Showing papers on "Reactivity (chemistry) published in 2005"
TL;DR: This workFloat their problematic reactions on water and to send observations of success or failure to us at onwater@scripps.edu for public dissemination with attribution.
Abstract: [*] Dr. S. Narayan, Dr. J. Muldoon, Prof. M. G. Finn, Prof. V. V. Fokin, Prof. H. C. Kolb, Prof. K. B. Sharpless Department of Chemistry and the Skaggs Institute of Chemical Biology The Scripps Research Institute 10550 North Torrey Pines Road La Jolla, CA 92037 (USA) Fax: (+ 1)619-554-6738 E-mail: sharples@scripps.edu [**] We thank Dr. Vladislav Litosh for carrying out preliminary work. Support from the National Institutes of Health, National Institute of General Medical Sciences (GM 28384), the National Science Foundation (CHE9985553), the Skaggs Institute for Chemical Biology, and the W. M. Keck Foundation is gratefully acknowledged. S.N. thanks the Skaggs Institute for a postdoctoral fellowship. We also thank Dr. Suresh Suri, Edwards Air Force Base, California, for a generous gift of quadricyclane. We urge our fellow chemists to float their problematic reactions on water and to send observations of success or failure to us at onwater@scripps.edu for public dissemination with attribution. Supporting information for this article is available on the WWW under http://www.angewandte.org or from the author. Angewandte Chemie
1,393 citations
TL;DR: Two widely studied nano-Fe0 preparations are characterized: one synthesized by reduction of goethite with heat and H2 and the other by reductive precipitation with borohydride, which exhibit corrosion potentials that are more negative than nano-sized Fe2O3, Fe3O4, micro-sizedFe0, or a solid Fe0 disk.
Abstract: There are reports that nano-sized zero-valent iron (Fe0) exhibits greater reactivity than micro-sized particles of Fe0, and it has been suggested that the higher reactivity of nano-Fe0 may impart advantages for groundwater remediation or other environmental applications. However, most of these reports are preliminary in that they leave a host of potentially significant (and often challenging) material or process variables either uncontrolled or unresolved. In an effort to better understand the reactivity of nano-Fe0, we have used a variety of complementary techniques to characterize two widely studied nano-Fe0 preparations: one synthesized by reduction of goethite with heat and H2 (FeH2) and the other by reductive precipitation with borohydride (FeBH). FeH2 is a two-phase material consisting of 40 nm α-Fe0 (made up of crystals approximately the size of the particles) and Fe3O4 particles of similar size or larger containing reduced sulfur; whereas FeBH is mostly 20−80 nm metallic Fe particles (aggregates ...
896 citations
TL;DR: A simple and green approach for synthesizing palladized iron (Fe-Pd) nanoparticles by applying a water-soluble starch as a stabilizer is developed, and the starched nanoparticles exhibited markedly greater reactivity when used for dechlorination of TCE or PCBs in water.
Abstract: Dechlorination of TCE and PCBs using bimetallic nanoparticles has received increasing interest in recent years. However, due to the extremely high reactivity, nanoparticles prepared using current methods tend to either react with surrounding media or agglomerate, resulting in the formation of much larger flocs and significant loss in reactivity. To overcome these drawbacks, we developed a simple and green approach for synthesizing palladized iron (Fe−Pd) nanoparticles. We modified the conventional methods by applying a water-soluble starch as a stabilizer. The starched nanoparticles displayed much less ag glomeration but greater dechlorination power than those prepared without a stabilizer. TEM analyses indicated that the starched nanoparticles were present as discrete particles as opposed to dendritic flocs for nonstarched particles. The mean particle size was estimated to be 14.1 nm with a standard deviation of 11.7 nm, which translated to a surface area of ∼55 m2 g-1. While starched nanoparticles remai...
782 citations
TL;DR: Findings of an investigation of the synthesis and assembly of core-shell nanoparticles with high monodispersity are described, revealing a decrease in magnetization and blocking temperature and an increase in coercivity for Fe(3)O(4)@Au, reflecting the decreased coupling of the magnetic moments.
Abstract: The ability to synthesize and assemble monodispersed core−shell nanoparticles is important for exploring the unique properties of nanoscale core, shell, or their combinations in technological applications. This paper describes findings of an investigation of the synthesis and assembly of core (Fe3O4)−shell (Au) nanoparticles with high monodispersity. Fe3O4 nanoparticles of selected sizes were used as seeding materials for the reduction of gold precursors to produce gold-coated Fe3O4 nanoparticles (Fe3O4@Au). Experimental data from both physical and chemical determinations of the changes in particle size, surface plasmon resonance optical band, core−shell composition, surface reactivity, and magnetic properties have confirmed the formation of the core−shell nanostructure. The interfacial reactivity of a combination of ligand-exchanging and interparticle cross-linking was exploited for molecularly mediated thin film assembly of the core−shell nanoparticles. The SQUID data reveal a decrease in magnetization ...
543 citations
TL;DR: Anatase-type TiO2 particles were synthesized by a two-step method consisting of the preparation of a solid precursor through hydrolysis of titanium alkoxide followed by heat treatment at different temperatures under ambient air as discussed by the authors.
Abstract: Anatase TiO2 particles were synthesized by a two-step method consisting of the preparation of a solid precursor through hydrolysis of titanium alkoxide followed by heat treatment at different temperatures under ambient air. This simple method led to a highly porous material with a 200 nm homogenous particle size, while the BET specific surface area and crystallite sizes evolved between 49 m2 g−1 to 223 m2 g−1 and from 17.0 nm to 6.3 nm, respectively. Their electrochemical performances clearly revealed the beneficial influence of the divided texture of anatase-type TiO2 on the reactivity with lithium, namely in terms of reversibility. Furthermore, we showed that the TiO2 texture strongly affects the extent of the solid solution domain. Finally, through a simple chemical titration it was possible to clearly quantify the capacitive/faradaic contributions of the electrochemical reaction.
382 citations
TL;DR: Reductive alpha-elimination and hydrogenolysis were concurrent pathways for compounds possessing chlorine substitution on one carbon only, forming less chlorinated ethanes, indicating PCA removal conforms to pseudo-first-order kinetics.
Abstract: Dechlorination of three chlorinated methanes (CCl4, CHCl3, CH2Cl2) was investigated using a nano-sized iron synthesized by borohydride reduction of Fe3+ under anaerobic conditions. When reacted with chlorinated methanes in batch reactors, nano-sized iron rapidly transformed CCl4 and CHCl3, but showed negligible reactivity toward CH2Cl2, giving the reactivity order of CCl4 > CHCl3 >> CH2Cl2. CH4 was observed along with CH2Cl2 in the reduction of CCl4 and CHCl3, and they are presumed to be generated via concerted reductive elimination steps involving carbene and charged radical species. Pathways for CH4 production from CCl4 and CHCl3 reductions are proposed. Evidence obtained from the study of several physicochemical factors including pH, initial concentration, hydrogen concentration, and metal loading indicates reduction of chlorinated methanes occurs via a direct electron transfer reduction mechanism, rather than an indirect mechanism involving reactive hydrogen species. In a comparative experiment with t...
367 citations
329 citations
TL;DR: Concentration profiles demonstrate that enol flame chemistry cannot be accounted for purely by keto-enol tautomerization, and currently accepted hydrocarbon oxidation mechanisms will likely require revision to explain the formation and reactivity of these unexpected compounds.
Abstract: 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.
301 citations
TL;DR: It has been nearly 20 years since the last major review on boron cations and developments in weakly coordinating anion technology suggest that this area of research is ripe for exploration.
Abstract: Boron cations are elusive and highly electrophilic species that play a key role in the chemistry of boron. Despite early interest in the chemistry of boron cations, until recently they have largely remained chemical curiosities. However, hints at harnessing their potential as potent electrophiles have begun to appear and developments in weakly coordinating anion technology suggest that this is an area of research that is ripe for exploration. It has been nearly 20 years since the last major review on boron cations; herein we summarize the progress in the area since that time.
295 citations
TL;DR: The reaction kinetics are found to have a strong chiral dependence, with rates being slowest for large-band gap species and increasing for smaller-bandgap nanotubes, and the chiral-dependent kinetics can be tuned to effectively freeze the reacted spectra at a fixed chiral distribution.
Abstract: Chiral selective reactivity and redox chemistry of carbon nanotubes are two emerging fields of nanoscience. These areas hold strong promise for producing methods for isolating nanotubes into pure samples of a single electronic type, and for reversible doping of nanotubes for electronics applications. Here, we study the selective reactivity of single-walled carbon nanotubes with organic acceptor molecules. We observe spectral bleaching of the nanotube electronic transitions consistent with an electron-transfer reaction occurring from the nanotubes to the organic acceptors. The reaction kinetics are found to have a strong chiral dependence, with rates being slowest for large-bandgap species and increasing for smaller-bandgap nanotubes. The chiral-dependent kinetics can be tuned to effectively freeze the reacted spectra at a fixed chiral distribution. Such tunable redox chemistry may be important for future applications in reversible non-covalent modification of nanotube electronic properties and in chiral selective separations.
290 citations
TL;DR: Study of the early steps of reaction of 2-microm sized particles of Bioglass, in solutions buffered with TRIS at different pH, by means of ICP-ES and FTIR spectroscopy showed that the process of silica formation is not separable from cation leaching from the glass, as well as the formation of the calcium phosphate rich layer.
TL;DR: HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not, for teaching and research institutions in France or abroad.
Abstract: HAL is a multi-disciplinary open access archive for the deposit and dissemination of scientific research documents, whether they are published or not. The documents may come from teaching and research institutions in France or abroad, or from public or private research centers. L’archive ouverte pluridisciplinaire HAL, est destinée au dépôt et à la diffusion de documents scientifiques de niveau recherche, publiés ou non, émanant des établissements d’enseignement et de recherche français ou étrangers, des laboratoires publics ou privés.
TL;DR: In this paper, a data processing tool was developed to generate the explicit gas-phase oxidation schemes of acyclic hydrocarbons and their oxidation products under tropospheric conditions and the protocol used to select the reaction products and the rate constants.
Abstract: . Organic compounds emitted in the atmosphere are oxidized in complex reaction sequences that produce a myriad of intermediates. Although the cumulative importance of these organic intermediates is widely acknowledged, there is still a critical lack of information concerning the detailed composition of the highly functionalized secondary organics in the gas and condensed phases. The evaluation of their impacts on pollution episodes, climate, and the tropospheric oxidizing capacity requires modelling tools that track the identity and reactivity of organic carbon in the various phases down to the ultimate oxidation products, CO and CO2. However, a fully detailed representation of the atmospheric transformations of organic compounds involves a very large number of intermediate species, far in excess of the number that can be reasonably written manually. This paper describes (1) the development of a data processing tool to generate the explicit gas-phase oxidation schemes of acyclic hydrocarbons and their oxidation products under tropospheric conditions and (2) the protocol used to select the reaction products and the rate constants. Results are presented using the fully explicit oxidation schemes generated for two test species: n-heptane and isoprene. Comparisons with well-established mechanisms were performed to evaluate these generated schemes. Some preliminary results describing the gradual change of organic carbon during the oxidation of a given parent compound are presented.
TL;DR: Reactivity and structural studies of unusual rhodium and iridium systems bearing two N-heterocyclic carbene (NHC) ligands are presented and DFT calculations show that these electrophilic metal centers are stabilized by pi-donation of the NHC ligands.
Abstract: Reactivity and structural studies of unusual rhodium and iridium systems bearing two N-heterocyclic carbene (NHC) ligands are presented. These systems are capable of intramolecular C−H bond activation and lead to coordinatively unsaturated 16-electron complexes. The resulting complexes can be further unsaturated by simple halide abstraction, leading to 14-electron species bearing an all-carbon environment. Saturation of the vacant sites in the 16- and 14-electron complexes with carbon monoxide permits a structural comparison. DFT calculations show that these electrophilic metal centers are stabilized by π-donation of the NHC ligands.
TL;DR: It is proposed that a direct electron transfer from surface-complexed phenol to the conduction band of TiO2 upon absorbing visible light initiates the oxidative degradation of phenolic compounds.
Abstract: The visible-light-induced degradation reaction of 4-chlorophenol (4-CP) was investigated in aqueous suspension of pure TiO2. Contrary to common expectations, 4-CP could be degraded under visible illumination (λ > 420 nm), generating chlorides and CO2 concomitantly. The observed visible reactivity was not due to the presence of trace UV light since the visible-light-induced reactions exhibited behaviors distinguished from those of UV-induced reactions. Dichloroacetate could not be degraded under visible light, whereas it degraded with a much faster rate than 4-CP under UV irradiation. The addition of tert-butyl alcohol, a common OH radical scavenger, did not affect the visible reactivity of 4-CP, which indicates that OH radicals are not involved. Other phenolic compounds such as phenol and 2,4-dichlorophenol were similarly degraded under visible light. The surface complexation between phenolic compounds and TiO2 appears to be responsible for the visible light reactivity. Diffuse reflectance UV−vis spectra ...
TL;DR: In this article, the authors report progress in the chemistry of synthetic systems related to the active site of iron-only hydrogenase and present a review of the literature related to these systems.
TL;DR: The experimentally observed stacking arrangement between consecutive bases in DNA and RNA/DNA double helices is shown to enhance their hydrogen bonding ability as opposed to gas phase optimized complexes, and the MEP minima on surfaces around the bases in experimental structures show that their hydrogen bonded capacity increases when taking more neighboring (intra-strand) stacking partners into account.
Abstract: The interplay between aromatic stacking and hydrogen bonding in nucleobases has been investigated via high-level quantum chemical calculations. The experimentally observed stacking arrangement between consecutive bases in DNA and RNA/DNA double helices is shown to enhance their hydrogen bonding ability as opposed to gas phase optimized complexes. This phenomenon results from more repulsive electrostatic interactions as is demonstrated in a model system of cytosine stacked offset-parallel with substituted benzenes. Therefore, the H-bonding capacity of the N3 and O2 atoms of cytosine increases linearly with the electrostatic repulsion between the stacked rings. The local hardness, a density functional theory-based reactivity descriptor, appears to be a key index associated with the molecular electrostatic potential (MEP) minima around H-bond accepting atoms, and is inversely proportional to the electrostatic interaction between stacked molecules. Finally, the MEP minima on surfaces around the bases in experimental structures of DNA and RNA–DNA double helices show that their hydrogen bonding capacity increases when taking more neighboring (intra-strand) stacking partners into account.
TL;DR: This methodology shows the particular reactivity of N-iminopyridinium ylides that provides access to substituted piperidines in good enantiomeric excesses.
Abstract: We have developed an efficient catalytic enantioselective hydrogenation of pyridine derivatives. Enhanced reactivity was possible by an optimization of the electronic properties of the catalyst through ligand modification. This methodology shows the particular reactivity of N-iminopyridinium ylides that provides access to substituted piperidines in good enantiomeric excesses.
TL;DR: In this article, the chemical reactivity of mass-selected small noble metal cluster ions in a temperature controlled radio frequency ion trap arrangement is investigated, in order to reveal the possible role of these small noble-metal cluster ions to act as gas-phase catalysts in the carbon monoxide combustion reaction.
TL;DR: Experiments with stereospecifically deuterated primary alcohol substrates rule out a "Wacker-type" mechanism involving anti oxypalladation and suggest that the reaction proceeds by syn oxypAlladation for both mono- and bidentate ligands, in contrast, cyclizations of deuterium-labeled carboxylic acid substrates undergo anti oxylating ligands.
Abstract: Oxidative cyclizations of a variety of heteroatom nucleophiles onto unactivated olefins are catalyzed by palladium(II) and pyridine in the presence of molecular oxygen as the sole stoichiometric oxidant in a nonpolar solvent (toluene). Reactivity studies of a number of N-ligated palladium complexes show that chelating ligands slow the reaction. Nearly identical conditions are applicable to five different types of nucleophiles: phenols, primary alcohols, carboxylic acids, a vinylogous acid, and amides. Electron-rich phenols are excellent substrates, and multiple olefin substitution patterns are tolerated. Primary alcohols undergo oxidative cyclization without significant oxidation to the aldehyde, a fact that illustrates the range of reactivity available from various Pd(II) salts under differing conditions. Alcohols can form both fused and spirocyclic ring systems, depending on the position of the olefin relative to the tethered alcohol; the same is true of the acid derivatives. The racemic conditions served as a platform for the development of an enantioselective reaction. Experiments with stereospecifically deuterated primary alcohol substrates rule out a “Wacker-type” mechanism involving anti oxypalladation and suggest that the reaction proceeds by syn oxypalladation for both mono- and bidentate ligands. In contrast, cyclizations of deuterium-labeled carboxylic acid substrates undergo anti oxypalladation.
TL;DR: Various types of aromatic and aliphatic sulfides are selectively oxidized to sulfoxides and sulfones in good to excellent yields using 30% H2O2 in the presence of catalytic amounts of a novel recoverable silica-based tungstate interphase catalyst at room temperature.
TL;DR: Nickel-catalyzed cross-coupling of Grignard reagents with aryl (poly)fluorides or ( poly)chlorides can be achieved efficiently in the presence of a new triarylphosphine ligand bearing a nearby hydroxy group.
Abstract: Nickel-catalyzed cross-coupling of Grignard reagents with aryl (poly)fluorides or (poly)chlorides can be achieved efficiently in the presence of a new triarylphosphine ligand bearing a nearby hydroxy group. The high reactivity and the unique chemoselectivity (ArF > ArOTf ≫ ArSR) of the catalysis have been attributed to synergy of nickel and magnesium atoms preorganized on the ligand, as has been surmised on the basis of theoretical modeling of the reaction mechanism.
TL;DR: In this article, the Ullman reaction and the hydrogenation of CO 2 to form CH 3 OH were tested using nanocrystalline particles of copper using solvated metal atom dispersion (SMAD) technique.
Abstract: Nanocrystalline particles of copper have been prepared using the solvated metal atom dispersion (SMAD) technique. Their chemical and catalytic reactivity have been tested in the Ullman reaction (aryl homocoupling) and the hydrogenation of CO 2 to form CH 3 OH. The powders obtained from different solvents have surface areas of 24–30 m 2 /g, particle sizes of 20–45 nm, and crystallite sizes of 8–12 nm. The order of activity found toward the aryl homocoupling reaction is Cu * /toluene > Cu * /THF > Cu * /pentane > Cu * /diglyme, with a maximum biphenyl yield of 90% at 150 °C after 6 h for the Cu * /toluene sample. For the CO 2 to CH 3 OH reaction, a maximum conversion of 80% was obtained for the Cu * /pentane/NC-ZnO sample. In general, the Cu * /nanocrystalline (NC)-ZnO samples show a larger conversion of CO 2 to methanol at 450 °C compared with NC-ZnO and NC-ZnO/CuO.
TL;DR: Ti(NMe(2))(4) has been used effectively as a precatalyst in a facile protocol for the intramolecular hydroamination of aminoalkenes to yield pyrrolidine and piperidine heterocyclic products with isolated yields up to 92%.
TL;DR: The reaction with various aromatic and heteroaromatic compounds revealed that the low mono/dialkylation selectivity was observed only for highly reactive aromatics, and the use of micromixing was found to be quite effective to improve the selectivity.
Abstract: Friedel−Crafts reactions of aromatic and heteroaromatic compounds with an N-acyliminium ion pool were studied. The reaction of 1,3,5-trimethylbenzene in a batch reactor gave rise to the selective formation of a monoalkylation product (69%). Presumably, the second alkylation is slower than the first alkylation because of the protonation of the monoalkylation product that decreases its reactivity. The reaction of 1,3,5-trimethoxybenzene, however, gave rise to the formation of both monoalkylation (37%) and dialkylation (32%) products. Disguised chemical selectivity due to faster reaction than mixing seems to be responsible for the lack of selectivity. The use of micromixing was found to be quite effective to solve this problem to increase the selectivity. The monoalkylation product was obtained in 92% yield together with a small amount of the dialkylation product (4%). The reaction with various aromatic and heteroaromatic compounds revealed that the low mono/dialkylation selectivity was observed only for hig...
TL;DR: In this paper, the authors reviewed issues concerning the reactivity of group 8 polypyridyl complexes from the perspective of recent research findings, which have led to considerable clarification of the reaction pathways.
TL;DR: A sensory polymeric material for the colorimetric sensing of cyanide in water has been developed based on the reactivity of this anion with the pyrylium cation.
TL;DR: In this article, a survey of the coordination chemistry and reactivity of these species is described, drawing comparisons with the well known N-heterocyclic carbene class of ligand where possible.
TL;DR: DFT calculations of C-H hydroxylation by a synthetic nonheme oxoiron(IV) oxidant supported by a neutral pentadentate N5 ligand show that this reagent is intrinsically more reactive than compound I of P450.
Abstract: DFT calculations of C−H hydroxylation by a synthetic nonheme oxoiron(IV) oxidant supported by a neutral pentadentate N5 ligand show that this reagent is intrinsically more reactive than compound I of P450. This nonheme iron oxidant is predicted to exhibit stereoselective reactions, strong solvent effect, and involve multistate reactivity with spin-state crossing.