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1.
Deerenberg S Schrekker HS van Strijdonck GP Kamer PC van Leeuwen PW Fraanje J Goubitz K 《The Journal of organic chemistry》2000,65(16):4810-4817
A series of chiral phosphine-phosphite ligands 1-6 have been synthesized and used in the enantioselective palladium-catalyzed reaction of rac-1,3-diphenyl-2-propenyl acetate with dimethyl malonate as nucleophile. Ligands 1a, 2, 3, 5a, 6a, and 6b have been synthesized starting from racemic tert-butylphenylphosphinoborane. The use of dynamically resolved Li phosphide (-)-sparteine provided the optically pure ligands. Crystals of the allylpalladium (6a) complex were obtained, suitable for X-ray crystal structure determination. The X-ray crystal structure of the allylpalladium (6a) complex revealed a longer palladium-carbon bond distance trans to the phosphine moiety indicating that the attack of the nucleophile takes place at the carbon trans to the phosphine moiety. This was confirmed by the fact that the phosphine moiety did not affect the enantioselectivity directly. Under mild reaction conditions, enantioselectivities up to 83% were obtained (25 degrees C) with ligand 1e. Systematic variation of the ligand bridge and the phosphite moiety showed that the configuration of the product is controlled by the atropisomerism of the biphenyl substituent at the phosphite moiety. The conformation of the biphenyl group, in turn, is controlled by the substituent at the chiral carbon in the bridge. Ligands with large bite angles yielded higher enantioselectivities. 相似文献
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Petra DG Kamer PC Spek AL Schoemaker HE van Leeuwen PW 《The Journal of organic chemistry》2000,65(10):3010-3017
A new class of efficient catalysts was developed for the asymmetric transfer hydrogenation of unsymmetrical ketones. A series of chiral N,S-chelates (6-22) was synthesized to serve as ligands in the iridium(I)-catalyzed reduction of ketones. Both formic acid and 2-propanol proved to be suitable as hydrogen donors. Sulfoxidation of an (R)-cysteine-based aminosulfide provided a diastereomeric ligand family containing a chiral sulfur atom. The two chiral centers of these ligands showed a clear effect of chiral cooperativity. In addition, aminosulfides containing two asymmetric carbon atoms in the backbone were synthesized. Both the sulfoxide-containing beta-amino alcohols and the aminosulfides derived from 1,2-disubstituted amino alcohols gave rise to high reaction rates and moderate to excellent enantioselectivities in the reduction of various ketones. The enantioselective outcome of the reaction was favorably affected by selecting the most appropriate hydrogen donor. Enantioselectivities of up to 97% were reached in the reduction of aryl-alkyl ketones. 相似文献
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Bakker R.J. Jaroszynski D.A. van der Meer A.F.G. Oepts D. van Amersfoort P.W. 《Quantum Electronics, IEEE Journal of》1994,30(7):1635-1644
The Free-Electron Laser for Infrared eXperiments (FELIX) offers a unique combination of short electron bunches and long wavelengths, i.e. a slippage parameter μc ranging up to 10. As a consequence, pronounced short-pulse effects can be observed. In this paper the experimental observation of two of these effects is discussed, namely the occurrence of limit-cycle oscillations and the feasibility of tuning of the micropulse duration. The stable limit-cycle oscillation of the macropulse power is due to a modulation of the optical micropulse shape. This is a consequence of a combination of high optical power and short pulses. The former causes synchrotron oscillations of the electrons and the effect is, therefore, closely related to spiking phenomena. The short-pulse nature of FELIX ensures that the oscillations do not evolve into the chaotic behavior normally associated with spiking and the sideband instability. Experimental results are compared with numerical simulations 相似文献
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Sandee AJ van Der Veen LA Reek JN Kamer PC Lutz M Spek AL van Leeuwen PW 《Angewandte Chemie (International ed. in English)》1999,38(21):3231-3235
By a sol-gel process a rhodium complex containing a diphosphane with a large natural P-Rh-P bite angle is covalently anchored in a silica matrix (see picture). The immobilized catalyst is a very selective hydroformylation catalyst that is completely and conveniently separated from the product and can be reused in numerous cycles. 相似文献
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Y.J. Jo J. Jaroszynski A. Yamamoto A. Gurevich S.C. Riggs G.S. Boebinger D. Larbalestier H.H. Wen N.D. Zhigadlo S. Katrych Z. Bukowski J. Karpinski R.H. Liu H. Chen X.H. Chen L. Balicas 《Physica C: Superconductivity and its Applications》2009,469(9-12):566-574
Here, we report an overview of the phase-diagram of single-layered and double-layered Fe arsenide superconductors at high magnetic fields. Our systematic magneto-transport measurements of polycrystalline SmFeAsO1-xFx at different doping levels confirm the upward curvature of the upper critical magnetic field Hc2(T) as a function of temperature T defining the phase boundary between the superconducting and metallic states for crystallites with the ab planes oriented nearly perpendicular to the magnetic field. We further show from measurements on single-crystals that this feature, which was interpreted in terms of the existence of two superconducting gaps, is ubiquitous among both series of single- and double-layered compounds. In all compounds explored by us the zero temperature upper critical field Hc2(0), estimated either through the Ginzburg–Landau or the Werthamer–Helfand–Hohenberg single gap theories, strongly surpasses the weak-coupling Pauli paramagnetic limiting field. This clearly indicates the strong-coupling nature of the superconducting state and the importance of magnetic correlations for these materials. Our measurements indicate that the superconducting anisotropy, as estimated through the ratio of the effective masses γ = (mc/mab)1/2 for carriers moving along the c-axis and the ab-planes, respectively, is relatively modest as compared to the high-Tc cuprates, but it is temperature, field and even doping dependent. Finally, our preliminary estimations of the irreversibility field Hm(T), separating the vortex-solid from the vortex-liquid phase in the single-layered compounds, indicates that it is well described by the melting of a vortex lattice in a moderately anisotropic uniaxial superconductor. 相似文献
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Brunetti E Shanks RP Manahan GG Islam MR Ersfeld B Anania MP Cipiccia S Issac RC Raj G Vieux G Welsh GH Wiggins SM Jaroszynski DA 《Physical review letters》2010,105(21):215007
Progress in laser wakefield accelerators indicates their suitability as a driver of compact free-electron lasers (FELs). High brightness is defined by the normalized transverse emittance, which should be less than 1π mm mrad for an x-ray FEL. We report high-resolution measurements of the emittance of 125 MeV, monoenergetic beams from a wakefield accelerator. An emittance as low as 1.1±0.1π mm mrad is measured using a pepper-pot mask. This sets an upper limit on the emittance, which is comparable with conventional linear accelerators. A peak transverse brightness of 5×101? A m?1 rad?1 makes it suitable for compact XUV FELs. 相似文献
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Rowlands-Rees TP Kamperidis C Kneip S Gonsalves AJ Mangles SP Gallacher JG Brunetti E Ibbotson T Murphy CD Foster PS Streeter MJ Budde F Norreys PA Jaroszynski DA Krushelnick K Najmudin Z Hooker SM 《Physical review letters》2008,100(10):105005
The generation of quasimonoenergetic electron beams, with energies up to 200 MeV, by a laser-plasma accelerator driven in a hydrogen-filled capillary discharge waveguide is investigated. Injection and acceleration of electrons is found to depend sensitively on the delay between the onset of the discharge current and the arrival of the laser pulse. A comparison of spectroscopic and interferometric measurements suggests that injection is assisted by laser ionization of atoms or ions within the channel. 相似文献