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Hart DJ 《Science (New York, N.Y.)》1984,223(4639):883-887
Organic chemists have begun to use intra- and intermolecular free-radical addition reactions to develop useful synthetic transformations. Carboncentered radicals can form bonds with electron-rich or electron-deficient alkenes, allenes, and acetylenes. Radical addition reactions can also be used to construct hindered carbon-carbon bonds. These characteristics, as well as the large number of functional groups that tolerate free-radical conditions contribute to the importance of such reactions in organic synthesis. 相似文献
3.
The gas-phase irradiation of CpRh(CO)(2) (Cp = eta(5)-C(5)H(5)) was examined in order to study the rates of reaction of the 16-electron intermediates presumed to be involved in the C-H oxidative addition of alkanes. "Naked" (unsolvated) CpRh(CO) was detected, and direct measurements of the rates of reaction of this very short-lived complex with alkane C-H bonds were made. Activation of C-H bonds occurs on almost every collision for alkanes of moderate size, and intermediates in which the alkanes are bound to the metal centers, without their C-H bonds being fully broken, are implicated as intermediates in the overall reaction. 相似文献
4.
In contrast to carbon, silicon does not readily form double bonds, and compounds containing silicon-silicon double bonds can usually be stabilized only by protection with bulky substituents. We have isolated a silicon analog of spiropentadiene 1, a carbon double-ring compound that has not been isolated to date. In the crystal structure of tetrakis[tri(t-butyldimethylsilyl)silyl]spiropentasiladiene 2, a substantial deviation from the perpendicular arrangement of the two rings is observed, and the silicon-silicon double bonds are shown to be distorted. Spectroscopic data indicate pronounced interaction between two remote silicon-silicon double bonds in the molecule. Silicon-silicon bonds may be more accessible to synthesis than previously assumed. 相似文献
5.
Bergman RG 《Science (New York, N.Y.)》1984,223(4639):902-908
Alkanes, although plentiful enough to be considered for use as feedstocks in large-scale chemical processes, are so unreactive that relatively few chemical reagents have been developed to convert them to molecules having useful functional groups. However, a recently synthesized iridium (lr) complex successfully converts alkanes into hydridoalkylmetal complexes (M + R-H --> R-M-H). This is a dihydride having the formula Cp(*)(L)lrH(2), where Cp(*) and L are abbreviations for the ligands (CH(3))(5)C(5) and (CH(3))(3)P, respectively. Irradiation with ultraviolet light causes the dihydride to lose H(2), generating the reactive intermediate Cp(*)lrL. This intermediate reacts rapidly with C-H bonds in every molecule so far tested (including alkanes) and leads to hydridoalkyliridium complexes Cp(*)(L)lr(R)(H). Evidence has been obtained that this C-H insertion, or oxidative addition, reaction proceeds through a simple three-center transition state and does not involve organic free radicals as intermediates. Thus the intermediate Cp(*)lrL reacts most rapidly with C-H bonds having relatively high bond energies, such as those at primary carbon centers, in small organic rings, and in aromatic rings. This contrasts directly with the type of hydrogen-abstraction selectivity that is characteristic of organic radicals. The hydridoalkyliridium products of the insertion reactions can be converted into functionalized organic molecules-alkyl halides-by treatment with mercuric chloride followed by halogens. Expulsion (reductive elimination) of the hydrocarbon from the hydridoalkyliridium complexes can be induced by Lewis acids or heat, regenerating the reactive intermediate Cp(*)lrL. Oxidative addition of the corresponding rhodium complexes Cp(*)RhL to alkane C-H bonds has also been observed, although the products formed in this case are much less stable and undergo reductive elimination at -20 degrees C. These and other recent observations provide an incentive for reexamining the factors that have been assumed to control the rate of reaction of transition metal complexes with C-H bonds-notably the need for electron-rich metals and the proximity of reacting centers. 相似文献
6.
1979~1983年从省内46个县、市各主要病区分离了将近200个稻白叶枯病菌株,根据它们在我国现行一套鉴别品种上的反应,可将其分为致病力强弱不同的四个菌系群。另外,我们利用国外已知功能抗性基因的鉴别品种,与我国鉴别品种一起进行比较试验,根据77个代表菌株在这些鉴别品种上的反应,将这些鉴别品种归为七个品种群,并将这些菌株相应地分为四个菌群。这一试验结果表示,我省稻白叶枯病菌的致病力可能与日本和菲律宾菌系的不同。基于上述试验,我们认为金刚30、窄叶青8号、农垦57、植科1号、南15、IR26和76C 凡7等品种,可作为我省稻白叶枯病菌系鉴定的一套试用鉴别品种。根据这套鉴别品种,可将我省稻白叶枯病菌区分为五个菌系群(0,Ⅰ、Ⅱ、Ⅲ和Ⅳ)和九个亚群,基中Ⅲ群和Ⅳ群菌株对水稻品种表现分化性侵染。 相似文献
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Jia C Piao D Oyamada J Lu W Kitamura T Fujiwara Y 《Science (New York, N.Y.)》2000,287(5460):1992-1995
Efficient electrophilic metalation of aromatic C-H bonds leading to new C-C bond formation through regio- and stereoselective addition to alkynes and alkenes has been realized by a catalytic amount (0.02 to 5 mole percent) of palladium(II) or platinum(II) compounds in a mixed solvent containing trifluoroacetic acid at room temperature. Various arenes undergo unexpected selective trans hydroarylation to terminal or internal C&cjs0812;C bonds inter- and intramolecularly with high efficiency (up to a turnover number of 4500 for palladium), especially for electron-rich arenes, giving thermodynamically unfavorable cis-alkenes, and the oxygen- and nitrogen-containing heterocycles. The simplicity, generality, and efficiency of this process should be very attractive to the possible industrial application for the functionalization of arenes. 相似文献
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Albrecht C Blank K Lalic-Mülthaler M Hirler S Mai T Gilbert I Schiffmann S Bayer T Clausen-Schaumann H Gaub HE 《Science (New York, N.Y.)》2003,301(5631):367-370
Direct quantification of biomolecular interaction by single-molecule force spectroscopy has evolved into a powerful tool for materials and life sciences. We introduce an approach in which the unbinding forces required to break intermolecular bonds are measured in a differential format by comparison with a known reference bond (here, a short DNA duplex). In addition to a marked increase in sensitivity and force resolution, which enabled us to resolve single-base pair mismatches, this concept allows for highly specific parallel assays. This option was exploited to overcome cross-reactions of antibodies in a protein biochip application. 相似文献
9.
Carbon-fluorine bonds are among the most unreactive functionalities in chemistry. Interest in their activation arises in part from the high global warming potentials of anthropogenic polyfluoroorganic compounds. Conversion to carbon-hydrogen bonds (hydrodefluorination) is the simplest modification of carbon-fluorine bonds, but efficient catalytic hydrodefluorination of perfluoroalkyl groups has been an unmet challenge. We report a class of carborane-supported, highly electrophilic silylium compounds that act as long-lived catalysts for hydrodefluorination of trifluoromethyl and nonafluorobutyl groups by widely accessible silanes under mild conditions. The reactions are completely selective for aliphatic carbon-fluorine bonds in preference to aromatic carbon-fluorine bonds. 相似文献
10.
For characterization of sequence and posttranslational modifications, molecular and fragment ion mass data from ionizing and dissociating a protein in the mass spectrometer are far more specific than are masses of peptides from the protein's digestion. We extend the approximately 500-residue, approximately 50-kilodalton (kD) dissociation limitation of this top-down methodology by using electrospray additives, heated vaporization, and separate noncovalent and covalent bond dissociation. This process can cleave 287 interresidue bonds in the termini of a 1314-residue (144-kD) protein, specify previously unidentified disulfide bonds between 8 of 27 cysteines in a 1714-residue (200-kD) protein, and correct sequence predictions in two proteins, one with 2153 residues (229 kD). 相似文献
11.
Adhesion of a biological cell to another cell or the extracellular matrix involves complex couplings between cell biochemistry, structural mechanics, and surface bonding. The interactions are dynamic and act through association and dissociation of bonds between very large molecules at rates that change considerably under stress. Combining molecular cell biology with single-molecule force spectroscopy provides a powerful tool for exploring the complexity of cell adhesion, that is, how cell signaling processes strengthen adhesion bonds and how forces applied to cell-surface bonds act on intracellular sites to catalyze chemical processes or switch molecular interactions on and off. Probing adhesion receptors on strategically engineered cells with force during functional stimulation can reveal key nodes of communication between the mechanical and chemical circuitry of a cell. 相似文献
12.
Boland JJ 《Science (New York, N.Y.)》1992,255(5041):186-188
Hydrogen can be used as an effective probe of the structure of semiconductor surfaces. Such surfaces consist of bonds with varying degrees of bond strain, and hydrogen can react with each selectively depending on the reaction conditions. This selectivity is derived from a reduced barrier to reaction associated with strained bonds. In this manner, hydrogen can be used to pick apart the surface one bond type at a time, thereby revealing the structure of even complex multilayer reconstructions. This method is used to directly show that the rest-layer of the Ge(111)-c(2 x 8) surface has a bulk structure. 相似文献
13.
Oligothymidylates: formation by thermal condensation of O 2 ,5'-cyclothymidine 3'-phosphate 总被引:1,自引:0,他引:1
Thermal activation of O(2),5'-cyclothymidine 3'-phosphate in solution and in the solid state led to the formation of thymidine oligonucleotides containing up to approximately 12 nucleotide units. Only the 3',5' internucleotide diester bonds were formed. This polymerization occurs without the addition of any activating agent or catalyst. 相似文献
14.
植物叶绿体基因工程与细胞核基因工程相比,具有许多独特的优势,如能够实现外源基因特异整合及高效表达、多基因共表达、外源基因不会随花粉扩散、没有位置效应和基因沉默等.目前已在16种植物中成功获得叶绿体转基因植株,改良了植物的农艺性状,特别是在烟草叶绿体中高效表达了40多种外源蛋白,包括多种抗体和疫苗.尽管如此,这项技术目前... 相似文献
15.
通过对我国1992年以来损失在1亿元以上的台风损失分布进行拟合,根据我国台风损失数据特征,选择一种能对具有尖峰、厚尾、偏态特征的分布进行较好拟合分布的g-h分布进行分析;在均衡定价理论的基础上构建一种台风巨灾债券利率定价模型,并利用相关数据进行定价分析,将模型应用于三种台风巨灾债券并计算出其利率。结果表明,无论是哪种类型的巨灾债券,由于利率均高于同期国债利率,对投资者来说都具有较大吸引力,是一种较为理想的巨灾风险创新产品。 相似文献
16.
Schrock RR 《Science (New York, N.Y.)》1983,219(4580):13-18
Several well-characterized transition metal catalysts contain a metal-carbon double bond or a metal-carbon triple bond. In other homogeneous (or heterogeneous) catalyst systems in which the metal is likely to be in a relatively high oxidation state, such as molybdenum(VI) or tungsten(VI), metal-carbon multiple bonds may play an important role. Some recent results suggest that even supposedly well understood reactions such as ethylene polymerization may actually involve catalysts that behave as if they contained a metal-carbon double bond instead of a metal-carbon single bond. The chemistry of metal-carbon double and triple bonds should eventually complement and perhaps. overlap the known chemistry of complexes containing metal-oxygen double bonds or metal-nitrogen triple bonds, respectively; unique catalytic reactions involving carbon, nitrogen, and oxygen ligands multiply bonded to transition metals are therefore possible. 相似文献
17.
Direct and selective replacement of carbon-hydrogen bonds with new bonds (such as C-C, C-O, and C-N) represents an important and long-standing goal in chemistry. These transformations have broad potential in synthesis because C-H bonds are ubiquitous in organic substances. At the same time, achieving selectivity among many different C-H bonds remains a challenge. Here, we focus on the functionalization of C-H bonds in complex organic substrates catalyzed by transition metal catalysts. We outline the key concepts and approaches aimed at achieving selectivity in complex settings and discuss the impact these reactions have on synthetic planning and strategy in organic synthesis. 相似文献
18.
Tebbe FN Harlow RL Chase DB Thorn DL Campbell GC Calabrese JC Herron N Young RJ Wasserman E 《Science (New York, N.Y.)》1992,256(5058):822-825
C(60) and liquid bromine react to form C(60)Br(24), a crystalline compound isolated as a bromine solvate, C(60)Br(24)(Br(2))(x), The x-ray crystal structure defines a new pattern of addition to the carbon skeleton that imparts a rare high symmetry. The parent C(60) framework is recognizable in C(60)Br(24), but sp(3) carbons at sites of bromination distort the surface, affecting conformations of all of the hexagonal and pentagonal rings. Twenty-four bromine atoms envelop the carbon core, shielding the 18 remaining double bonds from addition. At 150 degrees to 200 degrees C there is effectively quantitative reversion of C(60)Br(24) to C(60) and Br(2). 相似文献
19.
Freeman RG Grabar KC Allison KJ Bright RM Davis JA Guthrie AP Hommer MB Jackson MA Smith PC Walter DG Natan MJ 《Science (New York, N.Y.)》1995,267(5204):1629-1632
The self-assembly of monodisperse gold and silver colloid particles into monolayers on polymer-coated substrates yields macroscopic surfaces that are highly active for surface-enhanced Raman scattering (SERS). Particles are bound to the substrate through multiple bonds between the colloidal metal and functional groups on the polymer such as cyanide (CN), amine (NH(2)), and thiol (SH). Surface evolution, which can be followed in real time by ultraviolet-visible spectroscopy and SERS, can be controlled to yield high reproducibility on both the nanometer and the centimeter scales. On conducting substrates, colloid monolayers are electrochemically addressable and behave like a collection of closely spaced microelectrodes. These favorable properties and the ease of monolayer construction suggest a widespread use for metal colloid-based substrates. 相似文献
20.
We observed coherent proton tunneling in the cyclic network of four hydrogen bonds in calix[4]arene. The tunneling frequency of 35 megahertz was revealed by a peak in the magnetic field dependence of the proton spin-lattice relaxation rate measured with field-cycling nuclear magnetic resonance in the solid state at temperatures below 80 kelvin. The amplitude of the coherent tunneling peak grows with temperature according to a Boltzmann law with energy D/kB = (125 +/- 10) kelvin (where kB is Boltzmann's constant). The tunneling peak can be interpreted in the context of level crossings in the region where the tunneling frequency matches the proton Larmor frequency. The tunneling spectrum reveals fine structure that we attribute to coupling between the hydrogen bonds in the network. The characteristics of the tunneling peak are interpreted in the context of the potential energy surface experienced by the hydrogen atoms in the network. 相似文献