Highly asymmetric heterogeneous catalytic hydrogenation of isophorone on proline modified base-supported palladium catalysts

Mhadgut, Shilpa C.; Török, Marianna; Esquibel, Joseph; Török, Béla

doi:10.1016/j.jcat.2006.01.004

Cited by 41 publications

(43 citation statements)

References 41 publications

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“…8 Another system, the proline-directed asymmetric hydrogenation of isophorone ( Figure 1) and similar molecules, typically carried out with a Pd catalyst, has been investigated by Tungler and co-workers 9,10 who proposed that these reactions proceed by the same general mechanism as that which operates for asymmetric C=O hydrogenation -a reactive encounter between the adsorbed organic substrate and the adsorbed chiral agent (proline) in the presence of hydrogen adatoms. However, we have shown, 11 and others have confirmed, 12 that this interpretation is not correct. The metal surface merely carries out a racemic hydrogenation of adsorbed isophorone and the observed enantiomeric excess (e.e.)…”

Section: Introductionmentioning

confidence: 57%

Heterogeneously Catalyzed Asymmetric Hydrogenation of C═C Bonds Directed by Surface-Tethered Chiral Modifiers

et al. 2009

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The full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. observed enantiomeric excesses increased with increasing size of the alkyl group in the sulfide. This likely reflects varying degrees of ligand dispersion on the surface: bulky substituent groups hinder close approach of ligand molecules to each other, inhibiting close-packed island formation, favoring dispersion as separate molecules and leading to effective asymmetric induction. Conversely, small substituents favor island formation leading to very low asymmetric induction. Enantioselective reaction most likely involves initial formation of an enamine or iminium species, confirmed by use of an analogous tertiary amine, which leads to racemic product. Ligand rigidity and resistance to selfassembled monolayer formation are important attributes that should be designed into improved chiral modifiers.

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Section: Introductionmentioning

confidence: 57%

Heterogeneously Catalyzed Asymmetric Hydrogenation of C═C Bonds Directed by Surface-Tethered Chiral Modifiers

et al. 2009

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“…Thus we have shown, 74 and others have confirmed, 75 that interpretation in terms of an adsorbed 355 prochiral intermediate formed by a condensation reaction between proline and isophorone is not correct. 356…”

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confidence: 59%

Aspects of Heterogeneous Enantioselective Catalysis by Metals

2011

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Publisher's copyright statement:This document is the Accepted Manuscript version of a Published Work that appeared in nal form in Langmuir, copyright c American Chemical Society after peer review and technical editing by the publisher. To access the nal edited and published work see http://pubs.acs.org/doi/abs/10.1021/la200009w. Additional information:Use policyThe full-text may be used and/or reproduced, and given to third parties in any format or medium, without prior permission or charge, for personal research or study, educational, or not-for-pro t purposes provided that:• a full bibliographic reference is made to the original source • a link is made to the metadata record in DRO • the full-text is not changed in any way The full-text must not be sold in any format or medium without the formal permission of the copyright holders.Please consult the full DRO policy for further details. Some aspects of metal-catalyzed heterogeneous enantioselective reactions are reviewed with specific 23 reference to four different systems where the phenomena that control enantioselection appear to be very 24 different. In the case of glucose electro-oxidation it is clear that any intrinsic chirality present at the 25 metal surface plays a vital role. With -keto hydrogenation, achiral surfaces modified by the adsorption 26 of chiral agents become effective enantioselective catalysts and formation of extended arrays of chiral 27 species appears not to be of importance: instead a 1:1 docking interaction controlled by hydrogen 28 bonding between the adsorbed chiral modifier and the prochiral reactant determines the outcome. 29Hydrogen bonding also plays a central role in -ketoester hydrogenation, but here fundamental studies 30indicate that the formation of ordered arrays involving the reactant and chiral ligand is of importance. 31Asymmetric C=C hydrogenation, though relatively little studied, has the potential for major impact in 32 synthetic organic chemistry both at the laboratory scale and in the manufacture of fine chemicals and 33 pharmaceuticals. The structural attributes that determine whether or not a given chiral ligand is effective 34 have been identified; the ability to form strong covalent bonds with the metal surface while also 35 resisting hydrogenation and displacement by the strongly-adsorbing reactant under reaction conditions 36 are essential necessary conditions. Beyond these, ligand rigidity in the vicinity of the chirality center 37 coupled with resistance to SAM formation are critically important factors whose absence results in 38 racemic chemistry.

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“…We have refined this model and have pointed that strong proline adsorption is necessary to obtain high enantioselectivities [13]. Using this observation as a hypothesis led us to first observe that ultimately a secondary kinetic resolution is responsible for the excellent enantioselectivities [14]. A few months later (see publication dates in [15]) in a mechanistic study Lambert et al came to the same conclusion concerning the importance of the kinetic resolution [16].…”

Section: Introductionmentioning

confidence: 80%

“…Mean metal particle sizes of the catalysts have been determined [13,14] by high resolution transmission electron microscopy (JEOL 4000FX electron microscope) as described earlier [18]. The mean metal particle sizes and dispersion values are as follows: 5% Pd/BaCO 3 -4.3 nm (D = 0.20%), 5% Pd/Al 2 O 3 -4.1 nm (D = 0.21), 5% Pd/C-4.2 nm (D = 0.20).…”

Section: Methodsmentioning

confidence: 99%

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Nature of Proline-induced Enantiodifferentiation in Asymmetric Pd Catalyzed Hydrogenations: Is the Catalyst Really Indifferent?

et al. 2008

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The mode of enantioselection in the proline modified asymmetric hydrogenation of isophorone (3,5,5-trimethyl-2-cyclohexenone) on supported Pd catalysts has been studied. It is shown that several experimental factors, such as modifier structure and chemical nature of the catalyst support, strongly affect the outcome of the hydrogenations. Secondary kinetic resolution was found to be the major reason for obtaining high enantioselectivities on most catalysts. Extensive studies have been carried out to clarify the importance of the interaction of the prolinedihydroisophorone complex with the catalyst. The secondary kinetic resolution of dihydroisophorone was investigated under different conditions. First, racemicdihydroisophorone was studied using several (S)-proline modified supported Pd catalysts, then the individual enantiomers were subjected to a similar reaction on Pd/ BaCO 3 catalyst in the presence of (S)-proline. Our results provide convincing support for the heterogeneous enantioselection model under the current experimental conditions.

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Highly asymmetric heterogeneous catalytic hydrogenation of isophorone on proline modified base-supported palladium catalysts

Cited by 41 publications

References 41 publications

Heterogeneously Catalyzed Asymmetric Hydrogenation of C═C Bonds Directed by Surface-Tethered Chiral Modifiers

Heterogeneously Catalyzed Asymmetric Hydrogenation of C═C Bonds Directed by Surface-Tethered Chiral Modifiers

Aspects of Heterogeneous Enantioselective Catalysis by Metals

Nature of Proline-induced Enantiodifferentiation in Asymmetric Pd Catalyzed Hydrogenations: Is the Catalyst Really Indifferent?

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