On the Implications of Hemiketal Formation during Ethyl Pyruvate Hydrogenation in Alcohol Solvents

Bohnen, Frank M.; Gamez, A.; Blackmond, Donna G.

doi:10.1006/jcat.1998.2247

Cited by 5 publications

(3 citation statements)

References 19 publications

Supporting

Mentioning

Contrasting

Order By: Relevance

“…A primary example is the asymmetric hydrogenation of prochiral substrates, which is of major technological importance for the synthesis of fine chemicals, pharmaceuticals, and fragrances . In particular, the enantioselective hydrogenation of α-keto esters on chirally modified Pt surfaces, first observed by Orito et al in the late 1970s, has received considerable attention due to its notably high chiral efficiency. − In the case of the substrate ethyl pyruvate (EP), hydrogenation to ( R )-ethyl lactate (EL) over Pt catalysts modified with the alkaloid cinchonidine (CD) can be achieved with exceptionally high enantiomeric excess (Scheme ). , However, in spite of the many studies of this reaction, there remain several unanswered questions about how EP interacts with the Pt surface, the effect of surface structure on the reaction, and ultimately the mechanism of chiral discrimination.…”

Section: Introductionmentioning

confidence: 99%

Structure Sensitivity in Catalytic Hydrogenation at Platinum Surfaces Measured by Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS)

et al. 2016

View full text Add to dashboard Cite

The in situ combination of electrochemistry and shell-isolated nanoparticle enhanced Raman spectroscopy (SHINERS) has been used for the first time to investigate the surface structure sensitivity of asymmetric catalytic hydrogenation at single-crystal Pt electrodes. The adsorption and hydrogenation behavior of aqueous ethyl pyruvate (EP) at a range of modified and unmodified Pt{hkl} electrodes was measured both by cyclic voltammetry and by recording Raman spectra at hydrogen evolution potentials. Two primary surface intermediates were observed, including the previously reported half-hydrogenation state (HHS), formed by addition of a hydrogen atom to the keto carbonyl group, as well as a new species identified as intact chemisorbed EP bound in a μ2(C,O) configuration. The relative populations of these two species were sensitive to the Pt surface structure; whereas the μ2(C,O) EP adsorbate was dominant at pristine Pt{111} and Pt{100}, the HHS was only observed at these electrodes after the introduction of defects by electrochemical roughening. Intrinsically defective Pt{110} and kinked Pt{321} and Pt{721} surfaces exhibited behavior similar to that of electrochemically roughened basal surfaces, indicating the requirement for low coordination sites for observation of the HHS. Rationalization of the differing behaviors is given on the basis of density functional theory (DFT) calculations, which indicate that the μ2(C,O) EP adsorbate is considerably more stable on basal {111} than on {221} stepped surfaces. A mechanism is proposed in which the μ2(C,O)-bound species is a precursor to the HHS but the rate of the first hydrogen atom addition is slow, leading to a low steady-state population of the HHS at terrace sites. The implications of this in the context of enantioselective hydrogenation at chirally modified Pt are discussed.

show abstract

Section: Introductionmentioning

confidence: 99%

Structure Sensitivity in Catalytic Hydrogenation at Platinum Surfaces Measured by Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS)

et al. 2016

View full text Add to dashboard Cite

show abstract

“…Elucidation of the underlying surface mechanism of the enantioselective hydrogenation of α-ketoesters at cinchona-modified platinum surfaces represents a key challenge to our full understanding of chiral interactions at solid surfaces. − To obtain the best enantioselective excess (ee), the reaction itself is usually carried out using a freshly reduced alumina-supported Pt catalyst at room temperature and elevated hydrogen pressures using acetic acid as solvent and cinchonidine (CD) as the chiral modifier. When optimized, this procedure affords an ee of 97% of the ( R )-product (ethyl lactate (EL)) using the substrate ethyl pyruvate (EP) …”

Section: Introductionmentioning

confidence: 99%

“…Since its original discovery by Orito and co-workers over 30 years ago and in spite of numerous studies, − no overall consensus has been reached on a unified mechanism that can fully explain the entire range of reported phenomena associated with this reaction. Debate and argument surrounds, for example, the mode of interaction of the adsorbed alkaloid with the substrate, , the mechanism of hydrogen transfer to the substrate, , the reasons for rate acceleration upon addition of cinchona modifiers ,,, and whether or not the enantioselectivity of the reaction is governed by reaction at the surface of platinum or the bulk solution phase. − …”

Section: Introductionmentioning

confidence: 99%

Enantioselective Hydrogenation of α-Ketoesters: An in Situ Surface-Enhanced Raman Spectroscopy (SERS) Study

et al. 2011

View full text Add to dashboard Cite

Adsorption of ethyl pyruvate (EP) and methyl pyruvate (MP) has been studied on a SERS-active platinum electrode. Gas-phase experiments under a hydrogen atmosphere show that decarbonylation to form adsorbed CO together with formation of a half-hydrogenated state of MP/EP are the dominant surface reaction pathways occurring under these conditions. Similar findings were obtained using a hydrogen-evolving platinum electrode under potentiostatic control in 0.1 M aqueous sulfuric acid containing EP or MP. DFT calculations have been used to support the assignment of vibrational bands in the SERS obtained from the half-hydrogenated state intermediate. At hydrogen evolution potentials and using concentrations of MP and EP < 0.02M, adsorbed hydrogen adatoms were detected. At higher EP concentrations, hydrogen-atom formation was inhibited by the greater surface coverages of the half-hydrogenated state of EP/MP and to a lesser extent adsorbed CO derived from decarbonylation side reactions. Addition of the chiral modifier cinchonidine (CD) to the acidic electrolyte under conditions of hydrogen evolution resulted in significantly less CO production and a reduction in the intensity of all SERS peaks associated with the half-hydrogenated state. Most importantly, however, the presence of surface hydrogen, previously absent on the unmodified surface, was always observed irrespective of the EP concentration. The significance of this result in relation to the question of the origin of rate enhancement during heterogeneously catalyzed enantioselective hydrogenation of α-ketoesters is discussed.

show abstract

Enantioselective Hydrogenation Using Heterogeneous Modified Catalysts: An Update

Studer¹,

Blaser²,

2003

View full text Add to dashboard Cite

The state of the art for the enantioselective hydrogenation applying chirally modified heterogeneous catalysts is reviewed with emphasis on new developments between 1997 and 2002. Discussed are various combinations of metal–modifier–substrate which give enantioselectivities useful for synthetic applications. The three most important asymmetric catalysts types are nickel catalysts modified with tartaric acid, useful for β‐functionalized ketones with ees up to 98.6%, platinum catalysts modified with cinchona alkaloids and related modifiers, successful for α‐functionalized ketones with ees up to 98% and palladium catalysts modified with cinchona alkaloids which achieve ees up to 94% for selected activated CC bonds. Mechanistic investigations comprising surface science and spectroscopic studies often combined with computational modeling as well as kinetic studies are summarized and the various mechanistic models are discussed.

show abstract

On the Implications of Hemiketal Formation during Ethyl Pyruvate Hydrogenation in Alcohol Solvents

Cited by 5 publications

References 19 publications

Structure Sensitivity in Catalytic Hydrogenation at Platinum Surfaces Measured by Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS)

Structure Sensitivity in Catalytic Hydrogenation at Platinum Surfaces Measured by Shell-Isolated Nanoparticle Enhanced Raman Spectroscopy (SHINERS)

Enantioselective Hydrogenation of α-Ketoesters: An in Situ Surface-Enhanced Raman Spectroscopy (SERS) Study

Enantioselective Hydrogenation Using Heterogeneous Modified Catalysts: An Update

Contact Info

Product

Resources

About