(734b) Characterizing Active Sites on Chirally-Modified Supported Noble Metal Catalysts for Asymmetric Hydrogenation through Selective Poisoning
In heterogeneous asymmetric hydrogenations, enantioselectivity is only induced at chirally-modified sites, while remaining unmodified surface sites lead to racemic product.3The highly specific interplay between co-adsorbed chiral modifier and prochiral substrate further increases the complexity of such poisoning experiments, and various organosulfur compounds with different adsorption behavior are needed to shed light on the complex surface processes. However, such studies have the potential to provide valuable insight into the distribution and intrinsic catalytic performance of racemic and chirally modified sites. Furthermore, they might pave the way to optimization strategies for asymmetric heterogeneous hydrogenations.
The effect of a specific thiol on the catalytic performance was investigated by adding the thiol and the chiral modifier cinchonine simultaneously to the reaction slurry, prior to the substrate. Using operandoattenuated total reflection infrared spectroscopy, that is combining catalytic performance and spectroscopic measurement of the same catalyst sample, we were able to correlate the adsorption geometry and number of strongly bound organosulfur compounds at the active sites to the measured activity and enantioselectivity of the catalytic system. Our current results indicate that thiol adsorption has a strong impact on the catalytic activity, while leaving enantioselectivity unaffected. In contrast, the heterocycle thiophene disrupts the catalytic activity and the enantioselectivity. In general, the molecular structure of the organosulfur compound was found to play a crucial role for the poisoning behavior.
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