Showing papers on "Reactivity (chemistry) published in 1998"
796 citations
504 citations
TL;DR: An improved basis for predicting the rates of thiol-disulfide exchange reactions is provided and ways in which differences in the rate of competing reactions can be either minimized, to simplify the analysis of disulfide-coupled folding reactions, or enhanced, to favor formation of particular disulfides are suggested.
Abstract: Thiol-disulfide exchange reactions are required for many aspects of cellular metabolism including the folding of disulfide-bonded proteins, electron transfer, and numerous regulatory mechanisms. To identify factors influencing the rates of these reactions in polypeptides, the reactivities of Cys thiols in 16 model peptides were measured. For each of the peptides, which contained single Cys residues with thiol pKas ranging from 7.4 to 9.1, the rates of exchange with four disulfide-bonded compounds were measured. In reactions with two of the disulfide reagents, cystine and 2-hydroxyethyl disulfide, the peptide thiols displayed Bronsted correlations between reaction rate and pKa similar to those observed previously with model compounds (betanuc = 0.5 and 0.3, respectively). For two reagents with net charges, oxidized glutathione and cystamine, however, the apparent Bronsted coefficients were 0 and 0.8, respectively. These observations are in striking contrast with those obtained with model compounds, for which the Bronsted coefficients for the nucleophilic thiolates are largely independent of the disulfide-containing compound. The differences in the apparent Bronsted coefficients can be largely accounted for by electrostatic interactions between charged groups on the peptides and disulfide reagents and demonstrate that such interactions can play a dominant role in determining the rates of thiol-disulfide exchange in biological molecules. The results presented here provide an improved basis for predicting the rates of these reactions and suggest ways in which differences in the rates of competing reactions can be either minimized, to simplify the analysis of disulfide-coupled folding reactions, or enhanced, to favor formation of particular disulfides.
380 citations
TL;DR: High turnover numbers and up to 98% ee were obtained in the catalytic hydrogenation of unfunctionalized aryl-substituted olefins with iridium-phosphanyldihydrooxazole complexes 1 (see reaction scheme).
Abstract: High turnover numbers and up to 98% ee were obtained in the catalytic hydrogenation of unfunctionalized aryl-substituted olefins with iridium-phosphanyldihydrooxazole complexes 1 (see reaction scheme). The anion of the complex-for example, hexafluorophosphate or tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (BARF- )-has a remarkable effect on the reactivity and longevity of the catalyst.
368 citations
TL;DR: In this paper, chemical reactions taking place at elevated temperatures in a polymer-bonded lithiated carbon anode were studied by differential scanning calorimetry, and the influences of parameters such as degree of intercalation, number of cycles, specific surface area, and chemical nature of the binder were elucidated.
Abstract: Chemical reactions taking place at elevated temperatures in a polymer-bonded lithiated carbon anode were studied by differential scanning calorimetry. The influences of parameters such as degree of intercalation, number of cycles, specific surface area, and chemical nature of the binder were elucidated. It was clearly established that the first reaction taking place at ca. 120-140 °C was the transformation of the passivation layer products into lithium carbonate, and that lithiated carbon reacted with the molten binder via dehydrofluorination only at T > 300 °C. Both reactions strongly depend on the specific surface area of the electrodes and the degree of lithiation.
347 citations
TL;DR: In this article, the interaction of propylene at elevated temperature (>300 K) and elevated pressure (5 Torr) with cuprous and cupric oxide has been investigated with core level XPS, resonant photoemission, and temperature-programmed desorption.
Abstract: Cu2O is an efficient catalyst in the partial oxidation of propylene to acrolein, while propylene oxidation on CuO leads to complete combustion. The interaction of propylene at elevated temperature (>300 K) and elevated pressure (5 Torr) with cuprous and cupric oxide has been investigated with core level XPS, resonant photoemission, and temperature-programmed desorption. Reduction of the copper oxide surfaces was examined as a function of temperature and revealed that cupric oxide has a greater reactivity toward propylene oxidation than cuprous oxide (Ea = 5.9 versus 11.5 kcal/mol for cuprous oxide (24.7 and 48.1 kJ/mol)). This variable temperature oxidation of propylene was also monitored via core level and resonant photoemission and was found to occur by a similar mechanism on both surfaces. Reaction at lower temperature produces a surface intermediate which exhibits carbon 1s XPS peaks at 284.0 and 285.5 eV binding energy in a 2:1 intensity ratio. This is consistent with an allyl alkoxide surface specie...
336 citations
TL;DR: In this article, an experimental gasification study of Bowmans coal with steam and with carbon dioxide in a single-particle reactor operating at atmospheric pressure and at temperatures between 714 and 892°C was conducted.
335 citations
TL;DR: In this paper, the DFT-based descriptors "local softness" and "local hardness" are used as reactivity indices to predict the reactivity sequences (both intramolecular and intermolecular) of carbonyl compounds toward nucleophilic attack on them.
Abstract: The DFT-based reactivity descriptors “local softness” and “local hardness” are used as reactivity indices to predict the reactivity sequences (both intramolecular and intermolecular) of carbonyl compounds toward nucleophilic attack on them. The finite difference approximation is used to calculate local softness, whereas local hardness is approximated by −Vel/2N, where Vel is the electronic part of the molecular electrostatic potential. Both aldehydes and ketones, aliphatic and aromatic, have been selected as systems. Critical cases, e.g., C6H5CHCHCHO, CH3CHCHCHO, and CH2CHCHO, where a CC double bond is in conjugation with the CO group, are also considered. Two new reactivity descriptors are proposed, “relative electrophilicity” (sk+/sk-) and “relative nucleophilicity” (sk-/sk+), which will help to locate the preferable reactive sites. Our results show that local hardness can be used as a guiding parameter when constructing intermolecular reactivity sequences.
318 citations
TL;DR: In this paper, a chemically reasonable mechanistic model for the water oxidation function of photosystem I1 was proposed based on the nucleophilic attack by calcium-ligated hydroxide on an electrophilic 0x0 group ligated to high-valent manganese to achieve the critical 0-0 bond formation step.
Abstract: There has been much speculation concerning the mechanism for water oxidation by Photosystem 11. Based on recent work on the biophysics of Photosystem I1 and our own work on the reactivity of synthetic manganese complexes, we propose a chemically reasonable mechanistic model for the water oxidation function of this enzyme. An essential feature of the model is the nucleophilic attack by calcium-ligated hydroxide on an electrophilic 0x0 group ligated to high-valent manganese to achieve the critical 0-0 bond formation step. We also present a model for S-state advancement as a series of proton-coupled electron transfer steps, which has been proposed previously (Hoganson et. al., Photosynth. Res. 46, 177 (1995); Gilchrist et. al. Proc. Nat. Acad. Sci, USA. 92, 9545 (1995)), but for which we have developed model systems that allow us to probe the thermodynamics in some detail. One of the great unsolved mysteries in bioinorganic chemistry is the mechanism of water oxidation by the oxygen evolving complex (OEC) of Photosystem I1 (PS 11). This reaction is responsible for nearly all of the dioxygen on our planet and conceptually is the reverse reaction of respiration where dioxygen is converted back to water. Plants use an expansive airay of photopigments in Photosystem 11, four manganese ions, calcium and chloride to carry out these reactions. While intensively studied for many years, only now is a picture emerging as to how this fascinating and essential chemistry may result. The scope of this article is far too limited to allow for a detailed summary of previous studies in the field: therefore, interested readers are directed to recent reviews of this topic(ref. 1,2).
306 citations
Abstract: The reactivity of a new three carbon synthon, generated in situ from the reaction of 2,3-butadienoates or 2-butynoates with an appropriate phosphine as the catalyst, toward the electron-deficient imines is described. Triphenylphosphine-catalyzed reaction of methyl 2,3-butadienoate with N-sulfonylimines gave the single [3+2] cycloadduct in excellent yield; tributylphosphine-catalyzed reaction of methyl 2,3-butadienoate or 2-butynoate with N-tosylimines afforded the corresponding [3+2] cycloadduct as the major product along with a small amount of the three components adduct. Aliphatic N-tosylimines gave moderate yield for this reaction. In addition, a new phosphine-catalyzed cyclization reaction of dimethyl acetylenedicarboxylate with N-tosylimines is also described. A reaction mechanism is proposed. Further elaborations of the cycloaddition products and the synthesis of pentabromopseudilin using this method are exemplified.
283 citations
TL;DR: Differential scanning calorimetry (DSC) has been demonstrated to be a useful means of studying the reactivity of materials used in lithium-ion cells as discussed by the authors, and the results on the safety of lithium ion cells are discussed.
TL;DR: Comparing the behavior of the ionic reaction in the gas phase with that in solution is compared and the relation of structure and reactivity, and the effects of small numbers of solvent molecules on the reaction are reviewed.
Abstract: Nucleophilic displacement reactions (the SN2 reaction) of ions in the gas phase are a prototypical reaction system that allows a study of dynamics, mechanisms, and structure-energy relations. This article reviews aspects of the kinetics (especially the applicability of statistical reaction rate theory), the relation of structure and reactivity, and the effects of small numbers of solvent molecules on the reaction and compares the behavior of the ionic reaction in the gas phase with that in solution.
TL;DR: Highly dispersed titanium oxide photocatalysts anchored onto transparent plates of porous silica glass were successfully prepared by metal ion implantation, and their photocatalysistic reactivity for... as mentioned in this paper.
Abstract: Highly dispersed titanium oxide photocatalysts anchored onto transparent plates of porous silica glass were successfully prepared by metal ion implantation, and their photocatalytic reactivity for ...
TL;DR: In this paper, the tuning of donor reactivity during coupling reactions, such that we may avoid the lengthy protecting-group manipulations of classical carbohydrate synthesis, affords a strategy for the rapid assembly of large sugar systems.
Abstract: The concise preparation of complex oligosaccharides remains a significant challenge for synthetic organic chemistry. The tuning of donor reactivity during coupling reactions, such that we may avoid the lengthy protecting-group manipulations of classical carbohydrate synthesis, affords a strategy for the rapid assembly of large sugar systems. Competition reactions have been used to quantify the influence of protecting groups, monosaccharide type, and anomeric leaving groups on the reactivity of various glycosyl donors.
TL;DR: Changing the oxidation state of redox-active ligands is a powerful method for electrochemically controlling the reactivity and selectivity of bound transition metals.
Abstract: Changing the oxidation state of redox-active ligands is a powerful method for electrochemically controlling the reactivity and selectivity of bound transition metals (see drawing on the right). In some cases reaction rates can be increased by many orders of magnitude simply by removing an electron from a metal-ligand complex. In the case of polymeric redox-active ligands, a continuous range of properties can be achieved for the transition metal complex.
TL;DR: The development of molecular orbitals (MO) techniques and their applications to structural and reactivity problems in coordination chemistry has been greatly enhanced by the availability of density functional theory (DFT) methods as discussed by the authors.
TL;DR: In this article, the reactivity of the title compound as 4π component in inverse-type Diels-Alder reactions, including kinetic data, is reported. But the results are limited to 4-mono-and 4-5-disubstituted pyridazines.
TL;DR: The photocatalytic reactivity of these catalysts was investigated as a function of the Ti content for the liquid-phase oxidation of 1-octanol to 1 -octanal, and it was dramatically enhanced in regions of lower Ti content as mentioned in this paper.
Abstract: Titanium−silicon (Ti/Si) binary oxides having different Ti contents were prepared by the sol−gel method and used as photocatalysts. The photocatalytic reactivity of these catalysts was investigated as a function of the Ti content for the liquid-phase oxidation of 1-octanol to 1-octanal, and it was found to be dramatically enhanced in regions of lower Ti content. In situ photoluminescence, UV−vis reflectance, FT-IR, ESR, XAFS, XRD, and XPS spectroscopic investigations of these Ti/Si binary oxides indicated that the titanium oxide species are highly dispersed in the SiO2 matrixes and exist in a tetrahedral coordination exhibiting a characteristic photoluminescence spectrum due to the radiative decay from the charge-transfer excited state of the tetrahedrally coordinated titanium oxide species. The good parallel relationship between the yield of the photoluminescence and the specific photocatalytic reactivity of the Ti/Si binary oxides as a function of the Ti content clearly indicates that the high photocata...
TL;DR: In this article, the synthesis and several reactions of the stable silylene 1 (1,3-di-tert-butyl-2, 3-dihydro-1H-1, 3,2-diazasilol-2-ylidene) are described.
Abstract: The synthesis and several reactions of the stable silylene 1 (1,3-di-tert-butyl-2,3-dihydro-1H-1,3,2-diazasilol-2-ylidene) are described. X-ray crystal structures are given for two intermediates in the synthesis of 1, the diimine 7 and its dilithium salt 8, and for a byproduct, the spiro compound 10. 1 reacts with ethanol and water by inserting into the O−H bond, and with iodomethane by insertion into the C−I bond. The silanol from reaction of 1 with water condenses to a disiloxane, 13. From 1 and sulfur and selenium, cyclic compounds containing Si2E2 rings are obtained (E = S or Se). Crystal structures are presented for disiloxane 13 and for the sulfur and selenium cycloadducts, 15 and 17.
TL;DR: The overall reactivity is governed both by the intrinsic reactivity of the reactive species and its concentration, two factors which can be antagonistic and a non-linear Hammett correlation of the oxidative addition with the basicity of the phosphine is observed for monodentate ligands.
TL;DR: In this paper, the dimeric alkaline earth metal bis[bis(trialkylsilyl)phosphanides] were given as the minimum structures according to ab initio calculations for calcium and strontium for gaseous [M(PH2)2]2, whereas for barium the structure of the type M(μ-PH 2)4Ba has to be considered.
TL;DR: The development of enantioselective Lewis acid catalyzed carbonyl addition reactions of π-nucleophiles such as enolsilanes and allylstannanes is a topic of current interest.
Abstract: The development of enantioselective Lewis acid catalyzed carbonyl addition reactions of π-nucleophiles such as enolsilanes and allylstannanes is a topic of current interest.1 The extension of this general process to include simple olefinic nucleophiles via the carbonyl-ene reaction2 has important practical implications. In this context, Mikami and Nakai have reported a catalytic enantioselective ene reaction with glyoxylate esters;3 however, due to the limiting reactivity of the catalyst-glyoxylate complex,4 only nucleophilic 1,1-disubstituted olefins may be employed. We have recently reported that bidentate bis(oxazolinyl) (box) Cu(II) complexes 1-3 are effective enantioselective catalysts in Diels-Alder5 and aldol reactions6 with substrates that can participate in catalyst chelation. In this study, we demonstrate that
TL;DR: In this article, the reactivity of base-free titanium imido complex Cp*2TiNPh (1) (Cp* = pentamethylcyclopentadienyl) toward alkenes and alkynes was investigated.
Abstract: This paper describes the reactivity of the base-free titanium imido complex Cp*2TiNPh (1) (Cp* = pentamethylcyclopentadienyl) toward alkenes and alkynes. Complex 1 reacts with ethylene and acetylene to generate the azametallacycles Cp*2Ti(N(Ph)CH2CH2) (2) and Cp*2Ti(N(Ph)CHCH) (3), respectively. In the case of ethylene, the cycloaddition is readily reversible, and 2 was characterized spectroscopically under an ethylene atmosphere. We have also examined the reactivity of 1 toward phenyl- and trimethylsilylacetylene and have found that 1 activates the alkynyl C−H bond to give anilido−acetylide complexes Cp*2Ti(N(Ph)H)C⋮CR (6, R = Ph; 7, R = SiMe3). This reaction proceeds without observable intermediate metallacyclobutene complexes such as those observed previously for the reaction of the related oxo complex Cp*2Ti(O)pyr (pyr = pyridine) with terminal alkynes. Thermolysis of azametallacyclobutene 3 results in formation of the novel ring-activated complex Cp*(η5,η1-C5Me4CH2CHCH)TiN(Ph)H (5).
TL;DR: A rare, structurally characterized μ1,η1 -hydrazonato complex is a probable intermediate in the Pd-catalyzed N-arylation of hydrazones and proceeds efficiently and under mild conditions with chelating phosphane ligands.
Abstract: A rare, structurally characterized μ1 ,η1 -hydrazonato complex is a probable intermediate in the Pd-catalyzed N-arylation of hydrazones. Starting from aryl halides the reaction proceeds efficiently and under mild conditions with chelating phosphane ligands (L2 ); even Cs2 CO3 can be used as the base [Eq. (a)]. R=alkyl, aryl, MeCO, MeO; X=Br, I.
TL;DR: In this paper, the adsorption and dissociation of H2S and S2 on a series of oxide and metal/oxide surfaces have been studied using synchrotron-based high-resolution photoemission.
Abstract: The adsorption and dissociation of H2S and S2 on a series of oxide (Al2O3, Cr2O3, Cr3O4, Cu2O, ZnO) and metal/oxide (Cu/Al2O3, Cu/ZnO) surfaces have been studied using synchrotron-based high-resolution photoemission. H2S and S2 mainly interact with the metal centers of the oxides. At 300 K, H2S undergoes complete decomposition. The rate of decomposition on Al2O3 is much lower than those found on Cr3O4, Cr2O3, ZnO, and Cu2O. For these systems, the smaller the band gap in the oxide, the bigger its reactivity toward S-containing molecules. The results of ab initio SCF calculations for the adsorption of H2S, HS, and S on clusters that resemble the (0001) face of α-Al2O3, α-Cr2O3, and ZnO show that the S-containing species interact stronger with Cr or Zn than with Al centers. These theoretical results and the trends seen in the experimental data indicate that the reactivity of an oxide mainly depends on how well its bands mix with the orbitals of H2S or HS. The electrostatic interactions between the dipole of ...
TL;DR: In this article, the equilibrium constant for reaction (1) has been determined in neutral solution to be (1.4±0.1)×105 d mol-1, with forward and reverse rate constants, k1 and k-1.
Abstract: The equilibrium constant for reaction (1) has been determined in neutral solution to be (1.4±0.1)×105 d mol-1, with forward and reverse rate constants, k1 and k-1, of (8.5±0.7)×109 d mol-1 s-1 and (6.0±0.5)×104 s-1, respectively. At 25°C the rate constants for the reactions of ClNsbd and Cl2- were measured to be (2.5±0.3)×105 s-1 and (1.3±0.1)×103 s-1 with water and (6.5±0.6)×108 d mol-1 s-1 and ca. 0, within experimental error, with 2-methyl-propan-2-ol. Deviation from the equilibrium values of [ClNsbd] and [Cl2-] in the early stages of the reactions was investigated and shown to account for the discrepancy between the value of K1 determined here and a previous estimate from our laboratory.
TL;DR: In this article, the Ni-based catalysts for catalytic partial oxidation (CPO) of methane were investigated with four Ni based catalysts prepared through reduction of hydrotalcite-type precursors, in order to understand if selectivities and conversions could be kinetically controlled.
TL;DR: In this article, the size-dependent chemical reactivity of nickel clusters, selected and deposited with low energy (0.2 eV/atom) on thin MgO(100) films was investigated.
Abstract: Particles of nanometer size (nanoparticles) supported on well-characterized oxide surfaces are of particular interest to model the high complexity of real catalysts to answer questions such as the role of intrinsic size effects and the influence of the support.1,2 Model systems so far consisted of size-distributed nanoparticles deposited on oxide substrates,3-5 which do not allow an unambiguous determination of the cluster's chemical nature. Here, we report on the size-dependent chemical reactivity of nickel clusters, size selected and deposited with low energy (0.2 eV/atom) on thin MgO(100) films. Monodispersed Ni30 clusters show a higher reactivity for CO dissociation than Ni11 and Ni20. In particular, Ni30 clusters are extremely reactive and dissociate up to 10 CO molecules at temperatures below 280 K. Our results demonstrate that such small, supported clusters are unique for catalytic reactions not only due to their high surface-to-volume ratio but essentially because of the distinctive properties of ...
TL;DR: The photochemistry of a series of bis(acyl)phosphine oxides and the rate constants of the reactions of their phosphorus radicals with n-butylacrylate, thiophenol, bromotrichloromethane, oxygen, and methyl viologen have been investigated by laser flash photolysis.
Abstract: The photochemistry of a series of bis(acyl)phosphine oxides and the rate constants of the reactions of their phosphorus radicals with n-butylacrylate, thiophenol, bromotrichloromethane, oxygen, and methyl viologen have been investigated by laser flash photolysis. The results were compared to a mono(acyl)phosphine oxide ((2,4,6-trimethylbenzoyl)diphenylphosphine oxide). The variation in reactivity of the different phosphorus radicals was correlated with the degree of radical localization and s-character on the phosphorus atom, as reflected by the 31P hyperfine coupling constant, which was measured by time-resolved ESR. High 31P hyperfine splitting is observed for high degree of spin localization in a σ-orbital. It was shown that typical radical reactions (addition to monomers and oxygen, and atom abstractions) correlate well with the 31P hyperfine coupling. An inverse correlation was observed for electron-transfer reactions (reduction of methyl viologen), were a higher p-character of a localized orbital en...
TL;DR: In this paper, a review article profiles the advances in ruthenium chemistry for the synthesis of a new class of organometallic complexes containing conjugated carbon-rich chains.