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71.
del Mar Conejo M Fernández R del Río D Carmona E Monge A Ruiz C Márquez AM Sanz JF 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(18):4452-4461
The beryllocenes [Be(C(5)Me(4)H)(2)] (1), [Be(C(5)Me(5))(2)] (2), and [Be(C(5)Me(5))(C(5)Me(4)H)] (3) have been prepared from BeCl(2) and the appropriate KCp' reagent in toluene/diethyl ether solvent mixtures. The synthesis of 1 is facile (20 degrees C, overnight), but generation of decamethylberyllocene 2 demands high temperatures (ca. 115 degrees C) and extended reaction times (3-4 days). The mixed-ring beryllocene 3 is obtained when the known [(eta(5)-C(5)Me(5))BeCl] is allowed to react with K[C(5)Me(4)H], once more under somewhat forcing conditions (115 degrees C, 36 h). The structures of the three metallocenes have been determined by low-temperature X-ray studies. Both 1 and 3 present eta5/eta1 geometries of the slip-sandwich type, whereas 2 exhibits an almost regular, ferrocene-like, sandwich structure. In the mixed-ring compound 3, C(5)Me(5) is centrally bound to beryllium and the eta(1)-C(5)Me(4)H ring bonds to the metal through the unique CH carbon atom. This is also the binding mode of the eta(1)-ring of 1. To analyze the nature of the bonding in these molecules, theoretical calculations at different levels of theory have been performed on compounds 2 and 3, and a comparison with the bonding in [Be(C(5)H(5))(2)] has been made. As for the latter molecule, energy differences between the eta5/eta5 and the eta5/eta1 structures of 2 are very small, being of the order of a few kcal mol(-1). Constrained space orbital variations (CSOV) calculations show that the covalent character in the bonding is larger for [Be(C(5)Me(5))(2)] than for [Be(C(5)H(5))(2)] due to larger charge delocalization and to increased polarizability of the C(5)Me(5) fragment. 相似文献
72.
Tormos JR Wiley KL Seravalli J Nachon F Masson P Nicolet Y Quinn DM 《Journal of the American Chemical Society》2005,127(42):14538-14539
Secondary beta-deuterium kinetic isotope effects have been measured as a function of substrate concentration for recombinant human butyrylcholinesterase-catalyzed hydrolysis of acetyl-L3-thiocholine (L = 1H or 2H). The isotope effect on V/K is inverse, D3V/K = 0.93 +/- 0.03, which is consistent with conversion of the sp2 hybridized carbonyl carbon of the scissile ester bond of the E + A reactant state to a quasi-tetrahedral structure in the acylation transition state. In contrast, the isotope effect on Vmax under conditions of substrate activation is markedly normal, D3(betaVmax) = 1.29 +/- 0.06, an observation that is consistent with accumulation of a tetrahedral intermediate as the reactant state for catalytic turnover. Generally, tetrahedral intermediates for nonenzymatic ester hydrolyses are high-energy steady-state intermediates. Apparently, butyrylcholinesterase displays an unusual ability to stabilize such intermediates. Hence, the catalytic power of cholinesterases can largely be understood in terms of their ability to stabilize tetrahedral intermediates in the multistep reaction mechanism. 相似文献
73.
Marcelo O. Santiago Alzir A. Batista Marcio P. de Araújo Claudio L. Donnici Icaro de S. Moreira Eduardo E. Castellano Javier Ellena Sauli dos Santos Jr Salete L. Queiroz 《Transition Metal Chemistry》2005,30(2):170-175
A series of cis-{RuCl2(PPh3)2[4,4-(X)2-2,2-bipy]} [cis-chlorines; X=-H, -Me, -SMe, and (-Cl,-Me)] complexes have had their structures determined by single crystal X-ray diffraction. The geometry of these complexes, also determined in CH2Cl2 solution by 31P{1H}-n.m.r. spectroscopy, showed that the chemical shifts for the phosphorus atoms are slightly dependent on the pKa of the 4,4-(-X)2-2,2-bipy ligands. 相似文献
74.
The electrochemistry of monoclinic and tetragonal vanadium-doped zirconias (VZrO2), prepared from gel precursors with vanadium loadings ranging from 0.5 to 15 mol%, has been studied using abrasive-conditioned
graphite/polyester composite electrodes immersed in aqueous HCl and HClO4 solutions. Isolated vanadium centers form a solid solution in the zirconia lattice with a solubility limit close to 5 mol%.
Above 5 mol%, finely dispersed V2O5 is formed. Vanadium centers located at the boundary sites of the zirconia lattice display successive one-electron transfer
processes near to +0.25 and +0.10 V vs. SCE, whereas finely dispersed V2O5 yields three successive reduction processes at +0.46, +0.30, and +0.16 V vs. SCE. Electrochemical data indicate the presence
of both V5+ and V4+ centers in the lattice of monoclinic and tetragonal zirconias, the V5+/V4+ ratio decreasing as the vanadium loading increases.
Electronic Publication 相似文献
75.
McIntosh R Ellis D Gil-Lostes J Dalby KJ Rosair GM Welch AJ 《Dalton transactions (Cambridge, England : 2003)》2005,(10):1842-1846
Reduction of the tethered carborane 1,2-(CH2)3-1,2-closo-C2B10H10 followed by treatment with CoCl2/NaCp, [(p-cymene)RuCl2]2(p-cymene=C6H4MeiPr-1,4), (PMe2Ph)2PtCl2 or (dppe)NiCl2(dppe=Ph2PCH2CH2PPh2) affords reasonable yields of the new 13-vertex metallacarboranes 1,2-(CH2)3-4-Cp-4,1,2-closo-CoC2B10H10 (1), 1,2-(CH2)3-4-(p-cymene)-4,1,2-closo-RuC2B10H10 (2), 1,2-(CH2)3-4,4-(PMe2Ph)2-4,1,2-closo-PtC2B10H10 (3) and 1,2-(CH2)3-4,4-(dppe)-4,1,2-closo-NiC2B10H10 (4), respectively. All compounds were characterised spectroscopically and crystallographically. The cobalt and ruthenium species 1 and 2 have Cs symmetry in both solution and the solid state, having henicosahedral cage structures featuring a trapezoidal C1C2B9B5 face. The platinum and nickel compounds 3 and 4 have asymmetric docosahedral cage structures in the crystal (the more so for 4 than for 3) although both appear, by 11B and 31P NMR spectroscopy, to have Cs symmetry in solution. Low-temperature experiments on the more soluble platinacarborane could not freeze out the diamond-trapezium-diamond fluctional process that we assume is operating in solution, and we therefore conclude that this process has a relatively low activation barrier, probably <35 kJ mol-1. 相似文献
76.
Koji Yagi Veronica Garcia Ma. E. Rivas Javier Salas Andres Camargo Takeo Tabata 《Journal of inclusion phenomena and macrocyclic chemistry》1984,2(1-2):179-184
Some crown ethers were found to show significant antifungal activity against some wood-decay fungi, phytopathogenic fungi and eumycetes,Trichophytons for dermatomycosis. Their toxicity was evaluated by the paper disc method as well as by determining the values of ED50, i.e., the concentration which inhibits the mycelium growth by 50%. The fungi examined areTyromyces palustris, Picnoporous coccineus, Coriolous versicolor, Pyricularia filamentosa, Fusarium sp., Trichophyton rubrum, Trichophyton sp., etc. Among the 26 crown ethers tested, 3,5-di-t-butyl-benzo-15-crown-5 showed relatively high activity, the highest ED50 value of which being 8 M or 3 ppm. Other alkylbenzocrown ethers, dicyclohexyl crown ethers and Kryptofix 22DD also showed considerable activity. On the other hand, unsubstituted crown ethers, benzocrown ethers with a polar substituent, Kryptofix 222B and Kryptofix 221 were inactive. 相似文献
77.
Fernández EJ Laguna A López-De-Luzuriaga JM Mendizabal F Monge M Olmos ME Pérez J 《Chemistry (Weinheim an der Bergstrasse, Germany)》2003,9(2):456-465
The reactions of solutions of TlPF(6) and OPPh(3) in tetrahydrofuran or acetone with NBu(4)[AuR(2)] (R=C(6)Cl(5), C(6)F(5)) gave the new complexes [Au(C(6)Cl(5))(2)](2)[Tl(OPPh(3))][Tl(OPPh(3))(L)] (L=THF (1), acetone (2)) and the previously reported [Tl(OPPh(3))(2)][Au(C(6)F(5))(2)] (3). The crystal structures of complexes 1 and 2 display extended unsupported chains with short intermolecular interactions between alternating gold(I) and thallium(I) centres. Moreover, the Tl(I) centres show two different types of geometrical environments, such as pseudotetrahedral and distorted trigonal-bipyramidal, due to the presence of solvent molecules that act as ligands in the solid-state structure. Quasirelativistic and nonrelativistic ab initio calculations were performed to study the nature of the intermetallic Au(I)-Tl(I) interactions and are consistent with the presence of a high ionic contribution (80 %) and dispersion-type (van der Waals) interaction with a charge-transfer contribution (20 %) when relativistic effects are taken into account. All complexes are luminescent in the solid state at room temperature and at 77 K. Complexes 1 and 2 show site-selective excitation, probably due to the different environments around the Tl(I) centres. The DFT and time-dependent (TD)-DFT calculations are in agreement with the experimental excitation spectra for all complexes and confirm the site-selective excitation behaviour as a function of the Tl(I) geometrical environment. 相似文献
78.
Adams LA Aggarwal VK Bonnert RV Bressel B Cox RJ Shepherd J de Vicente J Walter M Whittingham WG Winn CL 《The Journal of organic chemistry》2003,68(24):9433-9440
A simple and high-yielding method for the preparation of cyclopropane amino acids is described. The novel method involves the one-pot cyclopropanation of readily available dehydroamino acids using aryl and unsaturated diazo compounds generated in situ from the corresponding tosylhydrazone salts. It was found that thermal 1,3-dipolar cycloaddition followed by nitrogen extrusion gave the cyclopropane amino acid derivatives with good E selectivity, while reactions in the presence of meso-tetraphenylporphyrin iron chloride gave predominantly the corresponding Z isomers. The synthetic utility of this process was demonstrated in the synthesis of (+/-)-(Z)-2,3-methanophenylalanine [(+/-)-(Z)-1], the anti-Parkinson (+/-)-(E)-2,3-methano-m-tyrosine [(+/-)-(E)-2], and the natural product (+/-)-coronamic acid [(+/-)-3]. 相似文献
79.
Muñoz-Muriedas J Perspicace S Bech N Guccione S Orozco M Luque FJ 《Journal of computer-aided molecular design》2005,19(6):401-419
Summary The use of a recently proposed hydrophobic similarity index for the alignment of molecules and the prediction of their differences
in biological activity is described. The hydrophobic similarity index exploits atomic contributions to the octanol/water transfer
free energy, which are evaluated by means of the fractional partitioning scheme developed within the framework of the Miertus-Scrocco-Tomasi
continuum model. Those contributions are used to define global and local measures of hydrophobic similarity. The suitability of this computational strategy is examined for two series of compounds
(ACAT inhibitors and 5-HT3 receptor agonists), which are aligned to maximize the global hydrophobic similarity using a Monte Carlo-simulated protocol.
Indeed, the concept of local hydrophobic similarity is used to explore structure–activity relationships in a series of COX-2 inhibitors. Inspection of
the 3D distribution of hydrophobic/hydrophilic contributions in the aligned molecules is valuable to identify regions of very
similar hydrophobicity, which can define pharmacophoric recognition patterns. Moreover, low similar regions permit to identify
structural elements that modulate the differences in activity between molecules. Finally, the quantitative relationships found
between the pharmacological activity and the hydrophobic similarity index points out that not only the global hydrophobicity,
but its 3D distribution, is important to gain insight into the activity of molecules.
J.M.M. and S.P. have contributed equally to this study. 相似文献
80.
Méndez I. Daniel Klimova Elena Perez F. Javier 《Journal of organometallic chemistry》2004,689(15):2503-2510
The Wittig olefination of ferrocene-containing chalcones leads to 1,3-dienes and their cyclodimers. The reaction of 1,3-dienes with dibromocarbene yields gem-dibromo(ferrocenylvinyl)cyclopropanes. Upon reductive dehalogenation with ethylmagnesium bromide they afford the corresponding monobromo derivatives. All the obtained compounds were characterized by 1H and 13C NMR, elemental analysis, and mass spectrometry. The structures of compounds 5a,b, 6g, 6h, and 7i were confirmed by X-ray diffraction analysis. 相似文献