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11.
Sebastian Schroeder Christina Strauch Niklas Gaelings Meike Niggemann 《Angewandte Chemie (International ed. in English)》2019,58(15):5119-5123
A new concept for selectivity control in carbocation‐driven reactions has been identified which allows for the chemo‐, regio‐, and stereoselective addition of nucleophiles to alkynes—assisted vinyl cation formation—enabled by a Li+‐based supramolecular framework. Mechanistic analysis of a model complex (Li2NTf2+?3 H2O) confirms that solely the formation of a complex between the incoming nucleophile and the transition state of the alkyne protonation is responsible for the resulting selective N addition to the vinyl cation. Into the bargain, a general, operationally simple synthetic procedure to previously inaccessible vinyl triflimides is provided. 相似文献
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Homogeneous hydrogenation of barbituric acid derivatives with parahydrogen yields a substantial increase of the (1)H NMR signals of the reaction products. These physiologically relevant compounds were hydrogenated at both ambient and elevated temperatures and pressures using a standard cationic rhodium catalyst. The resulting nonthermal nuclear spin polarization (hyperpolarization) is limited by the spin-lattice relaxation time T(1) of the corresponding nuclei in the products, being shorter than the time constant of the hydrogenation. The signal-to-noise ratio of the NMR spectra could be further increased upon signal averaging the antiphase PHIP signals of 25 successive scans following 30 degrees pulse experiments and a delay of 10 s. 相似文献
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The Courses of the Ammonolyses of the Ammonium Hexafluorometalates of Aluminum, Gallium, and Indium, (NH4)3MF6 (M = Al, Ga, In) The courses of the ammonolysis reactions of the ammonium hexafluorometalates (NH4)3MF6 (M = Al, Ga, In) were investigated with the aid of in‐situ powder diffractometry and differential thermal analysis. Under these conditions, the reaction of (NH4)3AlF6 with gaseous ammonia yields at about 360 °C AlF3 via the intermediates NH4AlF4, Al(NH3)2F3 and Al(NH3)F3. The ammonolysis of (NH4)3GaF6 produces GaN at about 400 °C. Depending upon the actual reaction conditions, the intermediates NH4GaF4 and Ga(NH3)F3 as well as their ammonia adducts NH4GaF4 · NH3 and Ga(NH3)2F3 and the amide‐ammoniate Ga(NH3)(NH2)F2 are observed. In the case of (NH4)3InF6 the intermediates (NH4)3InF6 · NH3 and In(NH3)F3 may exist; there are also indications for the reduction of In(III) to In(I) and for the existence of In(NH3)2F and InF as products of the ammonolysis of (NH4)3InF6. 相似文献
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Meike Fleischhammer 《Journal of solid state chemistry》2009,182(4):942-6907
Co-doped anatase and rutile bulk-samples prepared by the sol-gel technique are found to be paramagnetic at room-temperature. Only further annealing in Ar/H2 gas results in a ferromagnetic behavior. X-ray diffraction and electron-microscope studies reveal for low doping levels <4% the formation of Co-doped rutile samples and the formation of CoTiO3 as a new phase. Co3O4 can be detected in anatase samples with Co doping levels ?4%. The observed Co oxides are reduced by Ar/H2 to Co metal. The room-temperature ferromagnetism can therefore be traced back to a segregation of metallic Co. 相似文献
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Maschler Tobias Strmer-Stephan Bastian Morhard Jrg Stegmaier Thomas Tilebein Meike Griepentrog Hans W. 《Annals of Operations Research》2022,314(2):577-600
Annals of Operations Research - Selecting a vegetation layer design goes along with determining its future irrigation need. Therefore, it is essential to take a design decision that is minimising... 相似文献
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Let p(G) and c(G) denote the number of vertices in a longest path and a longest cycle, respectively, of a finite, simple graph G. Define σ4(G)=min{d(x
1)+d(x
2)+ d(x
3)+d(x
4) | {x
1,…,x
4} is independent in G}. In this paper, the difference p(G)−c(G) is considered for 2-connected graphs G with σ4(G)≥|V(G)|+3. Among others, we show that p(G)−c(G)≤2 or every longest path in G is a dominating path.
Received: August 28, 2000 Final version received: May 23, 2002 相似文献
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Microfluidic systems are capillary networks of varying complexity fabricated originally in silicon, but nowadays in glass and polymeric substrates. Flow of liquid is mainly controlled by use of electroosmotic effects, i.e. application of electric fields, in addition to pressurized flow, i.e. application of pressure or vacuum. Because electroosmotic flow rates depend on the charge densities on the walls of capillaries, they are influenced by substrate material, fabrication processes, surface pretreatment procedures, and buffer additives. Microfluidic systems combine the properties of capillary electrophoretic systems and flow-through analytical systems, and thus biochemical analytical assays have been developed utilizing and integrating both aspects. Proteins, peptides, and nucleic acids can be separated because of their different electrophoretic mobility; detection is achieved with fluorescence detectors. For protein analysis, in particular, interfaces between microfluidic chips and mass spectrometers were developed. Further levels of integration of required sample-treatment steps were achieved by integration of protein digestion by immobilized trypsin and amplification of nucleic acids by the polymerase chain reaction. Kinetic constants of enzyme reactions were determined by adjusting different degrees of dilution of enzyme substrates or inhibitors within a single chip utilizing mainly the properties of controlled dosing and mixing liquids within a chip. For analysis of kinase reactions, however, a combination of a reaction step (enzyme with substrate and inhibitor) and a separation step (enzyme substrate and reaction product) was required. Microfluidic chips also enable separation of analytes from sample matrix constituents, which can interfere with quantitative determination, if they have different electrophoretic mobilities. In addition to analysis of nucleic acids and enzymes, immunoassays are the third group of analytical assays performed in microfluidic chips. They utilize either affinity capillary electrophoresis as a homogeneous assay format, or immobilized antigens or antibodies in heterogeneous assays with serial supply of reagents and washing solutions. 相似文献
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Martin Bröring Prof. Dr. Stephan Link Meike Cordes Carsten D. Brandt 《无机化学与普通化学杂志》2007,633(2):211-218
A new hydrogen terminated 2,2′‐bidipyrrin ligand was prepared from a bipyrrole dialdehyde and 3,4‐diethylpyrrole by a POCl3 induced condensation and isolated as the nickel(II) chelate. Unexpectedly a side reaction occured when base‐deficient and aerobic conditions were chosen in the metalation step. This side reaction led to a novel pentapyrrolic nickel(II) complex with one externally bound pyrrole ring. Further studies showed that the reactions of 2,2′‐bidipyrrins with 3,4‐diethyl‐ or 2,3,4‐trimethylpyrrole and an oxidant resulted in a stepwise exchange of the terminal pyrrole moieties and, in the former case, the introduction of one additional pyrrole ring into one of the two meso positions of the open‐chain tetrapyrrole. 相似文献
20.
The product pattern found for the dimethyldioxirane-mediated oxidation of phenylethyne strongly depends on the reaction conditions. Dimethyldioxirane generated in situ from caroate (HSO(5)(-)) and acetone in acetonitrile-water furnishes phenylacetic acid as the main product. With solutions of dimethyldioxirane in acetone, mandelic acid and phenylacetic acid are mainly formed. The relative abundances of the two acids depend on the residual water present in the dimethyldioxirane-acetone solution. Application of thoroughly dried solutions of the reagent effects increased formation of mandelic acid. When phenylethyne is oxidized by dimethyldioxirane transferred into tetrachloromethane, to minimize traces of water even further, oligomeric mandelic acid is obtained. The results are rationalized by the initial formation of phenyloxirene, which is known to equilibrate with phenylformylcarbene and benzoylcarbene. Subsequent Wolff rearrangement produces intermediate phenylketene, which can be trapped by water as phenylacetic acid or suffer from further oxidation to the alpha-lactone of mandelic acid. The alpha-lactone can either react with water to yield mandelic acid or, under anhydrous conditions, to yield oligomeric mandelic acid. In addition to mandelic acid and phenylacetic acid phenylglyoxylic acid, benzoic acid and benzaldehyde are observed as reaction products. The formation of phenylglyoxylic acid by transfer of two oxygen atoms to the unrearranged carbon skeleton of phenylethyne followed by oxygen insertion into the aldehydic C-H bond of the intermediately formed phenylglyoxal is discussed. In a second pathway this acid is formed by partial oxidation of mandelic acid. Benzaldehyde and benzoic acid are explained as products of the oxidative degradation of the alpha-lactone by dimethyldioxirane. Under in situ conditions benzoic acid is also formed by caroate initiated oxidative decarboxylation of phenylglyoxylic acid and/or intermediate phenylglyoxal. 相似文献