Rene LeBlanc

In-Situ Surface Raman Investigations of Cinchonidine-Modified Platinum Catalysts for Enantioselective Hydrogenation Reactions

MELANIE SCHAAL

YARITZA LOPEZ

Production of optically pure chemicals is a costly and energy intensive proposition.  Heterogeneous catalytic approaches have the potential to create significant savings over current homogeneous techniques, in terms of both the ease of catalyst separation and reduced use of solvents.  The Orito reaction, involving the use of cinchonidine-modified platinum catalysts to perform enantioselective C=O bond hydrogenations (e.g., ethyl pyruvate hydrogenation), is one of the most well studied heterogeneous chiral catalysts.  Although there have been extensive kinetic studies of this system, there is limited understanding of the surface environment under reaction conditions.  However, the advent of in-situ spectroscopic methods is fast changing this situation.  Our work has involved the use of surface-enhanced Raman spectroscopy (SERS) to probe the surface of polycrystalline platinum in various solutions that contain varying concentrations of cinchonidine.  In addition, we have examined the effects of temperature and the presence of solution-phase hydrogen on cinchonidine adsorption.  Surfaces were prepared by electrodeposition of ultrathin platinum films onto roughened gold, which provided stable and intense SERS activity for performing these studies.

The vibrational properties of adsorbed cinchonidine on platinum in ethanol solutions at 25ºC have been probed in situ.  Based on the appearance and trends in the strong ring breathing mode at 1357 cm^-1, the modifier is strongly and irreversibly adsorbed through the quinoline portion of cinchonidine by p-bonding with the Pt surface.  Furthermore, analysis of both in-plane and out-of-plane vibrations suggests that the aromatic group of cinchonidine is tilted with respect to the surface.  The degree of tilt appears to increase as concentration increases over the range of cinchonidine liquid-phase concentrations examined here (0.03 to 1.2 mM).  The presence of an H-abstracted a-quinolyl species is also tentatively suggested by the appearance of a downshifted aromatic C=C stretching band not associated with adsorbed cinchonidine.  These findings are largely consistent with what has been observed previously for cinchonidine adsorption on Pt from different solvents using in-situ infrared spectroscopy.  Addition of hydrogen into the system results in enhanced Raman scattering from adsorbed cinchonidine.  Comparisons with SERS measurements of 10,11-dihydrocinchonidine adsorption in ethanol suggest that the H₂-induced changes likely result from hydrogenation of the vinyl group on cinchonidine.  The data suggest a more flat orientation of this species, resulting from increased interaction of the aromatic ring structure with the surface.  The results are consistent with kinetic studies of cinchonidine hydrogenation, which have implied much stronger adsorption of 10,11-dihydrocinchonidine as compared with cinchonidine.

Cinchonidine adsorption on platinum in ethanol was also examined as a function of temperature with SERS.  The temperature range chosen was from 30 to 70ºC, within which both the activity and selectivity of cinchonidine-modified Pt catalysts have been shown to change dramatically.  Platinum surfaces were modified with 260 µM cinchonidine in ethanol, and examined both in pure ethanol and in the modifying solution itself.  Adsorbed cinchonidine under pure ethanol was found to partially desorb as the temperature was raised, accompanied by an increase in the average tilt of the quinoline group with respect to the surface.  In contrast, the presence of solution-phase cinchonidine resulted in an increase in the cinchonidine surface coverage and average tilt as temperature was raised.  In both pure ethanol and in 260 mM cinchonidine, the presence of hydrogen causes a significant decrease in the alkaloid SERS bands at temperatures above 40ºC.  In addition, the average tilt of the quinoline group increases significantly at these elevated temperatures.  The temperature-dependence of 10,11-dihydrocinchonidine adsorption was also investigated, and is almost identical to that observed for cinchonidine in the presence of hydrogen.  This lends further support to the conclusion that cinchonidine is being hydrogenated on the Pt surface in the presence of hydrogen.  The significant changes observed on the cinchonidine-modified Pt surface above 40ºC correlate well with reported decreases in enantioselectivity and turn-over frequency at similar temperatures during ethyl pyruvate hydrogenation.

More recent work has extended these studies to other solvents (e.g., acetic acid) and explored the effects of adding a typical reactant substrate (e.g., ethyl pyruvate).  All of the results are being be interpreted with respect to the proposed reaction pathway for this important catalytic system, and are currently being written up in my dissertation. 

Papers in press:

1.  "In-situ Raman Investigation of Cinchonidine Adsorption on Polycrystalline Platinum in Ethanol," W. Chu, R. J. LeBlanc, and C. T. Williams, Catal. Comm. 3(12), 547-552 (2002).

2.  “Vibrational Band Assignments for the Chiral Modifier Cinchonidine:  Implications for Surface Studies,” W. Chu, R. J. LeBlanc, C. T. Williams, J. Kubota, and F. Zaera, J. Phys. Chem. B 107(51), 14365-14373 (2003).

3.  “Surface Raman Characterization of Cinchonidine-Modified Platinum in Ethanol: Effects of Liquid-Phase Concentration and Co-Adsorbed Hydrogen,” R. J. LeBlanc, W. Chu and C. T. Williams, J. Mol. Catal. A 212(1-2), 277-289 (2004).

4.  “Surface Raman Characterization of Cinchonidine-Modified Polycrystalline Platinum in Ethanol: Effects of Temperature and Comparison With 10,11-Dihydrocinchonidine,” R. J. LeBlanc and C. T. Williams, J. Mol. Catal. A 220(2), 207-214 (2004).      

5. "Molecular Dynamics Simulations of Cinchonidine-Modified Platinum in Ethanol: Comparisons with Surface Studies,” S. R. Calvo, R. J. LeBlanc, C. T. Williams and P. B. Balbuena, Surf. Sci. 563(1-3), 57-73 (2004).