Catalytic activation of carbon-fluorine bonds by a soluble transition metal complex

Homogeneous catalytic activation of the strong carbon-fluorine bonds under mild conditions was achieved with the use of rhodium complexes as catalysts. The catalytic reactions between polyfluorobenzenes and hydrosilanes result in substitution of fluorine atoms by hydrogen atoms and are chemo- and regioselective. With individual stoichiometric steps observed and combined, and with intermediates isolated and fully characterized (including crystal structures), these systems demonstrate the effectiveness of a rational approach to catalytic design.     

Fluorine in pharmaceuticals: looking beyond intuition

Fluorine substituents have become a widespread and important drug component, their introduction facilitated by the development of safe and selective fluorinating agents. Organofluorine affects nearly all physical and adsorption, distribution, metabolism, and excretion properties of a lead compound. Its inductive effects are relatively well understood, enhancing bioavailability, for example, by reducing the basicity of neighboring amines. In contrast, exploration of the specific influence of carbon-fluorine single bonds on docking interactions, whether through direct contact with the protein or through stereoelectronic effects on molecular conformation of the drug, has only recently begun. Here, we review experimental progress in this vein and add complementary analysis based on comprehensive searches in the Cambridge Structural Database and the Protein Data Bank.     

Enols are common intermediates in hydrocarbon oxidation

Models for chemical mechanisms of hydrocarbon oxidation rely on spectrometric identification of molecular structures in flames. Carbonyl (keto) compounds are well-established combustion intermediates. However, their less-stable enol tautomers, bearing OH groups adjacent to carbon-carbon double bonds, are not included in standard models. We observed substantial quantities of two-, three-, and four-carbon enols by photoionization mass spectrometry of flames burning representative compounds from modern fuel blends. Concentration profiles demonstrate that enol flame chemistry cannot be accounted for purely by keto-enol tautomerization. Currently accepted hydrocarbon oxidation mechanisms will likely require revision to explain the formation and reactivity of these unexpected compounds.     

Proton-catalyzed, silane-fueled Friedel-Crafts coupling of fluoroarenes

The venerable Friedel-Crafts reaction appends alkyl or acyl groups to aromatic rings through alkyl or acyl cation equivalents typically generated by Lewis acids. We show that phenyl cation equivalents, generated from otherwise unreactive aryl fluorides, allow extension of the Friedel-Crafts reaction to intramolecular aryl couplings. The enabling feature of this reaction is the exchange of carbon-fluorine for silicon-fluorine bond enthalpies; the reaction is activated by an intermediate silyl cation. Catalytic quantities of protons or silyl cations paired with weakly coordinating carborane counterions initiate the reactions, after which proton transfer in the final aromatization step regenerates the active silyl cation species by protodesilylation of a quaternary silane. The methodology allows the high-yield formation of a range of tailored polycyclic aromatic hydrocarbons and graphene fragments.     

Synthesis of Linear Acetylenic Carbon: The "sp" Carbon Allotrope

A carbon allotrope based on "sp" hybridization containing alternating triple and single bonds (an acetylenic or linear carbon allotrope) has been prepared. Studies of small (8 to 28 carbon atoms) acetylenic carbon model compounds show that such species are quite stable (130 degrees to 140 degrees C) provided that nonreactive terminal groups or end caps (such as tert-butyl or trifluoromethyl) are present to stabilize these molecules against further reactions. In the presence of end capping groups, laser-based synthetic techniques similar to those normally used to generate fullerenes, produce thermally stable acetylenic carbon species capped with trifluoromethyl or nitrile groups with chain lengths in excess of 300 carbon atoms. Under these conditions, only a negligible quantity of fullerenes is produced. Acetylenic carbon compounds are not particularly moisture or oxygen sensitive but are moderately light sensitive.     

竹材气相氟化处理的尺寸稳定性和防霉性能

  目的  以氟气作为改性剂,利用其较强的渗透性和反应活性与木材细胞形成化学键,达到长效存留和改性竹材的目的。  方法  将4年生的毛竹Phyllostachys edulis置于管式反应器中,通入质量百分比为25%的氟气,在150℃下反应4 h。为了进一步提高氟化效果,先用不同质量分数硫酸预处理竹材,再进行高温氟化处理。  结果  氟化反应主要发生在木质素上。氟化材红外光谱说明氟化处理材在739 cm-1处产生了表征碳氟键（C-F）的新峰,浓硫酸预处理氟化竹材在878和1 088 cm-1出现木质素苯环氟多取代（C-F_n）吸收峰。X射线光电子能谱中结合能为687.8 eV的C-F特征峰和689.2 eV的C-F₂特征峰证实了氟化处理竹材中C-F_n键的生成。热氟化试块在3次吸湿-干燥和吸水-干燥循环中平均抗胀率分别为19.1%和7.5%。硫酸预处理能进一步提高氟化材的尺寸稳定性,其中20 g·kg-1的硫酸预处理氟化材效果最为显著,平均抗胀率分别达31.0%和15.8%。防霉测试显示氟化处理材对木霉Trichoderma viride、青霉Penicillium citrinum和黑曲霉Aspergillus niger混合霉菌的抑制效果不明显,硫酸预处理后的氟化材防霉性能有所增加。  结论  竹材气相热氟化能对竹材内部进行改性,处理后的竹材尺寸稳定性和防霉性能均优于未处理竹材。利用氟气对竹材进行气相改性是一种渗透性强、反应活性高的竹材改性新技术。     

A stable bicyclic compound with two Si=Si double bonds

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.     

Ambient-temperature isolation of a compound with a boron-boron triple bond

Homoatomic triple bonds between main-group elements have been restricted to alkynes, dinitrogen, and a handful of reactive compounds featuring trans-bent heavier elements of groups 13 and 14. Previous attempts to prepare a compound with a boron-boron triple bond that is stable at ambient temperature have been unsuccessful, despite numerous computational studies predicting their viability. We found that reduction of a bis(N-heterocyclic carbene)-stabilized tetrabromodiborane with either two or four equivalents of sodium naphthalenide, a one-electron reducing agent, yields isolable diborene and diboryne compounds. Crystallographic and spectroscopic characterization confirm that the latter is a halide-free linear system containing a boron-boron triple bond.     

离子注入水稻干种子光声谱分析

<正> 中国科学院等离子体物理所与安徽省农科院水稻研究所合作,利用高技术——离子注入——于水稻种子诱变育种生物效应的研究,在物理学与生命科学的交叉上做出了有益的尝试。两年多的研究表明:离子注入水稻种子诱变,较其他方法诱变有突变率高、突变谱广的特点。作者认为:荷能粒子注入到水稻干种子中去,不仅有能量交换过程,引起生物体内很强的生理生化反应,而且发生了质量沉积过程,沉积的粒子同生物体分子基团直接键合,改变了生物体原来的分子基团结构。这一切,都会引起水稻种子遗传基因的突变。     

Organorhenium chemistry

As organometallic chemists have delved into the chemistry of rhenium, an amazing variety of unusual stable organorhenium compounds have been synthesized and novel reaction pathways have been uncovered. The broad range of stable organorhenium compounds can be traced to the availability of oxidation states from -III to VII and to the high strength of bonds to rhenium. New types of reaction mechanism are seen in response to the reluctance of rhenium to form coordinatively unsaturated complexes.     

Synthesis of imido analogs of the uranyl ion

Here we describe the synthesis of two imido analogs of the uranyl ion, UO(2+)2, in which the oxygens are replaced by divalent alkyl or aryl nitrogen groups: U(NtBu)2I2(THF)2 (1) and U(NPh)2I2(THF)3 (2) (where tBu is tert-butyl and THF is tetrahydrofuran). Both compounds have been fully characterized by standard analytical techniques, including x-ray crystallography, and the chemical bonding between the metal center and the nitrogen ligands was quantified by using hybrid density functional theory calculations. As expected for a uranyl analog, these complexes exhibit linear N-U-N linkages and very short U-N bonds. In addition, the theoretical calculations show strong involvement of the 5f and 6d electrons in the U-N bonding.     

Selective, nickel-catalyzed hydrogenolysis of aryl ethers

Selective hydrogenolysis of the aromatic carbon-oxygen (C-O) bonds in aryl ethers is an unsolved synthetic problem important for the generation of fuels and chemical feedstocks from biomass and for the liquefaction of coal. Currently, the hydrogenolysis of aromatic C-O bonds requires heterogeneous catalysts that operate at high temperature and pressure and lead to a mixture of products from competing hydrogenolysis of aliphatic C-O bonds and hydrogenation of the arene. Here, we report hydrogenolyses of aromatic C-O bonds in alkyl aryl and diaryl ethers that form exclusively arenes and alcohols. This process is catalyzed by a soluble nickel carbene complex under just 1 bar of hydrogen at temperatures of 80 to 120°C; the relative reactivity of ether substrates scale as Ar-OAr>Ar-OMe>ArCH(2)-OMe (Ar, Aryl; Me, Methyl). Hydrogenolysis of lignin model compounds highlights the potential of this approach for the conversion of refractory aryl ether biopolymers to hydrocarbons.     

Actinide organometallic chemistry

The stoichiometric and catalytic chemistry of metal-organic compounds having actinide-to-carbon bonds is in a stage of rapid growth. Chemical, structural, and bonding characteristics have been identified which differ in interesting and informative ways from those of d-block transition element compounds.     

链霉菌分解角蛋白的生化机制研究 Ⅲ含硫化合物对角蛋白酶作用机理和角蛋白降解的生化机制初步研究

硫酸钠、亚硫酸钠和巯基乙醇对角蛋白酶具有强烈的激活作用,使角蛋白酶活力分别依次提高了４,７和２０多倍,其主要表现作用于角蛋白酶中的二硫键还原酶。亚硫酸钠在０．０１ｍｏｌ／Ｌ浓度下不仅作用于二硫键还原酶,而且还作用于多肽水解酶。硫代硫酸钠对二硫键还原酶有强烈的抑制作用。角蛋白酶降解羽毛角蛋白首先是角蛋白酶中的二硫键还原酶使角蛋白中二硫键裂解产生变性角蛋白,然后在多肽水解酶的共同作用下,将变性角蛋白逐渐分解成多肽、寡肽和游离氨基酸,使角蛋白彻底分解。角蛋白酶中的二硫键还原酶是降解角蛋白的关键酶。     

Efficient activation of aromatic C-H bonds for addition to C-C multiple bonds

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.     

Heat inactivation of the relaxing site of actomyosin: prevention and reversal with dithiothreitol

Adenosine triphosphate and magnesium (MgATP) inhibit contraction by binding to a specific relaxing site on natural actomyosin gel. This inhibitory control site is distinct from the active sites where MgATP causes contraction.In high concentrations of MgATP, calcium triggers contraction by releasing the protein from substrate inhibition, allowing the contractile reactions to occur. Heating the protein for 5 minutes at 43 degrees C selectively inactivates the relaxing site. After this treatment, actomyosin with MgATP contracts as well without calcium as with it. That this effect of heat is prevented and reversed by dithiothreitol (an agent that reduces disulfide bonds) indicates that the structure of the relaxing site depends on certain labile sulfhydryl groups, which may be those of tropomyosin. When these are oxidized to disulfide bonds, the site loses its activity; when the disulfide bonds are reduced, the site regains its activity.     

Sex attractant of the codling moth: characterization with electroantennogram technique

trans-8, trans-10-Dodecadien-1-ol is a sex attractant of the codling moth, Laspeyresia pomonella. Antennal responses (electroantennograms) to a series of monounsaturated compounds were used in determining the location and configuration of the two double bonds. The synthetic compound is very attractive to male codling moths in the field.     

Low-temperature synthesis of zintl compounds with a single-source molecular precursor

Thermolysis of the heterobimetallic phosphinidene complex [Sb(PCy)3]2- Li6.6HNMe2 (Cy = C6H11) at 303 to 313 kelvin gives Zintl compounds containing (Sb7)3- anions. The complex thus constitutes a stable molecular single-source precursor to Zintl compounds and provides a potential low-temperature route to photoactive alkali metal antimonates. The new chemical reaction involved, which is driven thermodynamically by the formation of P-P bonds, has implications in the low-temperature synthesis of other technologically important materials (such as gallium arsenide).     

木材酸性液化条件下苯酚的作用机理

论述了酸性条件下木材液化中苯酚的作用机理.苯酚作为亲核试剂,引起了木材组分一些主要化学键的断裂,使得木材组分大分子降解为小分子;苯酚作为反应试剂,与其中一些降解生成的小分子反应生成具有酚类结构的化合物;苯酚作为溶剂,使生成的酚类结构化合物溶解,并减缓或阻止了已生成的活性小分子之间的缩聚反应.催化剂酸提供质子(H )以及酸性环境,使化学键的断裂更为容易,同时也改变了液化反应途径.     

A predictably selective aliphatic C-H oxidation reaction for complex molecule synthesis

Realizing the extraordinary potential of unactivated sp3 C-H bond oxidation in organic synthesis requires the discovery of catalysts that are both highly reactive and predictably selective. We report an iron (Fe)-based small molecule catalyst that uses hydrogen peroxide (H2O2) to oxidize a broad range of substrates. Predictable selectivity is achieved solely on the basis of the electronic and steric properties of the C-H bonds, without the need for directing groups. Additionally, carboxylate directing groups may be used to furnish five-membered ring lactone products. We demonstrate that these three modes of selectivity enable the predictable oxidation of complex natural products and their derivatives at specific C-H bonds with preparatively useful yields. This type of general and predictable reactivity stands to enable aliphatic C-H oxidation as a method for streamlining complex molecule synthesis.