Showing papers on "Reactivity (chemistry) published in 2001"
TL;DR: Mayr et al. as discussed by the authors used correlation analysis to determine the electrophilicity parameters E and N and s as defined by the equation log k(20 °C) = s(N + E).
Abstract: Twenty-three diarylcarbenium ions and 38 π-systems (arenes, alkenes, allyl silanes and stannanes, silyl enol ethers, silyl ketene acetals, and enamines) have been defined as basis sets for establishing general reactivity scales for electrophiles and nucleophiles. The rate constants of 209 combinations of these benzhydrylium ions and π-nucleophiles, 85 of which are first presented in this article, have been subjected to a correlation analysis to determine the electrophilicity parameters E and the nucleophilicity parameters N and s as defined by the equation log k(20 °C) = s(N + E) (Mayr, H.; Patz, M. Angew. Chem., Int. Ed. Engl. 1994, 33, 938−957). Though the reactivity scales thus obtained cover more than 16 orders of magnitude, the individual rate constants are reproduced with a standard deviation of a factor of 1.19 (Table 1). It is shown that the reactivity parameters thus derived from the reactions of diarylcarbenium ions with π-nucleophiles (Figure 3) are also suitable for characterizing the nucleoph...
565 citations
TL;DR: A homologous range of diphosphines based on rigid heterocyclic aromatic backbones of the xanthene-type with natural bite angles of approximately 100-134 degrees have been developed.
Abstract: The reactivity of organotransition metal complexes is dependent on the ligand environment of the metal. This Account describes the development and application of new diphosphine ligands, designed to induce large P−M−P angles in transition metal complexes. Aided by computational chemistry, a homologous range of diphosphines based on rigid heterocyclic aromatic backbones of the xanthene-type with natural bite angles of ∼100−134° have been developed. The special structure of the ligands has an enormous impact on stability and reactivity of various transition metal complexes. Highly active and selective catalysts have been obtained by influencing this reactivity.
405 citations
TL;DR: In this paper, a series of polyoxometalate/H2O2 systems were evaluated for dibenzothiophene oxidation using toluene solutions of the model compounds.
Abstract: Dibenzothiophene, 4-methyldibenzothiophene, and 4,6-dimethyldibenzothiophene are typical thiophenic sulfur compounds that exist in diesel fuels Using toluene solutions of the model compounds, experiments were carried out to compare the reactivity of the different dibenzothiophenes in oxidation reactions, a key step for oxidative desulfurizations A series of polyoxometalate/H2O2 systems were evaluated for dibenzothiophene oxidation The H2O2 solutions of phosphotungstic acid and its salt were very active catalyst systems for the model compound oxidation, while their molybdenum counterpart systems were much less active The H2O2 solutions of silicotungstic and silicomolybdic compounds were the least active catalyst systems for the reaction Oxidation reactivities decreased in the order of dibenzothiophene>4-methyldibenzothiophene>4,6-dimethyldibenzothiophene, the same reactivity trend that exists in HDS However, the oxidation of the dibenzothiophenes was achieved under mild reaction conditions and it was easy to increase reaction temperature or reaction time to achieve high oxidation conversions, even for the least reactive 4,6-dimethyldibenzothiophene Apparent activation energies of dibenzothiophene, 4-methyldibenzothiophene, and 4,6-dimethyldibenzothiophene oxidation were 538, 560, and 587 kJ/mol, respectively These activation energies indicated a decrease in reactivity of dibenzothiophenes as methyl substitutes increased at the 4 and 6 positions on dibenzothiophene rings Interestingly, in a formic acid/H2O2 system, the oxidation reactivity of the dibenzothiophenes showed the reverse trend, suggesting that steric hindrance might play a role when bulky polyoxoperoxo species, which likely form in a hydrogen peroxide solution, act as catalysts
390 citations
TL;DR: The results provide a basis for the rational design of peptides, where the kinetics and thus selectivity of protein/peptide conjugation with polymeric structures via Michael-type addition reactions can be controlled.
373 citations
TL;DR: In this article, the authors used transition-metal oxides to enhance surface electrochemical reactivity of Li-ion batteries and achieved a capacity of 700 mA h g(-1) with 100% capacity retention for up to 100 cycles and high recharging rates.
Abstract: Rechargeable solid-state batteries have long been considered an attractive power source for a wide variety of applications, and in particular, lithium-ion batteries are emerging as the technology of choice for portable electronics. One of the main challenges in the design of these batteries is to ensure that the electrodes maintain their integrity over many discharge-recharge cycles. Although promising electrode systems have recently been proposed, their lifespans are limited by Li-alloying agglomeration or the growth of passivation layers, which prevent the fully reversible insertion of Li ions into the negative electrodes. Here we report that electrodes made of nanoparticles of transition-metal oxides (MO, where M is Co, Ni, Cu or Fe) demonstrate electrochemical capacities of 700 mA h g(-1), with 100% capacity retention for up to 100 cycles and high recharging rates. The mechanism of Li reactivity differs from the classical Li insertion/deinsertion or Li-alloying processes, and involves the formation and decomposition of Li2O, accompanying the reduction and oxidation of metal nanoparticles (in the range 1-5 nanometres) respectively. We expect that the use of transition-metal nanoparticles to enhance surface electrochemical reactivity will lead to further improvements in the performance of lithium-ion batteries.
328 citations
323 citations
TL;DR: New, easily synthesized oligomeric analogues of 1 are reported that exhibit not only remarkably enhanced reactivity, but also significantly higher enantioselectivity relative to 1.
Abstract: Cooperative bimetallic catalysis has been documented in several recently reported asymmetric epoxide opening reactions that display second-order kinetic dependence on catalyst, specific requirements for multiple metal ions, and/or pronounced catalyst nonlinear effects.1 In such systems, one metal is proposed to serve as Lewis acid for epoxide activation and another as counterion for the nucleophile. In this mechanistic context, complexes that contain multiple metal centers in appropriate relative proximity and orientation can provide improved reactivity relative to monometallic catalysts. For example, chiral metal salen complexes such as 1 are effective catalysts for asymmetric epoxide ringopening reactions, and operate by a second-order mechanism.2 Linking these catalysts as dimers3 or to polymeric4 or dendrimeric5 frameworks leads to catalytic systems with similar enantioselectivity and substantially enhanced reactivity relative to 1. The synthetic utility of multimeric catalysts prepared in this manner is limited, however, by the inefficiency of their syntheses.6 We report here new, easily synthesized oligomeric analogues of 1 that exhibit not only remarkably enhanced reactivity, but also significantly higher enantioselectivity relative to 1.
308 citations
TL;DR: In this article, a blend of blended cements with 30% and 50% slag and water:solids ratios (w/s) of 0.50 and 0.35, respectively, were used for up to 6 months.
282 citations
TL;DR: Nucleus-independent chemical shifts (NICS) indicate that the more reactive inner rings actually are more reactive than the less reactive outer rings and even more aromatic than benzene itself.
276 citations
TL;DR: In this paper, a family of chelating amine bis(phenolate) zirconium dibenzyl complexes were synthesized in quantitative yields from a versatile family of amine−bis((2-hydroxyaryl)methyl) precursors, their X-ray structures solved, and their reactivity in the polymerization of 1hexene in the presence of B(C6F5)3 studied.
256 citations
TL;DR: Solid-state reactions that occur in drug substances and formulations include solid-state phase transformations, dehydration/desolvation, and chemical reactions, which are of particular interest in cases where the drug-substance may be unstable or react with excipients in the formulation.
TL;DR: The observed tolerance to poisons and controllable electrochemical reactivity present an alternative approach to the separation of olefins from complex streams.
Abstract: The complex Ni[S2C2(CF3)2]2reacts with light olefins, including ethylene and propylene, selectively and reversibly. The reaction is not poisoned by hydrogen gas, carbon monoxide, acetylene, or hydrogen sulfide, which are commonly present in olefin streams, presumably because olefin binding occurs through the sulfur ligand rather than the metal center. The reversible reaction of olefins with Ni[S2C2(CN)2]2 n ( n = 0, −1, −2) can be controlled electrochemically, where the oxidation state–dependent binding and release of olefins are fast on the electrochemical time scale. The observed tolerance to poisons and controllable electrochemical reactivity present an alternative approach to the separation of olefins from complex streams.
TL;DR: Competition experiments were used to determine that the 4-OH of a 2-deoxy-2-azidoglucose derivative is more reactive than that of the corresponding N-phthalimido glucose derivative which, in turn, is more easily glycosylated than the N-acetyl derivative.
Abstract: Competition experiments were used to determine that the 4-OH of a 2-deoxy-2-azidoglucose derivative is more reactive than that of the corresponding N-phthalimido glucose derivative which, in turn, is more easily glycosylated than the N-acetyl derivative. Glycosylation of the 4-OH groups of the N,N-diacetyl and N-acetyl-N-benzyl glucosamine was also found to be superior to that of the simple N-acetyl substance. The 3-O-picolinyl ether of a 4,6-O-benzylidene-protected N-acetylglucosamine was shown to have a strong intramolecular hydrogen bond to the adjacent acetamide group. This interaction does not persist in the 3-O-picolinyl-6-O-benzyl N-acetylglucosamine derivative, owing to a probable competing hydrogen bond between the 4-OH and the picolinyl ether. However, in the 3-O-picolinyl-4-O-benzyl N-acetylglucosamine regioisomer the picolinyl-acetamide hydrogen bond persists and leads to an enhancement of reactivity of the 6-OH, over and above that in the corresponding 3-O-benzyl ether, due to disruption of the typical intermolecular amide hydrogen bonding scheme. It is demonstrated that the picolinyl ether is readily removed by hydrogenolysis at atmospheric pressure and room temperature.
TL;DR: In this article, the substituent effect on the reactivity and selectivity of the ring-opening direction in the reaction of five-membered cyclic carbonates with n-hexylamine was investigated.
Abstract: This article focuses on the substituent effect on the reactivity and selectivity of the ring-opening direction in the reaction of five-membered cyclic carbonates with n-hexylamine. The reactivity of the cyclic carbonate and the formation selectivity of the adduct with a secondary hydroxyl group increased as a stronger electron-withdrawing group was introduced at the α-methylene of the cyclic carbonate. These results are discussed on the basis of the stability of intermediates, primary and secondary alcoholate anions, Mulliken charges on carbonyl carbon, and the bond lengths and orders of the OCO single bond. © 2001 John Wiley & Sons, Inc. J Polym Sci Part A: Polym Chem 39: 3678–3685, 2001
TL;DR: In this paper, the Ni−Au nanoparticle catalyst system is designed based on the detailed experimental and theoretical understanding of the alloying and the chemical reaction processes on single-crystal surfaces.
Abstract: We discuss the design of a Ni−Au nanoparticle catalyst system, which is based on the detailed experimental and theoretical understanding of the alloying and the chemical reaction processes on single-crystal surfaces. The alloy formation and structure of Ni−Au catalysts supported on SiO2 and on MgAl2O4 are simulated by Monte Carlo schemes as well as experimentally studied by a combination of in situ X-ray absorption fine structure, transmission electron microscopy, and in situ X-ray powder diffraction. On-line mass spectrometry is used to follow the reactivity of the catalyst and thermogravimetric analysis provided information on the deposition rate of carbon during steam reforming of n-butane. The simulations and the experiments give evidence for the formation of a Ni−Au surface alloy on the Ni particles for both supports. The Ni−Au catalysts exhibiting the surface alloy are active for steam reforming and are more resistant toward carbon formation than the pure Ni catalyst. Blocking of highly reactive Ni ...
TL;DR: According to the direct electron-transfer property and enhanced peroxidase activity of Hb in the membrane, a Hb/SP Sephadex membrane-based H2O2 biosensor is prepared, with a linear range approximately 5.0 x 10(-6) to 1.6 x 10 (-4) mol/L.
Abstract: Hemoglobin can exhibit a direct electron-transfer reaction after being entrapped in a SP Sephadex membrane A pair of stable and well-defined redox waves are obtained at a hemoglobin−SP sephadex mo
TL;DR: Montmorillonite/polyimide (3,3,4,4‘-benzophenone tetracarboxylic dianhydride-4, 4‘)-oxydianiline, BTDA-ODA) nanocomposites displaying different morphological, thermal, and mechanical characteristics as mentioned in this paper.
Abstract: Montmorillonite/polyimide (3,3‘,4,4‘-benzophenone tetracarboxylic dianhydride-4,4‘-oxydianiline, BTDA-ODA) nanocomposites displaying different morphological, thermal, and mechanical characteristics...
TL;DR: The control of reactivity to achieve specific syntheses is one of the overarching goals of organic chemistry as discussed by the authors, and the control of reactive reactivity is a major challenge in organic chemistry.
Abstract: The control of reactivity to achieve specific syntheses is one of the overarching goals of organic chemistry.[...]
TL;DR: Correlation between EXAFS and (13)C NMR data suggests, in short, two main surface structures having different environments for the methyl group: [Al](3)-OZrCp(CH(3), 1, and Cp(2)Zr(3))(2, 2), 2, with partially dehydroxylated silica, silica-alumina, and alumina surfaces to probe the relationship between catalyst structure and polymerization activity.
Abstract: The reactions of Cp*Zr(CH3)3, 1, and Cp2Zr(CH3)2, 2, with partially dehydroxylated silica, silica−alumina, and alumina surfaces have been carried out with careful identification of the resulting su...
TL;DR: These are the first direct reaction rate measurements of nucleophilic addition to the parent o-quinone methide, and a thermal and photochemical reversibility of the alkylation process opens a new perspective for the use and application of such adducts as o-QM molecular carriers.
Abstract: o-Quinone methide (1) has been produced in water both thermally and photochemically from (2-hydroxybenzyl)trimethylammonium iodide (2). Michael addition reactions of 1 to various amines, and sulfides, including amino acids and glutathione have been carried out, obtaining alkylated adducts (3-16) in fairly good to quantitative yields. The reaction rate and selectivity of 1 toward nitrogen and sulfur nucleophiles, in competition with the hydration reaction, have been investigated at different pH by laser flash photolysis technique. The observed reactivity spans 7 orders of magnitude on passing from water (kNu = 5.8 M-1 s-1) to the most reactive nucleophile (2.8 x 10(8) M-1 s-1, 2-mercaptoethanol under alkaline conditions). These are the first direct reaction rate measurements of nucleophilic addition to the parent o-quinone methide (1). Competition experiments provided strong kinetic support to the involvement of free 1 as an intermediate in both thermal and photochemical reactions. Furthermore, several alkylation adducts regenerate 1 either by heating (9, 10, 13, and 14) or by irradiation (9, 11-13, 16). Such a thermal and photochemical reversibility of the alkylation process opens a new perspective for the use and application of such adducts as o-QM molecular carriers.
TL;DR: The gas phase and aqueous thermochemistry and reactivity of nitroxyl (nitrosyl hydride, HNO) were elucidated with multiconfigurational self-consistent field and hybrid density functional theory calculations and continuum solvation methods.
Abstract: The gas phase and aqueous thermochemistry and reactivity of nitroxyl (nitrosyl hydride, HNO) were elucidated with multiconfigurational self-consistent field and hybrid density functional theory calculations and continuum solvation methods. The pKa of HNO is predicted to be 7.2 ± 1.0, considerably different from the value of 4.7 reported from pulse radiolysis experiments. The ground-state triplet nature of NO− affects the rates of acid-base chemistry of the HNO/NO− couple. HNO is highly reactive toward dimerization and addition of soft nucleophiles but is predicted to undergo negligible hydration (Keq = 6.9 × 10−5). HNO is predicted to exist as a discrete species in solution and is a viable participant in the chemical biology of nitric oxide and derivatives.
TL;DR: Four new indices are defined: a potential energy surface that results from the second-order truncation of the Taylor series in the external potential about some reference, Upsilon(R(1),R(2),.
Abstract: In a recent paper [J. Am. Chem. Soc. 2000, 122, 2010], the authors explored variational principles that help one understand chemical reactivity on the basis of the changes in electron density associated with a chemical reaction. Here, similar methods are used to explore the effect changing the external potential has on chemical reactivity. Four new indices are defined: (1) a potential energy surface that results from the second-order truncation of the Taylor series in the external potential about some reference, Upsilon(R(1),R(2),.,R(M)()); (2) the stabilization energy for the equilibrium nuclear geometry (relative to some reference), Xi; (3) the flexibility, or "lability", of the molecule at equilibrium, Lambda; and (4) the proton hardness, Pi, which performs a role in the theory of Bronsted-Lowry acids and bases that is similar to the role of the chemical hardness in the theory of Lewis acids and bases. Applications considered include the orientation of a molecule in an external electric field, molecular association reactions, and reactions between Bronsted-Lowry acids and bases.
TL;DR: In this paper, the selectivity of the amorphous carbon burning reaction was studied as a function of the reaction temperature by transmission electron microscopy, and the advantages of different methods are as follows: although oxidation by KMnO4 in an acidic suspension provides nanotubes free of amorphou carbon, and can easily be controlled, it must be followed by filtration and cc HCl treatment in order to dissolve the MnO2 formed during the reaction.
Book•
01 Jan 2001
TL;DR: In this paper, the authors describe the discovery and discovery of Dihydrogen Coordination, and the synthesis and activation of D2/H2 complexes on metal centers.
Abstract: Preface. 1. Introduction. 2. Background and Discovery of Dihydrogen Coordination. 3. Synthesis and General Properties of Dihydrogen Complexes. 4. Bonding and Activation of Dihydrogen on Metal Centers: Theory Versus Experiment. 5. Structural and NMR Studies of Dihydrogen Complexes. 6. Intramolecular Dynamics of H2/Hydride Ligand Systems. Hydrogen Rotation, Exchange, and Quantummechanical Effects. 8. Vibrational Studies of Coordinated H2. 9. Reactions and Acidity of H2 Complexes. 10. Activation of Hydrogen and Related Small Molecules by Metalloenzymes and Sulfur Ligand Systems. 11. Coordination and Activation of Si-H, Ge-H, and Sn-H Bonds. 12. C-H Bond Coordination and Activation. 13. Coordination and Activation of B-H and Other X-H and X-Y Bonds.
TL;DR: Dimethyl carbonate (DMC) is a valuable methylating reagent which can replace methyl halides and dimethylsulfate in the methylation of a variety of nucleophiles.
Abstract: Dimethylcarbonate (DMC) is a valuable methylating reagent which can replace methyl halides and dimethylsulfate in the methylation of a variety of nucleophiles. It couples tunable reactivity and unprecedented selectivity toward mono-C- and mono-N-methylation in the reactions of acidic CH 2 and primary aromatic amines, respectively. In addition, it is a pro- totype example of a green reagent, since it is nontoxic, made by a clean process, and biodegradable, and it reacts in the presence of a catalytic amount of base thereby avoiding the formation of undesirable inorganic salts as by-products. Other remarkable reactions are those where DMC behaves as an oxidant: cyclic ketones are transformed into α,ω-dimethyl esters with a reaction of atom efficiency of 1.0.
TL;DR: Preliminary investigations on Friedel-Crafts acylation further substantiate the argument that the interaction of the Lewis acidic species [Al(2)Cl(7)](-) of the ionic liquid with the formed HCl during the sulfonylation reaction, which is evidenced by the control experiment.
Abstract: 1-Butyl-3-methylimidazolium chloroaluminate ionic liquids have been employed as an unconventional reaction media and as Lewis acid catalyst for Friedel−Crafts sulfonylation reaction of benzene and substituted benzenes with 4-methyl benzenesulfonyl chloride. The substrates exhibited enhanced reactivity, furnishing almost quantitative yields of diaryl sulfones, under ambient conditions. Studies concerning the effect of Lewis acidity of the ionic liquid on the initial extent of conversion of this reaction has been carried out. 27Al NMR spectroscopy has been exploited as a tool to investigate the mechanistic details of the reaction. 27Al NMR spectral studies show the predominance of [Al2Cl7]- species in [bmim]Cl-AlCl3, N = 0.67, acidic ionic liquid in the presence of 4-methyl benzenesulfonyl chloride, and after the reaction with the aromatic hydrocarbon, [AlCl4]- species predominates. This change in speciation of aluminum can be attributed to the interaction of the Lewis acidic species [Al2Cl7]- of the ionic ...
TL;DR: In this article, non-purely thermal specific effects are evidenced and discussed in terms of reac- tion medium and mechanisms, taking into account the variations in polarity of the systems.
Abstract: Typical applications of solvent-free conditions and microwave activation are described. Non-purely thermal specific effects are evidenced and discussed in terms of reac- tion medium and mechanisms, taking into account the variations in polarity of the systems.
TL;DR: In this article, 1,2-bis[3-(1,3-dioxan-2-one-5-yl)-propylthio]ethane (B6CC) and B5CC with 4,9dioxadioxadodecane-1,12-diamine were carried out.
Abstract: Polyadditions of 1,2-bis[3-(1,3-dioxan-2-one-5-yl)-propylthio]ethane (B6CC) and 1,2-bis[4-(1,3-dioxolan-2-one-4-yl)-butylthio]ethane (B5CC) with 4,9-dioxadodecane-1,12-diamine were carried out. The reactivity of B6CC with the diamine was higher than that of B5CC. Additionally, B6CC gave a higher molecular weight polymer than B5CC. The reaction-rate constants of B6CC and B5CC at 30, 50, and 70 °C were evaluated as 0.70, 0.89, and 1.07 L/mol · h; and 0.03, 0.06, and 0.10 L/mol · h in N,N-dimethylacetamide (initial reagent concentration 0.5 M), respectively. The activation energies were estimated to be 9.2 and 24.9 kJ/mol in the reactions of B6CC and B5CC with the diamine, respectively. © 2001 John Wiley & Sons, Inc. J Polym Sci A: Polym Chem 39: 860–867, 2001