A Generalized Two-Point H-Bonding Model for Catalytic Stereoselective Hydrogenation of Activated Ketones on Chirally Modified Platinum
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- 作者：Sté ; phane Lavoie ; Marc-André ; Laliberté ; Israel Temprano ; and Peter H. McBreen
- 刊名：Journal of the American Chemical Society
- 出版年：2006
- 出版时间：June 14, 2006
- 年：2006
- 卷：128
- 期：23
- 页码：7588 - 7593
- 全文大小：330K
- 年卷期：v.128,no.23(June 14, 2006)
- ISSN：1520-5126
文摘
The asymmetric hydrogenation of 
hars/alpha.gif" BORDER=0>-ketoesters on cinchona-modified supported platinum particlesis a prototype reaction in heterogeneous chiral catalysis. The catalysis literature shows that the reaction ishighly metal-specific, that it displays rate-enhancement with respect to the racemic reaction on thenonmodified surface, and that the observed stereoselectivity is a sensitive function of substrate and modifierstructure. This set of observations has proven difficult to rationalize within the context of existing modelsfor the mechanism of the Orito reaction. The most widely discussed mechanistic models are based on theformation of chemisorbed 1:1 complexes through H-bonding between the quinuclidine function of thecinchona modifier and the prochiral, keto-carbonyl, function of the substrate. Recent surface science studies,as well as advances in the area of C-H···O hydrogen bonding, suggest that chemisorption-inducedpolarization may lead to an aromatic-carbonyl H-bonding interaction between the aromatic anchor of themodifier and the coadsorbed substrate. By specifying that the aromatic C-H···O interaction is to the prochiralcarbonyl and that it is accompanied by a H-bonding interaction between the ester carbonyl and thequinuclidine function, we show that it is possible to rationalize essentially all of the catalysis literature forthe Orito reaction in terms of a single molecular mechanism. The generality of the proposed mechanisticmodel is demonstrated by addressing data from the literature for a representative range of substrates,modifiers, solvents, and metals. Results of catalytic tests on an asymmetric diketone substrate are presentedin support of the model.
hars/alpha.gif" BORDER=0>-ketoesters on cinchona-modified supported platinum particlesis a prototype reaction in heterogeneous chiral catalysis. The catalysis literature shows that the reaction ishighly metal-specific, that it displays rate-enhancement with respect to the racemic reaction on thenonmodified surface, and that the observed stereoselectivity is a sensitive function of substrate and modifierstructure. This set of observations has proven difficult to rationalize within the context of existing modelsfor the mechanism of the Orito reaction. The most widely discussed mechanistic models are based on theformation of chemisorbed 1:1 complexes through H-bonding between the quinuclidine function of thecinchona modifier and the prochiral, keto-carbonyl, function of the substrate. Recent surface science studies,as well as advances in the area of C-H···O hydrogen bonding, suggest that chemisorption-inducedpolarization may lead to an aromatic-carbonyl H-bonding interaction between the aromatic anchor of themodifier and the coadsorbed substrate. By specifying that the aromatic C-H···O interaction is to the prochiralcarbonyl and that it is accompanied by a H-bonding interaction between the ester carbonyl and thequinuclidine function, we show that it is possible to rationalize essentially all of the catalysis literature forthe Orito reaction in terms of a single molecular mechanism. The generality of the proposed mechanisticmodel is demonstrated by addressing data from the literature for a representative range of substrates,modifiers, solvents, and metals. Results of catalytic tests on an asymmetric diketone substrate are presentedin support of the model.