Role of Guiding Groups in Cinchona-Modified Platinum for Controlling the Sense of Enantiodifferentiation in the Hydrogenation of Ketones
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- 作者：Fatos Hoxha ; Lucia Kö ; nigsmann ; Angelo Vargas ; Davide Ferri ; Tamas Mallat ; Alfons Baiker
- 刊名：Journal of the American Chemical Society
- 出版年：2007
- 出版时间：August 29, 2007
- 年：2007
- 卷：129
- 期：34
- 页码：10582 - 10590
- 全文大小：258K
- 年卷期：v.129,no.34(August 29, 2007)
- ISSN：1520-5126
文摘
Systematic structural variations of cinchona-type modifiers used in the platinum-catalyzedhydrogenation of ketones give insight into the adsorption mode of the modifier and its interaction with thesubstrate on the platinum surface under truly in situ conditions. The performance of a new modifier, O-(2-pyridyl)-cinchonidine, is compared to that of O-phenyl-cinchonidine and cinchonidine (CD). In thehydrogenation of ethyl pyruvate, ketopantolactone, and 2-methoxyacetophenone, CD gives the (R)-alcoholin excess. Introduction of the bulky O-phenyl group favors the (S)-enantiomer, whereas upon replacementof the phenyl by a 2-pyridyl group the (R)-alcohol is again the major product. This finding is particularlystriking, because the two ether groups have virtually identical van der Waals volumes. A catalytic studyincluding the nonlinear behavior of modifier mixtures, and attenuated total reflection infrared spectroscopyof the solid-liquid interface in the presence of hydrogen, revealed the adsorption mode and strength ofthe modifiers on Pt. Theoretical calculations of the modifier-substrate interactions offered a feasibleexplanation for the different role of the bulky ether groups: repulsion by the phenoxy and attraction by the2-pyridoxy groups. Simulation of the interaction of o-pyridoxy-CD with ketopantolactone on a model Ptsurface suggests that formation of two N-H-O-type H-bonds-involving the quinuclidine and pyridine Natoms, and the two keto-carbonyls in the substrate-controls the adsorption of the substrate during hydrogenuptake. This mechanistic study demonstrates the potential of insertion of suitable substituents into CD andtheir influence on adsorption and stereocontrol on the platinum surface.