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291.
Siri O Taquet JP Collin JP Rohmer MM Bénard M Braunstein P 《Chemistry (Weinheim an der Bergstrasse, Germany)》2005,11(24):7247-7253
When a 2,5-diamino-1,4-benzoquinonediimine C6H2(=NR)2(NHR)2 (2) is used as a bridging ligand, new dinickel(II) complexes [(acac)Ni[mu-C6H2(=NPh)4]Ni(acac)] (3a: R=Ph) and [(acac)Ni[mu-C6H2(=NCH2tBu)4]Ni(acac)] (3b: R=CH2tBu) are obtained; upon one-electron oxidation of these complexes delocalized mixed-valence compounds are formed. An X-ray diffraction study on 3b reveals equalization of the bond lengths within each of the ligand 6 systems and a lack of conjugation between them. The oxidized state in 3b+ involves both the bridging quinonoid ligand and the metal centers, with a major contribution coming from the bridging ligand. Electrochemical and spectroscopic methods were used to study the influence of the N-substituents of the tetranitrogen donor ligands 2. In this combined experimental and theoretical (DFT) study, it is also shown that the electronic structure within the dinickel system can be altered by addition of a coordinating ligand such as pyridine. The latter favors the high-spin configuration with semi-occupied metal-centered orbitals, leading to a metal-metal interaction in the mixed-valence Ni(II)-Ni(III) 3b+ system. 相似文献
292.
The rheological behavior of concentrated alumina-coated TiO2 slurries has been investigated in connection with the type of surface counterions (monovalent cations: X = Li+, Na+, TMA+) in the absence and in the presence of polyacrylic acid (PAA). The study has been conducted in a pH range of 4-10 and with ionic strengths lower than 0.01 M. The pH and ionic strength were adjusted with XOH and XCl, respectively. The surface properties have been investigated by titration of surface counterions and the apparent yield stress has been measured using a dynamic stress rheometer. It appears from the results that the pH at the maximum yield stress and the magnitude of the yield stress depend on the nature of the counterion. The yield stress measurements were also conducted in the presence of PAA (0.5 segment/nm2) adsorbed on the particle surface. In that case, the mineral surface and adsorbed polymer were neutralized with XOH. The results show that the dispersion efficiency depends on the polymer counterion. In general, it is found that the maximum yield stress and the corresponding counterion surface density both follow the sequence TMA+ < Na+ < Li+. The adsorption of PAA apparently amplifies the effects observed with the corresponding cation. An electrostriction effect of the hydration layer at the interface is suggested in order to explain the increasing yield stress as the surface density of Li+ increases. The so-called structure-making/structure-breaking model explains the yield stress reduction with the TMA+ surface density. 相似文献
293.
(-)-Siccanin (1), a natural product possessing significant antifungal properties, was synthesized enantioselectively via a biomimetic route. This synthetic route features two sequential radical cyclizations: a Ti(III)-mediated radical cyclization of epoxyolefin 48 to construct the B-ring, and a Suarez reaction to establish the tetrahyrofuran ring. Chiral chroman moiety of siccanin was prepared based on our recent development of the Pd-catalyzed asymmetric allylic alkylation (AAA) of phenol trisubstituted allyl carbonates. Several other members of the siccanin family were also synthesized including siccanochromenes A (2), B (3), E (6), F (7), and the methyl ether of siccanochromene C (55). These studies may shed light on the biosynthesis of this novel family of compounds. 相似文献
294.
Trost BM Shen HC Dong L Surivet JP Sylvain C 《Journal of the American Chemical Society》2004,126(38):11966-11983
The Pd-catalyzed asymmetric allylic alkylation (AAA) of phenol allyl carbonates serves as an efficient strategy to construct the allylic C-O bond allowing access to chiral chromans in up to 98% ee. The effect of pH and the influence of olefin geometry, as well as substitution pattern on the ee and the absolute configuration of the chiral chromans were explored in detail. These observations suggest a mechanism involving the cyclization of the more reactive pi-allyl palladium diastereomeric intermediate as the enantiodiscriminating step (Curtin-Hammett conditions). This methodology led to the enantioselective synthesis of the vitamin E core, the first enantioselective total synthesis of (+)-clusifoliol and (-)-siccanin, and the synthesis of an advanced intermediate toward (+)-rhododaurichromanic acid A. 相似文献
295.
Jean-Philippe Labrousse 《Rendiconti del Circolo Matematico di Palermo》1980,29(2):161-258
The present paper is concerned with the study of a new class of linear operators on a Hilbert space: the class of quasi-Fredholm operators, which contains many operators already studied in the litterature (in particular semi-Fredholm operators). An operatorA is said to be quasi-Fredholm of degreed, if the following conditions are satisfied:
- For alln greater thand, R(A n )∩N(A)=R(A d )∩N(A);
- N(A)∩R(A d ) is closed inH;
- R(A)+N(A d ) is closed inH.
- A is quasi-Fredholm iff there exists a direct decomposition ofH into the sum of two subspacesH 1 andH 2 which are invariant underA and such that the restriction ofA toH 1 is quasi-Fredholm of degree 0 and the restriction ofA toH 2 is nilpotent (Kato decomposition).
- A is quasi-Fredholm iff there exists a neighborhoodD of 0 in C such that for all λ≠0 in that neighborhoodA?λI has a generalized inverse which is meromorphic inD?{0} (The generalized inverse is holomorphic inD iffA is of degree 0).
296.
We present a new approach to the variational relaxation of functionals
of the type:where
is a continuous function with growth conditions of order p≥1 but not necessarily convex. We essentially study the case when μ is the k-dimensional Hausdorff measure restricted to a suitable piece of a k-dimensional smooth submanifold of
. 相似文献
297.
Thermal elimination of sulfenic acid from enantiopure beta,beta'-disubstituted bis-sulfoxides allows the stereoselective synthesis of enantiopure acyclic beta,beta'-disubstituted vinylsulfoxides. This mild and stereospecific synthesis provides either ( E) or ( Z) vinylsulfoxides in high yields and is compatible with acid or base sensitive functional groups. 相似文献
298.
Xu L Regnier T Lemiègre L Cardinael P Combret JC Bouillon JP Blanchet J Rouden J Harrison-Marchand A Maddaluno J 《Organic letters》2008,10(5):729-732
An unprecedented enantioselective deprotonation/conjugate elimination sequence, which transforms an allylic meso-dioxepane into a chiral diene, is described. The best desymmetrization conditions (ee up to 70%) involve s-BuLi and sparteine at -78 degrees C in THF. 相似文献
299.
Aleksandar Donev Alejandro L. Garcia Jean-Philippe Péraud Andrew J. Nonaka John B. Bell 《Current Opinion in Electrochemistry》2019
We apply fluctuating hydrodynamics to strong electrolyte mixtures to compute the concentration corrections for chemical potential, diffusivity, and conductivity. We show these corrections to be in agreement with the limiting laws of Debye, Hückel, and Onsager. We compute explicit corrections for a symmetric ternary mixture and find that the co-ion Maxwell-Stefan diffusion coefficients can be negative, in agreement with experimental findings. 相似文献
300.
Highly efficient nonlinear filter for femtosecond pulse contrast enhancement and pulse shortening 总被引:1,自引:0,他引:1
Jullien A Rousseau JP Mercier B Antonucci L Albert O Chériaux G Kourtev S Minkovski N Saltiel SM 《Optics letters》2008,33(20):2353-2355
We propose a highly efficient scheme for temporal filters devoted to femtosecond pulse contrast enhancement. The filter is based on cross-polarized wave generation with a spatially suger-Gaussian-shaped beam. In a single nonlinear crystal scheme the energy conversion to the cross-polarized pulse can reach 28%. We demonstrate that the process enables a significant spectral broadening. For an efficiency of 23% the pulse shortening is estimated to 2.2, leading to an intensity transmission of the nonlinear filter of 50%. 相似文献