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21.
Chiral Modification of Platinum by Co‐Adsorbed Cinchonidine and Trifluoroacetic Acid: Origin of Enhanced Stereocontrol in the Hydrogenation of Trifluoroacetophenone 下载免费PDF全文
Fabian Meemken Prof. Dr. Alfons Baiker Dr. Sebastian Schenker Prof. Dr. Konrad Hungerbühler 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(5):1298-1309
Cinchonidine (CD) adsorbed onto a platinum metal catalyst leads to rate acceleration and induces strong stereocontrol in the asymmetric hydrogenation of trifluoroacetophenone. Addition of catalytic amounts of trifluoroacetic acid (TFA) significantly enhances the enantiomeric excess from 50 to 92 %. The origin of the enantioselectivity bestowed by co‐adsorbed CD and TFA is investigated by using in situ attenuated total reflection infrared spectroscopy and modulation excitation spectroscopy. Molecular interactions between the chiral modifier (CD), acid additive (TFA) and the trifluoro‐activated substrate at the solid–liquid interface are elucidated under conditions relevant to catalytic hydrogenations, that is, on a technical Pt/Al2O3 catalyst in the presence of H2 and solvent. Monitoring of the unmodified and modified surface during the hydrogenation provides an insight into the phenomenon of rate enhancement and the crucial interactions of CD with the ketone, corresponding product alcohol, and TFA. Comparison of the diastereomeric interactions occurring on the modified surface and in the liquid solution shows a striking difference for the chiral preferences of CD. The spectroscopic data, in combination with calculations of molecular structures and energies, sheds light on the reaction mechanism of the heterogeneous asymmetric hydrogenation of trifluoromethyl ketones and the involvement of TFA in the diastereomeric intermediate surface complex: the quinuclidine N atom of the adsorbed CD forms an N?H?O‐type hydrogen‐bonding interaction not only with the trifluoro‐activated ketone but also with the corresponding alcohol and the acid additive. Strong evidence is provided that it is a monodentate acid/base adduct in which the carboxylate of TFA resides at the quinuclidine N‐atom of CD, which imparts a better stereochemical control. 相似文献
22.
Philipp Rotering Lukas F. B. Wilm Janina A. Werra Dr. Fabian Dielmann 《Chemistry (Weinheim an der Bergstrasse, Germany)》2020,26(2):406-411
Electron-rich tertiary phosphines are valuable species in chemical synthesis. However, their broad application as ligands in catalysis and reagents in stoichiometric reactions is often limited by their costly synthesis. Herein, we report the synthesis and properties of a series of phosphines with 1-alkylpyridin-4-ylidenamino and 1-alkylpyridin-2-ylidenamino substituents that are accessible in a very short and scalable route starting from commercially available aminopyridines and chlorophosphines. The determination of the Tolman electronic parameter (TEP) value reveals that the electron donor ability can be tuned by the substituent pattern at the aminopyridine backbone and it can exceed that of common alkylphosphines and N-heterocyclic carbenes. The potential of the new phosphines as strong nucleophiles in phosphine-mediated transformations is demonstrated by the formation of Lewis base adducts with CO2 and CS2. In addition, the coordination chemistry of the new phosphines towards CuI, AuI, and PdII metal centers has been explored, and a convenient procedure to introduce the most basic phosphine into metal complexes starting from air-stable phosphonium salt is described. 相似文献
23.
Alexander V. Fejzagić Sebastian Myllek Fabian Hogenkamp Julian Greb Prof. Dr. Jörg Pietruszka Dr. Thomas Classen 《ChemistryOpen》2020,9(9):959-966
Screening for an interesting biocatalyst and its subsequent kinetic characterization depends on a reliable activity assay. In this work, a fluorometric assay based on the halogenation of 4-methyl-7-diethylamino-coumarin was established to monitor haloperoxidase-activity. Since haloperoxidases utilize hydrogen peroxide and halide ions to halogenate a broad range of substrates by releasing hypohalous acids, a direct quantification of haloperoxidase-activity remains difficult. With the system presented here, 3-bromo-4-methyl-7-diethylaminocoumarin is preferentially formed and monitored by fluorescence measurements. As starting material and product share similar spectroscopical properties, a two-dimensional calibration ap-proach was utilized to allow for quantification of each compound within a single measurement. To validate the system, the two-dimensional Michaelis-Menten kinetics of a vanadium-dependent chloroperoxidase from Curvularia inaequalis were recorded, yielding the first overall kinetic parameters for this enzyme. With limits of detection and quantification in the low μm range, this assay may provide a reliable alternative system for the quantification of haloperoxidase-activity. 相似文献
24.
Lanyue Zhang Zhaochen Jiang Jochen Mellmann Fabian Weigler Fabian Herz Andreas Büick Evangelos Tsotsas 《Particuology》2021,(3):171-182
Particle distribution in the cross-section of the flighted rotating drum(FRD)is critical to the analysis of heat and mass transfer between gas and solids.In thi... 相似文献
25.
The attempt was undertaken to describe analytically isothermal resistivity measurement curves of Al Zn and Al Zn Mg alloys by means of the Johnson-Mehl-Avrami equation. The paper gives a survey of results obtained based on the application of the Extented Slope Analysis. Comparison was performed to results of fitting calculations (Linear Regression). It can be shown that the intersection behaviour after the Extented Slope Analysis leads to reasonable results if rough evaluation is required in a first stage of an investigation. Furthermore this method leads to reliable starting parameters and starting length of the fitting intervals. Additionally the second process with Al Zn Mg alloys can be estimated by use of the Extented Slope Analysis. 相似文献
26.
27.
Fabian Jachmann 《Solid State Communications》2007,142(4):212-216
In a thin film of superconducting Y Ba2Cu3O7 the impact of surface acoustic waves (SAWs) traveling on the piezoelectric substrate is investigated. A pronounced interaction between the ultrasonic waves and the vortex system in the type II superconductor is observed. The occurrence of a SAW-induced dc voltage perpendicular to the sound path is interpreted as dragging of vortices by the piezoacoustic SAW, which acts as a conveyor for the flux quanta. The antisymmetry of this voltage with respect to the magnetic field directly evidences the induced, directed flux motion. This dynamic manipulation of vortices can be seen as an important step towards flux-based electronic devices. 相似文献
28.
Stutzki F Jansen F Eidam T Steinmetz A Jauregui C Limpert J Tünnermann A 《Optics letters》2011,36(5):689-691
Ytterbium-doped large-pitch fibers with very large mode areas are investigated in a high-power fiber amplifier configuration. An average output power of 294?W is demonstrated, while maintaining robust single-mode operation with a mode field diameter of 62?μm. Compared to previous active large-mode area designs, the threshold of mode instabilities is increased by a factor of about 3. 相似文献
29.
Fabian Wenger 《Zeitschrift für Physik B Condensed Matter》1995,98(2):171-185
We show that weak static random potentials have pronounced effects on the quasiparticle states of a 2Dd-wave superconductor close to a node. We prove that the vertex correction coming from the simplest crossed diagram is important even for a nonmagnetic potential. The leading frequency and momentum dependent logarithmic singularities in the self-energy are calculated exactly to second order in perturbation theory. The self-energy corrections lead to a modified low energy density of states which depends strongly on the type of random potential and which can be measured in experiments. There is an exceptional case for a potential with extremely local scatterers and opposite nodes separated by (, ) where an exact cancelation takes place eliminating the leading frequency dependent singularity in the simplest crossed diagram. A comparison of the perturbative results with a self-consistent CPA (coherent potential approximation) for the nonmagnetic disorder reveals qualitative differences in the self-energy at the smallest energies which are due to the neglectance of vertex corrections in CPA. 相似文献
30.
Journal of Theoretical Probability - We investigate the space-time regularity of the local time associated with Volterra–Lévy processes, including Volterra processes driven by $$\alpha... 相似文献