Enantioselective Hydrogenation Of Methyl Acetoacetate Research Articles

Combined Experimental and Theoretical Study of Methyl Acetoacetate Adsorption on Ni{100}

The enantioselective hydrogenation of methyl acetoacetate (MAA) over modified Ni-based catalysts is a key reaction in the understanding of enantioselective heterogeneous catalysis as it represents the only example of this class of reactions catalyzed by base metals. Yet, there is very little molecular-level information available about the adsorption complex formed by the reactants on Ni surfaces. Here, we report a combined experimental and theoretical study of the adsorption of MAA on the Ni{100} surface. X-ray photoelectron spectroscopy shows that MAA forms stable multilayers at low temperatures, which desorb between 200 and 220 K. At higher temperatures a single chemisorbed layer is formed, which decomposes between 300 and 350 K. Density functional theory modeling predicts an enolate species with bidentate coordination as the most stable chemisorbed species. Comparison of photoelectron spectroscopy and X-ray absorption data with simulations using this adsorption model show good qualitative and quantitat...

Ligand-Acceleration by a Chiral Modifier in the Enantioselective Hydrogenation of Methyl Acetoacetate on a Raney Nickel Catalyst: Effect of a Modifier Configuration

Abstract Reaction rate and enantioselectivity of asymmetric hydrogenation of methyl acetoacetate were studied over Raney nickel catalysts modified with (R,R)-tartaric acid, malic acid, or succinic acid to reveal the impacts of the modifier configuration. Catalysts comodified with two different acids were also examined to confirm the conclusions. From analysis of the enantiomer ratio of the hydrogenation product and initial reaction rate, tartaric acid (TA) was found to have dual functions as the modifier during the hydrogenation; effective suppression of the racemic catalysis on bare Ni surface and extensive enantiodifferentiating ligand acceleration by adsorbed TA. It was demonstrated that each adsorbed chiral modifier molecule independently takes part in the enantiospecific hydrogenation.

Heterogeneous Enantioselective Hydrogenation: pH Dependence and Interplay between Catalytic Efficacy and Surface Composition

Abstract The performance of a catalytic system consisting of metallic Ni powder, tartaric acid (TA), and NaBr in the enantioselective hydrogenation of methyl acetoacetate was strongly influenced by the pH of TA solution upon chiral modification, which is attributable to the pH-induced change in the surface composition of Ni catalyst as unambiguously confirmed by X-ray photoelectron spectroscopy for the first time.

Tartaric acid–Ni supported catalysts obtained from hydrotalcite-like compounds: Effects of catalyst preparation variables on enantioselectivity

The catalytic properties of tartaric acid–nickel supported catalysts obtained from hydrotalcite-like compounds have been assessed in the enantioselective hydrogenation of methyl acetoacetate. Ni particle size above a minimum threshold of ca. 20nm was found to influence enantioselectivity. For materials with similar Ni crystallite size, e.e. progressively lowers as Mg is incorporated to Ni/Al, whereas incorporation of Zn improves e.e. for all Ni/Zn ratios. In general, activity is higher on Ni/Mg/Al than on Ni/Zn/Al series. Moreover, the synthetic method to obtain hydrotalcite-like compounds affects the catalytic properties of the resultant catalysts. For a series of materials of the same composition, the urea hydrolysis method leads to catalysts with enantiodifferentiation ability, whereas the coprecipitation method does not. Variables of reaction during chiral modification, such as pH and tartaric acid concentration, proved not to have a major effect on enantioselectivity. Furthermore, no direct correlation between the amount of tartaric acid adsorbed onto, or Ni leached from the surface, and the enantiodifferentiation ability of the catalysts was found.

Graphite oxide as support for the immobilization of Ru-BINAP: Application in the enantioselective hydrogenation of methylacetoacetate

Abstract Graphite oxide (GO), previously functionalized with an alkoxysilane or with an isocyanate, and a chiral amine, was used as support for the immobilization of the homogeneous Ru-BINAP complex. The functionalized supports and the hybrid catalysts were characterized by XRD, NMR, TG, DRIFTS and XPS techniques. Among the hybrid catalysts the more active and enantioselective in the hydrogenation of methyl acetoacetate is the sample prepared over the non-functionalized GO. The catalytic performance can be interpreted in terms of the metal complex interacting with the oxygen functional groups or with the chiral amine at its different locations, anchored covalently onto the Cl pendant positions or via acid–base interaction at the oxygen functional groups.

ChemInform Abstract: The Enantioselective Hydrogenation of Methyl Acetoacetate Over Supported Nickel Catalysts. Part 1. The Modification Procedure.

ChemInform Abstract: The Enantioselective Hydrogenation of Methyl Acetoacetate Over Supported Nickel Catalysts. Part 1. The Modification Procedure.

ChemInform Abstract: Role of Catalyst Activation in the Enantioselective Hydrogenation of Methyl Acetoacetate Over Silica-Supported Nickel Catalysts

ChemInform Abstract: Role of Catalyst Activation in the Enantioselective Hydrogenation of Methyl Acetoacetate Over Silica-Supported Nickel Catalysts

Highly enantioselective Ru-catalyzed asymmetric hydrogenation of β-keto ester in ionic liquid/methanol mixtures

The enantioselective hydrogenation of methyl acetoacetate (MAA) was studied in detail using a ruthenium-monodentate binaphthophosphepine complex in a homogeneous solution formed by different ionic liquids and methanol. Remarkably, ionic liquid additives did not only open an attractive way for catalyst recycling in repetitive batch-mode but also led to significantly increased catalytic activity compared to pure methanol. Enantioselectivities up to 95% have been achieved in mixed ionic liquid/methanol systems which are comparable to the values obtained in pure methanol.

Highly efficient enantio-selective hydrogenation of methyl acetoacetate over chirally modified Raney nickel catalytic system

A new method of preparing a tartaric acid modified Raney nickel (TA-MRNi-NaBr) catalytic system has been studied. Raney nickel was modified with ( R, R)-tartaric acid. Then, methanol, in which an appropriate amount of NaBr dissolved, was used as the hydrogenation reaction medium. This new catalytic system showed high efficiency and high durability in enantio-selective hydrogenation of methyl acetoacetate (MAA). The influence of the important modifying parameter (modifying pH) and reaction variables (solvent type, the types of inorganic salts and their amount, reaction temperature and hydrogen initial pressure) on the hydrogenation rate and optical yield were systematically investigated. The high optical yield of 85% was achieved at pressure (0.6 MPa) nearly 17-fold lower than those previously used to achieve such results, and the hydrogenation rate was enhanced significantly. For comparison, the conventional TA-NaBr-MRNi catalyst was also prepared. Characterization data confirmed that compared with the conventional TA-NaBr-MRNi catalyst, the improved TA-MRNi-NaBr catalyst had lower weight ratio of Al–Ni, higher total surface area and more acid-corrosion on its surface, which could lead to the high efficiency of the TA-MRNi-NaBr catalytic system.

Highly Efficient Enantioselective Hydrogenation of Methyl Acetoacetate over an Improved Tartaric Acid‐Modified Raney Nickel Catalyst.

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF.

Highly Efficient Enantio-selective Hydrogenation of Methyl Acetoacetate over an Improved Tartaric Acid-modified Raney Nickel Catalyst

Abstract Using methanol as the reaction medium (an appropriate amount of sodium bromide was directly added to it), the highly efficient enantio-selective hydrogenation of methyl acetoace-tate (MAA) was achieved over a tartaric acid-modified Raney nickel catalyst. 85% ee was attained under the comparatively mild conditions (0.6 MPa, 60 °C, 1 h).

In situ Chiral Modification of Nickel Catalysts for Enantioselective Hydrogenation of Methyl Acetoacetate

Chiral modifications of SiO2-supported and Raney nickel catalysts were studied. The catalysts were employed in asymmetric hydrogenation of methyl acetoacetate (MAA) to (R)- and (S)-enantiomers of methyl 3-hydroxybutanoate. The effects of modification parameters such as type and concentration of modifier; presence of a co-modifier and other additives, pH of modification solution on the enantioselectivity of MAA hydrogenation were discussed. Characteristic features of the in situ modification of Ni/SiO2 were also evaluated and the results obtained were compared with the conventional (premodification) approach. Parameters for the conventional and in situ methods were optimised in a series of experiments for both types of catalysts. The in situ modified Ni/SiO2 was found specifically suitable for repeated use due to virtually no decrease in selectivity and activity.

The kinetics of enantioselective hydrogenation of methyl acetoacetate using a modified Raney nickel catalyst

The kinetics of enantioselective hydrogenation of methyl acetoacetate using Raney nickel modified by (2R,3R)-(+)-tartaric acid were studied, thereby focusing on the effects of the reaction conditions on the enantioselectivity. Reactions were carried out in a liquid phase under atmospheric and increased hydrogen pressure. The reaction course was described by the exponential kinetics model and by Langmuir-Hinshelwood kinetics assuming the existence of two types of active sites on the catalyst surface, selective and non-selective. Using Langmuir-Hinshelwood kinetics, the acquired parameters were used for discussion of the effects of the reaction conditions on the enantioselectivity and the mechanism of the enantioselective hydrogenation.

Preparation of tartaric acid modified Raney nickel catalysts: study of modification procedure

Chiral modification of Raney nickel using (2 R,3 R)-tartaric acid was studied. The prepared catalyst was used in enantioselective hydrogenation of methylacetoacetate (MAA) to methyl-(3 R)-hydroxybutyrate. The influence of the most important modification parameters, such as pH, temperature, time, concentration of the modifier and the presence of a co-modifier, on the optical yield of MAA hydrogenation was systematically investigated. From the data obtained, a considerable influence of modifying conditions on the resulting enantioselectivity of the catalyst was evident. The optical yield increased with an increasing of the modifying temperature and time. Dependencies of the optical yield on the tartaric acid concentration and on the modifying pH passed through a maximum. Therefore, there exists an optimal value of modifying pH, at which a minimal catalyst amount is leached to the modifying solution. Furthermore, significant adsorption of tartaric acid and subsequent complex formation of nickel and tartaric acid occurs on the catalyst surface. It was found that the presence of sodium bromide in the modifying solution resulted in a lower degree of the nickel leaching, and a decrease of the residual aluminum content in the catalyst increased the optical yield of the reaction.

Enantioselective hydrogenation of methyl acetoacetate catalyzed by nickel supported on activated carbon or graphite

Heterogeneous enantioselective catalysts based on nickel, tartaric acid and NaBr supported on activated carbon or graphite were characterized and tested in the asymmetric hydrogenation of methyl acetoacetate (MAA). Nickel crystallite size depended on the type of carbon support, the preparation conditions and reduction temperature of nickel. Enantioselectivity increased significantly with increasing nickel crystallite size. High nickel loading (up to 85 wt.%) and large crystal size (<5000 Å) were examined. Operating conditions such as temperature, hydrogen pressure and catalyst to substrate ratio were varied over a wide range to achieve high enantiomeric excess values. Enantiomeric excess as high as 91% was obtained with nickel on graphite.

Creating Chiral Surfaces for Enantioselective Heterogeneous Catalysis: R,R-Tartaric Acid on Cu(110)

One of the most successful ways of inducing enantioselectivity in a heterogeneous catalytic system is by the adsorption of chiral “modifier” molecules on the reactive metal surface. However, little is known about the nature of the active sites present on the modified metal surface and how such modifiers bestow chirality to an achiral metal surface. In this paper we report the behavior of R,R-tartaric acid adsorption on a Cu(110) surface using high-resolution surface analytical techniques. R,R-Tartaric acid is known to be an extremely successful modifier molecule for the enantioselective hydrogenation of methyl acetoacetate, the simplest β-keto ester, to the R-enantiomer of the product molecule methyl 3-hydroxybutyrate. A combination of low-energy electron diffraction (LEED), scanning tunneling microscopy (STM), and Fourier transform reflection−absorption infrared spectroscopy (FT-RAIRS) techniques has allowed us to demonstrate that a complicated adsorption phase diagram exists for this system. A rich vari...

X-ray photoelectron spectroscopy and infrared spectroscopy studies for the mechanism of the enantioselective hydrogenation of methyl acetoacetate over mixed nickel–cerium oxides

Mixed nickel cerium oxides have been used for studying the enantioselective hydrogenation of methyl acetoacetate (MAA) in methyl (3)-hydroxybutyrate (MHB). X-ray photoelectron spectroscopy (XPS) and infrared spectroscopy (FTIR) provide interesting information about the adsorption of both the reagent and the modifying agent (viz: tartaric acid). From the XPS results, we can assume that the presence of Ce 3+ species, the amount of which is depending on the Nickel extent, stabilises a complex tartarate salt. The whole information lead to a new model with a stable six-membered cycle, in which the methylenic group of the reagent is involved.

A kinetic treatment of heterogeneous enantioselective catalysis

Kinetic data are reported for the liquid phase racemic hydrogenation and enantioselective hydrogenation of methyl acetoacetate (MAA) over Ni/SiO 2 and tartaric acid (TA)-treated Ni/SiO 2 , respectively, conducted in a slurry-type reactor. The contribution of mass transport to the overall reaction rate was considered by studying the effects of varying a number of process variables, such as catalyst particle size, catalyst weight, stirring speed and reaction temperature. Reaction conditions are identified wherein contributions due to mass transfer are slight and the hydrogenation proceeds under surface kinetic control. Reaction orders with respect to MAA, hydrogen and the product, methyl 3-hydroxybutyrate (MHB), were determined and activation energies over the temperature range 318 ⩽ T ⩽ 383 for the bare and TA-modified supported nickel are presented. The reaction rates were observed to pass through maxima as the MAA concentration was increased. The rate data are fitted to a Langmuir–Hinshelwood model where both reactants adsorb competitively on the surface; the experimentally determined rates agree to ±15% with the values predicted by the kinetic model. Modification of the catalyst with TA not only induced enantioselectivity but also promoted the rate of hydrogenation and this effect is discussed in terms of surface interactions and the proposed kinetic model. The ratio of MAA concentration in bulk solution to that at the catalyst surface under reaction conditions is examined and the dependence of enantioselectivity, in the case of the TA-treated sample, on MAA concentration is presented.

The role of catalyst activation in the enantioselective hydrogenation of methyl acetoacetate over silica-supported nickel catalysts

A range of Ni–SiO2 catalysts have been prepared by homogeneous precipitation–deposition and impregnation techniques and modified with aqueous solutions of R-(+)-tartaric acid (TA) to induce enantioselectivity in the asymmetric hydrogenation of a prochiral β-ketoester (methyl acetoacetate) to a β-hydroxyester ((R)-(−)-methyl 3-hydroxybutyrate). The effects of catalyst precursor preparation, nickel loading, and precalcination on the reduction of the precursor were examined and the nature of the resultant nickel particle size distribution is described. The influence of metal–support interactions on the uptake of TA was investigated and data on the corrosive metal leaching and the consequent changes in metal dispersion are presented. Modification of the catalysts with TA not only induced enantioselectivity, but also increased the turnover frequency by up to 15 times that observed for the unmodified surface. The amount of TA adsorbed and the fractional surface coverage by TA is related to the degree of enantiodifferentiation. The effects of variations in supported nickel metal particle sizes (in the overall range of 1.4–13.6 nm) on the reaction rate and enantioselectivity were examined; while the selectivity was independent of metal particle size, the rate of hydrogenation was found to be structure sensitive and a correlation between reaction rate sensitivity and particle size is presented.

The enantioselective hydrogenation of methyl acetoacetate over supported nickel catalysts: I. The modification procedure

Enantioselective Ni/SiO 2 catalysts have been prepared by modification with ( R)-(+)-tartaric acid (TA) and used in the asymmetric hydrogenation of methyl acetoacetate (MAA) to ( R)-(−)-methyl-3-hydroxybutyrate (MHB). The effects on the overall asymmetric activity of systematically varying modification conditions, namely time, temperature, TA concentration, and pH, are discussed. The amounts of TA adsorbed have been correlated with conversion and selectivity and an optimum surface coverage of 0.2 was identified. The nature of the interaction of TA with the surface nickel metal is considered and data on the corrosive leaching and the changes in metal dispersion are presented. The consequences of adding NaBr as a subcomponent of the modification solution are examined and the results are discussed in terms of surface coverage and nonselective site poisoning. The results obtained reveal the interrelated effects of each modification variable on surface TA coverage and ultimately on enantiomeric selectivity.