Enantioselection on Heterogeneous Noble Metal Catalyst: Proline-Induced Asymmetry in the Hydrogenation of Isophorone on Pd Catalyst
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- 作者：Laura Rodr铆guez-Garc铆a ; Konrad Hungerb眉hler ; Alfons Baiker ; Fabian Meemken
- 刊名：Journal of the American Chemical Society
- 出版年：2015
- 出版时间：September 23, 2015
- 年：2015
- 卷：137
- 期：37
- 页码：12121-12130
- 全文大小：542K
- ISSN：1520-5126
文摘
In the (S)-proline-mediated asymmetric hydrogenation of isophorone (IP) on supported Pd catalyst, excellent enantioselectivity is achieved, with an enantiomeric excess of up to 99%. The role of the heterogeneous catalyst has been the subject of a controversial debate, and the current mechanistic understanding cannot explain the observed enantioselectivity of this catalytic system. The lack of in situ information about the role of the heterogeneous catalyst has prompted us to investigate the surface processes occurring at the methanol鈥揚d catalyst interface using attenuated total reflection infrared spectroscopy. Time-resolved monitoring of the homogeneous solution and of the catalytic solid鈥搇iquid interface coupled with catalytic data provides crucial information on the catalytically relevant enantiodifferentiating processes. While the condensation of IP and the corresponding chiral product 3,3,5-trimethylcyclohexanone with the chiral amine is connected to the enantiodifferentiation, it was found that the crucial enantioselectivity-controlling steps take place on the metal surface, and the reaction has to be classified as heterogeneous asymmetric hydrogenation. The presented spectroscopic and catalytic results provide strong evidence for the existence of two competing enantioselective processes leading to opposing enantioselection. Depending on surface coverage of the Pd catalyst, the reaction is controlled either by kinetic resolution ((S)-pathway) or by chiral catalysis ((R)-pathway). Steering the hydrogenation on the (R)-reaction pathway requires sufficient concentration of IP-(S)-proline condensate, as this chiral reactive intermediate becomes the most abundant surface species, inhibiting the competing kinetic resolution. The unraveled (R)-reaction pathway emphasizes an intriguing strategy for inducing chirality in heterogeneous asymmetric catalysis.