ved cinchonidine. At high concentration (10(-4)-10(-3)M) all three species coexist on the Pt surface. A slow transition from an adsorbate layer with a high fraction of a-H abstracted cinchonidine to one with a high fraction o f N lone pair bonded cinchonidine is observed with the cinchonidine concent ration being the driving force for the process. The reverse transition in t he absence of dissolved cinchonidine is fast. Cinchonidine competes with so lvent decomposition products for adsorption sites on the Pt, which may cont ribute to the observed solvent dependence of the heterogeneous enantioselec tive hydrogenation of ketones by cinchonidine-modified Pt.