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排序方式: 共有121条查询结果,搜索用时 109 毫秒
51.
52.
Highly Modular P?OP Ligands for Asymmetric Hydrogenation: Synthesis,Catalytic Activity,and Mechanism
Héctor Fernández‐Pérez Dr. Steven M. A. Donald Dr. Ian J. Munslow Dr. Jordi Benet‐Buchholz Feliu Maseras Prof. Anton Vidal‐Ferran Prof. 《Chemistry (Weinheim an der Bergstrasse, Germany)》2010,16(22):6495-6508
A library of enantiomerically pure P? OP ligands (phosphine–phosphite), straightforwardly available in two synthetic steps from enantiopure Sharpless epoxy ethers is reported. Both the alkyloxy and phosphite groups can be optimized for maximum enantioselectivity and catalytic activity. Their excellent performance in the Rh‐catalyzed asymmetric hydrogenation of a wide variety of functionalized alkenes (26 examples) and modular design makes them attractive for future applications. The lead catalyst incorporates an (S)‐BINOL‐derived (BINOL=1,1′‐bi‐2‐naphthol) phosphite group with computational studies revealing that this moiety has a dual effect on the behavior of our P? OP ligands. On one hand, the electronic properties of phosphite hinder the binding and reaction of the substrate in two out of the four possible manifolds. On the other hand, the steric effects of the BINOL allow for discrimination between the two remaining manifolds, thereby elucidating the high efficiency of these catalysts. 相似文献
53.
Ruy Fabila-Monroy David Flores-Peñaloza Clemens Huemer Ferran Hurtado Jorge Urrutia David R. Wood 《Graphs and Combinatorics》2012,28(3):365-380
For a graph G and integer k ≥ 1, we define the token graph F k (G) to be the graph with vertex set all k-subsets of V(G), where two vertices are adjacent in F k (G) whenever their symmetric difference is a pair of adjacent vertices in G. Thus vertices of F k (G) correspond to configurations of k indistinguishable tokens placed at distinct vertices of G, where two configurations are adjacent whenever one configuration can be reached from the other by moving one token along an edge from its current position to an unoccupied vertex. This paper introduces token graphs and studies some of their properties including: connectivity, diameter, cliques, chromatic number, Hamiltonian paths, and Cartesian products of token graphs. 相似文献
54.
Prosenjit Bose Erik D. Demaine Ferran Hurtado John Iacono Stefan Langerman Pat Morin 《Discrete and Computational Geometry》2007,37(3):325-339
Let P be a simple polygon with m vertices, k of which are reflex, and which contains r red points and b blue
points in its interior. Let n = m + r + b. A ham-sandwich geodesic is a shortest path in P between two points on the boundary
of P that simultaneously bisects the red points and the blue points. We present an O(n log k)-time algorithm for finding a
ham-sandwich geodesic. We also show that this algorithm is optimal in the algebraic computation tree model when parameterizing
the running time with respect to n and k. 相似文献
55.
Inside Cover: Enantiopure Narrow Bite‐Angle POP Ligands: Synthesis and Catalytic Performance in Asymmetric Hydroformylations and Hydrogenations (Chem. Eur. J. 47/2014)
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56.
Enantiopure Narrow Bite‐Angle POP Ligands: Synthesis and Catalytic Performance in Asymmetric Hydroformylations and Hydrogenations
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Dr. Héctor Fernández‐Pérez Dr. Jordi Benet‐Buchholz Prof. Dr. Anton Vidal‐Ferran 《Chemistry (Weinheim an der Bergstrasse, Germany)》2014,20(47):15375-15384
Herein is reported the preparation of a set of narrow bite‐angle P–OP ligands the backbone of which contains a stereogenic carbon atom. The synthesis was based on a Corey–Bakshi–Shibata (CBS)‐catalyzed asymmetric reduction of phosphomides. The structure of the resulting 1,1‐P–OP ligands, which was selectively tuned through adequate combination of the configuration of the stereogenic carbon atom, its substituent, and the phosphite fragment, proved crucial for providing a rigid environment around the metal center, as evidenced by X‐ray crystallography. These new ligands enabled very good catalytic properties in the Rh‐mediated enantioselective hydrogenation and hydroformylation of challenging and model substrates (up to 99 % ee). Whereas for asymmetric hydrogenation the optimal P–OP ligand depended on the substrate, for hydroformylation, a single ligand was the highest‐performing one for almost all studied substrates: it contains an R‐configured stereogenic carbon atom between the two phosphorus ligating groups, and an S‐configured 3,3′‐diphenyl‐substituted biaryl unit. 相似文献
57.
A variety of methyl esters are converted into the corresponding propyl esters upon treatment with l-propanol and chlorotrimethylsilane. Among them acyclic aliphatic esters have the best conversion rate. 相似文献
58.
Alfredo García Ferran Hurtado Clemens Huemer Javier Tejel Pavel Valtr 《Computational Geometry》2009,42(9):913-922
Let S be a set of n4 points in general position in the plane, and let h<n be the number of extreme points of S. We show how to construct a 3-connected plane graph with vertex set S, having max{3n/2,n+h−1} edges, and we prove that there is no 3-connected plane graph on top of S with a smaller number of edges. In particular, this implies that S admits a 3-connected cubic plane graph if and only if n4 is even and hn/2+1. The same bounds also hold when 3-edge-connectivity is considered. We also give a partial characterization of the point sets in the plane that can be the vertex set of a cubic plane graph. 相似文献
59.
Given a pair of matrices (A, B) we study the Lipschitz stability of its controlled invariant subspaces. A sufficient condition is derived from the geometry
of the set formed by the quadruples (A, B, F, S) where S is an (A, B)-invariant subspace and F a corresponding feedback. 相似文献
60.
It is shown that square free set theoretic involutive non-degenerate solutions of the Yang-Baxter equation whose associated permutation group (referred to as an involutive Yang-Baxter group) is abelian are retractable in the sense of Etingof, Schedler and Soloviev. This solves a problem of Gateva-Ivanova in the case of abelian IYB groups. It also implies that the corresponding finitely presented abelian-by-finite groups (called the structure groups) are poly-Z groups. Secondly, an example of a solution with an abelian involutive Yang-Baxter group which is not a generalized twisted union is constructed. This answers in the negative another problem of Gateva-Ivanova. The constructed solution is of multipermutation level 3. Retractability of solutions is also proved in the case where the natural generators of the IYB group are cyclic permutations. Moreover, it is shown that such solutions are generalized twisted unions. 相似文献