| RESEARCH |
Christopher T. Williams, an assistant professor of chemical engineering at the University of South Carolina, Columbia, uses surface-enhanced Raman spectroscopy (SERS) to study catalytic systems. Stereoselective chemical reactions generally fall in the domain of solution-phase (homogeneous) catalysis. But in a few cases, such as the hydrogenation of -ketoesters studied by Williams, surface-mediated reactions can proceed with a high degree of enantioselectivity (C&EN, March 25, 2002, page 43). In the best known example of that type of reaction, ethyl pyruvate is converted to ethyl lactate over an alumina-supported platinum catalyst that's treated with cinchonidine, a chiral alkaloid. Cinchonidine functions as a chiral template (also referred to as a modifier) that steers the reaction toward (R)-ethyl lactate with an enantiomeric excess of roughly 95%. Williams noted that several observations regarding that stereoselective reaction have been made from kinetic studies. For example, it's known that enantiopure products are formed with the greatest selectivity when cinchonidine is present in a narrow concentration range (roughly one molecule per 10 platinum atoms). It's also known that the modifier itself is hydrogenated during reaction and that activity and enantioselectivity drop off quickly when the reaction is carried out above roughly 50 °C. But the reasons for the observed behavior aren't well understood, he said. To gain insight into the reaction mechanism, Williams, postdoctoral associate Wei Chu, and graduate student Rene J. LeBlanc prepared test specimens and probed their vibrational transitions using SERS. The samples were made by electrodepositing thin films of platinum onto roughened gold electrodes and then submerging the electrodes in ethanol solutions of cinchonidine. From the Raman spectra, which were interpreted with the help of quantum mechanical calculations, the South Carolina group found that cinchonidine adsorbs on the catalyst surface through -bonding interactions between its quinoline unit (fused-ring portion) and platinum [Catal. Commun., 3, 547 (2002)]. The spectra also reveal the effect of cinchonidine concentration on the molecule's surface orientation. By comparing the relative intensities of spectral bands corresponding to an in-plane quinoline C-C stretching motion with an out-of-plane C-H wagging motion, Williams and coworkers deduced that as the modifier concentration is increased, the tilt angle between the quinoline ring and the surface also increases--meaning the molecule "stands up" on the surface. These findings, which are consistent with surface studies conducted by other researchers, indicate that as the surface gets crowded, cinchonidine deviates from its optimal orientation, which lowers enantioselectivity. In related experiments, the South Carolina team recorded dramatic increases in the cinchonidine SERS signal and a change in the modifier's surface orientation when hydrogen was bubbled into the solution. Williams explained that under those conditions, the vinyl function on the quinuclidine moiety is hydrogenated, thereby converting the modifier to dihydrocinchonidine. He added that the reaction causes the quinoline ring of the adsorbed molecule to adopt a more parallel orientation to the surface, which, in turn, enhances the Raman signal. Switching to the effects of temperature, Williams reported that raising the temperature above 50 °C causes the modifier to desorb from platinum and alters its surface orientation. Given cinchonidine's finicky reaction preferences, it's not surprising therefore that enantioselectivity falls when conditions get too hot. |
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