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931.
Highly selective asymmetric hydrogenation using a three hindered quadrant bisphosphine rhodium catalyst 总被引:1,自引:0,他引:1
Hoge G Wu HP Kissel WS Pflum DA Greene DJ Bao J 《Journal of the American Chemical Society》2004,126(19):5966-5967
A concise synthesis of both enantiomers of ligand 2 and rhodium complex 5 is presented. The crux of the synthesis is a chiral HPLC separation of the enantiomers of 4. Rhodium complex 5 possesses three hindered quadrants in the steric environment within which a substrate binds. Evidence is presented that this configuration leads to high enantioselectivity (>99% ee) for rhodium-catalyzed asymmetric hydrogenation of alpha-acetamido dehydroamino acids, 6a-e. High enantioselectivities are also reported for the hydrogenation of a substrate precursor, 8, of pharmaceutical candidate, pregabalin. Advantages for large-scale hydrogenation of 8 using catalyst 5a vs Rh-Me-DuPhos are discussed. 相似文献
932.
J.?M.?L’HermiteEmail author V.?Blanchet A.?Le?Padellec B.?Lamory P.?Labastie 《The European Physical Journal D - Atomic, Molecular, Optical and Plasma Physics》2004,28(3):361-366
This paper describes the spectroscopy of Na3F both in the frequency and time domains. The photoionization efficiency curve shows two thresholds, associated to two isomers. The excited electronic states of the C2v isomer have been probed by photodepletion spectroscopy, and the results are used to analyze a time-resolved study of photoexcited Na3F, probed by photoionization. The pump-probe signal clearly shows damped oscillations, the period of which is fitted to
fs, close to twice the previously measured bending mode of Na2F [1], while the relaxation time is
fs.Received: 31 October 2003, Published online: 6 January 2004PACS:
36.40.Qv Stability and fragmentation of clusters - 33.50.-j Fluorescence and phosphorescence; radiationless transitions, quenching (intersystem crossing, internal conversion) 相似文献
933.
LIU Yimin HUANG Gangming & BAO Chengguang . Department of Physics Zhongshan University Guangzhou China . Department of Physics Shaoguan University Shaoguan China 《中国科学G辑(英文版)》2004,47(5):521-530
1 Introduction Quantum dots (QDs), often referred to as artificial atoms, are currently under in-tense study because they provide ideal structures used in optical-electronic microdevices, so they are essential in developing microtechniques. They are also essential in the aca-demic aspect, because rich information on microstructures can be extracted both theo-retically and experimentally. Since the early fabrication of the QDs, external magnetic field has been used to control their propertie… 相似文献
934.
Separation and detection of proteins have been realized on nonionic surfactant-modified poly(dimethylsiloxane) (PDMS) microfabricated devices with end-column amperometric detection. The hydrophobic PDMS channels are turned into hydrophilic ones after being modified with Brij35 and facilitate the separation of proteins. The coating can remarkably reduce the adsorption of large protein molecules and is stable in the range of pH 6-12. The detection of proteins in such channels needs less rinsing time and thus efficiency is raised. Even large molecules of proteins can also be detected with better reproducibility and enhanced plate numbers. The relative standard deviation (RSD) of the migration time for glucose oxidase (GOD) is 2.2% (n = 19). Separation of GOD and myoglobin has been developed in modified channels. Predominant operational variables, such as the coating conditions, the concentration of surfactant and buffer, are studied in detail. 相似文献
935.
Tran NE Lambrakos SG Moore PG Ashraf Imam M Dulcey CS 《The Journal of chemical physics》2004,120(21):10257-10262
A prototype case study is presented that examines the level of hydrogen content in H-SWNTs using the Surface Plasmon Resonance technique. The damping effect and the angular shift in the resonance minimum of an SWNT-gold interface due to the presence of hydrogen is analyzed using a parametric model, which is based on the concept of an effective permittivity. The new approach provides for a non-invasive analysis of the level of hydrogen content in H-SWNTs and is potentially extendable to other carbon-based hydrogen storage materials. 相似文献
936.
Supplementing our recent work on UV-absorbing anionic surfactants, new protocols based on frontal analysis continuous capillary electrophoresis (FACCE) were developed for the investigation of the aggregation threshold of non-UV absorbing anionic, cationic and neutral surfactants, and exemplified with sodium dodecyl sulfate (SDS), tetradecyltrimethylammonium bromide (TTABr) and Brij 35. Contrary to UV-absorbing surfactants, the critical micelle concentration (CMC) determination of non-UV absorbing surfactants requires the use of a marker providing adequate detection capabilities. UV-absorbing markers were selected, according to the charge of the studied surfactant (neutral for SDS and TTABr, anionic for Brij 35). In all cases, the free marker concentration was quantified as a function of the total surfactant concentration. In addition, a modified implementation of FACCE, that we called vacancy FACCE (VFACCE), was employed for the case of the neutral surfactant. VFACCE entails first filling the capillary with the system components to be studied in the background electrolyte, next continuously introducing the plain BGE electrokinetically. The salient theoretical features of FACCE and VFACCE were compared. These new protocols were successfully applied to yield reliable CMC values within short operational time and with low sample consumption. 相似文献
937.
Poriel C Ferrand Y Le Maux P Rault-Berthelot J Simonneaux G 《Inorganic chemistry》2004,43(16):5086-5095
Anodic oxidation of free base and manganese complexes of tetraspirobifluorenylporphyrins leads to the coating of the working electrode by insoluble electroactive poly(9,9'-spirobifluorene-free and manganese porphyrin) films which electrochemical behavior and physicochemical properties are described. After removal from the electrode, the manganese-complexed polymers were evaluated as catalysts for the oxidation of alkenes by iodobenzene diacetate or iodosylbenzene. The results show that the reactions proceeded very efficiently at room temperature with good yields. The electrosynthesized polymer catalysts can be recycled by simple filtration and reused even up to the eighth cycle without loss of activity and selectivity. These results represent an important improvement over those previously described for manganese-porphyrin-catalyzed epoxidation reactions. 相似文献
938.
Tran KM McAnoy AM Bowie JH 《European journal of mass spectrometry (Chichester, England)》2004,10(4):441-448
Stable (CC13CHO)- may be formed in the chemical ionisation ion source of a VG ZAB 2HF mass spectrometer by the SN2(Si) reaction between Me3SiC[triple bond]C13CHO and F-. Vertical (Franck-Condon) one-electron oxidation of (CC13CHO)- in the first of the tandem collision cells of a VG ZAB 2HF mass spectrometer gives CC13CHO. Some of these neutrals have sufficient excess energy to effect rearrangement to HCC13CO, some of which are energised and to decompose to HCC. and 13CO. Thus the neutral rearrangement exclusively involves H migration: no products from O migration are detected. The corresponding two-electron oxidation of (CC13CHO)- gives mainly unrearranged (CC13CHO)+. A minority of these cations are energised and rearrange by H and O migration to yield (HCC13CHO)+ and (OCC13CHO)+ respectively. All experimental observations are backed up by molecular modelling at the CCSD(T)/aug-cc-pVDZ//B3LYP/6-31G(d) level of theory. 相似文献
939.
Le Gal J Latapie L Gressier M Coulais Y Dartiguenave M Benoist E 《Organic & biomolecular chemistry》2004,2(6):876-883
Synthetic pathways to a range of novel semi-rigid tetradentate ligands, with sulfur, amido or amino donor groups, designed to coordinate technetium 99m have been developed. The technetium-99m complexes have been prepared and their stabilities in serum and by metathesis reaction via cysteine exchange reactions were compared. HPLC comparison of two (99m)Tc-complexes and their rhenium analogues led to the first proof of the nature of our radioactive complexes. 相似文献
940.
Allan RD Greenwood JR Hambley TW Hanrahan JR Hibbs DE Itani S Tran HW Turner P 《Organic & biomolecular chemistry》2004,2(12):1782-1788
Reaction of sodium azide with 4-methyl-3,5,6-tribromopyridazine results in the formation of 3,5,6-triazide intermediate which could cyclise to give two possible bicyclic products while ab initio calculations show that the formation of a tricyclic compound is extremely energetically unfavourable. However, experimentally, only one major product is isolated. The structure of this unstable product has been conclusively established by X-ray crystallography as 3,5-diazido-4-methyl[1,5-b]tetrazolopyridazine confirming theoretical predictions. 相似文献