In situ characterization of the adsorption of cinchona chiral modifiers on platinum surfaces

Abstract

Graphic The adsorption of cinchona chiral modifiers on platinum surfaces has been characterized with surface-sensitive techniques. The uptakes of quinoline and lepidine, models for the aromatic moiety of the cinchona, were first studied on Pt(1 1 1) single crystal surfaces and under ultrahigh vacuum by reflection–absorption infrared spectroscopy (RAIRS). Adsorption at low (∼100 K) temperatures is molecular and without any preference for a distinct molecular geometry. Temperature programmed desorption (TPD) experiments indicated extensive dehydrogenation starting at about 250 K, and infrared data obtained at room temperature corroborated the conversion of some of the aromatic CH bonds to a more aliphatic environment. An in situ RAIRS characterization of the adsorption of cinchonidine from carbon tetrachloride solutions onto a polycrystalline platinum substrate proved that in hydrogen-treated solvents the decomposition of the adsorbate is inhibited, so that the cinchona retains its molecular character on the surface at room temperature. Three adsorption regimes were identified depending on the concentration of cinchonidine in solution, namely, no observable adsorption at low concentrations, bonding via a flat-lying quinoline moiety in the intermediate concentration range, and a tilted adsorption geometry near saturation. The optimum activity and enantioselectivity reported previously in catalytic processes for intermediate concentrations can then be associated with the orientation of the aromatic ring parallel to the surface plane. In a separate study, it was determined that hydrogen plays a unique role in cinchonidine adsorption, initially conditioning the surface for the uptake (presumably by reduction and removal of surface contaminants), and later hydrogenating the quinoline fragment of the modifier and facilitating its desorption. Finally, different solvents were shown to behave differently in these systems. Specifically, adsorbed cinchonidine remains on the platinum surface after flushing with non-polar solutions such as cyclohexane, but can be easily removed by more polar solvents such as dichloromethane. This behavior correlates well with both the solubility of cinchonidine and the performance of the cinchona/platinum catalyst as a function of the nature of the solvent.