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161.
Asymmetric additions of alkyl radicals, generated from R3B, to chiral 2H-azirine-3-carboxylates offer a new entry to enantio-enriched aziridines, and proceed with high diastereoselectivity when using 8-phenylmenthol as chiral auxiliary. 相似文献
162.
163.
Karl-Siegfried Boos Anne Rudolphi Stefan Vielhauer Andreas Walfort Dieter Lubda Friedhelm Eisenbeiß 《Fresenius' Journal of Analytical Chemistry》1995,352(7-8):684-690
The direct and repetitive injection of untreated biological fluids (e.g., hemolyzed blood, plasma, serum, cell culture and tissue homogenates) onto an HPLC-system and the subsequent analysis of low-molecular weight compounds (e.g. drugs, xenobiotics, metabolites) is rendered possible by a coupled-column configuration and special precolumn packings. For this purpose a new family of chemically and enzymatically tailored reversed-phase packing materials have been prepared. The LC-integrated sample clean-up with these restricted access (bimodal) phases is based on the complete nonadsorptive size exclusion of macromolecules (e.g. proteins) and on the simultaneous dynamic partitioning of the target molecules. The bonded phase which exclusively covers the internal pore surface of a glyceryl-modified silica is a butyryl-(C-4), capryloyl-(C-8) or stearoyl-(C-18) moiety. These ligands allow a classical reversed-phase or ion-pair chromatography during the sample work-up step. The capacity of the n-alkyl phase is comparable with conventional silica based RP-materials. The broad hydrophobic retentive capability of these packings allows the extraction of a wide variety of compounds of biomedical interest. The electroneutral and hydrophilic particle exterior (glyceryl-residues) was generated using either soluble or immobilized enzymes (lipase, esterase) which cleave the fatty acid esters exclusively at the outer surface. Unwanted macromolecular components of a sample (e.g. proteins) are quantitatively eluted in the void volume due to the restricted access given by the pore size (6 nm) and the nonadsorptive external diol coverage. The lifetime of a precolumn (25 × 4 mm I.D.) packed with these novel bimodal, i.e. RP-SEC phases exceeds more than 200 injections of 500 l plasma. In addition to the synthesis, this paper describes an application of each of these Alkyl-Diol Silica (ADS) precolumn packings in fully automated coupled-column HPLC systems for the analysis of drugs and endogenous compounds in different biological matrices.Dedicated to Professor Dr. Dr. h.c. mult. J.F.K. Huber on the occasion of his 70th birthday 相似文献
164.
Torsten Barfels Bernd Schmidt Andreas von Czarnowski Hans-Joachim Fitting 《Mikrochimica acta》2002,139(1-4):11-16
For investigation of the luminescent center profile cathodoluminescence measurements are used under variation of the primary
electron energy E
0 = 2…30 keV. Applying a constant incident power regime (E
0·I
0 = const), the depth profiles of luminescent centers are deduced from the range of the electron energy transfer profiles dE/dx.
Thermally grown SiO2 layers of thickness d = 500 nm have been implanted by Ge+-ions of energy 350 keV and doses (0.5–5)1016 ions/cm2. Thus Ge profiles with a concentration maximum of (0.4 – 4) at% at the depth of dm≅240 nm are expected. Afterwards the layers have been partially annealed up to T
a = 1100 °C for one hour in dry nitrogen. After thermal annealing, not only the typical violet luminescence (λ = 400 nm) of
the Ge centers is strongly increased but also the luminescent center profiles are shifted from about 250 nm to 170 nm depth
towards the surface. This process should be described by Ge diffusion processes, precipitation and finally Ge nanocluster
formation. Additionally, a Ge surface layer is piled-up extending to a depth of roughly 25 nm. 相似文献
165.
Karappulli P. Balakrishnan Thomas A. Kaden Liselotte Siegfried Andreas D. Zuberbühler 《Helvetica chimica acta》1984,67(4):1060-1069
The complexation of Cu(I) and Cu(II) by a series of 12-, 14- and 16-membered macrocyclic ligands 1–6 containing the N2S2 donor set has been studied potentiometrically, spectrophotometrically and voltammetrically. In the case of Cu(II), mononuclear complexes CuL2+ with stability constants of 1010–1015 are formed. In addition, partially hydrolyzed species Cu(L)OH+ are observed at pH > 10 for the 12-membered ligands. For Cu(I), beside the specis CuL+ with stabilities of 1012–1014, the unexpected formation of protonated species CuLH2+ was detected. In contrast to the well-known general trends in coordination chemistry, the stability of these protonated species increases relative to that of the complexes with the neutral ligand when the ring size and concomitantly the distance between neighbouring donor atoms is decreased. From the stability constants of the Cu(I)- and Cu(II)-complexes the redox potentials have been calculated and are compared to the values of E1/2 obtained by cyclic voltammetry. Despite the identical donor set the Cu(II)/Cu(I) redox potentials of the complexes are spanning a range of 340 mV or six orders of magnitude in relative stability, reflecting the importance of subtle differences in steric requirements. 相似文献
166.
Guldi DM Gouloumis A Vázquez P Torres T Georgakilas V Prato M 《Journal of the American Chemical Society》2005,127(16):5811-5813
Induction of self-organization between zinc phthalocyanine (ZnPc) and C60 moieties in a novel amphiphilic ZnPc-C60 salt results in uniformly nanostructured 1-D nanotubules. Their photoreactivity, in terms of ultrafast charge separation (i.e., approximately 1012 s-1) and ultraslow charge recombination (i.e., approximately 103 s-1), is remarkable. In addition, the observed ZnPc*+-C60*- lifetime of 1.4 ms implies, relative to that of the monomeric ZnPc-C60 ( approximately 3 ns), an impressive stabilization of 6 orders of magnitude. 相似文献
167.
The carbodiphosphorane CO2 adduct O2CC(PPh3)2 ( 1a ) reacts with [(CO)5W(THF)] and [(CO)3W(NCEt)3] to produce the complexes [(CO)5W{η1‐O2CC(PPh3)2}] ( 2 ) and [(CO)4W{η2‐O2CC(PPh3)2}] ( 3 ), respectively. Whereas in 2 the betain‐like ligand is coordinated at the tungsten atom in a monodentate manner, in 3 it acts as a chelating ligand with formation of a WO2C four‐membered ring. As a by‐product during the reaction with the acetonitrile adduct also some crystals of the hydrolysis product [HC(PPh3)2]2[W6O19] · 3C2H4Cl2 (4 · 3C2H4Cl2) were isolated. All compounds could be characterized by X‐ray analyses and the usual spectroscopic methods. 相似文献
168.
We describe a route for the polymer supported total synthesis of the cyclic bisbibenzyls of the isoplagiochin type found in liverworts. TentaGel® resins were used as solid support for a sequence involving Suzuki, Wittig and hydrogenation protocols. The polymer linked intermediates could be characterized by HR-MAS NMR. This route is to be extended to the synthesis of small libraries of differently halogenated derivatives. 相似文献
169.
170.
Hans Bock Claudia Arad Christian Nther Zdenek Havlas Ilka Gbel Andreas John Markus Kleine 《Helvetica chimica acta》1995,78(4):866-878
Electron Transfer and Ion Pair Formation Single Crystal Structure of Bis(sodium 1,1′-biphenyl-2-thiolate-diglyme): An Intermediate in the Reductive Ring Opening of Dibenzothiophene On Na-metal reduction of dibenzothiophene, the five-membered sulfur ring opens to form a colorless 1,1′-biphenyl-2-thiolate sodium salt, which, according to its single-crystal structure determination, is a dimer containing a four-membered, twice diglyme-solvated ring (diglyme···Na⊕?SR)2. Additional measurements provide the following information: cyclic voltammetry in aprotic MeCN solution shows one quasi-reversible electron transfer at E = ?2.58 V. The dibenzothiophene radical anion can be generated in aprotic THF solution at a K mirror and characterized by an 81-line ESR spectrum and its simulation. This blue species is also the first UV/VIS detectable one before the solution changes via green (due to blue + yellow color mixing) to yellow, yielding across an isosbestic point a second and diamagnetic compound. All of the above results suggest a consecutive two-electron reduction followed by an intersystem protonation, M + (e?) → M.? (blue) + (e?) → (M??, yellow?) + (H⊕) → MH? (colorless), to yield the crystallized and structurally characterized reaction intermediate. The diglyme-solvated sodium-salt dimer provides a basis for a quantum-chemical discussion of some facets of the most likely microscopic reduction pathway. 相似文献