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71.
Rh(π-C3H5)(PF3)3 (I), reacts with trifluoroacetic acid to form propene and [Rh(CF3COO)(PF3)2]2 (II). I reacts with t-butyl bromide to give [RhBr(PF3)2]2 and a mixture of propene and 2-methyl-1-propene and with n-propyl bromide to give propene and [RhBr(PF3)2]2. Rh(π-C3H5)(PPh3)2 (III), and t-butyl bromide yield propene and 2-methyl-1-propene. In these reactions a mechanism involving β-hydrogen abstraction and hydrogen migration via the metal to carbon is proposed. When III reacts with Me3SnCl the Me3Sn—moiety migrates intact to the π-allyl group. I reacts with acetyl chloride to give propene, [RhCl(PF3)2]2 and the carbonyl rhodium complex Rh2Cl2(PF3)3(CO). II does not apparently undergo phosphine ligand exchange unlike the analogous halogeno-bridged dimers. 相似文献
72.
Meiler J Meusinger R Will M 《Journal of chemical information and computer sciences》2000,40(5):1169-1176
Nine different artificial neural networks were trained with the spherically encoded chemical environments of more than 500000 carbon atoms to predict their 13C NMR chemical shifts. Based on these results the PC-program "C_shift" was developed which allows the calculation of the 13C NMR spectra of any proposed molecular structure consisting of the covalently bonded elements C, H, N, O, P, S and the halogens. Results were obtained with a mean deviation as low as 1.8 ppm; this accuracy is equivalent to a determination on the basis of a large database but, in a time as short as known from increment calculations, was demonstrated exemplary using the natural agent epothilone A. The artificial neural networks allow simultaneously a precise and fast prediction of a large number of 13C NMR spectra, as needed for high throughput NMR and screening of a substance or spectra libraries. 相似文献
73.
O-GlcNAcylation of serine and threonine residues is a dynamic and essential post-translational modification involved in signaling pathways in eukaryotes. Studies of O-GlcNAcylation would be aided by small-molecule inhibitors of O-GlcNAc transferase (OGT), the sole enzyme know to mediate this modification, but discovery of such molecules has been hampered by poor expression of cloned OGT and lack of suitable high-throughput screens. This Communication describes the development an expression system to access large amounts of the catalytic domain of OGT and the implementation of a fluorescence-based substrate analogue displacement assay that has led to the discovery of a set of OGT inhibitors. This work lays the foundation for both structural and functional analysis of the catalytic domain of OGT. 相似文献
74.
Thomson RK Zahariev FE Zhang Z Patrick BO Wang YA Schafer LL 《Inorganic chemistry》2005,44(24):8680-8689
Easily prepared and highly modular organic amide proligands have been used to synthesize a series of new bis(amidate)-bis(amido) Ti and Zr complexes via protonolysis. These complexes have been structurally characterized by NMR spectroscopy and X-ray crystallography. The solid-state molecular structures of these complexes indicate that the amidate ligands bind to the metal centers in an exclusively bidentate fashion, resulting in discrete monomeric species. Geometric isomerism in these species is highly dependent upon the steric characteristics of the proligands utilized in the synthesis. In solution, these complexes are observed to isomerize on the NMR time scale, with one isomer being predominant. Bonding in the bis(amidate)-bis(amido) complexes was investigated by DFT calculations. The geometric isomers predicted by theory matched the experimentally observed results, within experimental error. The orbitals associated with amidate-metal bonding are energetically well below the frontier orbitals. The HOMO in these complexes is a pi orbital associated with amido ligand-to-metal bonding character, while the LUMO in all cases is a vacant d orbital on the metal center. 相似文献
75.
Geng J Ducati C Shephard DS Chhowalla M Johnson BF Robertson J 《Chemical communications (Cambridge, England)》2002,(10):1112-1113
A nanoscale ruthenium/gold bimetallic cluster of clusters has been used as a molecular precursor to produce pure ruthenium nanoparticles (seeds) as catalysts for the growth of carbon nanohorns (CNHs). 相似文献
76.
Supramolecular isomerism in coordination compounds: nanoscale molecular hexagons and chains 总被引:1,自引:0,他引:1
Abourahma H Moulton B Kravtsov V Zaworotko MJ 《Journal of the American Chemical Society》2002,124(34):9990-9991
A nanoscale supramolecular hexagon, 1, and its supramolecular isomeric chain structure, 2, have been prepared from self-assembly of 5-NO2-bdc and Cu(II) cations. The hexagon is neutral and soluble and has outer and inner diameters of 3.1 and 0.8 nm, respectively. 相似文献
77.
The results of (low energy) photoelectron spectroscopy render possible a better appreciation of the “Nature of the Chemical Bond”. The application of this new experimental method is demonstrated utilizing representative compounds of the nonmetal elements, and a close symbiosis delineated with molecular orbital models. In particular, general consequences are discussed concerning electron deficiency, σ- and π-interactions, electron pair delocalization, and substituent effects or geometric perturbations. Photoelectron spectroscopic ionization energies permit evaluation of parameters for specified molecular groups, allow correlation with numerous other experimental data, and are didactically valuable in the teaching of general chemistry. 相似文献
78.
Initially in this review the sodalite framework [T12O24]6– (T=Al, Si] is modeled with regular tetrahedra and disordered T atoms. Equations are given for calculating atomic coordinates from the unit cell parameter a and the T—O distancet; the expansion or contraction of the sodalite-cage is related quantitatively to changes ina through the cooperative twists of TO4 tetrahedra about 4 axes and changes in < TOT bridge angles. The fully expanded cage has=0° and the maximum value ofa. The equations are general for any framework formed by isomorphous substitution of T by atoms other than Al, Si and for any SiAl ratio. The model and equations are extended to the zeolite A framework, which can be built from fully expanded sodalite cages. With the cooperative tilt of the TO4 tetrahedra of zeolite A, described by Depmeier, the major variable features of the zeolite A framework are explained quantitatively. The faujasite framework has twisted-cages (>0), as in sodalite examples, and is quantitatively modeled most conveniently from sodalite examples with similar-cage contents. The review is extended to structures with T-ordering and distorted tetrahedra. Methods are given for estimating a for sodalites from a knowledge of the cavity contents, especially the sizes of cations and anions, and so on, present. Ways of predicting cavity sites in zeolite A as a function of cation size are presented, and the principal cavity sites in the faujasite-cage region are discussed. Finally the review considers isomorphous replacement of T atoms (Si or Al) by B, Be, Fe, Ga, Ge, and P; many of these substituted frameworks are stabilized by templates, or guest molecules, which reside in the cavities. Templates also stabilize Si, Al frameworks with high SiAl ratios. The modeling approach reviewed here is tested on a range of isomorphously substituted frameworks isotypic with sodalite; observed and calculated values of twist and 12O24]12– with distorted tetrahedra; these are structures with Al-O-Al bridges, which violate Loewenstein's Rule. 相似文献
79.
Tondi D Morandi F Bonnet R Costi MP Shoichet BK 《Journal of the American Chemical Society》2005,127(13):4632-4639
Bacterial expression of beta-lactamases is the most widespread resistance mechanism to beta-lactam antibiotics, such as penicillins and cephalosporins. There is a pressing need for novel, non-beta-lactam inhibitors of these enzymes. One previously discovered novel inhibitor of the beta-lactamase AmpC, compound 1, has several favorable properties: it is chemically dissimilar to beta-lactams and is a noncovalent, competitive inhibitor of the enzyme. However, at 26 microM its activity is modest. Using the X-ray structure of the AmpC/1 complex as a template, 14 analogues were designed and synthesized. The most active of these, compound 10, had a K(i) of 1 microM, 26-fold better than the lead. To understand the origins of this improved activity, the structures of AmpC in complex with compound 10 and an analogue, compound 11, were determined by X-ray crystallography to 1.97 and 1.96 A, respectively. Compound 10 was active in cell culture, reversing resistance to the third generation cephalosporin ceftazidime in bacterial pathogens expressing AmpC. In contrast to beta-lactam-based inhibitors clavulanate and cefoxitin, compound 10 did not up-regulate beta-lactamase expression in cell culture but simply inhibited the enzyme expressed by the resistant bacteria. Its escape from this resistance mechanism derives from its dissimilarity to beta-lactam antibiotics. 相似文献
80.
[reaction: see text] A palladium-catalyzed arylation of allylic acetates followed by beta-acetoxy elimination was shown to produce Heck-type coupling products. Optimal reaction conditions employed ligand-free palladium on carbon in the presence of tetrabutylammonium chloride, a trialkylamine base, and water. 相似文献