Heterogeneous enantioselective hydrogenation of 2-pyrones over cinchona-modified palladium

Huck, Wolf-Rüdiger; Mallát, Tamás; Baiker, Alfons

doi:10.1039/b108944g

Cited by 65 publications

(43 citation statements)

References 5 publications

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“…Again this is in line with a well-known fact: to obtain good enantioselectivities in hydrogenation reactions over Pd much higher CD concentrations are needed than on Pt. 22,23 Due to the competition with reactant and solvent molecules a high concentration of CD is needed to cover a significant amount of the surface with CD molecules. On Pt, however, the interaction of CD with the surface is so strong that small amounts of CD are enough to compete with other potential adsorbates.…”

Section: Discussionmentioning

confidence: 99%

Time-Lapse STM Studies of Diastereomeric Cinchona Alkaloids on Platinum Metals

et al. 2006

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The adsorption of cinchonidine (CD) and cinchonine (CN) on Pt(111) and Pd(111) single crystals has been investigated by means of scanning tunneling microscopy (STM) in an ultrahigh vacuum system. In timelapse series the mobilities of different adsorption species have been determined on a single molecule basis and with varying hydrogen background pressures in the system. The diastereomeric cinchona alkaloids, CD and CN, which are widely used as chiral modifiers of platinum group metals in catalytic enantioselective hydrogenation, showed similar adsorption modes and diffusion behavior on Pt(111), except that the flatly adsorbed CN molecules which were free (not in a dimer/cluster) were significantly more mobile than their CD analogues. CD adsorbed on Pd(111) showed similar adsorption modes as observed on Pt(111) but at considerably higher mobility of the flatly absorbed species already in the low-pressure region. The observed adsorption behaviors are discussed in the context of independent ATR-IR measurements and theoretical calculations. Special emphasis is put on the nonlinear effect observed in hydrogenation reactions with CD/ CN mixtures. Our observations corroborate that this effect is mainly a consequence of the different adsorption strengths of CD and CN on Pt.

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Section: Discussionmentioning

confidence: 99%

Time-Lapse STM Studies of Diastereomeric Cinchona Alkaloids on Platinum Metals

et al. 2006

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“…[23] The two groups of important cinchona alkaloids, which afford the opposite enantiomers in excess over Pt and Pd, are shown in Figure 1.…”

Section: Introductionmentioning

confidence: 99%

Non‐Linear Effect of Modifier Composition on Enantioselectivity in Asymmetric Hydrogenation over Platinum Metals

2003

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Prominent non-linear behavior was observed when mixtures of cinchona alkaloids were applied as chiral modifiers in enantioselective hydrogenations over Pt/Al 2 O 3 and Pd/TiO 2 . The phenomenon is traced to differences in the strength and geometry of alkaloid adsorption on the metal surface. In ethyl pyruvate hydrogenation under close to ambient conditions the weaker adsorbing quinidine (QD) outperformed the generally preferred modifier cinchonidine (CD) and afforded the highest ee (96 ± 98%) at 1 ± 5 bar. In the partial hydrogenation of 4-methoxy-6-methyl-2-pyrone to the dihydro derivative 4 CD gave 73% ee to (S)-4 and QD provided 72% ee to (R)-4, and still the alkaloid mixture containing less than 5 mol % CD afforded 15% ee to (S)-4. This nonlinear behavior may be advantageous in asymmetric synthesis as low purity chiral compounds can be applied as efficient modifiers for Pt or Pd.

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“…An important new application of cinchona modified Pd is the enantioselective hydrogenation of 4-alkoxy-and 4-methyl-derivatives of 2-pyrone. 123 These studies demonstrate that the potential of chirally modified Pd is much broader than has been commonly considered and its efficiency is comparable with those of Pt-cinchona and Ni-tartarate systems. 124 The asymmetric hydrogenation of substituted 2-pyrones has gained increasing interest in pharmaceutical chemistry, due to the importance of the structure of 2-pyrone and its partially hydrogenated derivatives in natural and synthetic biologically active compounds.…”

Section: 102-107mentioning

confidence: 67%