Chiral Modification of Platinum by Co‐Adsorbed Cinchonidine and Trifluoroacetic Acid: Origin of Enhanced Stereocontrol in the Hydrogenation of Trifluoroacetophenone

Meemken, Fabian; Baiker, Alfons; Schenker, Sebastian; Hungerbühler, Konrad

doi:10.1002/chem.201303261

Cited by 19 publications

(35 citation statements)

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“…[10] Recently, we investigated the adsorption-desorption behavior of TFAP and CD on the Pt catalyst applying modulation excitation spectroscopy and phase-sensitive detection revealing the formation of a diastereomeric complex between TFAP and CD involving a N À H À O-type hydrogen bond. [11] In another in situ spectroscopic study on the asymmetric hydrogenation of KPL, we found evidence for an additional stereodirecting interaction between the alcoholic OH group of CD and a carbonyl C=O group of the cyclic a ketoester, which appears to be relevant for explaining the excellent enantioselection achieved with this substrate. [12] While these in situ IR studies have been carried out in a similar chemical environment as encountered in the catalytic reaction, the methods applied captured only a snapshot of the surface at quasi-steady state and neither measurable catalytic turnover nor optical yield could be observed.…”

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confidence: 73%

Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy

2014

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confidence: 73%

Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy

2014

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“…Protic solvents lead to the protonation of the nucleophilic quinuclidine N atom of cinchona alkaloids, which is involved in the N-HÁ Á ÁO binding of the modifier-substrate complex. 50 Crucial molecular interactions between the chiral modifier (CD), acid additive (TFA) and the reactant trifluoroacetophenone at the catalyst surface were elucidated under reaction conditions. Fig.…”

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confidence: 99%

Crucial aspects in the design of chirally modified noble metal catalysts for asymmetric hydrogenation of activated ketones

Baiker

2015

Chem. Soc. Rev.

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In view of the importance of optically pure chiral products there is ample reason to develop methods that facilitate their efficient production. Compared to the mostly applied homogeneous catalysts based on transition metals coordinated to suitable chiral ligands, heterogeneous chiral catalysts could offer several features that are beneficial for practical application such as stability, ease of catalyst separation and regeneration as well as straightforward access to continuous process operation. Various strategies have been developed for imparting chirality to catalytic active surfaces, among which the chiral modification of active metal surfaces by adsorption of suitable chiral organic compounds has so far been among the most successful. In this tutorial review lessons learned from research on asymmetric hydrogenation on chirally modified noble metals will be presented. Key aspects for the design of such catalysts will be elucidated using chirally modified platinum catalysts for the asymmetric hydrogenation of α-activated ketones as an example.

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“…Up to date, most mechanistic studies of the surface processes relevant to asymmetric hydrogenation on chirally modified platinum catalysts have been carried out ex situ9 or under conditions relatively far from those encountered in practice 10. Recently, we investigated the adsorption–desorption behavior of TFAP and CD on the Pt catalyst applying modulation excitation spectroscopy and phase‐sensitive detection revealing the formation of a diastereomeric complex between TFAP and CD involving a NHO‐type hydrogen bond 11. In another in situ spectroscopic study on the asymmetric hydrogenation of KPL, we found evidence for an additional stereodirecting interaction between the alcoholic OH group of CD and a carbonyl CO group of the cyclic α ketoester, which appears to be relevant for explaining the excellent enantioselection achieved with this substrate 12.…”

Section: Methodsmentioning

confidence: 99%

Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy

2014

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Surface processes occurring at the catalytic chiral surface of a cinchona-modified Pt catalyst during the asymmetric hydrogenation of activated ketones have been monitored for the first time using operando ATR-IR spectroscopy. Fundamental information about this catalytic system could be gained, including the chiral modification process of the catalyst, the surface interaction of reactant ketone with preadsorbed chiral modifier, the role of hydrogen as well as the influence of the product enantiomers in the catalytic cycle. The formation of a diastereomeric transient surface complex between ketone and chiral modifier was found to be related to the ketone consumption. Among the studied activated ketones, a correlation between stereoselection and the strength of the intermolecular hydrogen bond was identified. Dissociated hydrogen from the catalytic surface is found to play a crucial role in the formation of the diastereomeric surface complex.Catalysis on chiral surfaces can offer an interesting alternative to homogeneous asymmetric catalysis with chiral transition metal complexes because of the inherent advantages regarding stability, handling, separation, regeneration, and recyclability of the solid catalysts. [1] These properties make them also better amenable to continuous processes. [2] Among the various strategies for designing a heterogeneous enantioselective catalyst [3] one of the most successful is to chirally modify a metal catalyst by addition of a strongly adsorbing chiral compound, the so-called chiral modifier. In the hydrogenation of activated ketones on Pt catalysts chiral modification of the catalytic surface with cinchona alkaloids leads to remarkable activity and enantioselectivity with an enantiomeric excess (ee) of up to 98%. [4] Unfortunately, the performance of this catalytic system is highly sensitive to the molecular structures of the ketone and chiral modifier, and even the nature of the solvent and the Institute for Chemical and Bioengineering Hçnggerberg, HCI, 8093 Zürich (Switzerland)

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Chiral Modification of Platinum by Co‐Adsorbed Cinchonidine and Trifluoroacetic Acid: Origin of Enhanced Stereocontrol in the Hydrogenation of Trifluoroacetophenone

Cited by 19 publications

References 90 publications

Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy

Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy

Crucial aspects in the design of chirally modified noble metal catalysts for asymmetric hydrogenation of activated ketones

Monitoring Surface Processes During Heterogeneous Asymmetric Hydrogenation of Ketones on a Chirally Modified Platinum Catalyst by Operando Spectroscopy

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