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141.
Antonio L. Braga Helmoz R. Appelt Claudio C. Silveira Ludger A. Wessjohann Paulo H. Schneider 《Tetrahedron》2002,58(52):10413-10416
The enantioselective alkynylation reaction of aldehydes with alkynes and diethylzinc, catalyzed by chiral disulfide–oxazolidine ligands, provides a simple, practical and inexpensive method to access chiral propargylic alcohols in good yields and satisfactory ee's. 相似文献
142.
Stripelike domains of Langmuir monolayers formed by surfactants with partially fluorinated lipid anchors (F-alkyl lipids) are observed at the gas-liquid phase coexistence. The average periodicity of the stripes, measured by fluorescence microscopy, is in the micrometer range, varying between 2 and 8 microm. The observed stripelike patterns are stabilized due to dipole-dipole interactions between terminal- CF(3) groups. These interactions are particularly strong as compared with nonfluorinated lipids due to the low dielectric constant of the surrounding media (air). These long-range dipolar interactions tend to elongate the domains, in contrast to the line tension that tends to minimize the length of the domain boundary. This behavior should be compared with that of the lipid monolayer having alkyl chains, and which form spherical microdomains (bubbles) at the gas-liquid coexistence. The measured stripe periodicity agrees quantitatively with a theoretical model. Moreover, the reduction in line tension by adding traces (0.1 mol %) of cholesterol results, as expected, in a decrease in the domain periodicity. 相似文献
143.
144.
Crystal Structures of MgCrO4-type Li2VCl4 and Spinel-type Li2MgCl4 and Li2CdCl4 The crystal structures of the ternary lithium chlorides Li2MCl4 (M = Mg, V, Cd) have been determined firstly by X-ray single-crystal experiments. Li2MgCl4 and Li2CdCl4 crystallize in an inverse spinel structure (space group Fd3 m, Z = 8, a = 1 040.1(2) and 1 062.06(9) pm, structural parameters u = 0.25699(2) and 0.2550(1), R = 1.7 and 3.7% for 218 and 211 unique reflections). The Li? Cl distances of the tetrahedrally coordinated Li+ ions are significantly greater than calculated with Shannon's crystal radii ( > 238 ± 1 instead of 233 pm). Contrary to the results of X-ray powder data reported in the literature, Li2VCl4 crystallizes in the distorted spinel structure of MgCr2O4 type (space group F4 3m, Z = 8, a = 1 037.49(2) pm, R = 5.9% for 217 unique reflections). The decrease of the site symmetry of the octahedrally coordinated ions (V2+, Li+) from 3 m to 3m resulting in contracted and widened tetrahedral M4 entities of the spinel structure is obviously caused by V? V metal—metal bonds (shortest V? V distance 366.2(7) pm). 相似文献
145.
146.
147.
The effect of fluctuations and anisotropy on the transition from the normal to the superconducting state are studied. Neglecting magnetic fluctuations, which is justified as long as the Ginzburg-Landau parameter 1, the critical behavior belongs to thexy-universality class including superfluid helium. Since (t)=0
t
2/3, wheret=1-T/T
c
, upon approachingT
c
further, the intrinsic fluctuating magnetic field might change the nature of the transition. Concentrating on thexy-regime, we derive with the aid of the helicity modulus a universal relation betweenT
c
and the amplitudes of the phase correlation length and penetration depth. We also extend the universal critical point amplitude relations to the case of superconductors with uniaxial mass anisotropy. Our analysis of recent specific heat and excess dc conductivity measurement suggest that for both static and dynamic properties three-dimensional critical behavior has been observed. The -like specific heat singularity points to criticalxy-behavior. Further evidence is provided in terms of the universal amplitude relations, providing estimates for the amplitudes of the correlation lengths for the magnitude and phase of the order parameter and the London penetration depth. We find remarkable agreement with experiment and the correlation volume is comparable to that in superfluid helium. 相似文献
148.
149.
Preparation, Characterization and Reaction Behaviour of Sodium and Potassium Hydridosilylamides R2(H)Si—N(M)R′ (M = Na, K) — Crystal Structure of [(Me3C)2(H)Si—N(K)SiMe3]2 · THF The alkali metal hydridosilylamides R2(H)Si—N(M)R′ 1a‐Na — 1d—Na and 1a‐K — 1d‐K ( a : R = Me, R′ = CMe3; b : R = Me, R′ = SiMe3; c : R = Me, R′ = Si(H)Me2; d : R = CMe3, R′= SiMe3) have been prepared by reaction of the corresponding hydridosilylamines 1a — 1d with alkali metal M (M = Na, K) in presence of styrene or with alkali metal hydrides MH (M = Na, K). With NaNH2 in toluene Me2(H)Si—NHCMe3 ( 1a ) reacted not under metalation but under nucleophilic substitution of the H(Si) atom to give Me2(NaNH)Si—NHCMe3 ( 5 ). In the reaction of Me2(H)Si—NHSiMe3 ( 1b ) with NaNH2 intoluene a mixture of Me2(NaNH)Si—NHSiMe3 and Me2(H)Si—N(Na)SiMe3 ( 1b‐Na ) was obtained. The hydridosilylamides have been characterized spectroscopically. The spectroscopic data of these amides and of the corresponding lithium derivatives are discussed. The 29Si‐NMR‐chemical shifts and the 29Si—1H coupling constants of homologous alkali metal hydridosilylamides R2(H)Si—N(M)R′ (M = Li, Na, K) are depending on the alkali metal. With increasing of the ionic character of the M—N bond M = K > Na > Li the 29Si‐NMR‐signals are shifted upfield and the 29Si—1H coupling constants except for compounds (Me3C)(H)Si—N(M)SiMe3 are decreased. The reaction behaviour of the amides 1a‐Na — 1c‐Na and 1a‐K — 1c‐K was investigated toward chlorotrimethylsilane in tetrahydrofuran (THF) and in n‐pentane. In THF the amides produced just like the analogous lithium amides the corresponding N‐silylation products Me2(H)Si—N(SiMe3)R′ ( 2a — 2c ) in high yields. The reaction of the sodium amides with chlorotrimethylsilane in nonpolar solvent n‐pentane produced from 1a‐Na the cyclodisilazane [Me2Si—NCMe3]2 ( 8a ), from 1b‐Na and 1‐Na mixtures of cyclodisilazane [Me2Si—NR′]2 ( 8b , 8c ) and N‐silylation product 2b , 2c . In contrast to 1b‐Na and 1c‐Na and to the analogous lithium amides the reaction of 1b‐K and 1c‐K with chlorotrimethylsilane afforded the N‐silylation products Me2(H)Si—N(SiMe3)R′ ( 2b , 2c ) in high yields. The amide [(Me3C)2(H)Si—N(K)SiMe3]2·THF ( 9 ) crystallizes in the space group C2/c with Z = 4. The central part of the molecule is a planar four‐membered K2N2 ring. One potassium atom is coordinated by two nitrogen atoms and the other one by two nitrogen atoms and one oxygen atom. Furthermore K···H(Si) and K···CH3 contacts exist in 9 . The K—N distances in the K2N2 ring differ marginally. 相似文献
150.