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排序方式: 共有6161条查询结果,搜索用时 390 毫秒
991.
Jianhua Xie Weiling Kong Xiaocheng Wang Wenju Bai Lixin Wang Qilin Zhou 《Frontiers of Chemistry in China》2009,4(3):299-306
The asymmetric hydrogenation of the conformationally flexible racemic α-substituted acyclic dialkyl ketones via dynamic kinetic
resolution (DKR) has been developed by using Ru-SDPs/diamine catalysts. Chiral alcohols were produced in high yields with
good to excellent enantioselectivities (85%–97% ee) and diastereoselectivities (up to 97:3). This hydrogenation reaction provided
a new approach to the synthesis of the key intermediate of J-104118. 相似文献
992.
993.
He-Xiang Bai Jing-Mei Wang Shi-Jing Xia Quan-Rui Wang 《Journal of chemical crystallography》2011,41(3):316-321
Abstract
A novel pentaheterocyclic ring system derived from (5α)-cholestan-3-one, i.e. [1R-[1α(R*),3aβ,3bα,5aβ,12aα,12bβ,14aα]]-1-(1,5-dimethylhexyl)-1,2,3,3a,3b,4,5,11,12,12a,12b,13,14,14a-tetradecahydro-8,12a,14a-trimethyl-9-(2,4,6-trichlorophenyl)-cyclopenta[5,6]naphtho[2,1-d][1,2,4]triazolo[1,5-a]azepinium hexachloroantimonate (6) has been synthesized via the reverse-electron-demand 1,3-dipolar cycloaddition of the 1-aza-2-azoniaallene cation 4 to the triple bond of acetonitrile followed by ring enlargement. The structure of 6 was determined by NMR, IR and high-resolution mass spectra, and unequivocally confirmed by X-ray crystallographic analysis. The title compound crystallizes in monoclinic class under the space group P2-1 with a = 8.163(3) ?, b = 11.214(4) ?, c = 24.191(9) ?, α = 90°, β = 97.740(5)°, and γ = 90°. The five-membered triazole ring is essentially planar and aromatic, while the seven-membered azepine ring is not planar, but adopts a chair-like conformation. 相似文献994.
Dongbin Dang Haiyan Li Guangshui Zheng Yan Bai 《Journal of chemical crystallography》2011,41(11):1612-1615
Abstract
A new Ag(I) complex Ag(HL)2(NO3) 1 has been synthesized with 3,5-diphenylpyrazole (HL) and characterized by IR spectrum, elemental analysis and X-ray crystallography. The crystallographic data for the complex 1: triclinic P[`\text1] P{\bar{\text{1}}} , a = 10.4369(17) ?, b = 10.4837(18) ?, c = 12.963(2) ?, α = 69.318(2)°, β = 86.733(3)°, γ = 88.263(2)°, V = 1324.7(4) ?3, Z = 2, Dc = 1.530 Mg m−3, F(000) = 620, μ = 0.803 mm−1, R 1 = 0.0251, wR 2 = 0.0638. Ag(I) atom is bound to two nitrogen atoms from two ligands. In solid state there are π···π interactions and C–H···π interactions between adjacent units to generate 2D supramolecular structure. The title complex shows strong luminescent emission at 389 and 467 nm in solid state at room temperature. 相似文献995.
The simplified methodology for obtaining the Lie algebra structures of nonlinear evolution equations
The technique for obtaining the prolongation structure of differential equations is simplified. This new simplified method is used to obtain the Lie algebra structure of the Burgers–KdV equation. 相似文献
996.
By using elementary symmetric functions, this paper presents an explicit representation for the Lagrangian numerical differentiation formula as well as the error estimate for local approximation. And we also point out that the numerical differentiation formula constructed by Li [J.P. Li, General explicit difference formulas for numerical differentiation, J. Comput. Appl. Math. 183 (2005) 29-52] is actually a special case of the Lagrangian numerical differentiation formula to approximate the values of the derivatives at the nodes. 相似文献
997.
Precondition plays a critical role in the numerical methods for large and sparse linear systems. It is also true for nonlinear algebraic systems. In this paper incomplete Gröbner basis (IGB) is proposed as a preconditioner of homotopy methods for polynomial systems of equations, which transforms a deficient system into a system with the same finite solutions, but smaller degree. The reduced system can thus be solved faster. Numerical results show the efficiency of the preconditioner. 相似文献
998.
999.
Motivated by the pooling designs over the incidence matrices of matchings with various sizes of the complete graph K2n considered by Ngo and Du [Ngo and Du, Discrete Math. 243 (2003) 167–170], two families of pooling designs over the incidence matrices oft-cliques (resp. strongly t-cliques) with various sizes of the Johnson graph J(n,t) (resp. the Grassmann graph Jq(n,t)) are considered. Their performances as pooling designs are better than those given by Ngo and Du. Moreover, pooling designs associated with other special distance-regular graphs are also considered. 相似文献
1000.
在原始对偶内点算法的设计和分析中,障碍函数对算法的搜索方法和复杂性起着重要的作用.本文由核函数来确定障碍函数,设计了一个求解半正定规划问题的原始-对偶内点算法.这个障碍函数即可以定义算法新的搜索方向,又度量迭代点与中心路径的距离,同时对算法的复杂性分析起着关键的作用.我们计算了算法的迭代界,得出了关于大步校正法和小步校正法的迭代界,它们分别是O(√n log n 10g n/ε)和O(√n log n/ε),这里n是半正定规划问题的维数.最后,我们根据一个算例,说明了算法的有效性以及对核函数的参数的敏感性. 相似文献