An in Situ Attenuated Total Reflection Infrared Study of a Chiral Catalytic Solid−Liquid Interface:  Cinchonidine Adsorption on Pt

Abstract

An in situ attenuated total reflection study of the chiral solid-liquid interface created by cinchonidine adsorption on a Pt/Al(2)O(3) model catalyst is presented. Experiments were performed in the presence of dissolved hydrogen, that is under conditions used for the heterogeneous enantioselective hydrogenation of alpha-functionalized ketones. Cinchonidine adsorbs via the quinoline moiety. The adsorption mode is coverage dependent and several species coexist on the surface. At low concentration (10(-6)M) a predominantly flat adsorption mode prevails. At increasing coverage two different tilted species, alpha-H abstracted and N lone pair bonded cinchonidine, are observed. The latter is only weakly bound and in a fast dynamic equilibrium with dissolved 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 alpha-H abstracted cinchonidine to one with a high fraction of N lone pair bonded cinchonidine is observed with the cinchonidine concentration being the driving force for the process. The reverse transition in the absence of dissolved cinchonidine is fast. Cinchonidine competes with solvent decomposition products for adsorption sites on the Pt, which may contribute to the observed solvent dependence of the heterogeneous enantioselective hydrogenation of ketones by cinchonidine-modified Pt.

Asymmetric Hydrogenation and Transfer Hydrogenation of Ketones

“…The adsorption of cinchonidine on the Pt/Al2O3 in the presence of a solvent and H2 is strongly concentration dependent. The quinolone moiety of the modifier is responsible for the absorption on the Pt surface (Ferri & Bürgi, 2001). An inversion of the enantioselectivity occurs in the asymmetric hydrogenation of the activated ketones by changing the solvent composition, including water and acid additives (von Arx et al, 2001b;Bartók et al, 2002).…”

Heterogeneous Enantioselective Hydrogenation over Cinchona Alkaloid Modified Platinum: Mechanistic Insights into a Complex Reaction

“…Furthermore, the loss in enantioselectivity when blocking the quinuclidine N of the modifier would indicate that the quinuclidine N is involved in the formation of the "chiral footprint". However, in situ infrared 23,24 and Raman spectroscopies 25 of cinchonidine adsorption, as well as near edge X-ray absorption fine structure spectroscopy (NEXAFS), of 10,11-dihydrocinchonidine on Pt-(111) in ultrahigh vacuum 26 indicate that the modifier is anchored via the quinoline ring. Thus in the light of currently available experimental findings a "chiral footprint" mechanism seems improbable for the Pt-cinchona alkaloid system.…”

“…37 Later a detailed analysis of the spectra based on comparison with model compounds such as quinoline and pyridine and ab initio calculations were presented. 23 The orientation of the quinoline moiety with respect to the surface was determined from the orientation of the dynamic dipole moment associated with several vibrations of the molecule. This analysis lead to the following picture (Figure 4): At low concentrations, an adsorption mode prevails where the quinoline ring is oriented preferentially but not completely flat with respect to the surface.…”

“…Negative bands are due to solvent decomposition products that are displaced from the surface by cinchonidine. 23 The right panel shows adsorbed cinchonidine species as deduced from ATR spectroscopy.…”

Asymmetric heterogeneous catalytic hydrogenation; is it a useful tool for the synthetic organic chemist?

“…The researchers use the most sophisticated techniques and molecular modelling to explore the surface phenomena during asymmetric hydrogenations [12][13][14][15][16][17] . As a consequence, a breakthrough can be expected, first in understanding of the mechanism, later in the efficiency of enantioselective heterogeneous catalysed hydrogenations.…”