Hydrogenation Of Β-keto Esters Research Articles

Synthesis and characterization of a high-purity chiral 5,5'-disulfonato-BINAP ligand and its application in asymmetric hydrogenation of β-keto esters

Abstract 2,2′-Bis(diphenylphosphino)-1,1′-binaphthyl (BINAP) is a widely used chiral phosphine ligand, and the sulfonated BINAP is one of the important water-soluble BINAP derivatives. In this article, we report an improved oleum sulfonation method for the synthesis of the high-purity 5,5′-disulfonato-(S)-BINAP ligand with an industrially acceptable high yield. By optimizing the sulfonation conditions and improving the workup process, the yield of 5,5′-disulfonated products was enhanced to 73 %. Furthermore, an acetonitrile extraction protocol was developed for the purification of sulfonated products by efficiently removing phosphine oxides, by which the content of phosphine oxide can be controlled below 3 %. The structure of 5,5′-disulfonato-(S)-BINAP was characterized with the elemental analysis, FT-IR, 1H NMR, 13C NMR, 31P NMR, 1H-1H COSY, 1H-13C HSQC, 1H-13C HMBC, and HR-MS, which unambiguously confirmed that the sulfo groups are introduced at the 5,5′-position of the binaphthyl ring skeleton in (S)-BINAP. The prepared high-purity 5,5′-disulfonato-(S)-BINAP was evaluated in the Ru-catalyzed asymmetric hydrogenation of β-keto esters, which exhibited high catalytic activity, enantioselectivity and excellent catalytic stability.

Observing the in situ chiral modification of Ni nanoparticles using scanning transmission X-ray microspectroscopy

Enantioselective heterogeneous hydrogenation of CO bonds is of great potential importance in the synthesis of chirally pure products for the pharmaceutical and fine chemical industries. One of the most widely studied examples of such a reaction is the hydrogenation of β-ketoesters and β-diketoesters over Ni-based catalysts in the presence of a chiral modifier. Here we use scanning transmission X-ray microscopy combined with near-edge X-ray absorption fine structure spectroscopy (STXM/NEXAFS) to investigate the adsorption of the chiral modifier, namely (R,R)-tartaric acid, onto individual nickel nanoparticles. The C K-edge spectra strongly suggest that tartaric acid deposited onto the nanoparticle surfaces from aqueous solutions undergoes a keto-enol tautomerisation. Furthermore, we are able to interrogate the Ni L2,3-edge resonances of individual metal nanoparticles which, combined with X-ray diffraction (XRD) patterns showed them to consist of a pure nickel phase rather than the more thermodynamically stable bulk nickel oxide. Importantly, there appears to be no “particle size effect” on the adsorption mode of the tartaric acid in the particle size range ~90–~300nm.

XPS studies of the effects of modification pH on the interaction of methylacetoacetate with ( S)-aspartic acid-modified Ni surfaces

The adsorption, from aqueous solution, of (S)-aspartic acid onto polycrystalline Ni was investigated as a function of pH via X-ray photoelectron spectroscopy in order to investigate why the hydrogenation of β-ketoesters over aspartic acid-modified Ni catalysts exhibits a very sharp maximum in enantioselectivity at modification pH 5. Following similar modification and washing procedures to those used in the real catalyst preparation, it was found that, under the optimum preparation conditions, the coverage of aspartic acid on the Ni surface drops below detectable levels. The implications for understanding the mechanism of the enantioselective hydrogenation reaction are discussed.

Surface Chemistry Underpinning Enantioselective Heterogeneous Catalysis: Supramolecular Self-Assembly of Chiral Modifiers and Pro-Chiral Reagents on Ni{111}

One of the most heavily studied examples of enantioselective heterogeneous catalysis is the hydrogenation of β-ketoesters over chirally modified Ni catalysts. We use scanning tunneling microscopy to investigate the interaction of the simplest β-ketoester, methylacetoacetate, with Ni{111} surfaces premodified with (S)-glutamic acid. The behavior of methylacetoacetate is strongly dependent on the initial modifier coverage. At an intermediate (S)-glutamic acid coverage, two distinct domains are identified which correspond to ordered arrangements of glutamate and methylacetoacetate in different stoichiometric ratios. The implications of our findings for enantioselective catalysis are discussed.

ChemInform Abstract: Synthesis of (3S,4S)-4-Hydroxy-2,3,4,5-tetrahydropyridazine-3- carboxylic Acid, Component of Luzopeptin A.

Abstract The enantioselective synthesis of (3 S , 4 S )-4-hydroxy-2, 3, 4, 5-tetrahydropyridazine-3-carboxylic acid 1 is described. The two stereogenic centers in anti relationship are obtained by sequential enantio and chemoselective hydrogenation of β-ketoester in presence of chiral ruthenium catalyst and diastereoselective amination of β-hydroxyester with di t -butylazodicarboxylate.

Highly Enantioselective Ru‐Catalyzed Hydrogenation of β‐Keto Esters Using Electron‐Donating Bis(trialkylphosphine) Ligand‐TangPhos.

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The influence of Au on the adsorption of methylacetoacetate on Ni—a study with medium energy ion scattering and reflection absorption infrared spectroscopy

The hydrogenation of β-ketoesters over chirally modified Ni catalysts is one of the most successful examples of enantioselective heterogeneous catalysis. One of the factors limiting the enantioselectivity of this process is the fact that bare (achiral) Ni sites are active for the racemic hydrogenation reaction. In this work, we investigate the interaction of the simplest β-ketoester, methylacetoacetate, with bimetallic Ni/Au surfaces in order to examine whether the addition of Au to the Ni catalyst has the potential to enhance the enantioselectivity of the catalytic reaction. Using reflection absorption infrared spectroscopy (RAIRS), we probe the molecular conformation of methylacetoacetate on the bimetallic surface as a function of pre-annealing temperature. We show that the presence of Au strongly influences the adsorption of methylacetoacetate which adsorbs predominantly as the enolate. Using medium energy ion scattering (MEIS), we found that once the surface composition of Ni reaches ∼20% or above, the sticking probability of methylacetoacetate increases significantly and methylacetoacetate adsorption results in further segregation of Ni to the surface. The implications for enantioselective heterogeneous catalysis are discussed.

Asymmetric Hydrogenation of Furan- containing Ketones over Tartaric Acid-modified Raney Nickel Catalyst

The hydrogenation of β-keto esters containing a furan unit at the conjugated position to the (3-carbonyl group was carried out over a chiral heterogeneous catalyst, tartaric acid-modified Raney nickel. The hydrogenation first proceeds at the carbonyl to give optically active alcohols, but a simple substrate undergoes further hydrogenation of the furan part to produce a diastereomeric mixture of alcohols having a tetrahydrofuran moiety. This over-reduction was efficiently suppressed by substitutions of a substituent at the furan part. The optical yield at the hydroxy group is in the range of 40-90%.

Enantioselective hydrogenation of β-keto esters catalyzed by chiral binaphthylbisphosphine ruthenium complexes

The catalytic activity and the enantioselectivity manifested by cationic chiral binaphthylbisphosphine ruthenium complexes in asymmetric hydrogenation of β-keto esters were studied. The effects of the nature of the solvent, the reaction temperature, the pressure, addition of acids, and the reagent ratio on the yield and the degree of enantiomeric enrichment of the reaction products were examined. For hydrogenation of ethyl 4-chloroacetoacetate to form (R)- or (S)-enantiomers of ethyl 4-chloro-3-hydroxybutyrate, conditions were found which allow one to quantitatively prepare this valuable synthon with high enantiomeric purity (97–99%) at a low concentration of the catalyst (the ratio substrate: Ru=10000).

Enantioface-Differentiating (Asymmetric) Hydrogenation of β-Ketoester with Modified Raney Nickel Catalyst (MRNi). XXXI. A Comparative Study of Reaction Rates and Optical Yields

Abstract Enantioface-differentiating hydrogenations of methyl acetoacetate over (S)-alanine-, (S)-2-aminobutyric acid, (S)-valine-, (S)-leucine-, (S) -malic acid-, and (R,R)-tartaric acid-MRNi were carried out under atmospheric pressure of hydrogen and the effects of the modifying reagent on the rates of the formation of enantiomer, vS and vR, were investigated. The rates of hydrogenation, v=vS+vR, were the same for all the amino acid-MRNi and also for all the hydroxy dicarboxylic acid-MRNi. However, the vR⁄vs ratios were different. These results are explained in terms of the independence of the rate-determining and enantioface-differentiating steps in the reaction pathway of hydrogenation. The rate-determining step of hydrogenation was shown to be the hydrogen addition to the adsorbed substrate, while the enantioface-differentiating step was expected to occur prior to the rate-determining step.