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排序方式: 共有1943条查询结果,搜索用时 421 毫秒
41.
Chaoran Xu Jianglin Qiao Shunxi Dong Yuqiao Zhou Xiaohua Liu Xiaoming Feng 《Chemical science》2021,12(15):5458
Heterocycles have been widely used in organic synthesis, agrochemical, pharmaceutical and materials science industries. Catalytic three-component ylide formation/cycloaddition enables the assembly of complex heterocycles from simple starting materials in a highly efficient manner. However, asymmetric versions remain a yet-unsolved task. Here, we present a new bimetallic catalytic system for tackling this challenge. A combined system of Rh(ii) salt and chiral N,N′-dioxide–Sm(iii) complex was established for promoting the unprecedented tandem carbonyl ylide formation/asymmetric [4 + 3]-cycloaddition of aldehydes and α-diazoacetates with β,γ-unsaturated α-ketoesters smoothly, affording various chiral 4,5-dihydro-1,3-dioxepines in up to 97% yield, with 99% ee. The utility of the current method was demonstrated by conversion of products to optically active multi-substituted tetrahydrofuran derivatives. A possible reaction mechanism was provided to elucidate the origin of chiral induction based on experimental studies and X-ray structures of catalysts and products.Catalytic asymmetric tandem carbonyl ylide formation/[4 + 3]-cycloaddition of β,γ-unsaturated α-ketoesters, aldehydes and α-diazoacetates was achieved by using a bimetallic rhodium(ii)/chiral N,N′-dioxide–Sm(iii) complex catalyst. 相似文献
42.
不同软段结构聚氨酯-酰亚胺嵌段共聚物的合成及热性能研究 总被引:4,自引:2,他引:4
用二步法合成了不同软段 (PPO ,PEG ,PEPA)聚氨酯 酰亚胺 (PUI)嵌段共聚物 ,FTIR光谱表征了所有合成PUI分子主链均含有酰亚胺链段 ,并研究了PUI嵌段共聚物的热性能受软段类型及长度的影响 .DSC研究表明聚酯型PUI的软硬段之间的相容性比聚醚型PUI好 ,随相同软段分子量的增加 ,PUI体系的软硬段兼容性变差 ,并显示了相分离的特征 ;热失重 (TGA)研究得出不同软段的PUI样品的热稳定性大小顺序为 :PEPA PUI >PEG PUI>PPO PUI ;动态力学 (DMTA)研究给出了所合成的PUI样品在 5 0~ 2 0 0℃范围内均出现了较长的模量平台显示出有较好的耐热性 ,且随硬段含量的升高其储能模量不断增强 相似文献
43.
Hu Chuanjiang Duan Chunying Meng Qingjin Liu Yongjiang Lu Changsheng Reng Xiaoming Chen Yijun Cao Mi 《Transition Metal Chemistry》2000,25(2):141-144
The mixed-valence molybdovanadate compound Na2(NH4)4[VIVVV
8Mo)O28] · 10H2O [Vanadata(6-)tetradeca--oxotetra-3-oxodi-6-oxoheptaoxo(oxomolybdate) nonatetrammonium disodium, decahydrate] has been synthesized from sodium molybdate(VI) dihydrate and sodium metavanadate dihydrate in aqueous solution by adding NH2OH · HCl. The molecular structure has been determined by X-ray diffraction and is based on the isopolydecavanadate structure. The molybdate atom is crystallographically disordered over 6MO6 octahedral sites. The e.s.r. spectrum clearly indicates that one vanadium atom has the oxidation number +4. 相似文献
44.
The mass spectrometry of a group of inorganic oxidizers was studied using the electrospray ionization technique. It was found that a series of cluster ions were predominant in both positive- and negative-ion mode, allowing for the characterization of the investigated oxidizers. The identity of the recorded cluster ions was further confirmed by using some isotopically labeled compounds and tandem mass spectrometry with collision-induced dissociation. The use of electrospray ionization mass spectrometry for positive identification of major oxidizer components in explosive formulations was demonstrated by three samples of forensic interest. 相似文献
45.
Xiaoming Ren Jingli Xie Youcun Chen Reinhard Karl Kremer 《Journal of Molecular Structure》2003,660(1-3):139-146
The two ion-pair complexes, [pyH]2[Zn(mnt)2] (1) and [4,4′-bipyH2]-[Zn(mnt)2] (2), were synthesized, where mnt2− denotes maleonitriledithiolate, and [pyH]+, [4,4′-bipyH2]2+ represent pyridinium and diprotonated 4,4′-bipyridinium, respectively. Their single crystal structures show that there are strong bifurcated H-bonding interactions between the cations of the pyridinium derivative and the [Zn(mnt)2]2− anions in both 1 and 2. The bifurcated H-bonding interactions between the N–H of the pyridiniums and the CN groups of the mnt2− ligands give rise to a 2D layered H-bonding network, the adjacent layers come together in such way as mutual embrace to give a tight pack, thus 2D hydrogen-bonding sheets further develop into 3D H-bonding networks through weak C–HS and ππ stacking interactions in 1. As for 2, the cations and anions connect into several types of H-bonding macrorings ([2+2], [3+3] and [4+4]), these H-bonding macrorings fuse to extend into 2D layered structure, the interpenetration between [3+3] and [4+4] type H-bonding macrorings in the adjacent layers give further rise to novel 3D extended H-bonding networks, in which there are clearly parallel stacks of cations and the chelate rings of anions. 相似文献
46.
Zhigang Liu Renxian Zhou Xiaoming Zheng 《天然气化学杂志》2007,16(2):167-172
The CuO-CeO2 catalyst prepared by chelating method has a superior catalytic performance for the preferential oxidation of CO in rich hydrogen, compared with the CuO-CeO2 catalyst prepared by coprecipitation method. The CO conversions over these catalysts, at 120℃and 120000 ml/(g-h) in the absence of CO2 and H2O, are 99.6% and 88.6%, respectively, and the selectivity of O2 over these catalysts is very close (i.e. 51.3% and 55.8%, respectively). The influence of certain factors such as hydrogen concentration, carbon monoxide concentration, H2O, O2/CO ratios, and space velocity on the catalytic performance of CuO-CeO2 catalyst prepared by chelating method is also studied. The results show that the addition of hydrogen and H2O has a negative effect on the catalytic performance of CuO-CeO2 catalyst, however, the variation of space velocity and the O2/CO ratio causes a comparatively slight influence. 相似文献
47.
Lu C Wei F Wu N Huang L Zhao X Jiao X Luo C Cao W 《Langmuir : the ACS journal of surfaces and colloids》2004,20(3):974-977
A kind of hybrid multilayer film based on mercaptobenzoic acid-capped Au nanoparticles (MBA-Au-NPs) and photoreactive nitrodiazoresin (NDR) has been fabricated via electrostatic self-assembly. Upon exposure to UV light, the initial ionic bonds between the layers of the film convert into covalent bonds and the film stability toward polar solvents, salt, or surfactant solutions increases significantly. The micropatterned NDR/MBA-Au-NP film with the covalently linked architecture was formed by selecting exposure of the film through a photomask and later developed in sodium dodecyl sulfate (SDS) aqueous solution. The metallic Au-NP micropatterns, furthermore, are produced by sintering the micropatterned NDR/MBA-Au-NP film at 550 degrees C, at which the organic components are removed completely. The well-defined micropatterns were characterized with atomic force microscopy (AFM), field emission scanning electron microscopy (FE-SEM), microscope with a charge-coupled device (CCD) camera, and X-ray photoelectron spectroscopy (XPS). 相似文献
48.
49.
Drag force on a metallic or nonmetallic spherical particle exposed to a plasma flow is studied for the extreme case of a free-molecule regime. Analytical expressions are derived for the drag components due to, respectively, atoms, ions, and electrons and for the total drag on the whole sphere due to all the gas species. It has been shown that the drag is proportional to the square of the particle radius or the drag coefficient is independent of the particle radius. At low gas temperatures with a negligible degree of ionization, the drag is caused mainly by atoms and could be predicted by using the well-known drag expression given in ordinary-temperature rarefied gas dynamics. On the other hand, the drag is caused mainly by ions at high plasma temperatures with a great degree of ionization. The contribution of electrons to the total drag is always negligible. Ignoring gas ionization at high plasma temperatures would overestimate the particle drag. There is a little difference between metallic and nonmetallic spheres in their total drag forces, with a slightly higher value for a metallic sphere at high plasma temperatures, but usually such a small difference could be neglected in engineering calculations. The drag increases rapidly with increasing gas pressure or oncoming speed ratio. For a two-temperature plasma, the drag increases at low electron temperatures but decreases at high electron temperatures with the increase in the electron/heavy-particle temperature ratio.Nomenclature
C
d
Drag coefficient
-
e
Elementary charge
-
f
D,F
D
Local and total drag (N/m
2 andN)
-
f
–
Velocity distribution function for incident gas particles
-
f
+
Velocity distribution function for reflected gas particles
-
k
Boltzmann's constant
-
m
Gas particle mass (kg)
-
n
Number density of gas species (m
–3)
-
P
–,P
+
Surface pressure due to incident and reflected gas particles
-
R
0
Sphere radius (m)
-
S
Speed ratio,S
j=U/(2kT
j/mj)1/2
-
T
e,T
h
Electron and heavy-particle (atom, ion) temperature
-
T
w
Wall temperature
-
U
Oncoming plasma flow velocity
-
v
x, vy, vz
Velocity components of gas particles in thex, y, andz directions (m/sec)
-
v
Thermal motion speed of gas particles,v
j
=(8kT
j
/m
j
)1/2
-
v
ze
Smallestv
z of electrons which could reach the sphere surface,v
ze=(2e/m
e)1/2 (m/sec)
-
v
zw
Value ofv
z of ions or electrons as arriving at the sphere surface (m/sec)
-
Center angle
-
Gas density (kg/m3)
-
Shear stress (N/m2)
-
Absolute value of the floating potential (V)
- ,
Local and total particle fluxes incident to the surface
- a
Atoms
- e
Electrons
- h
Heavy particles
- i
Ions
-
j
jth gas species
- m
Metallic sphere
- mn
Nonmetallic sphere
A preliminary version of this paper was presented at the Eighth International Symposium on Plasma Chemistry held in Tokyo, September 1987. 相似文献
50.
Wood TE Dalgleish ND Power ED Thompson A Chen X Okamoto Y 《Journal of the American Chemical Society》2005,127(16):5740-5741
Helical zinc(II) complexes of bis(dipyrromethene)s bearing homochiral amide substituents were synthesized. Analysis of the products by chiral HPLC showed two diastereomeric major products and showed that dipyrromethene double-nuclear helicates are stereochemically stable and do not interconvert. Circular dichroism (CD) studies showed that the complexation reactions had proceeded with modest diastereomeric excesses. Analysis of an analogous symmetric zinc(II) bis(dipyrromethene) lacking homochiral substituents that could act as chromophores discounted induced CD by the chiral auxiliaries. 相似文献