bluuebunny/crossref_metadata_2025 huggingface dataset:
Novel experimental findings in the B–Z system employing mixed substrates
Authors: Jayalakshmi V. , Ramaswamy R.
Published: May 2006

Certain novel features of the Belousov–Zhabotinsky (B–Z) system employing different mixed substrates and Mn(II) as the catalyst are presented. Malic acid is the common substrate together with one of malonic, citric, cyanoacetic, maleic, acetic, oxalic, or tartaric acid as the second substrate. The correlation of the oscillatory behaviour with values of exchange current density establishes the oscillatory control by the redox couple, namely, Mn(III)/Mn(II) and (or) Br2/Br−. Each substrate when used alone gives rise to an oscillatory behaviour with characteristic features that can be compared with the system containing mixed substrate. The combination of substrates provides different modes of oscillatory behaviour such as entrainment, independent, partial inhibition, or complete inhibition. These observations are rationalized in terms of relevant steps involved in the mechanism of the reaction. Key words: mixed substrate, exchange current density, entrainment, independent, inhibition.
Calculation of phase diagrams and thermodynamic properties of 18 binary common-ion systems of Na,K,Ba//F,MoO4,WO4
Authors: Sangster James Malcolm
Published: May 2006

Phase diagram and thermodynamic data of 18 binary common-ion molten salt systems in Na,K,Ba//F,MoO4,WO4 were optimized by computer algorithm. The phase diagram data as well as single-salt data were retrieved from an extensive critical literature search. Expressions for the excess properties of solution phases and thermodynamic properties of intermediate compounds were thereby obtained. These data were used to generate a "best" phase diagram for each binary system considered. Key words: molten salts, phase diagrams, thermodynamic properties.
Structural studies of organoboron compounds LXIV. Boron chelate formation with glycolohydroxamic acids, and the crystal and molecular structure of 5-(1-hydroxycyclohexyl)-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-4-cyclopentane
Authors: Kliegel Wolfgang , Schumacher Ute , Rettig Steven J. , Trotter James
Published: May 2006

The syntheses of representative diphenyl- and difluoroboron chelates of 2-hydroxycarbohydroxamic acids (glycolohydroxamic acids) are reported. Crystals of 5-(1-hydroxycyclohexyl)-2,2-diphenyl-1,3-dioxa-4-azonia-2-borata-4-cyclopentene, 2a, are triclinic, a = 11.5717(5), b = 11.7262(4), c = 14.3529(5) Å, α = 90.277(6)°, β = 110.497(5)°, γ = 96.814(5)°, Z = 4, space group [Formula: see text] The structure was solved by direct methods and refined by full-matrix least-squares procedures to R = 0.042 and Rw = 0.056 for 5318 reflections with I ≥ 3σ(F2). Compound 2a is the first N-unsubstituted hydroxamatoborate derivative to be structurally characterized by an X-ray crystallographic analysis. The solid state structure of 2a consists of centrosymmetric, hydrogen-bonded tetrameric units that also feature intermolecular [Formula: see text](aromatic) interactions. Bond lengths include: B—O(N) = 1.557(2) and 1.553(2) Å, B—O(N) = 1.558(2) and 1.566(2),Å B—C(phenyl) = 1.592(3)–1.601(3) Å. Key words: hydroxamatoborate, organoboron compound, crystal structure.
Energétique des liaisons inter et intramoléculaires dans les trois isomères de l'aminophénol
Authors: Sabbah Raphaël , Gouali Meriem
Published: May 2006

The prsesent work is concerned with a thermodynamic study of the three aminophenol isomers (general formula: C6H7NO). It was achieved using four techniques: combustion calorimetry of small amounts of substance (a few milligrams), sublimation calorimetry, differential thermal analysis, and heat capacity measurements. From this study, it was possible: to determine the enthalpies of combustion, sublimation, and fusion of these compounds; to discuss the relative stability of the three molecules; to determine the intermolecular enthalpy bonds; to determine the experimental resonance energies and to compare them with the theoretical values; to determine the atomization enthalpies and to compare them with the values calculated from the energetical contributions previously determined in our laboratory; to consider the existence of an intramolecular hydrogen bond in the ortho isomer. Key words: thermodynamics; thermochemistry; calorimetry; differential thermal analysis; 2-aminophenol or ortho-aminophenol; 3-aminophenol or meta-aminophenol; 4-aminophenol or para-aminophenol; enthalpy of combustion, of sublimation, of fusion, of atomization, enthalpy of inter and intramolecular bonds; hydrogen bond; resonance energy; triple point temperature.
The reactions of O(3P) with some carboxylic acids and esters
Authors: Caravan P. , Budge Suzanne M. , Roscoe John M.
Published: May 2006

The reactions of O(3P) with formic acid, acetic acid, methyl acetate, ethyl acetate, n-propyl acetate, and isopropyl acetate have been studied kinetically as a function of temperature. The rate constants were measured in a discharge-flow system under pseudo-first-order conditions with a large excess of the organic reagent. No reaction could be found between O(3P) and either formic acid or acetic acid in the temperature range 300–420 K. The acetate esters, on the other hand, reacted with O(3P) at readily measurable rates over the same temperature range. The temperature dependence of the rate constants for these reactions in the temperature range 300–580 K is given, in the units L mol−1s−1, by:[Formula: see text]The lack of measurable reaction with the carboxylic acids, the systematic increase in the preexponential factor with increasing size of the ester group, and the observation of a significant yield of acetic acid among the products of the reactions with the esters indicate that attack by O(3P) is predominantly at the ester function. Linear free energy correlations are used to compare the results for the esters with those for reactions of O(3P) with other organic compounds containing oxygen. Key words: kinetics, mechanism, reactions of O(3P) with esters.
Solvent effect on preferred protonation sites in nicotinate and isonicotinate anions
Authors: Hallé Jean-Claude , Lelievre Jacques , Terrier François
Published: May 2006

Potentiometric determinations of the two successive acidities of nicotinic and isonicotinic acids (AH2+) have been carried out in aqueous dimethyl sulfoxide mixtures containing up to 95% Me2SO by volume. In both systems, the results reveal that the addition of Me2SO induces a proton transfer from the pyridinium ring to the carboxylate group, the tautomeric equilibrium between the neutral forms of the two acids being displaced toward the zwitterionic form (AH±) in aqueous solution, but toward the molecular form (AH°) in Me2SO. An analysis of the data by means of Hammett relationships previously established for benzoic acids over the whole range of H2O/Me2SO mixtures allowed the four microscopic acidity constants as well as the tautomeric equilibrium constant KTpertaining to the complete ionization scheme of the two acids to be determined. At 20 °C, there are equal populations of the tautomeric AH° and AH±species in the mixtures containing 38 and 47% Me2SO for the nicotinic and isonicotinic systems, respectively. Hammett relationships describing the ionization behaviour of a number of substituted pyridinium cations in H2O/Me2SO mixtures are also discussed. Possible reasons accounting for the peculiar effects exerted by the NH2, CONH2, and COOH substituents on the process are suggested. Key words: nicotinic and isonicotinic acids, substituted pyridines, acidities, tautomeric equilibrium, protonation sites, water – dimethyl sulfoxide mixtures.
Correlation analysis of reactivity in the addition of substituted benzylamines to β-nitrostyrene
Authors: Jalani Neeta , Kothari Seema , Banerji Kalyan K.
Published: May 2006

The kinetics of addition of a number of ortho-, meta-, and para-substituted benzylamines to β-nitrostyrene (NS) in acetonitrile have been studied. The reaction is first order with respect to NS. The order with respect to the amine is higher than one. It has been shown that the reaction follows two mechanistic pathways, uncatalyzed and catalyzed by the amine. The Arrhenius activation energy for the catalyzed path is negative, indicating the presence of a pre-equilibrium (k1, k−1) leading to the formation of a zwitterion. The values of the rate constant, k1, for the nucleophilic attack have been determined for 28 benzylamines. The rate constant k1was subjected to correlation analysis using Charton's LDR and LDRS equations. The polar regression coefficients are negative, indicating the formation of a cationic species in the transition state. The reaction is subject to steric hindrance by ortho substituents. Key words: nucleophilic addition, benzylamines, correlation analysis, kinetics, alkene.
Standard state heat capacities of aqueous electrolytes and some related undissociated species
Authors: Hepler Loren G. , Hovey Jamey K.
Published: May 2006

Uses of heat capacities of solutions of electrolytes are reviewed, with a particular emphasis on the standard state partial molar heat capacities and their applications to calculations of the effects of temperature on equilibrium constants, electrode potentials, enthalpies, and entropies. Methods of obtaining these standard partial molar heat capacities are summarized, followed by comparisons of values obtained in different ways. Many of the "best" such heat capacities are collected and then used as the basis for establishing single-ion heat capacities based on the convention that CpO(H+) = 0, followed by illustrations of the convenient use of these quantities. Finally, there is brief discussion of theoretical analysis of these standard partial molar heat capacities in relation to ion–solvent interactions. Key words: heat capacities, electrolytes; aqueous solutions, heat capacities; thermodynamics, electrolytes.
Studies on the interaction of selenite and selenium with sulphur donors. Part 4. Thiosulfate
Authors: Rahim Shermin , Milne John
Published: May 2006

Raman and Se-77 NMR spectroscopy confirm that when selenous acid is reduced by thiosulfate in water selenopentathionate and tetrathionate are formed.[Formula: see text]Depending upon the stoichiometry and pH, two isomers of the selenopentathionate ion, O- and S-bonded, are formed. Insufficiently acid solutions cause decomposition to selenium and tetrathionate ion.[Formula: see text]Fresh solutions prepared from crystalline sodium selenopentathionate and water undergo slow decompositon. NMR and Raman spectra show the presence of both the O-bonded and S-bonded linkage isomers. The O-bonded isomer facilitates the formation of tetrathionate. Addition of thiosulfate to selenotrithionate solution or sulfite to selenopentathionate solution yields trithionate with no indication of dithionate or tetrathionate formation. This suggests that simple S—S bond formation at selenium does not occur but that there may be direct attack of the incoming ligand on the attached ligand. Key words: selenite, thiosulfate, selenopentathionate, Se-77 NMR, Raman spectroscopy, linkage isomerism.
Theoretical exammation of the Diels–Alder reaction of 1,3-butadiene with cyclopentadiene and 2H-phosphole
Authors: Bachrach Steven M. , Perriott Laureta M.
Published: May 2006

All Diels–Alder reactions between 1,3-butadiene and cyclopentadiene or 2H-phosphole have been examined at the MP4SDQ/6-31G*//HF/6-31G* level. There is remarkable similarity between the two systems. The thermodynamic product is the bicyclo[4.2.0]nonadiene while the kinetic product is the norbornene product. There is a slight kinetic preference for the endo addition and for the butadiene to be in the s-trans conformation. Except for the case where butadiene is the diene component and addition is endo, the reactions are concerted and synchronous. In these other two cases, the reaction is stepwise with a diradical intermediate.Key words: phosphole, Diels–Alder reaction, topological electron density analysis.
Electrostatic properties of ammonium fluoride and deuterated ice-Ih
Authors: Beek C.G. Van , Overeem J. , Ruble J.R. , Craven B.M.
Published: May 2006

New single crystal X-ray diffraction data have been collected at reduced temperature (120 K) and high resolution (sinθ/λ ≤ 1.34 Å−1) for ammonium fluoride and for heavy ice-Ih, (D2O) in order to determine the detailed electron density distribution in these structures. The crystals are isostructural. Ammonium fluoride, NH4F, is hexagonal with space group P63mc, a = 4.4365(3), c = 7.1672(5) Å, Z = 2. Ice-Ih, D2O, is hexagonal with space group P63/mmc, a = 4.4950(5), c = 7.3170(10) Å, Z = 4. The X-ray data were collected with Nb-filtered MoKα radiation (λ = 0.7093 Å). Refinements with Stewart's rigid pseudoatom model gave R(F2) = 0.016 for 292 reflections for NH4F and R(F2) = 0.023 for 296 reflections for ice-Ih. Hydrogen nuclear positions and mean-square displacements were taken from previous neutron diffraction studies. The structures each contain two crystallographically distinct H(D) atoms, namely, those with N-H or O-D directed along the c-axis and those that are directed tetrahedrally with respect to c. For NH4F, maps of the total electron density, the total electrostatic potential, and the values of the Laplacian at the bond critical points show that the bonding interactions involving both kinds of H atom are the same within experimental error. Similar results are obtained for ice-Ih although there are complications and limitations owing to the disorder in the structure. Nevertheless, the pseudoatom model enables the electron density distribution to be determined for an ordered water molecule isolated from the crystal structure. The estimated molecular dipole moment for water in ice-Ih, is 2.1 debye. Key words: electron density, bond critical points, hydrogen bonding, water dipole moment.
Study of NO and CO dissociation on the (100) Cu surface using density functional theory and the topological analysis of the electronic density and its Laplacian
Authors: Aray Yosslen , Rodríguez Jesús
Published: May 2006

Molecular orbital ab initio Hartree–Fock, post-Hartree–Fock at the MP2 and QCISD levels, and density functional theory calculations of the dipole moment, the topology of the electronic density, ρ(r), and its Laplacian, [Formula: see text], for CO and NO molecules are reported. The results obtained confirm that density functional methods provide remarkably good electronic properties and a good description of the topology of ρ(r) and [Formula: see text]. The Becke exchange functional with the correlation functional of Lee, Yang, and Parr was used to calculate the electronic density of the (100) Cu surface. Topological analysis of ρ(r) shows that the crystal graph corresponds to square pyramids between the atoms of the top of the surface and the atoms of the second layer The topological analysis of [Formula: see text] shows that the atomic graph of the Cu surface exhibits one (3,−3) local charge concentration surrounded by four (3,+1) local charge depletion points. Additionally, there is a (3,+3) local depletion in the midpoint between each of four contiguous Cu atoms corresponding to the active site for the adsorption of the (3,−3) local charge concentration on the C atom of the CO or the N atom of the NO molecule. The larger value of the [Formula: see text] at the nonbonded charge concentration on the atoms and the geometrical configuration of these critical points favor the interaction of the NO over the CO molecule with the (100) Cu surface. This result is in accord with the known reaction barriers for these molecules. Key words: density functional theory, Laplacian of the electronic density, (100) Cu surface, carbon monoxide, nitrogen monoxide, molecular graph, atomic graph.
Approximate kinetic energy density functionals generated by local-scaling transformations
Authors: Ludeña E.V. , López-Boada R. , Pino R.
Published: May 2006

Different stages in the development of density functional theory are succinctly reviewed for the purpose of tracing the origin of the local-scaling transformation version of density functional theory. Explicit kinetic energy functionals are generated within this theory. These functionals are analyzed in terms of several approximations to the local-scaling function and are applied to a few selected first-row atoms. Key words: density functional theory, kinetic energy density functionals, local-scaling transformations, explicit kinetic energy functionals, kinetic energy of first-row atoms.
Analysis of molecular polarizabilities and polarizability derivatives in H2, N2, F2, CO, and HF, with the theory of atoms in molecules
Authors: Gough Kathleen M. , Yacowar Margaret M. , Cleve Richard H. , Dwyer Jason R.
Published: May 2006

Ab initio molecular orbital calculations have been performed on the title molecules at the SCF-HF and MP2 levels to obtain molecular polarizabilities and the derivatives associated with bond stretch. The wave functions from these calculations have been analyzed with the theory of atoms in molecules (AIM). Both the polarizability and its derivative are successfully reconstructed from AIM terms representing the transfer of charge between atoms (CT = charge transfer) and the rearrangement of charge within an atomic basin (AD = atomic dipole). The results for the diatomics are compared to each other and to the alkanes studied previously. Equilibrium polarizabilities are qualitatively explained with reference to atomic electronegativity and type of bonding. While derivatives of the mean molecular polarizability differ by a factor of two at most, individual contributions vary by an order of magnitude. The derivatives along the bond axis for H2 are ΔCT = 2.06 and ΔAD = −0.62 × 10−30 C m/V, while for N2 they are ΔCT = 13.77 and ΔAD = −10.00 × 10−30 C m/V. The common feature observed is that as the induced dipole due to charge transfer increases, the induced dipole due to changes in the atomic dipole also increases and opposes it. Key words: diatomic molecules, molecular polarizability, molecular polarizability derivative, theory of atoms in molecules, ab initio molecular orbital calculations.
Excited state carbon acid dissociation and competing photorearrangements of 5H-dibenzo[a,c]cycloheptene derivatives
Authors: Budac. David. , Wan Peter
Published: May 2006

The first examples of dissociating excited state carbon acids were reported by our group. A necessary structural feature is the 5H-dibenzocycloheptene ring system where the incipient carbanion is formally an antiaromatic system in S0. In this work, structure–reactivity studies of the excited singlet state carbon acid dissociation and competing formal di-π-methane rearrangement of several 5H-dibenzo[a,c]cycloheptene derivatives have been carried out in order to gain more insights into the photochemistry displayed by these compounds. Photolysis of 7-deuterio-5H-dibenzo[a,c]cycloheptene (9) in aqueous solution showed that the photogenerated carbanion is allylically delocalized. Derivative 7 was found to be less reactive than 3 with respect to carbon acid dissociation whereas 8 was unreactive. Ethanolamine (in CH3CN) was found to be an effective base in catalyzing carbon acid dissociation for 3, 7, and 9, as indicated by higher yields of deuterium incorporation and rates of fluorescence quenching. Binaphthyl derivatives 10 and 11 displayed contrasting photobehaviour. Photolysis of binaphthyl 11 resulted in only efficient (ΦP = 0.47) formal di-π-methane rearrangement under all conditions and no evidence was found for carbon acid dissociation, even in the presence of ethanolamine as base. On the other hand, the formal di-π-methane reaction was very inefficient for binaphthyl 10 (Φp < 0.001). Due to the conformational rigidity inherent in 10, the methylene protons at the 3-position are NMR resolvable as pseudo-axial and pseudo-equatorial protons. Photolysis in the presence of ethanolamine (in D2O–CH3CN) resulted in stereoselective deprotonation of the pseudo-axial proton (Φex ≈ 0.02), as indicated by deuterium exchange studies. The results show that excited state carbon acid dissociation is an observable general reaction of dibenzo and dinaphtho cycloheptenes only if more favourable photochemical pathways are not competing. Key words: excited state carbon acid, di-π-methane rearrangement, carbanion, stereoelectronic effect, fluorescence quenching.
3,4-Disubstituted derivatives of 1,2,5-thiadiazole 1,1-dioxide. Ethanol addition reactions and electroreduction of 3-methyl-4-phenyl and 3,4-dimethyl derivatives in acetonitrile and ethanol solvents
Authors: Caram José Alberto , Mirífico María Virginia , Aimone Silvia Lucía , Vasini Enrique Julio
Published: May 2006

3-Methyl-4-phenyl-1,2,5-thiadiazole 1,1-dioxide (TMP), as well as 3,4-dimethyl-1,2,5-thiadiazole 1,1-dioxide (TMM), react with ethanol (EtOH), which adds to one of their C = N double bonds. The equilibrium constants for the addition reaction are measured in mixed acetonitrile (ACN) – EtOH solvents by means of UV spectroscopy in the case of TMP, and by 13C NMR spectroscopy in the case of TMM, since TMM presents only terminal UV absorption. Both equilibrium constants are also estimated through cyclic voltammetry (CV) experiments. In the case of TMP, the ethanol molecule adds to the C = N bond located on the methyl-substituted side of the substrate, according to 13C NMR spectroscopy and CV results. The electroreduction characteristics of the substrates and their ethanol addition products are studied using CV techniques in ACN, EtOH, and CAN–EtOH solvent mixtures. The radical anion formed by the first electron transfer to TMM is unstable and decomposes rapidly while that corresponding to TMP undergoes a relatively slow homogeneous second-order reaction with the substrate (k = 3 × 102 M−1 s−1). The equilibrium constant for EtOH addition and the voltammetric properties of the substrates are compared with those of the previously studied 3,4-diphenyl derivative (TPP). Key words: electrochemistry, thiadiazoles, structure–reactivity relations, kinetics.
Selection of new Fe(III)/Fe(II) chelating agents as catalysts for the oxidation of hydrogen sulfide to sulfur by air
Authors: Martell Arthur E. , Motekaitis Ramunas J. , Chen Dian , Hancock Robert D. , McManus Derek
Published: May 2006

The removal and oxidation to sulfur of H2S, contained in a broad range of sour gas streams including natural gas, by a Fe(III)/Fe(II) chelate system imposes several requirements on the iron chelate that is employed as the redox catalyst. The solution of the catalyst must be neutral or mildly alkaline to efficiently absorb H2S from the gas passed through the solution. The stability of the iron(III) chelate must be high enough that solid Fe(OH)3 not precipitate at the pH employed. Also the stability of the iron(II) chelate must be high enough to prevent the precipitation of FeS from the reaction mixture. The difference in stabilities of the Fe(III) and Fe(II) chelates must be low enough so that the Fe(III) form can be reduced by HS− and that the Fe(III) chelate can be regenerated by aeration of the solution at alkaline pH. An unexpected result of this study is that chelating agents designed for high stability of iron(III) chelates do not form satisfactory redox catalysts. Ligands that form suitable iron complexes as catalysts have both oxygen and nitrogen donors. All complexes investigated are readily reoxidized from the ferrous to the ferric chelate by air. Key words: iron chelates, stability constants, redox potentials, hydrogen sulfide, redox catalysts.
Direct syntheses of pentakis(trifluoromethyl)cyclopentadienide salts and related systems
Authors: Chambers Richard D. , Roche Alex J. , Vaughan Julian F.S.
Published: May 2006

The well-established synthesis of heptafluorobut-2-ene 2 from hexachlorobutadiene 1 and potassium fluoride has been further investigated, and novel dienes 3 and a triene 4 have been observed. Remarkably, the potassium salt of pentakis(trifluoromethyl)cyclopentadienide 7 has been isolated from this system. Salt 7 has also been obtained directly from 2. Cyclopentadiene 6 has been isolated from salt 7 by distillation from sulphuric acid. Bi-and tri-cyclic analogues of 7 have also been obtained, i.e., 9 and 10, by reaction of 2 with perfluorocyclopentene. Key words: heptafluorobut-2-ene, pentakis(trifluoromethyl)cyclopentadienide, 5H-pentakis(trifluoromethyl)cyclopentadiene.
Organoplatinum(IV) derivatized vinyl monomers and polymers prepared by oxidative addition
Authors: Achar Sudhir , Puddephatt Richard J. , Scott John D.
Published: May 2006

Organoplatinum(IV) complexes of general formula [PtXMe2R(NN)], containing vinyl substituents, have been prepared by oxidative addition of RX, (RX = methyl 2-(bromomethyl)acrylate, 2-(bromomethyl)acrylic acid, 2-bromoethyl methacrylate, acryloyl chloride, and chloromethylstyrene), to [PtMe2(NN)], NN = 2,2′-bipyridine or 4,4′-di-tert-butyl-2,2′-bipyridine. Polymers with organoplatinum(IV) substituents have been prepared either by free radical polymerization of the organoplatinum(IV) derivatized monomers or by free radical polymerization of the organic monomers, followed by the oxidative addition of the C-X substituents of these polymers to [PtMe2(NN)]. In the latter method, it is generally not possible to metallate all the C—X bonds, but a high degree of platinum incorporation can be achieved. Key words: organoplatinum, polymer, oxidative addition, vinyl.
Oxidative addition of a B—B bond by an iridium(I) complex: molecular structure of mer-cis-[Ir(PMe3)3Cl(Bcat)2]
Authors: Dai Chaoyang , Stringer Graham , Marder Todd B. , Baker R. Thomas , Scott Andrew J. , Clegg William , Norman Nicholas C.
Published: May 2006

The B—B bond in B2Cat2 (cat = 1,2-O2C6H4) is readily activated by [Ir(PMe3)3Cl(COE)] (COE = η2-cyclooctene) (1) to yield a novel iridium(III) bis(boryl) complex mer-cis-[Ir(PMe3)3Cl(Bcat)2] (2). This complex is the first structurally characterized iridium bis(boryl) compound, and is the first metal bis(boryl) complex for which two distinct boryl group environments are observed in solution. Colourless crystals of 2 are monoclinic, P21/n, with unit cell dimensions a = 8.9638(7) Å, b = 32.197(2) Å, c = 9.3980(7) Å, and β = 90.385(2)°. Key words: iridium, boron, diboron, boryl, oxidative addition, X-ray structure, trimethylphosphine.
Nucleophilic addition to di- and poly-iron arene complex cations
Authors: Abd-Ei-Aziz Alaa S. , Armstrong Debbie A. , Bernardin Shelly , Hutton Harold M.
Published: May 2006

Hydride and cyanide addition to a series of di- and polycyclopentadienyliron arene complex cations with etheric bridges is described. Reaction of the di-iron complexes with sodium borohydride resulted in the formation of a number of adducts.p-Methyl- and o,o-dimethylphenoxybenzene cyclopentadienyliron complexes were used as models in this study to allow for the characterization of the analagous di-iron complexes. The use of HH COSY and CH COSY NMR techniques enabled us to identify the isomeric nature of these adducts. The hydride addition results indicated that the etheric substituent had the predominant effect over the methyl group, leading to a higher addition ratio to the meta-, followed by the ortho-, then the para-positions. It was also clear that in the di-iron system, the hydride addition to each complexed arene ring took place independently. The addition of the cyanide anion to di- and poly-iron arene systems was more selective than that of the hydride anion. Reaction of sodium cyanide with p-methyl- or o-methyl-substituted arene complexes led to the formation of one adduct, with the cyanide being added to the meta position to the etheric bridges. However, cyanide addition to the di-iron complex, with a methyl substituent attached at the meta position of each complexed arene, led to the formation of a mixture of adducts. Cyanide addition to the poly-iron system with p-substituted arenes proved to be very selective, allowing for the formation of one adduct. Oxidative demetallation with 2,3-dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) produced the uncomplexed polyaromatic ethers with cyano groups in a very good yield. Key words: cyclopentadienyliron, arene, nucleophilic addition, hydride, cyanide.
Structure and bonding of organosilicon compounds containing silicon–silicon and silicon–germanium bonds: an X-ray absorption fine structure study
Authors: Xiong Jian Z. , Jiang Detong , Dixon Craig E. , Baines Kim M. , Sham T.K.
Published: May 2006

Si K-edge X-ray Absorption Fine Structures (XAFS) spectroscopy has been used to study the structure and bonding of a series of highly symmetric organosilicon compounds, Si(CH3)4, Ge[Si(CH3)3]4, Si[Ge(CH3)3]4, and Si[Si(CH3)3]4, in the gas phase. It was found that there is a significant difference in the absorption coefficient in the near-edge region between local Si–Si interactions in these compounds and long-range Si–Si interactions in crystalline solids and that the Si—Si bond length in the molecule Si[Si(CH3)3]4 is 2.364(10) Å and the Si—Ge bond lengths in Si[Ge(CH3)3]4 and Ge[Si(CH3)3]4 are 2.38(3) Å and 2.396(15) Å, respectively. These results and their comparison to Si—Si and Si—Ge interatomic distances in crystalline solids are discussed. Key words: XAFS, EXAFS, organosilicon, Si—Ge bond length.
A revised structure for the piperidine alkaloid andrachamine
Authors: Mill Sibel , Hootelé Claude
Published: May 2006

A new method for the synthesis of trans-2,6-disubstituted piperidine derivatives is described. The transformation of cyclic α-methoxycarbamates 5 and 6 affords trans ketones 17 and 18. The synthesis of diols 1–4 from 17 and 18 has shown that the structure proposed in the literature for the piperidine alkaloid andrachamine is incorrect. A reexamination of the original spectral data of this alkaloid suggested that it is a meso 2,6-disubstituted piperidine derivative. Unambiguous syntheses of 23 and 24 and comparison with a sample of andrachamine have established that this alkaloid possesses structure 23. Key words: N-acyliminium, piperidine, alkaloid, andrachamine.
Acid-catalyzed hydrolysis of 4-diazo-isothiochroman-3-one. Comparison with the acyclic analog and the corresponding oxygen system
Authors: Jefferson E.A. , Kresge A.J. , Paine S.W.
Published: May 2006

The acid-catalyzed hydrolysis of the cyclic diazothiolactone, 4-diazoisochroman-3-one (3) was found to occur with the hydronium-ion isotope effect, [Formula: see text] and to give the ring-contracted product, 1,3-dihydrobenzo[c]thiophene-1-carboxylic acid (4). This shows that protonation of the diazo carbon atom occurs in the rate-determining step and that the reaction also involves migration of the thio group. The hydronium-ion catalytic coefficient for this reaction, [Formula: see text], is 45 times less than that for hydrolysis of its acyclic thio ester analog, S-methyl phenyldiazothioacetate (5). Semiempirical AM1 molecular orbital calculations support the idea that this difference in reactivity is the result of increased delocalization of negative charge into the aromatic ring in the case of the cyclic substrate, which reduces the negative charge on the diazo carbon atom and makes it less susceptible to protonation. Key words: hydrolysis, diazoalkanes, charge delocalization, AM1 calculations, thio group migration.
Reactions of dialkoxycarbenes with tethered triple bonds to form heterocyclic compounds
Authors: Kassam Karim , Warkentin John
Published: May 2006

Dialkoxycarbenes with a tethered triple bond, generated from the corresponding oxadiazolines, undergo an intramolecular cyclization with the tethered alkyne moiety to give 3,3-dialkoxyvinylcarbene/1,3-dipole intermediates. The regioselectivity of the carbene cyclization is highly dependent upon the nature of the alkyne substituent. In the cases where an exocyclic vinylcarbene is generated, the vinylcarbene intermediate can undergo a synthetically useful [3 + 2] cycloaddition with either an appropriate olefin or an alkyne. This approach allows the rapid construction of some interesting heterocyclic ring systems. Keywords: carbene, dialkoxycarbene, vinylogous dialkoxycarbene, heterocycle.
1996 Clara Benson Award Lecture The kinetics of the reaction of carbon with oxygen
Authors: Back M.H.
Published: May 2006

The results of studies of the kinetics of the reaction of oxygen with thin films of carbon are described with special emphasis on the role, in the mechanism of the reaction, of the stable complex formed between oxygen and the carbon surface. A key part of the proposed mechanism is the reaction between gaseous oxygen and the stable complex to form the product carbon dioxide. Measurements of the total pressure and the products formed as a function of time of the reaction have allowed estimates of the rate constants for the elementary reactions that make up the mechanism. This in turn has led to the application of computer modelling to the mechanism of the reaction and to the development of a more detailed mechanism involving two types of reactive sites on the carbon surface. Keywords: carbon, oxygen, kinetics, mechanism, computer modelling.
Equilibrium acidities of some carbon acids in acetonitrile
Authors: Galezowski W. , Stanczyk M. , Jarczewski A.
Published: May 2006

The pK a values for C-acids (4-nitrophenylnitroalkanes, 4-nitrophenyl-(4-X-phenyl)cyanomethanes, 2,4,6-trinitrotoluene, and 9-cyanofluorene) in acetonitrile have been measured. These values were determined spectrophotometrically in buffer solutions consisting of a strong amine base and its picrate or perchlorate using known potentiometric pKa's of amines, and they range from 19 for bis(4-nitrophenyl)cyanomethane to 25.1 for ethyl bis(4-nitrophenyl)acetate. Keywords: C-acids, pKa values, equilibrium, reactivity, proton transfer reaction.
The steric course of some electrophilic additions to the tetrahydropyridazine ring moiety of benzo[g]pyridazino[1,2-b]-phthalazine-6, 13-dione derivatives. II.
Authors: Cano María Del Carmen , Gómez-Contreras Fernando , Sanz Ana María , Yunta María Josefa Rodriguez
Published: May 2006

The stereochemical features involved in electrophilic additions to the ring A moiety in diazatetracyclic analogues of anthracyclinones have been investigated. The 1-methyl and 1,3-dimethyl derivatives 2a and 2b were selected as the substrates and made to react with bromine azide, iodine azide, and NBS–EtOH in order to evaluate the influence of the C-1 substituent on the product distributions. Electrophilic attack at the C-2/C-3 double bond occurs mainly in the less hindered anti fashion. The nucleophilic step governs the stereochemistry of the reaction products. Conformational factors are predominant over the steric and electronic effects originated by the ring A substituents, since axial antiparallel attack of the nucleophile on the intermediate epihalonium ion is clearly preferred to the equatorial approach. Keywords: diazatetracycles, Diels–Alder reaction, electrophilic addition, anthracyclinone analogues, pyridazine ring.
Electronic absorption spectra and structure of hydroxycoumarin derivatives and their ionized forms
Authors: Traven Valery F. , Vorobjeva Larisa I. , Chibisova Tatjana A. , Carberry Edward Andrew , Beyer Noelle Jean
Published: May 2006

Electronic absorption spectra of 18 hydroxycoumarin derivatives and their ionized forms have been studied. Close agreement between experimental and the PPP CI calculated electron absorption band energies has been found in most cases. Strong polarization of the carbonyl function of the pyrone ring in the 7-hydroxycoumarin derivatives, H-bonding between the hydroxyl group and neighboring substituent in the ortho-substituted hydroxycoumarins, as well as their tautomeric transformations, have been suggested in the discussion of the electronic absorption spectra of the hydroxycoumarin derivatives. In accord also with calculational results, ionization of the hydroxyl function leads to a bathochromic shift of the longest-wavelength absorption bands in the spectra of 7-hydroxycoumarin derivatives. The ionization has no effect on the electronic absorption of the 4-hydroxycoumarin derivatives. Relative stabilities of the tautomeric forms of hydroxycoumarin derivatives and their ionized forms have also been compared by MNDO calculations. Keywords: hydroxycoumarins, intramolecular H-bonding, ionization, electronic absorption spectra, keto–enol tautomerism.
Conversion of methane to ethylene
Authors: Miremadi B.K. , Colbow K. , Morrison S. Roy
Published: May 2006

A lithium aluminate – MgO catalyst has been found to convert methane to ethylene with a high selectivity. The rate of conversion increased when a MoO3 co-catalyst was used to remove the poisoning products. It is shown that for optimum conversion and selectivity to ethylene, the oxygen pressure should be at an intermediate value, high enough to provide active sites but low enough to avoid ethylene oxidation. Thus the oxygen should be "bled-in" along the catalyst bed. In demonstration of these concepts we have shown a 28.6% CH4 conversion with 63.2% ethylene selectivity and 9.8% ethane, producing a C2 yield of about 21.4%. Keywords: methane oxidation, methane conversion, ethylene production, oxidation catalyst, aluminate catalyst.
Influence of working electrodes in Belousov–Zhabotinsky oscillatory system
Authors: Jayalakshmi V. , Ramaswamy R.
Published: May 2006

The potential oscillatory profiles recorded simultaneously in the same B–Z system using platinum, glassy carbon, and graphite electrodes are different in their characteristics. The variance in the amplitude and potential range is a result of the difference in the potential response of the electrode to the different redox couples. The results obtained by employing cyanoacetic acid, syringic acid, malonic acid, citric acid, methylacetoacetate, or vanillin as the substrate and Ce(III), Mn(II), or Ferroin as the catalyst are presented. The response of the working electrode to the various redox couples is correlated to the value of the exchange current density. Keywords: potential oscillatory profile, redox couple, working electrode, potential response, exchange current density.
Synthesis of 4,4-bis(2-methylphenyl)-3-butenyl (and butyl) analogs of 4-phenyl-1,4- and 6-phenyl-1,6-dihydropyridine-3-carboxylic acids and their evaluation as neuronal GABA-uptake inhibitors
Authors: Iqbal Nadeem , Wei Zhong-Yong , Baker Glen B. , Knaus Edward E.
Published: May 2006

Treatment of 3-[2-(4,4-dimethyl-4,5-dihydrooxazolin-2-yl)]-4-phenyl-1,4-dihydropyridine (13) with NaH–DMSO, and then reaction with 1,1-bis(2-methylphenyl)-4-bromobutane (12c) afforded 1-[4,4-bis(2-methylphenyl)butyl]-3-[2-(4,4-dimethyl-4,5-dihydrooxazolin-2-yl)]-4-phenyl-1,4-dihydropyridine (14). Reaction of methyl nicotinate with 2.1 equivalents 12c or 1,1-bis(2-methylphenyl)-4-bromo-1-butene (11b) afforded 4,4-bis(2-methylphenyl)butyl 1-[4,4-bis(2-methylphenyl)butyl]pyridinium-3-carboxylate bromide (17) or 4,4-bis(2-methylphenyl)-3-butenyl 1-[4,4-bis(2-methylphenyl)-3-butenyl]pyridinium-3-carboxylate bromide (18), respectively. The nonregioselective reaction of the pyridinium salts (17/18) with PhMgCl in THF at −23 °C using a catalytic amount of CuI afforded a mixture of isomeric 4-phenyl-1,4-dihydropyridyl (21 or 22) and 6-phenyl-1,6-dihydropyridyl (27 or 28) products in a ratio of approximately 1:1. All attempts to hydrolyze the 4,4-bis(2-methylphenyl)butyl or 3-butenyl ester moiety of 21/22 or 27/28 to a carboxyl group resulted in decomposition products. In contrast, the corresponding 3-(2-cyanoethyl) esters (23, 24, 29, 30) were readily converted to the corresponding carboxyl analogs (25, 26, 31, 32) via a β-elimination reaction of acrylonitrile using the non-nucleophilic base DBU. The 4-phenyl-1,4-dihydropyridyl (14, 25, 26) and 6-phenyl-1,6-dihydropyridyl (27/28 or 31/32) compounds inhibited the in vitro uptake of [3H]GABA into striatal prisms in the 21–44% range at a 10−4 M test compound concentration, relative to the reference drug nipecotic acid (87% inhibition). Structure–activity correlations showed the dihydropyridyl C-3 substituent was a determinant of [3H]GABA uptake where the potency order was CO2H > 2-(4,4-dimethyl-4,5-dihydrooxazolin-2-yl) > CO2(CH2)3CH-(o-tolyl)2 and CO2(CH2)2CH=C-(o-tolyl)2. Compounds possessing C-3 and (or) N-1 CO2(CH2)3CH-(o-tolyl)2 substituents were generally more potent than analogs having CO2(CH2)2CH=C-(o-tolyl)2 substituents. In general, 1,6-dihydropyridyl compounds were more potent than the corresponding 1,4-dihydropyridyl isomers. Keywords: 1,4- and 1,6-dihydropyridines, GABA-uptake inhibitors.
A simple synthesis of (±)-1,2,3,6-tetrahydro-2,3′-bipyridine (anatabine) and (±)-3-(2-piperidinyl)pyridine (anabasine) from lithium aluminum hydride and pyridine
Authors: Yang Chi-Ming , Tanner Dennis D.
Published: May 2006

The hydrolysis of a pyridine solution of lithium tetrakis(N-dihydropyridyl)aluminate (LDPA), which was prepared at 0 °C, yields a mixture of 1,4-, 1,2-, and 2,5-dihydropyridines (DHPs) in a ratio of 26:37:38. The subsequent reversible base-catalyzed condensation of a 1:1 mixture of 1,2- and 2,5-DHPs carried out in the presence of oxygen affords an 89% yield of (±)-anatabine. When the reaction mixture is allowed to stand in the presence of oxygen, anabasine is slowly formed from anatabine by the reaction of the residual DHPs. Anatabine can also be converted into (±)-anabasine by catalytic hydrogenation. Keywords: lithium aluminum hydride, pyridine, anatabine, anabasine.
A synthesis of α-cuparenone based on symmetry considerations
Authors: Ho Tse-Lok , Chang May-Hua
Published: May 2006

A ring expansion approach to α-cuparenone was accomplished in 5 or 6 steps. The key intermediate is 3-methyl-3-(4-methylphenyl)cyclobutanone, which was derived from a [2 + 2]cycloaddition of dichloroketene on a styrene derivative, followed by dechlorination. Keywords: symmetry, α-cuparenone, ring expansion, sesquiterpene.
Bisindolylbenzenoids from ascostromata of Petromycesmuricatus
Authors: Ooike Mikiko , Nozawa Koohei , Kawai Ken-ichi , Udagawa Shun-ichi
Published: May 2006

New bisindolylbenzenoids designated petromurins A (1), B (2), C (3), and D (4) were isolated from the ascostromata of Petromycesmuricatus. These structures were determined by chemical and spectroscopic methods. Keywords: Petromycesmuricatus; bisindolylbenzenoids; petromurin A, B, C, and D; ascostromata; Aspergillusmuricatus.
Assembly of tricyclic compounds that include the spiro[4.4]nonane subunit
Authors: Liu Pei-Ying , Wu Yong-Jin , Burnell D. Jean
Published: May 2006

Spiroannulation of ketals derived from unsaturated cyclohexenone derivatives by the Lewis acid-catalyzed reaction of 1,2-bis((trimethylsilyl)oxy)cyclobutene 5 led to spiro[4.5]decene-diones 9 and 30. These were transformed into spiro[4.4]nonene derivatives 25 and 35/36 via similar sequences involving ozonolysis and aldol ring reclosure. Reduction of an annular double bond allowed facile closure of a third ring, also by an aldol reaction. This work led to a single oxygenated angular triquinane 28 from 9, and to the tricyclo[6.2.1.01,5]undecane derivative 41 from 30. An oxatricyclo[5.3.1.01,5]undecane derivative 19 was also obtained from 9. Keywords: spiroannulation, geminal acylation, sesquiterpene synthesis.
Diels–Alder adducts of ortho-benzoquinones: rearrangements and further transformations
Authors: Carlini Rina , Higgs Kerianne , Taylor Nicholas , Rodrigo Russell
Published: May 2006

ortho-Benzoquinones substituted with an electron-withdrawing group (EWG = CO2Me, COSMe, COCH3, CHO) at C-3 or C-4 react as dienophiles at the C3—C4 double bond in Diels–Alder reactions with several dienes, with predictable regiochemistry. The adducts undergo migration of the "angular" EWG substituent with concomitant aromatization to produce substituted catechols. The bicyclic products can be oxidized in situ and annelated by a further Diels–Alder reaction to yield 9,10-phenanthraquinone systems. When (2E)-2,4-pentadienol is employed as the diene in the second cycloaddition, reaction of the alcoholic hydroxyl group with one of the quinone carbonyl groups results in the production of tetracyclic lactols 22a, 22b, and 23. Keywords: benzoquinone, Diels–Alder, rearrangement, phenanthraquinone, phenanthrofuran.
Synthesis of thyroxine: biomimetic studies
Authors: Bell Natalie V. , Bowman W. Russell , Coe Paul F. , Turner Andrew T. , Whybrow Del
Published: May 2006

The biomimetic oxidative coupling of the ethyl ester of N-acetyl-3,5-diiodotyrosine (1) to yield the ethyl ester of N-acetylthyroxine (2) has been investigated. A putative mechanism involving phenolic coupling to yield an intermediate aryloxydienone (7) followed by an E2 elimination for loss of the side chain has been proposed. Oxidative couplings with analogous 4-substituted 3,5-diiodophenols indicate that a number of mechanisms are possible; these include quinone methide intermediates and SN2 substitutions in the intermediate aryloxydienones. Rearomatization of the intermediate aryloxydienones is a strong driving force for the loss of the side chains. The results indicate that 3,5-diiodo-4-aryloxydienones are good leaving groups in E2 and SN2 mechanisms. The synthetic method provides a facile synthesis of thyroxine analogues from readily available 4-substituted 3,5-diiodophenols. Keywords: diiodotyrosine, phenolic coupling, phenoxyl radicals, thyroxine.
Chemoselective intramolecular annulation of 3-alkylindolines into dihydro or tetrahydrofuro[2,3-b]indoles
Authors: Suárez-Castillo Oscar R. , García-Velgara Manuel , Morales-Ríos Martha S. , Joseph-Nathan Pedro
Published: May 2006

3-Alkyl-2-hydroxyindolines, conveniently prepared from 2-hydroxyindolenines and a Grignard reagent, cyclize in the aprotic solvent tetrahydrofuran to afford tetrahydro-3-cyano-2-oxofuro[2,3-b]indoles, while in the protic solvent methanol the chemoselectivity changed to give dihydro-2-amino-3-carbomethoxyfuro[2,3-b]indoles. The steric effect of the alkyl group on the reactivity of 3-alkyl-2-hydroxyindolines is discussed for both processes. The ring transformation of tetrahydro-3-cyano-2-oxofuro[2,3-b]indoles into dihydro-2-amino-3-carbomethoxyfuro[2,3-b]indoles via γ-lactone imines is also discussed. Keywords: furo[2,3-b]indoles, α-cyano-γ-lactones, chemoselectivity, ring transformation, β-enamino esters.
3-Phosphorylated 1,5-hexadienes as precursors for vinylic or 3-silanylvinylic phosphonates
Authors: Probst Michael F. , Modro Agnes M. , Modro Tomasz A.
Published: May 2006

3-(Diethylphosphono) substituted 1,5-hexadienes are prepared by the α-regioselective allylation of allylic phosphonates. The products can be converted into vinylic phosphonates via two types of reactions. The first involves Cope rearrangement yielding the terminal vinylphosphonates of the 1,5-hexadiene system. The second involves γ-silanylation leading to the formation of the 1-silanyl-3-phosphono derivatives of the 2,5-hexadiene skeleton. Keywords: allylic and vinylic phosphonates, nucleophilicity of allylphosphonate carbanions, electrophilic silanylation, Cope rearrangement of alkenylphosphonates.
Difunctional derivatives of bis(m-phenylene)-32-crown-10
Authors: Gibson Harry W. , Nagvekar Devdatt S.
Published: May 2006

Optimization studies of the condensation of methyl 3,5-dihydroxybenzoate (1) with tetra(ethylene glycol) dichloride (3) resulted in improvement of the yield of the 1 + 1 cyclization product, 5-carbomethoxy-m-phenylene-16-crown-5 (5), to 67% (HPLC), but no improvement in the yield (28%, HPLC) of the desired 2 + 2 product, bis(5-carbomethoxy-m-phenylene)-32-crown-10 (4). However, after optimization, a two-step procedure provided improved yields of 4 and this procedure was generalized to afford other difunctional monomers. Condensation of substituted resorcinols with 3 and subsequent transformations yielded substituted (R) 3,5-bis(11-chloro-3,6,9-trioxaundecyl-oxy)benzenes (7, 9–14). Reaction of dihalides 7 (R = COOCH3), 13 (R = CHO), and 12 (R = CH2OSi(Me)2-t-Bu) with methyl 3,5-dihydroxybenzoate (1) produced bis(5-carbomethoxy-m-phenylene)-32-crown-10 (4) (43%), 5-carbomethoxy-m-phenylene-5′-formyl-m′-phenylene-32-crown-10 (15) (32%), and the lactone (16a) (18%, derived from the initially formed 5-hydroxymethyl-m-phenylene-5′-carbomethoxy-m′-phenylene-32-crown-10 (16)), respectively. Subsequent reactions gave the corresponding diacid (17), bis(hydroxymethyl) (19), bis(bromomethyl) (20), diacetyl (18), diformyl (21), bis(p-nitrophenoxymethyl) (22), and di(acetoxymethyl) (23) derivatives. Keywords: cyclization, functionalized bis(m-phenylene) crown ethers.