Nature of Proline-induced Enantiodifferentiation in Asymmetric Pd Catalyzed Hydrogenations: Is the Catalyst Really Indifferent? (original) (raw)
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Journal of Molecular Catalysis A: Chemical, 2009
Enantioselective hydrogenation of isophorone in the presence of (S)-proline was examined over a series of Pd/MgO catalysts with varying Pd particle size. It was found that the Pd surfaces not only supply chemisorbed hydrogen required for the hydrogenation of isophorone but also be involve in the enantiodifferentiating step. Moreover, it was observed for the first time that the configuration of the chiral product could be tuned by varying the size of Pd particle such that (R)-TMCH rather than (S)-TMCH was obtained over the Pd/MgO catalysts with large Pd particles. Two competitive reaction pathways during the enantioselective hydrogenation of isophorone were proposed depending on the size of Pd particle. One involves the enantioselective hydrogenation of the reaction intermediates formed on large Pd particle with size exceeding 10 nm leading to the excess formation of (R)-TMCH enantiomer, whereas the other invokes the primary hydrogenation of isophorone to racemic TMCH over Pd particle with size smaller than 4 nm, followed by the kinetic resolution process leaving the (S)-TMCH enantiomer in excess.
Langmuir, 2007
The adsorption rates onto a range of platinum single-crystal surfaces of key species involved in the proline-directed heterogeneous enantioselective hydrogenation of isophorone were investigated by electrochemical means. Specifically, the uptakes of the prochiral reactant (isophorone), the chiral hydrogenation product (3,3,5-trimethylcyclohexanone), and the chiral directing agent ((R)-and (S)-proline) were examined. The effects of R,S chiral kink sites on the adsorption of (R,S)-proline were also studied. The reactant adsorbs ∼10 5 times faster than the chiral modifier so that under conditions of competitive adsorption the latter is entirely excluded from the metal surface. Supplementary displacement and reaction rate measurements carried out with practical Pd/carbon catalysts show that under certain reaction conditions isophorone quickly displaces preadsorbed proline from the metal surface. Thus both kinetics and thermodynamics ensure that the chiral modifier can play no role in any surface-mediated process that leads to enantiodifferentiation. These results are fully consistent with the recent proposal 1 that the crucial step leading to enantiodifferentiation occurs in the solution phase and not at the metal surface. In addition, it is found that there is no preferred diastereomeric interaction between (R,S)-proline and R,S step kink sites on Pt{643} and Pt{976}, implying that such sites do not play a role in determining the catalytic behavior of supported metal nanoparticles.
Journal of Catalysis, 2010
The kinetic resolution of 3,5,5-trimethyl cyclohexanone (TMCH) and asymmetric hydrogenation of isophorone (3,5,5-trimethyl cyclohex-2-enone, IP) were investigated on different Pd catalysts in the presence of (S)-proline (Pr). It could be proven that in isophorone hydrogenation the optically active TMCH was formed not only by kinetic resolution but also through asymmetric C@C hydrogenation. The activity and stereoselectivity of different Pd catalysts depended on the support material, preparation method, and reaction conditions as well, confirming our assumption that enantiodifferentiation takes also place on the catalyst surface and not only in the homogeneous liquid phase condensation reaction.
Journal of Catalysis, 2010
The kinetic resolution of 3,5,5-trimethyl cyclohexanone (TMCH) and asymmetric hydrogenation of isophorone (3,5,5-trimethyl cyclohex-2-enone, IP) were investigated on different Pd catalysts in the presence of (S)-proline (Pr). It could be proven that in isophorone hydrogenation the optically active TMCH was formed not only by kinetic resolution but also through asymmetric C@C hydrogenation. The activity and stereoselectivity of different Pd catalysts depended on the support material, preparation method, and reaction conditions as well, confirming our assumption that enantiodifferentiation takes also place on the catalyst surface and not only in the homogeneous liquid phase condensation reaction.
The Journal of Physical Chemistry C, 2010
according to the journal that you are submitting your paper to) 2 ABSTRACT Asymmetric catalysis is of paramount importance in organic synthesis and in current practice is achieved by means of homogeneous catalysts. The ability to catalyze such reactions heterogeneously would have a major impact both in the research laboratory and in the production of fine chemicals and pharmaceuticals, yet heterogeneous asymmetric hydrogenation of C=C bonds remains hardly explored.