• Einführung • Für die industrielle Fertigung sind optisch reine Produkte nötig • Experimentell s... more • Einführung • Für die industrielle Fertigung sind optisch reine Produkte nötig • Experimentell schwierig: die in situ-Untersuchung von Grenzschichten • Wie lagern sich chirale Moleküle (Modifikatoren) möglichst selektiv an das Platin an? • Kasten 1: Einige Erläuterungen zur Stereochemie • Kasten 2: Einige Erläuterungen zur Spektroskopie • Verwendete Literatur • Internet
Interaction complexes between cinchonidine modifier and methyl pyruvate reactant proposed for the... more Interaction complexes between cinchonidine modifier and methyl pyruvate reactant proposed for the enantioselective hydrogenation over platinum catalysts have been calculated using ab initio methods. For s-trans-methyl pyruvate it was found that the complex yielding (R)-methyl lactate upon hydrogenation was more stable than the corresponding pro-(S) complex. The calculated energy difference of 1.8 kcal/mol corresponds to an enantiomeric excess of 92%, in good agreement with experiment. For the analogous complexes of s-cis-methyl pyruvate the energy difference is only 0.2 kcal/mol in favour of pro-(R), corresponding to 17% enantiomeric excess. Due to the larger dipole moment of the s-cis conformer of methyl pyruvate its hydrogen-bonded complexes with cinchonidine are considerably more stable than the corresponding s-trans complexes. However, the predicted low enantiomeric excess for the s-cis conformer is in contrast with experiment. Possible reasons for this behaviour are discussed.
Platinum particles supported on graphite have been investigated by scanning tunneling microscopy ... more Platinum particles supported on graphite have been investigated by scanning tunneling microscopy (STM). For one monolayer thick Pt particles the individual Pt atoms form a characteristic intensity pattern due to a mismatch between the Pt and graphite lattice. Based on density functional theory calculations and model structures of Pt on graphite it is argued that the observed STM imaging contrast has its origin in the tip induced elastic deformation of the graphite underneath the Pt particle. The Pt±graphite potential is much stier than the graphite±graphite potential. The calculations furthermore indicate that Pt adsorption is favored over top rather than hole sites and that the barrier for diusion is very low. Ó
A new modifier, 2-phenyl-9-deoxy-10,11-dihydrocinchonidine, has been synthesized for the enantios... more A new modifier, 2-phenyl-9-deoxy-10,11-dihydrocinchonidine, has been synthesized for the enantioselective hydrogenation of ketopantolactone and α-ketoesters over chirally modified Pt/alumina. The results indicate flat adsorption of cinchonidine with the quinoline ring oriented parallel to the surface and, furthermore, give some insight into the conformation of the modifier within the transition state complex. Comparison of the structures and catalytic behaviors of 9-deoxycinchonidine and the new modifier allows to exclude the previously proposed perpendicular or tilted adsorption of the quinoline ring via the N atom.
The mechanism of enantiodifferentiation in the hydrogenation of alkenoic acids over cinchona-modi... more The mechanism of enantiodifferentiation in the hydrogenation of alkenoic acids over cinchona-modified Pd has been investigated using the tiglic acid → 2-methyl-butanoic acid transformation as test reaction. Application of simple derivatives of cinchonidine, modified at the (C-9)-OH and/or the quinuclidine nitrogen, proved that both functional groups are involved in the enantiodiscriminating step. Addition of a strong base (1,8-diazabicyclo[5.4.0]undec-7-ene, DBU) to tiglic acid prior to hydrogenation revealed that one cinchonidine molecule interacts with a dimer of tiglic acid on the metal surface. Ab initio calculations corroborate the existence of an energetically favored acid dimer-cinchonidine intermediate stabilized by hydrogen bonding, involving both the OH and the quinuclidine nitrogen of cinchonidine.
ChemInform Abstract: ChemInform Abstract: Design of New Chiral Modifiers for Heterogeneous Enantioselective Hydrogenation: A Combined Experimental and Theoretical Approach
Manganese oxide-silica mixed oxide aerogels with different morphological and chemical properties ... more Manganese oxide-silica mixed oxide aerogels with different morphological and chemical properties were prepared using the sol-gel method and ensuing extraction of the solvent with supercritical CO 2 . Two types of manganese precursor, varying hydrolysis conditions of the silica and manganese precursors, influence of base addition for gelation, and calcination temperatures were investigated. Base addition had a strong effect on textural properties, producing high-surface-area, mesoporous aerogels, whereas these properties were only marginally affected by kind of manganese precursor used. The presence of different manganese oxide species was evidenced by X-ray diffraction, Raman and diffuse reflectance infrared Fourier transform spectroscopy, and temperature-programmed reduction. Mn 4+ , Mn 3+ , and Mn 2+ oxide species were found after calcination at 600 • C in air. Sol-gel processing with manganese(II) nitrate resulted in highly dispersed mixed oxides. Basic gelation of these sols strongly influenced the state of the manganese, leading to crystallites of hausmannite and to amorphous Mn 5 O 8 in the calcined samples. Aerogels derived from the less reactive Mn(III) (acac) 3 did not contain any manganese oxide crystallites when prepared under the same basic conditions. The catalytic performance of the aerogels in the selective oxidation of ammonia strongly depended on the state of the manganese. Samples containing crystalline Mn 3 O 4 were more active than amorphous aerogels with dispersed manganese oxide species and afforded high selectivity to N 2 O. The presence of amorphous Mn 5 O 8 further increased the activity and the selectivity to nitrous oxide, reaching 74% at 360 • C. Nitrogen formation was found to be related to the amount of strongly Lewis-bound ammonia. The amorphous aerogels showing more Lewis-bound ammonia produced mainly nitrogen below 480 • C, affording a selectivity of 78% at 360 • C. c 2002 Elsevier Science (USA) Key Words: manganese oxide-silica aerogels; selective oxidation of ammonia; supercritical drying; hausmannite; Mn 5 O 8 ; Brønsted and Lewis acidity.
The relation between the electronic structure of α-substituted ketones and their reactivity in th... more The relation between the electronic structure of α-substituted ketones and their reactivity in the racemic and enantioselective platinum-catalyzed hydrogenation has been investigated using a combined theoretical and experimental approach. A correlation between the keto carbonyl orbital energy and the hydrogenation rate has been found, which rationalizes the effect of the substituent on the rate of hydrogenation. The uncovered relationship between the keto carbonyl orbital energy and the hydrogenation rate provides a rational explanation for the often observed rate acceleration that occurs when cinchona-modified platinum is used as a enantioselective hydrogenation catalyst. The previously suggested model for enantiodiscrimination based on the different stability of the diastereomeric complexes formed between the reactant and the cinchona modifier is discussed in the light of the new kinetic findings. c 2002 Elsevier Science (USA)
Mixed gels of vanadia and silica: Structural properties and catalytic behavior in selective reduction of nitric oxide with ammonia
Abstract Mixed gel catalysts of vanadia and silica containing 0.1, 1, 10, 20, and 50 wt% of vanad... more Abstract Mixed gel catalysts of vanadia and silica containing 0.1, 1, 10, 20, and 50 wt% of vanadia were prepared from alkoxides of the constituents. The catalysts were investigated with regard to their physical and chemical properties, and their activity in the selective ...
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