Asymmetric Hydrogenation Of Α-ketoesters Research Articles

Efficient Asymmetric Hydrogenation ofα-Ketoesters Catalyzed by Cinchona Alkaloid and Ionic Liquid Co-stabilized Pt Nanoparticles

Platinum nanoparticles(Pt NPs)were easily prepared in imidazolium-based ionic liquids by simple H2 reduction of PtO2 in the presence of cinchona alkaloid stabilizers.TEM showed that well-dispersed Pt NPs of about 1.78 nm were obtained.With the modification of cinchona alkaloids,the Pt NPs catalysts exhibited excellent activity and enantioselectivity in the asymmetric hydrogenation ofα-ketoesters.With the stabilization and modification of cinconidine,the ethyl pyruvate asymmetric hydrogenation could reach up to 4640 h^-1 TOF,and the ee value ofα-ketoesters heterogeneous enantioselective hydrogenation could reach 70.0%~80.1%.Studies showed that the characteristics of the chiral center,conformation as well as steric hindrance of the stabilizer and/or the modifier synergistically affected the asymmetric catalytic performance.The combination of BMIMPF6 and acetic acid solvents was also necessary for achieving high activity and enantioselectivity.Catalysts can be reused several times without significiant loss of activity and enantioselectivity.

An unexpected effect of water on the asymmetric hydrogenation of α-ketoesters on platinum nanoparticles confined in carbon nanotubes

The influence of water on asymmetric hydrogenation of α-ketoesters on platinum nanoparticles confined in the channels of carbon nanotubes was investigated. As high as 95% enantioselectivity is achieved in the asymmetric hydrogenation of ethyl pyruvate in acetic acid/water solution compared with 86% enantioselectivity obtained in anhydrous acetic acid. The adsorption experiment confirmed that the presence of water can enrich the chiral modifier cinchonidine in the channels 3900 times of that in solution and also enhance the ability of the channels to discriminate between the reactant and product molecules with only small hydrophobic/hydrophilic difference that kinetically favors the catalytic reaction in the channels of carbon nanotubes.

Ordered Mesoporous Carbons with Ia3d Symmetry Supported Pt Catalyst for Efficient Asymmetric Hydrogenation

Pt nanoparticles supported on CMK-8 ordered mesoporous carbons (OMCs) with Ia3d symmetry after chirally modified with cinchonidine (CD) proved to be highly efficient for asymmetric hydrogenation of α-ketoesters. Up to 34,819 h−1 TOF with 75 % ee was furnished for the enantioselective hydrogenation of ethyl 2-oxo-4-phenylbutyrate with CD-modified Pt/CMK-8 catalyst. To the best of our knowledge, these results obtained with chirally modified Pt/CMK-8 catalyst are the best ones among those with Pt catalysts supported on carbon materials. It is suggested that both the physical structure features of CMK-8 OMCs and the chemical nature of Pt catalyst are beneficial for the asymmetric hydrogenation.

Preface

The influence of water on asymmetric hydrogenation of α-ketoesters on platinum nanoparticles confined in the channels of carbon nanotubes was investigated. As high as 95% enantioselectivity is achieved in the asymmetric hydrogenation of ethyl pyruvate in acetic acid/water solution compared with 86% enantioselectivity obtained in anhydrous acetic acid. The adsorption experiment confirmed that the presence of water can enrich the chiral modifier cinchonidine in the channels 3900 times of that in solution and also enhance the ability of the channels to discriminate between the reactant and product molecules with only small hydrophobic/hydrophilic difference that kinetically favors the catalytic reaction in the channels of carbon nanotubes.

Chirally Modified Platinum Nanoparticles Stabilized by Dendritic Core-Multishell Architectures for the Asymmetric Hydrogenation of Ethyl Pyruvate

In this paper we present the asymmetric hydrogenation of α-keto esters with platinum nanoparticles homogeneously stabilized in dendritic core-multishell architectures. The main focus lies on recycling and metal leaching, because little is reported so far about these aspects. It is shown that the stabilizing polymer allows for the efficient modification of the Pt surface with the chiral alkaloid cinchonidine, thereby inducing enantioselectivity and enhancing the reaction rate in the asymmetric hydrogenation of ethyl pyruvate. After optimization of the reaction conditions 63% ee for (R)-ethyl lactate was obtained. During recycling it was found that this value could even be increased upon ultrafiltration of the catalyst prior to use. Recycling was accomplished for 10 cycles with stable activity and enantioselectivity (∼73% ee) in the first eight runs. Aggregation of the initially well dispersed nanoparticles was observed by transmission electron microscope (TEM) analysis, leading to reduced conversion after the 8 th cycle, but metal leaching into the product has been observed only in the very first run.

Highly Enantioselective Hydrogenation of α-Keto Esters Catalyzed by Ru-Tunephos Complexes

Various enantiomerically pure α-hydroxy esters were synthesized by asymmetric hydrogenation of α-keto esters catalyzed by Ru-C n -Tunephos complex. Up to 97.1% ee has been achieved for both α-aryl and α-alkyl substituted α-keto esters.

The structure and reactivity of quinoline overlayers and the adsorption geometry of lepidine on Pt{1 1 1}: model molecules for chiral modifiers in enantioselective hydrogenation

NEXAFS, variable temperature STM and TPR have been used to study model adsorbates of relevance to heterogeneous chiral hydrogenation on platinum surfaces. In both the presence and absence of co-adsorbed hydrogen, quinoline molecules are uniformly though randomly distributed on Pt {1 1 1} at 300 K. At 350 K, irreversible surface reactions denude the Pt terraces of modified adsorption sites. Under conditions of hydrogen starvation, extensive polymerisation of quinoline occurs. In the presence of hydrogen, polymerisation is accompanied by hydrogenolysis. In both cases, the {1 1 1} terraces are depleted of quinoid-modified adsorption sites. Quinoline lies approximately flat at 300 K ( α∼15°), bonded to the surface predominantly via the aromatic π system. This adsorption geometry remains largely unaffected by heating to 360 K, although the data suggest that molecular tilt increases somewhat with surface coverage ( α∼20°). Lepidine exhibits a greater tilt angle than quinoline on Pt {1 1 1} ( α∼32°), the likely result of steric hindrance due to the substituent at the C4 ′ position––a characteristic that is present in all known efficient chiral modifiers for the reaction of interest. These findings are discussed with respect to the mechanism of chiral induction and the origin of enantioselectivity collapse in the asymmetric hydrogenation of α-ketoesters on Pt catalysts.

Some aspects of the chemistry of platinum carbonyl clusters

Different aspects of the work carried out on the Chini-clusters are reviewed here. These are: (1) the uses of platinum clusters for the synthesis of hetero-metal clusters, (2) platinum clusters as mediators in pH-driven transport of sodium ions and electrons through a liquid membrane, and (3) asymmetric hydrogenation of α-ketoesters with cluster derived heterogeneous catalysts.

The asymmetric hydrogenation of α-keto esters catalyzed by rhodium(I) complexes with chiral diphosphine ligands. On the catalytic cycles and the mechanism of asymmetric induction

Asymmetric hydrogenations of n-propyl pyruvate and ketopantoyl lactone catalyzed by rhodium complexes with (—) DIOP and BPPM were carried out under a variety of conditions. It was found that i) the Schrock-Osborn mechanism was not operative in these reactions at all since no acceleration of the reaction rate by the addition of water (1%) was observed, ii) there was a clear difference between cationic and neutral (in situ) rhodium catalysts in their enantioselectivity, and iii) there was a remarkable solvent effect on the extent and direction of asymmetric induction. Possible mechanisms are discussed on the basis of these results.

キナアルカロイド修飾白金‐アルミナ触媒によるα‐ケトエステルの不斉水素化

The asymmetric hydrogenation of α-keto esters using the Pt-Al2O3 catalyst modified with cinchona alkaloid has been studied. The hydrogenation of methyl pyruvate in benzene containing a small amount of quinine with the catalyst modified by quinine gave (R)-(+)-methyl lactate in 86.8% optical yield. (R)-(-)-Ethyl mandelate was obtained in 83.9% optical yield by hydrogenation of ethyl benzoylformate with the catalyst modified by cinchonidine in benzene or diethyl ether without any additive.