Hydrogenation Of Methyl Pyruvate Research Articles

Examination of the surface chemistry of activated carbon on enantioselective hydrogenation of methyl pyruvate over Pt/C catalysts

Pt/C catalysts were prepared on oxidized activated carbons. Different oxidation treatments were used both in liquid and gas phase using HNO 3 and 5% O 2/N 2 as the respective oxidizing agents. Thus, different surface chemical properties were achieved. The enantioselective hydrogenation of methyl pyruvate with (−)-cinchonidine as modifier revealed that the catalytic activity is dependent on the concentration of the surface oxygen remaining after activation as measured by XPS. It is suggested that it is a result of the increase in the carbon hydrophilicity that would enable the reactants to go through the catalyst. It is also suggested that the active sites created by the decomposition of the functional groups would allow hydrogen chemisorption and, therefore, the CO polarization via hydrogen bond interaction. The chemical nature of the surface functional groups did not seem to influence either the catalytic activity or enantioselectivity.

A semiempirical study of the conformational behavior of cinchonidine and its interaction with methyl pyruvate

Hydrogenation of methyl pyruvate on a palladium or platinum surface in the presence of cinchona alkaloids leads to a high degree of enantiodifferentiation. In the present study, the semiempirical AM1 and PM3 methods are employed to perform a detailed analysis of the conformational behavior of cinchonidine and to study its interaction with methyl pyruvate. Nine different minima were located on the potential energy surface for cinchonidine by both the AM1 and the PM3 methods. Some barriers to interconversion between them are relatively high; however, it is always possible to connect two minima through barriers lower than 3.0 kcal/mol so most of the minima can interact with the substrate. The interaction between cinchonidine and methyl pyruvate was calculated by placing methyl pyruvate near the cinchonidine molecule in different orientations and optimizing the final complex. The calculated interaction energy is lower than 3.5 kcal/mol and is predominantly due to van der Waals noncovalent interactions. An analysis of the structure of possible pro-R and pro-S complexes indicates that interaction between cinchonidine and methyl pyruvate alone is not enough to induce enantiodifferentiation.

Improvement in the enantioselective hydrogenation of methyl pyruvate over platinum clusters by addition of rare earth cations

The modification of some rare earth metal cations to polymer-stabilized Pt colloidal clusters leads to significant increase in both the ee and the activity in the homogeneous enantioselective hydrogenation of methyl pyruvate to (R)-(+)-methyl lactate. Compared to ee (72.5%) in the absence of rare earth cations, the highest ee (88.0%) in ethanol solvent is obtained by adding a suitable amount of Yb3+ into the Pt colloidal cluster catalyst. The modification effect of rare earth cations is also effective in other alcohol and water solvents.

Asymmetric hydrogenation of methyl pyruvate using platinum carbonyl cluster supported on an anion exchanger as the catalyst

Anionic platinum carbonyl clusters supported on quaternary amine functionalized cross-linked polystyrene, i.e., anion exchangers, are effective precatalysts for the hydrogenation of methyl pyruvate to methyl lactate. Kinetic data has been obtained for anion exchangers with different quaternary groups. Highest observed rate constant and enantioselectivity are obtained with cinchonine funtionalized resin. The kinetic data also indicates saturation kinetics. The decarbonylated used catalyst could be recarbonylated to give a material spectroscopically (infrared) equivalent to freshly anchored cluster.

Orientation and Conformation of Methyl Pyruvate on Ni(111)

The molecular orientation and conformation of methyl pyruvate on Ni(111) was studied in the temperature range 105−220 K. The full monolayer formed at 105 K was found to be almost exclusively in the bidentate cis-conformation, with the molecular plane oriented perpendicular to the surface. In contrast, the low coverage layer at 105 K was found to be composed of a mixture of trans- and cis-methyl pyruvate. However, direct exposure at 200−220 K yielded exclusively cis-bidentate adsorption at all coverages. The observation of the preferred cis-bidentate species is at odds with the adsorption geometry and conformation usually assumed in rationalizations of the enantioselective hydrogenation of methyl pyruvate on chiral-compound modified platinum metal particle catalysts.

Enantioselective hydrogenation: chemistry of the modification of platinum by cinchonidine

Effective modification of 6.3% Pt/silica (EUROPT-1) by the alkaloid cinchonidine to achieve fast enantioselective hydrogenation of methyl pyruvate to methyl lactate or of butane-2,3-dione to 3-hydroxybutan-2-one is shown to require adsorption of the alkaloid in competition with a strong co-adsorbate. Comparison of experiments in which catalyst is modified under aerobic conditions and under anaerobic conditions indicates that, normally, oxygen present as air dissolved in solvent and organic reactant functions as the strong co-adsorbate. Alternative co-adsorbates that facilitate effective modification include nitrous oxide, ethyne, propyne and buta-1,3-diene; ineffective co-adsorbates include propene and carbon monoxide. Product analysis shows that co-adsorbate propyne is removed as propene and propane in the early stages of reaction, and this creates the surface conditions necessary for fast enantioselective hydrogenation of pyruvate ester or alkadione. Modification under propyne provides reaction rates and values of the enantiomeric excess under standard conditions (10 bar, 293 K) that are comparable to, or higher than those obtained with normal aerobic modification. The ability to modify catalysts under reducing conditions has provided, for the first time, a reliable route to the preparation of enantioselective Ru catalysts.

Enantioselective hydrogenation of methyl pyruvate over polymer-stabilized and supported iridium clusters

The enantioselective hydrogenation of methyl pyruvate was studied over finely dispersed polyvinylpyrrolidone-stabilized iridium (PVP-Ir) and the corresponding alumina-supported iridium cluster (Al 2O 3-Ir) modified with cinchonidine. It was found that the modifier, apart from inducing enantioselectivity, has a considerable rate-promoting effect on this reaction with the average rate increased by a factor of 2–3.5 over both the two kinds of catalysts. Favorable reaction parameters were 283 K, 4.0 MPa and alcohols or acetic acid as solvents. Similar to what had been reported in Pt/cinchona system, the highest value of the enantiomeric excess (34.1%) was obtained in acetic acid, accompanied by a marked enhancement of the reaction rate.

Chiral environments at alkaloid-modified platinum surfaces

Molecular environments at enantioselective sites are considered with reference to the adsorption of cinchonidine, epiquinidine, brucine, and oxycodone onto 6.3% Pt/silica (EUROPT-1), using the hydrogenation of methyl pyruvate and of butane-2,3-dione as molecular probes. The mode of action of cinchonidine is briefly reviewed, and the ineffectiveness of epiquinidine is interpreted. Pt modified by the strychnos alkaloid brucine is active for the hydrogenation of methyl pyruvate at 10 bar pressure and 293 K giving an enantiomeric excess of up to 20% in favour of S-lactate, but is not enantioselective for butane-2,3-dione hydrogenation. Modelling shows that brucine adsorbed at a step at a Pt surface forms a cavity which provides for the selective enantioface adsorption of methyl pyruvate, but not of butane-2,3-dione. The model locates the site for alkaloid adsorption and the site for enantioselective hydrogenation on the same terrace of metal atoms. Pt modified by the morphine alkaloid oxycodone is enantioselective for the hydrogenation of both methyl pyruvate and butane-2,3-dione at 10 bar pressure and 293 K, the enantiomeric excess being typically 15% with respect to R-product in each case. Oxycodone, like brucine, appears to require a step site for adsorption but in this case modelling indicates that sites for alkaloid adsorption and for hydrogenation are on different terraces. The activity and enantioselectivity exhibited by cinchonidine-modified Pt is inhibited when modification and reaction are conducted under anaerobic conditions. The hypothesis that co-adsorption of alkaloid and oxygen limits the coverage of the former and that access to enantioselective sites becomes possible only after removal of adsorbed-oxygen in the early stages of reaction is tested. Modification has been carried out under atmospheres of propyne or of buta-1,3-diene as co-adsorbent; the resulting catalysts show high activity and enantioselectivity after removal of the co-adsorbent in the early stages of pyruvate hydrogenation. Thus, optimisation of Pt catalysts for enantioselective reaction should take into account both metal particle geometry and alkaloid concentration.

XPS and LEED studies of 10,11-dihydrocinchonidine adsorption at Pt(111). Implications for the role of cinchona alkaloids in enantioselective hydrogenation

The interaction of 10,11-dihydrocinchonidine (DHC) with a Pt(111) surface has been studied using X-ray photoelectron spectroscopy (XPS), ultraviolet photoelectron spectroscopy (UPS) and low-energy electron diffraction (LEED). From a saturated ethanolic solution DHC is chemisorbed in multilayers whereas from more dilute solutions adsorption is limited to a monolayer. When exposed to ethanol, the multilayers are removed leaving a strongly chemisorbed monolayer of DHC. A comparison of the LEED patterns of the structurally related compounds naphthalene, quinoline, and DHC shows that only naphthalene forms an ordered adlayer (3 × 3) whereas quinoline and DHC are adsorbed in a disordered state and without decomposition. The enhanced hydrogenation rate and the optical yield for the methyl pyruvate hydrogenation reaction at a cinchona alkaloidmodified EUROPT-1 catalyst, and the absence of ordered adsorption of the alkaloid on a Pt(111) surface, support the 1 : 1 interaction model for the alkaloid and pyruvate.Data are also reported for the interaction of Pt(111) with ethanol, both in the liquid and vapour state; dissolved oxygen is shown to influence the surface chemistry and lead to the formation of surface acetate species.

Enantioselective Hydrogenation: III. Methyl Pyruvate Hydrogenation Catalyzed by Alkaloid-Modified Iridium

Enantioselective hydrogenation of methyl pyruvate, MeCOCOOMe to methyl lactate, MeCH(OH)COOMe, is catalyzed in solution at room temperature by supported iridium catalysts modified with cinchona alkaloids. Modification with cinchonidine or quinine yields R-lactate in excess, whereas modification with cinchonine or quinidine favors S-lactate formation. Ir/SiO 2 catalysts (20%) calcined at 393 to 573 K and reduced at 523 to 593 K were highly active for racemic hydrogenation in the absence of a modifier (rates typically 1.8 mol h −1 g −1 cat) and were comparably active when modified with cinchonidine but gave an enantiomeric excess of about 30%. Use of higher calcination or reduction temperatures led to substantially inferior activity and selectivity. The high rates recorded for both racemic and enantioselective reactions are dependent on the catalysts being activated before use by a procedure involving exposure of the catalyst to air after the initial reduction. Use of a Cl-free precursor gave an Ir/SiO 2 catalyst (20%) of superior activity but inferior enantioselectivity. Ir/CaCO 3 (5%) was more active for racemic hydrogenation than for enantioselective hydrogenation, but provided the highest value of the enantiomeric excess 39%. Kinetics of reaction are reported. Exchange of H for D in 10,11-dihydrocinchonidine at room temperature over Ir/CaCO 3 occurred in the quinoline moiety but not in the quinuclidine ring system, indicating that the alkaloid was adsorbed to the Ir surface via the interaction of its π-electron system. For both silica-supported and calcium carbonate-supported Ir, the presence of chloride ion in the catalyst was advantageous for the achievement of enantioselectivity.

Recently published work on EUROPT-1, a 6% Pt/SiO 2 reference catalyst

EUROPT-1 is a 6% Pt/SiO 2 catalyst which has been thoroughly characterised with respect to its composition, structure, chemisorption behaviour and catalytic properties. Extensive further work on this catalyst has been published and this is now reviewed. Debye function analysis of X-ray diffraction spectra suggest platinum particles to be cubooctahedral, although preliminary extended X-ray absorption fine structure results do not accord with this model. Further isotherms for carbon monoxide, hydrogen and oxygen chemisorption have been reported and enthalpies of adsorption for the last two gases measured. Limited results on structure-insensitive reactions such as exchange of methane with deuterium and alkene hydrogenation are available. Methane homologation as well as skeletal reactions of alkanes in the presence of hydrogen represent structure-sensitive reactions. Some further work on ethane, propane and n-butane is reported; detailed results on n-hexane and methylcyclopentane include changes in rates and selectivities as a function of hydrogen and hydrocarbon pressure. They are interpreted in terms of availability of surface hydrogen and hydrogen effects on the formation of carbonaceous deposits of various degree of dehydrogenation. Effects of sintering have also been studied, and active centres for various reactions have been tentatively suggested. On the whole, EUROPT-1 shows behaviour which is characteristic of platinum catalysts containing small platinum particles but in some limited respects it shows unusual and unexpected properties. Reactions of some oxygenated compounds are described briefly. Catalytic properties of EUROPT-1 can be modified by metals, oxides, sulphur and nitrogen; treatment with cinchonidine converts it into an enantioselective catalyst for the hydrogenation of methyl pyruvate. Finally the importance of selecting appropriate reference supported metal catalyst (s) is stressed and the future role for EUROPT-1 as a possible candidate is pointed out; results must however be compared under carefully defined conditions if they are to have real value.

Platinum-catalysed enantioselective hydrogenation: effects of low coverage of modifier

The template mechanism for the interpretation of the enantioselective hydrogenation of methyl pyruvate to optically active methyl lactate is briefly reviewed. Evidence in support of this mechanism is provided by investigation of the reaction under conditions where the alkaloid modifier (cinchonidine) is present at concentrations corresponding to sub-monolayer quantities when adsorbed on the platinum active phase of EUROPT-1.

Carbonyl cluster derived polystyrene supported platinum for asymmetric hydrogenation of α-ketoesters

Ion-pairing of anionic carbonyl clusters with cinchona alkaloid groups on cross-linked polystyrene is a viable method for the synthesis of asymmetric catalysts for the hydrogenation of methyl pyruvate.

Origin of enhanced rate in the platinum-catalysed enantioselective hydrogenation of methyl pyruvate

The remarkable 20-fold rate enhancement that accompanies the achievement of enantioselectivity in Pt-catalysed methyl pyruvate hydrogenation at 293 K and 10 bar pressure has been shown to be the consequence of three distinct roles played by the cinchona alkaloid modifier. The modifier promotes the rate (i) by its action as an alicyclic N-base, (ii) by its ability when adsorbed to facilitate the establishment of an enhanced surface coverage of adsorbed hydrogen, and (iii) by its effect as an aromatic adsorbate on the apparent specific activity of platinum. These effects have been demonstrated in experiments in which various N-bases have been used to promote the racemic rate of methyl pyruvate hydrogenation at 293 K, and in which H 2/D 2 exchange at 195 K has been used to estimate variations in the surface coverage of adsorbed hydrogen. The importance of the three effects is: (i) > (ii) ≈ (iii) and their combined effect accounts for the observed enhancement. It is proposed that the N-bases stabilise one of the half-hydrogenated states by H-bonding.

ChemInform Abstract: Enantioselective Hydrogenation. Part 2. Variation of Activity and Optical Yield with Experimental Variables in Methyl Pyruvate Hydrogenation Catalyzed by Cinchona‐Modified Platinum/Silica (EUROPT‐1) .

AbstractChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

Enantioselective hydrogenation: II. variation of activity and optical yield with experimental variables in methyl pyruvate hydrogenation catalyzed by cinchona-modified platinum/silica (EUROPT-1)

The enantioselective hydrogenation of methyl pyruvate, McCOCOOMe, to methyl lactate, MeCH(OH)COOMe, preferentially in the R-(+)-form has been catalyzed in a stirred reactor over the temperature range 253 to 348 K and over the pressure range 10 to 110 bar by 6.3%.Pt/silica (EUROPT-1) modified by cinchonidine and dihydrocinchonidine. Variations of activity and of optical yield are reported as a function of (a) modification procedure, (b) reaction procedure, and (c) temperature. The dependence of optical yield on conversion and hydrogen pressure is also reported. The achievement of high optical yield (ca. 74 to 80%, i.e., 87 to 90% R-(+)-, 13 to 10% S-(-)-ester) is critically dependent on the choice of modifier, the modification procedure, and temperature, whereas conversion, hydrogen pressure, and the presence or absence of solvent are of lesser importance. The observations are interpreted in terms of the mechanism proposed in Part I. Preliminary experiments with a flow reactor show unexpectedly high opti cal yields.

ChemInform Abstract: Enantioselective Hydrogenation. Part 1. Surface Conditions During Methyl Pyruvate Hydrogenation Catalyzed by Cinchonidine‐Modified Platinum/Silica (EUROPT‐1)

ChemInform Abstract: Enantioselective Hydrogenation. Part 1. Surface Conditions During Methyl Pyruvate Hydrogenation Catalyzed by Cinchonidine‐Modified Platinum/Silica (EUROPT‐1)

Enantioselective hydrogenation I. Surface conditions during methyl pyruvate hydrogenation catalyzed by cinchonidine-modified platinum/silica (EUROPT-1)

An investigation has been made of surface conditions during the enantioselective hydrogenation of methyl pyruvate, McCOCOOMe, to R-(+)-methyl lactate, McCH(OH)COOMe, at 10 bar pressure catalyzed by 6.3% Pt/silica EUROPT-1 modified by cinchonidine. Isotherms for the adsorption of cinchonidine and of methyl pyruvate at 298 K are reported. During modification, cinchonidine adsorbs both on the platinum active phase and on the silica support; co-adsorption of ethanol with cinchonidine on the platinum enhances the effectiveness of the modifier. Methyl pyruvate is strongly chemisorbed and forms physically adsorbed multilayers on EUROPT-1. Cinchonidine promotes the rate of methyl pyruvate hydrogenation at 293 K by a factor of 25. Some characteristics of the reaction of cinchonidine with adsorbed hydrogen are reported. The effectiveness of cinchonidine both as a modifier and as a promoter collapses above 322 K. The kinetics of methyl pyruvate hydrogenation are reported as are the optical yields obtained over the catalyst preconditioned in four ways before modification. The best activity and enantioselectivity were observed over EUROPT-1 reduced at 373 K from which hydrogen had been desorbed before modification. A mechanism is proposed which interprets both the observed enantioselectivity and the promoting effect of cinchonidine on the rate of reaction.

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.