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211.
Dr. Paul Benndorf Dipl.‐Chem. Jochen Kratsch Larissa Hartenstein Dipl.‐Chem. Corinna M. Preuss Prof. Dr. Peter W. Roesky 《Chemistry (Weinheim an der Bergstrasse, Germany)》2012,18(45):14454-14463
The treatment of the recently reported potassium salt (S)‐N,N′‐bis‐(1‐phenylethyl)benzamidinate ((S)‐KPEBA) and its racemic isomer (rac‐KPEBA) with anhydrous lanthanide trichlorides (Ln=Sm, Er, Yb, Lu) afforded mostly chiral complexes. The tris(amidinate) complex [{(S)‐PEBA}3Sm], bis(amidinate) complexes [{Ln(PEBA)2(μ‐Cl)}2] (Ln=Sm, Er, Yb, Lu), and mono(amidinate) compounds [Ln(PEBA)(Cl)2(thf)n] (Ln=Sm, Yb, Lu) were isolated and structurally characterized. As a result of steric effects, the homoleptic 3:1 complexes of the smaller lanthanide atoms Yb and Lu were not accessible. Furthermore, chiral bis(amidinate)–amido complexes [{(S)‐PEBA}2Ln{N(SiMe3)2}] (Ln=Y, Lu) were synthesized by an amine‐elimination reaction and salt metathesis. All of these chiral bis‐ and tris(amidinate) complexes had additional axial chirality and they all crystallized as diastereomerically pure compounds. By using rac‐PEBA as a ligand, an achiral meso arrangement of the ligands was observed. The catalytic activities and enantioselectivities of [{(S)‐PEBA}2Ln{N(SiMe3)2}] (Ln=Y, Lu) were investigated in hydroamination/cyclization reactions. A clear dependence of the rate of reaction and enantioselectivity on the ionic radius was observed, which showed higher reaction rates but poorer enantioselectivities for the yttrium compound. 相似文献
212.
Mohrig JR Beyer BG Fleischhacker AS Ruthenburg AJ John SG Snyder DA Nyffeler PT Noll RJ Penner ND Phillips LA Hurley HL Jacobs JS Treitel C James TL Montgomery MP 《The Journal of organic chemistry》2012,77(6):2819-2828
As part of a comprehensive investigation on the stereochemical aspects of base-catalyzed 1,2-elimination reactions, we have studied a set of acyclic carbonyl substrates that react by an irreversible E1cB mechanism with largely anti stereospecificity. (2)H NMR data show that these reactions using KOH in EtOH/H(2)O under non-ion-pairing conditions produce a minimum of 85-89% anti elimination on stereospecifically labeled tert-butyl (2R*,3R*)- and (2R*,3S*)-3-(3-trifluoromethylphenoxy)-2,3-(2)H(2)-butanoate, S-tert-butyl (2R*,3R*)- and (2R*,3S*)-3-(3-trifluoromethylphenoxy)-2,3-(2)H(2)-butanethioate, and the related ketones, (4R*,5R*)- and (4R*,5S*)-5-(3-trifluoromethylphenoxy)-4,5-(2)H(2)-3-hexanone. With both diastereomers of each substrate available, the KIEs can be calculated and the innate stereoselectivities determined. The elimination reactions of the β-3-trifluoromethylphenoxy substrates occur by E1cB mechanisms with diffusionally equilibrated enolate-anion intermediates. Thus, it is clear that anti elimination does not depend solely upon concerted E2 mechanisms. Negative hyperconjugation provides a satisfactory explanation for the anti stereospecificity exhibited by our carbonyl substrates, where the leaving group activates the anti proton, leading to the enolate intermediate. The activation of the anti proton by negative hyperconjugation may also play a role in the concerted pathways of E2 mechanisms. We have also measured the rates of the hydroxide-catalyzed elimination reactions of butanoate, thiobutanoate, and ketone substrates in EtOH/H(2)O, with β-tosyloxy, acetoxy, and 3-trifluoromethylphenoxy nucleofuges. 相似文献
213.
Hydrogen bonded complexes between two different aliphatic alcohols exhibit donor/acceptor isomerism. In a supersonic jet expansion, the less stable isomer can isomerize to the more stable isomer if the energy difference is sufficiently large and the barrier sufficiently low. We show by FTIR jet spectroscopy that this is progressively the case for methanol/methanol-d1, methanol/ethanol, and methanol/tert-butyl alcohol, until no metastable donor/acceptor isomer persists in the expansion. Collisional relaxation experiments, 18O labeling and quantum chemical calculations are used to assign the spectra. Differences between energetical and spectroscopic acceptor strengths are discussed. 相似文献
214.
Hegedüs M Kovács G Módos K Rontó G Lammer H Panitz C Fekete A 《Journal of photochemistry and photobiology. B, Biology》2006,82(2):94-104
The experiment "Phage and Uracil Response" (PUR) will be accommodated in the EXPOSE facility of the International Space Station (ISS). Its objective is to examine and quantify the effect of specific space conditions on bacteriophage T7 and isolated T7 DNA thin films. In order to define the environmental and technical requirements of the EXPOSE, the samples were subjected to the Experiment Verification Test (EVT). During EVT the samples were exposed to selected space conditions: high vacuum (10(-4) to 10(-6) Pa) and UV-C radiation (254 nm) alone and in combination. Characteristic changes in the absorption spectrum, in the electrophoretic pattern of DNA/phage and the decrease of the amount of PCR products have been detected indicating the damage of isolated and intraphage T7 DNA. Intraphage DNA is more sensitive to simulated space parameters than isolated T7 DNA in thin layers as well. We obtained substantial evidence that DNA lesions accumulate throughout exposure, and the amount of damage depends on the thickness of the layers. According to our preliminary results, the damages by exposure to conditions of dehydration and UV irradiation are larger than the sum of vacuum alone, or radiation alone case, suggesting a synergistic action of space vacuum and UV radiation with DNA being the critical target. 相似文献
215.
Corinna R. Hess Dr. Thomas Weyhermüller Dr. Eckhard Bill Dr. Karl Wieghardt Prof. 《Angewandte Chemie (International ed. in English)》2009,48(20):3703-3706
Mixed doubles : The dimeric complex [{Fe(tim)}2] (see structure, tim=2,3,9,10‐tetramethyl‐1,4,8,11‐tetraazacyclotetradeca‐1,3,8,10‐tetraene) represents an unprecedented complex containing an unsupported Fe? Fe bond. The crystal structure confirms the presence of reduced tim units, thus indicating ligand redox activity. Spectroscopic and computational studies establish a triplet ground state for [{Fe(tim)}2] and suggest a mixed‐valence compound with respect to both the Fe ions and the ligands.
216.
Corinna Naundorf Daniela Ferrari Giovanni Rojas Gerhard Fink Markus Klapper Klaus Müllen 《大分子反应工程》2009,3(8):456-466
The influence of organic supports on the polymerization behavior of post‐metallocene catalysts is studied and compared with similarly supported titanium and zirconium metallocenes. The effects of the immobilization, activation, and polymerization process were studied by video microscopy, laser confocal fluorescence microscopy, SEM, and TEM. A model for the polymerization process for a catalyst supported on latex particles was developed from the results obtained. Organic supports based on latex particles are easily adjustable for different catalysts due to the versatile functionalization of the surfaces and can be applied to different types of olefin polymerization catalysts. They can be considered as an alternative to SiO2 or MgCl2 supports.
217.
Niklas Gangloff Corinna Fetsch Robert Luxenhofer 《Macromolecular rapid communications》2013,34(12):997-1001
The nucleophilic living ring‐opening polymerization of N‐substituted glycine N‐carboxyanhydrides using solid‐phase synthesis resins is reported. By variation of experimental parameters, products with near Poisson distributions are obtained. As opposed to reversible deactivation radical polymerization, the living polymerization is demonstrated to be viable to high monomer conversion and through multiple monomer addition steps. Successful preparation of a multiblock copolypeptoid is proof for a highly living and robust character of the solid‐phase peptoid polymerization.
218.
The focus of this paper is the utilization of improved stresses on the surface of a construction for error estimation. The improved stresses are derived by using stress boundary conditions. They are applied for the calculation of recovered stresses required by the Zienkiewicz–Zhu error indicator. This approach leads to an enhancement of the error estimation on the boundary. (© 2008 WILEY-VCH Verlag GmbH & Co. KGaA, Weinheim) 相似文献