bluuebunny/crossref_metadata_2025 huggingface dataset:
Coupled transport of iodide ions during interdiffusion of aqueous HCl and NaOH
Authors: Leaist Derek G.
Published: April 2011

Stokes diaphragm cells have been used to study multicomponent diffusion in aqueous HCl–NaOH–NaI mixtures at 25 °C. The nine quaternary diffusion coefficients for the mixture containing 0.025 mol L−1 of each electrolyte are reported. The measured coefficients are used to analyze coupled transport of iodide ions generated by interdiffusion of aqueous HCl and NaOH. The analysis shows that the gradient in concentration of NaOH is twice as effective at producing flow of iodide ions relative to the HCl gradient, and that the NaI gradient substantially alters the rates of diffusion of HCl and NaOH components. Equations are developed for predicting multicomponent transport coefficients that describe coupled transport of ions driven by interdiffusion of strong acids and bases.
Thermal isomerizations of protonated bicyclo[3.1.0]hexenones
Authors: Childs Ronald F. , George Baha E.
Published: April 2011

The thermal isomerization of a wide range of protonated methyl substituted bicyclo[3.1.0]hex-3-en-2-ones to protonated phenols has been examined using triflic acid as a strong acid solvent. The rate constants and activation energies of these isomerizations have been determined. The barriers to the isomerizations were shown to be dependent on the number and position of the methyl substituents. The results show that three different mechanisms are needed to account for these isomerizations, two of which involve a preliminary circumambulatory rearrangement prior to ring opening and the other process involving a direct ring opening of the initial protonated bicyclohexenone to give an intermediate meta-protonated phenol.
The importance of ground state conformation in the photoisomerizations of protonated enones
Authors: Childs Ronald F. , Mahendran Mailvaganam , Blackburn Christopher , Antoniadis Gregory
Published: April 2011

The conformations of several protonated enones have been determined by low temperature 13C nuclear magnetic resonance spectroscopy. Information on the ground state conformation has been used to understand the differences in the photochemistry of these systems. Photoisomerizations of 4 and 5 have been examined. In the case of 4, irradiation leads only to the formation of the Z isomer 16; protonated dihydrofuran 13 was not observed. In the case of 5, both the Z isomer 17 and protonated dihydrofuran 14 were observed. The protonated dihydrofuran 14 was produced from the start of the reaction. It is concluded from these and previous results that for the photocyclization to occur the starting cation must exist in an s-cis conformation in the ground state.
Synthesis of a β-diketone from a dithioacetal. A model study for the synthesis of an ionomycin fragment
Authors: Shelly Kevin P. , Weiler Larry
Published: April 2011

The anions of dithioacetals react with epoxides to give β-hydroxydithioketals, which can be oxidized and hydrolyzed to β-diketones. This sequence is applied to a model study of the synthesis of the C-5 to C-15 fragment of the antibiotic ionomycin.
Products and reaction paths in the liquid phase oxidation of trans-1,2-dichloroethene with oxygen
Authors: Griesbaum Karl , Hayes Michael P. , Werli Vera
Published: April 2011

Ultraviolet initiated liquid phase oxidation of neat trans-1,2-dichloroethene (1a) with oxygen at 30 °C afforded ca. 45% of the combined Cl-products CO, CO2, and phosgene as well as nine oxygenated and nine non-oxygenated chlorinated organic products. Major oxygenated products were cis- and trans-2,3-dichlorooxirane (6a, b) and 1,2,2-trichloroethyl formate (13); minor products were e.g., meso and rac bis(1,2,2-trichloroethyl) ether (11c, d) and dichloromethyl 1,2,2-trichloroethyl ether (12). The mode of formation of all products is rationalized by a unified reaction scheme.
Nuclear analogs of β-lactam antibiotics. Synthesis of 6,6-disubstituted acylaminopenems
Authors: Banville Jacques , Lapointe Philippe , Belleau Bernard , Menard Marcel
Published: April 2011

The preparation of 6α-methyl-2-methyl-6β-phenoxyacetamidopenem-3-carboxylate, 6α-methoxy-2-methyl-6β-phenoxyacetamidopenem-3-carboxylate, and 6α-methoxy-2-methyl-6β-phenylmalonylamidopenem-3-carboxylate from penicillin V and 6-aminopenicillanic acid is described. These penems have been isolated and characterized as their sodium or potassium salt. The chemical stability of the above compounds was determined as their half-life in aqueous buffer. In this way, it was found that the 6α-methyl analog was more stable than the parent 6-monosubstituted acylaminopenem while the remaining analogs were of comparable stability.
Equilibrium and kinetic studies of a chemical model of active transport: Membrane transport of potassium salts by cryptand carriers driven by potassium ion complexation
Authors: Fyles Thomas M. , Hansen Steven P.
Published: April 2011

A theoretical framework of energy transduction in membranes was used as a point of departure for the design of a simple chemical model of primary active transport (reaction pumping). In the model, cryptand carriers transport potassium salts across a bulk liquid membrane, driven by the complexation of potassium by a dicarboxylate crown ether in one aqueous phase. Both the equilibrium position and kinetic behavior of the model system could be predicted from the theoretical framework. Experimental verification of the predictions confirms the fitness of the theoretical framework and provides a design tool for the formulation of new membrane transport systems.
The conformational behaviour of 5Jm(F,CH) in 3,5-difluoro derivatives of toluene, ethylbenzene, and cumene. Applications
Authors: Schaefer Ted , Takeuchi Craig S. , Sveinson Steven Eric
Published: April 2011

The CNDO/2 and INDO MO FPT values for 5Jm(F,CH3) in 3-fluoro- and 3,5-difluorotoluene are exactly reproduced by A cos2 θ + B sin2 θ + C sin2 (θ/2). Here θ is the angle by which the α C—H bond twists out of the benzene plane. Adjustment of A,B, and C to give an agreement with experiment for 3,5-difluorotoluene yields an equation, best considered empirical, which is tested by 5J(F,CH) in 3,5-difluoroethylbenzene and 3,5-difluoroisopropylbenzene. The equation reproduces 5J(F,CH) in these two compounds, the values of [Formula: see text] being derived from 6Jp(H,CH), the spin–spin coupling constants over six bonds between the α and para ring protons. 5J(F,CH) is obtained for the asymmetrical compounds, 2,3-difluorobenzylidene diacetate and 2-bromo-5-fluorobenzylidene diacetate. It is shown how 5J(F,CH) in the latter can discriminate between two conformers, each of which, on the basis of 6J(H,CH), will be characterized by a large degree of torsion about the [Formula: see text] bond.
Étude des courants périodiques observés lors de la dissolution électrochimique du nickel en présence d'ions iodure et chlorure en milieu aqueux
Authors: Le Li Ben , Vittori Olivier
Published: April 2011

Electrodissolution of nickel in the presence of chloride and iodide ions has been investigated using low sweep voltammetry (< 2 mV/s). It has been observed that the rate of dissolution of nickel is related to the concentration of chloride, and that addition of iodide modifies the shape of the curves. Oscillations of the current are noticed after a delay shortened by increasing the iodide concentration. These oscillations occur over a wide range of potential provided the iodide concentration is kept low. Increasing this concentration suppresses the oscillations, while two peaks appear, the more cathodic becoming progressively larger. Addition of some surfactants is discussed and progressively larger. Addition of some surfactants is discussed and only SOBS (sodium p-octylbenzene sulfonate) suppresses the oscillations even at low concentration, while starch acts progressively from 1 to 40 g/L. Changes of color of the nickel surface during the oscillations suggests a preferential dissolution at the grain boundaries, explaining the black color observed alternatively with a metallic appearance, the nickel being at this time covered with finely divided particles that dissolve quickly. Iodide, appearing only occasionally at such anodic potentials, is probably involved in the dissolution process.
Some phase relationships between basic bismuth chlorides in aqueous solutions at 25 °C
Authors: Taylor  , Lopata Vincent
Published: April 2011

not available
Solvatation des électrons en excès dans les solvants polaires: corrélation entre les valeurs de εmax, W1/2 et EAmax
Authors: Jay-Gerin J.-P. , Ferradini C.
Published: April 2011

On the basis of data found in the literature, it is shown that a correlation exists between the molar extinction coefficient at the maximum optical absorption of the solvated electron (εmax), its width at half height (W1/2), and the energy corresponding to that maximum (EAmax) Keywords: solvated electrons, polar solvants, optical absorption spectra. [Journal translation]
Thiophenophane metal complexes. III. Preparation, X-ray structure, and properties of 2,5,7,10-tetrathia[12] (2, 5) thiophenophane, L, and [Ag2L2]2+, its dinuclear silver complex cation
Authors: Lucas C. Robert , Liu Shuang , Newlands Michael J. , Charland Jean-Pierre , Gabe Eric J.
Published: April 2011

Preparations of the thiophenophane C12H18S5(L3), and its binuclear silver(I) complex cation [Ag2(L3)2]2+ are described. The 13C and 1H nmr spectra of L3 and the ir and fast atom bombardment mass spectra of the complex are reported. X-ray structures of L3 and [Ag2(L3)2](ClO4)2 have been determined. For L3: space group P21/n, a = 10.8866(2), b = 9.4419(4), c = 14.9608(2) Å, β = 97.132(1)°, Z = 4, Rf = 0.057, Rw = 0.045. For [Ag2(L3)2] [ClO4]2: space group P21/n,a = 9.2717(3), b = 19.8697(2), c = 10.2654(4) Å, β = 100.592(2)°, Z = 2, Rf = 0.036, Rw = 0.030. The effect of structure on the ligand's ability to form complexes is discussed. The centrosymmetric dinuclear cation has slightly distorted trigonal bipyramidal coordination geometry about each silver. The silver atoms are linked by one bridging thioether sulfur from each ligand. The remaining three sites on each silver are occupied by two terminal thioether sulfurs from one ligand molecule and one from the other ligand molecule. The complex is remarkably stable photochemically and chemically but under certain specific conditions reacts with loss of ligand. This stability is discussed with respect to structure. Keywords: binuclear silver(I) thiophenophane complex.
Direct NMR evidence for the equivalent participation of L-phenylalanine and L-tyrosine in the biosynthesis of the intermolecular Diels–Alder type adducts of prenylchalcone and prenylated 2-arylbenzofuran in Morus alba cell cultures
Authors: Hano Yoshio , Nomura Taro , Ueda Shinichi
Published: April 2011

L-[3-13C]Phenylalanine and L-[3-13C]tyrosine were administered to Morus alba cell cultures, to produce intermolecular Diels–Alder type adducts of a prenylchalcone and a 2-arylbenzofuran such as chalcomoracin (1) as well as the adducts of two molecules of prenylchalcones such as kuwanon J (2) in high yields. The 13C NMR spectra of 1 and 2 isolated from the cultures revealed that both amino acids were incorporated intact into chalcomoracin (1) and kuwanon J (2). This is the first example of direct NMR evidence for the almost equivalent incorporation of phenylalanine and tyrosine into the shikimate metabolites. This finding suggests the participation of a biosynthetic route from phenylalanine via trans-cinnamate to p-coumarate and from tyrosine to p-coumarate in this plant.
Photochemistry of a model lignin compound. Spin trapping of primary products and properties of an oligomer
Authors: Barclay L.R.C. , Cromwell G.R. , Hilborn J.W.
Published: April 2011

α-Guaiacoxylacetoveratrone, 1, was selected as a model of a lignin structure component for photolysis and free radical spin trapping studies. The 3,4-dimethoxyphenacyl radical, Ar-COĊH2, 2, thermally generated from α-bromoacetoveratrone with trimethyltin, was spin trapped by phenyl-tert-butylnitrone (PBN), and the spin-trapped radical identified by ESR and mass spectroscopy. Spin trapping by PBN of radical photoproducts from 1 gave a mixture of trapped radicals, according to ESR. These were separated as their hydroxylamines and identified by liquid chromatography/mass spectroscopy (LC/MS) under chemical ionization (CI) conditions. This LC/MS (CI) method identified the same phenacyl fragment, 2 (reduced), as well as a fragment tentatively attributed to a spin-trapped guaiacoxyl radical, chemically reduced to the hydroxylamine. Extended direct photolysis of 1 yields colored paramagnetic oligomers. The synthesis of 1-(3,4-dimethoxyphenyl)-2-(4-hydroxy-3-methoxyphenylethanone(10), a rearranged, recombination photoproduct of 1, is described. A pathway for formation of oligomers as unstable dimers of 10 is presented.
Investigations of the biosynthesis of the phytotoxin coronatine
Authors: Parry Ronald J. , Mhaskar Sunil V. , Lin Ming-Teh , Walker Alan E. , Mafoti Robson
Published: April 2011

The biosynthesis of the phytotoxin coronatine has been investigated by administration of isotopically labeled precursors to Pseudomonas syringae pv. glycinea. The structure of coronatine contains two moieties of distinct biosynthetic origin, a bicyclic, hydrindanone carboxylic acid (coronafacic acid) and a cyclopropyl α-amino acid (coronamic acid). Investigations of coronafacic acid biosynthesis have shown that this compound is a polyketide derived from three acetate units, one butyrate unit, and one pyruvate unit. The two carbonyl oxygen atoms of coronafacic acid were found to be derived from the oxygen atoms of acetate. Additional experiments are described that rule out some possible modes for assembly of the polyketide chain. Coronamic acid is shown to be derived from L-isoleucine via the intermediacy of L-alloisoleucine. Examination of the mechanism of the cyclization of L-alloisoleucine to coronamic acid revealed that the formation of the cyclopropane ring takes place with the removal of only two hydrogen atoms from the amino acid, one at C-2 and the other at C-6. The nitrogen atom at C-2 of L-alloisoleucine is shown to be retained. On the basis of these observations, a mechanism is postulated for the cyclization reaction that involves the diversion of an enzymatic hydroxylation reaction into an oxidative cyclization. Finally, a precursor incorporation experiment with deuterium-labeled coronamic acid demonstrated that free coronamic acid can be efficiently incorporated into coronatine. This observation indicates that the cyclization of L-alloisoleucine to coronamic acid can occur before formation of the amide bond between coronafacic acid and coronamic acid.
Septatisine, a novel diterpenoid alkaloid from Aconitum septentrionale Koelle
Authors: Joshi Balawant S. , Sayed Hanaa M. , Ross Samir A. , Desai Haridutt K. , Pelletier S. William , Cai Ping , Snyder John K. , Aasen Arne J.
Published: April 2011

Septatisine 3, an atisane-type diterpenoid alkaloid, was isolated from the roots of Aconitum septentrionale Koelle. Its structure was established on the basis of homonuclear 1H COSY, LRCOSY, NOESY, DNOE, fixed-evolution HETCOR, and selective INEPT techniques.
The formation of corydaline and related alkaloids in Corydalis cava in vivo and in vitro
Authors: Rueffer Martina , Bauer Wolfgang , Zenk Meinhart H.
Published: April 2011

The biosynthesis of corydaline and thalictricavine has been investigated by application of [8-3H]-labelled palmatine and berberine to Corydalis cava bulb tissue. In both cases, high incorporation into the respective (14R,13S)corydaline and -thalictricavine was observed. (S)-[14-3H, 8-14C]Scoulerine was also transferred into corydaline with all of the tritium being lost, indicating that a redox reaction had taken place in the transition of the (14S) to the (14R) product. A partly enriched protein fraction catalyzed the reduction of both protoberberine precursors to the 7,8-dihydro intermediate, which was subsequently methylated at C-13 at the expense of S-adenosylmethionine (SAM) and reduced at C-14 by a B-type reaction with NADPH as reductant. The identity of the product was unequivocally established by NMR, MS, and CD measurements. This biosynthetic route is in full agreement with previously published predictions based on in vivo experiments.
ortho and remote metalation – cross coupling strategies. Total synthesis of the naturally occurring fluorenone dengibsinin and the azafluoranthene alkaloid imeluteine
Authors: Fu J. -m. , Zhao B. -p. , Sharp M. J. , Snieckus V.
Published: April 2011

The total synthesis of the naturally occurring fluorenone, dengibsinin (7a), and the azafluoranthene alkaloid, imeluteine (30e), is described. Using combined ortho metalation – cross coupling sequences that terminate in Friedel–Crafts (18b → 6d) and remote metalation (21a,b → 6c,d) reactions, the synthesis of fluorenone dimethyl ethers 6c and 6d is reported. 6c and 6d were shown not to be identical to dengibsinin dimethyl ether and dengibsin dimethyl ether, respectively, derived from the natural products. This work, together with synthetic and structural evidence from Sargent and Talapatra, led to the reassignment of structures for dengibsinin (7a) and dengibsin (7b). Using the metalation – cross coupling approach, the synthesis of the reassigned dengibsinin (7a) is presented. Two alternate unsuccessful approaches to imeluteine are briefly described (31 + 32 and 33 + 34). The successful approach incorporates the remote metalation – double cyclization, 43 → 44, as the key step.
Carbocation-like reactivity patterns in X'-substituted-4-biphenylylnitrenium ions
Authors: Ren Daniel , McClelland Robert A
Published: April 2011

4-Azido-X'-substituted biphenyls (X' = 4'-MeO, 4'-Me, 4'-F, 3'-Me, 4'-Cl, H, 3'-MeO, 3'-Cl, 4'-CF3) have been prepared and subjected to 248 nm flash photolysis irradiation in 20:80 acetonitrile:water. Transient X'-substituted 4-biphenylylnitrenium ions 10 (Ar C6H4-N + H) are observed, with lifetimes ranging from 0.6 ms (4'-MeO) to 26 ns (4'-CF3). These cations are quenched by azide ion, with values of kaz ranging from 6 to 10 x 109 M-1 s-1, with the majority in the range (9-10) x 109. This near constant kaz provides further evidence that arylnitrenium ions are quenched by azide ion at the diffusion limit. The solvent reactivities, plotted in a single-parameter Hammett plot against σ sup + (X), exhibit a poor correlation, with the points for the para π-electron donors deviating from the correlation line based on the other substituents in the direction of requiring a more negative substituent parameter. The data are more satisfactorily fit to the two-parameter Yukawa-Tsuno equation; the parameter r + obtained in this fit is 2.8. Thus the resonance interaction of the para π -donor X'-substituents with the positive charge of the cation is underestimated by σ sup + , a situation that has previously been observed with benzylic-type carbenium ions. The conclusion is made that, in their reaction with water, 4-biphenylylnitrenium ions behave like benzyl cations bearing two additional stabilizing vinyl groups, i.e., as if they had the structure Ar-C + (C = C)2. The inherent reactivity and the pattern of the aryl substituent effects are in fact similar to those in the carbocation series Ar-C + (Ph)2.Key words: nitrenium, nitrene, aryl azide, photochemistry, lifetime.
Transverse compression and the secondary H/D isotope effects in intramolecular SN2 methyl-transfer reactions
Authors: Wolfe Saul , Kim Chan-Kyung , Yang Kiyull , Weinberg Noham , Shi Zheng
Published: April 2011

Using ab initio molecular orbital theory mainly at the 3-21 + G level, intramolecular SN2 methyl transfer between two oxygens confined within a rigid template is found to proceed exclusively by a high energy retention mechanism when the oxygens are separated by three or four bonds, and by a high energy inversion mechanism when the oxygens are separated by six bonds. Both mechanisms exist when the oxygens are separated by five bonds. The CH3/CD3 kinetic isotope effects are normal (1.21-1.34) in the retention processes and inverse (0.66-0.81) in the inversion reactions. In the case of inversion, compression of C-H bonds of the transition state by structural effects in the plane perpendicular to the O-C-O plane increases the inverse isotope effect. The retention barriers are high because retention is inherently unfavorable, even when pericyclic stabilization of the transition state is possible. The inversion barriers are high because a rigid template cannot accommodate a linear O-CH3-O structure, and the O-C-O bending vibration is stiff (the Eschenmoser effect). Using a novel design strategy, a nonrigid template has been found in which the barrier and the CH3/CD3 kinetic isotope effect are the same as in an intermolecular reaction.Key words: Eschenmoser effect, isotope effect, compression, SN2, sigmatropic rearrangement.
1,2-Bis(1-aryl-3-alkyltriazen-3-yl)ethanes and related compounds
Authors: Hooper Donald L , Pottie Ian R , Vacheresse Marc , Vaughan Keith
Published: April 2011

A series of novel bistriazenes, the 1,2-bis(1-aryl-3-methyltriazen-3-yl)ethanes, Ar-N T N-NMe-CH2CH2-NMe-N T N-Ar, have been synthesized by diazonium coupling with N,N'-dimethylethylenediamine. These bistriazenes are stable crystalline compounds and have been unequivocally characterized by IR and NMR spectroscopy (1H and 13C), and elemental analysis. The structures of two compounds in the series have been confirmed by X-ray crystallography. The 1H NMR spectra show significant line broadening of the N-methyl resonances arising from the restricted rotation around the N2-N3 bond of the triazene units. The presence of strongly electron-withdrawing groups on the aryl ring restricts the rotation to the point where the N-methyl signals of the rotamers are distinct even at room temperature; four resonances of the N-methyl signal are clearly evident and these can be assigned to the anti-anti, syn-syn, and syn-anti conformations of the bistriazene. Diazonium coupling with N,N'-diethylethylenediamine affords the N,N'-diethyl homologues of the bistriazenes, which have been similarly characterized. As model compounds to assist in spectroscopic analysis, a series of related triazenes, the 1-(1-aryl-3-methyltriazen-3-yl)-N,N-dimethyl-2-ethanamines, were prepared by diazonium coupling with N,N,N'-trimethylethylenediamine. These dialkyltriazenes exist mainly as oils, but characterization was achieved by IR, 1H NMR, and 13C NMR spectroscopy, also showing the presence of two rotamers in solution when strongly electron-withdrawing substituents are bonded to the aryl moiety.Key words: triazene, bistriazene, diazonium, ethylenediamine, molecular dynamics, NMR.
Influence des ultrasons sur la diastéréosélectivité. Synthèse d'imidazolidine-4-one chirales
Authors: Hubert Cathy , Garrigues Bernard
Published: April 2011

A three-step synthesis of imidazolidine-4-one has been realized. Sodium dithionite was a very efficient reagent for the last step, the reduction of a ketone. The reaction is slow under the normal conditions of heating and its diastereoselectivity is poor. Under the influence of ultrasound, the selectivity and the yield are very good. The use of sodium dithionite makes it possible to work in water and to avoid the use of hydrides.Key words: sodium dithionite, ultrasound, imidazolidine-4-one.
Article
Authors: Muchall Heidi M , Werstiuk Nick H , Choudhury Biswajit
Published: April 2011

Photoelectron (PE) spectra of two stable carbenes 7 and 8 have been recorded and the spectra have been interpreted with the aid of eigenvalues and eigenvectors taken from Becke3LYP calculations. For the carbene series 6-8, the lone pair on the carbene carbon atom is the HOMO. The first adiabatic ionization potential (IP) of eight electronically quite different carbenes has been calculated using semiempirical PM3 and ab initio HF, Becke3LYP, and Becke3PW91 methods (3-21G(*) and 6-31+G* basis sets) as well as the CBS-4 model. For the first vertical IP, the HAM/3, Becke3LYP, and Becke3PW91 methods have been employed. CBS-4 and DFT calculations show excellent agreement with experimental values. Considering both accuracy and speed, the method of choice for the prediction of first ionization potentials of carbenes seems to be Becke3LYP/6-31+G*//Becke3LYP/3-21G(*).Key words: carbenes, electronic structure, first ionization potential, photoelectron spectroscopy, quantum chemical calculations.
A conformational study of (1S, 2R, 5S)-( + )-menthol using vibrational circular dichroism spectroscopy
Authors: McCann Jennifer L , Rauk Arvi , Wieser Hal
Published: April 2011

We report the experimental and predicted absorption and vibrational circular dichroism (VCD) spectra of (1S, 2R, 5S)-(+)-menthol. The harmonic force fields and atomic polar tensors (APTs) were obtained for 10 conformers using density functional theory (DFT) with the Becke3LYP functional and the 6-31G** basis set. The atomic axial tensors (AATs) were obtained with restricted Hartree-Fock (RHF) theory and a 6-31G basis set. VCD intensities were predicted from vibronic coupling theory (VCT). The predicted absorption and VCD spectra based on an equilibrium mixture of conformers corresponded well with the experimental spectra, indicating a reasonable geometry and frequency analysis for each of the conformers. The largest contributions towards the equilibrium population derive from conformers in which the cyclohexane ring occupies a chair conformation, the isopropyl group is directed away from the hydroxyl group, and only the hydroxyl group assumes different conformations.Key words: vibrational circular dichroism (VCD), menthol, conformational analysis.
Article
Authors: Karatas Mehmet , Koni Serap , Dogan Ilknur
Published: April 2011

Sterically hindered N-(o-aryl)-rhodanines (a) (N-(o-aryl)-2-thioxo-4-thiazolidinones) have been synthesized and the N-(o-tolyl) and N-(o-chlorophenyl) derivatives have been converted to their dioxo analogs (b) (N-(o-aryl)-2,4-thiazolidine- diones). The chirality of the compounds in their conformational ground states was proved by detection of the diastereotopic protons or methyl groups at C-5 of the heteroring using 1H and 13C NMR spectroscopy. In the presence of the optically active auxiliary (S)-(+)-1-(9-anthryl)-2,2,2-trifluoroethanol the enantiomers of the racemic mixtures formed diastereomeric association complexes via H-bonding.Key words: chirality, chiral auxiliary, rotational isomerism, N-(o-aryl)-rhodanines, N-(o-aryl)-2,4-thiazolidinediones, 1H and 13C NMR.
Article
Authors: Tanaka Kelly SE , Bennet Andrew J
Published: April 2011

A bicyclo[2.2.2] analogue of the glycosidase inhibitor 1-deoxynojirimycin, 2,6-anhydro-1-deoxymannojirimycin (6), has been synthesized in nine steps from 1,5-anhydro-D-glucitol. This conformationally restricted aza-sugar has been shown to inhibit, albeit weakly, several glycosidase enzymes.Key words: glycosidase, inhibitor, bicyclic, deoxymannojirimycin, deoxynojirimycin.
1,3-Diphenylsulphonylcyclobutane: an unprecedented case of a higher stability of the trans isomer of 1,3-disubstituted cyclobutane
Authors: Sella Ariel , Cohen Shmuel , Hoz Shmaryahu
Published: April 2011

Equilibration of 1,3-diphenylsulphonylcyclobutane in t-BuOK/t-BuOH shows that the trans isomer is more stable than is the cis isomer by 2.1 kcal/mol. This is in contradistinction to the general case of 1,3-disubstituted cyclobutanes, where the bulky substituent prefers the cis geometry in order to avoid 1,3-diaxial interactions. The effect was found to be enthalpic in nature. Ab initio 3-21G calculations resulted in an energy difference of 2.6 kcal between the two isomers, in favor of the trans isomer. The cyclobutane rings were found to be nearly planar. While this may explain the higher stability of the trans isomer, it is not clear why the system avoids steric interaction this way, rather than by adopting the puckered conformation with the two bulky substituents at equatorial positions.
Kinetics of thermal racemization of (2S,3S)-1-13C-1,2,3-d3-cyclopropane followed by vibrational circular dichroism spectroscopy
Authors: Freedman Teresa B , Hausch Diane L , Cianciosi Steven J , Baldwin John E
Published: April 2011

Vibrational circular dichoism spectra recorded for (2S,3S)-1-13C-1,2,3-d3-cyclopropane and for mixtures of it and the three related stereoisomers prepared through gas-phase thermal stereomutation reactions at 407°C lead to the rate constant for racemization: kα = (4k1 + 4k12) = (3.12 ± 0.04) x 10-5 s-1. This and the rate constant measured for geometrical equilibration between the two chiral and the two achiral stereoisomers of 1-13C-1,2,3-d3-cyclopropane, ki = (8k1 + 4k12) = (4.63 ± 0.20) x 10-5 s-1, give two equations in two unknowns, and allow one to solve for one-center (k1) and two-center (k12) epimerization rate constants for cyclopropane stereomutations. They are nearly equal, a clear indication of closely competitive reaction pathways.Key words: cyclopropane stereomutations, thermal epimerizations, chirality through deuterium and carbon-13 labeling, vibrational circular dichroism.
Article
Authors: Mitchell Reginald H , Lau Danny YK , Mak Thomas CW , Wu Bo-mu , Anklin Clemens
Published: April 2011

The [18]paracyclo[14]annulene, 6, a trans-10b,10c-dimethyldihydropyrene bridged by an 18 atom chain at the 2,7-positions, was prepared in 33% yield from 1,14-tetradecanedioyl dichloride and 2,7-bis(hydroxymethyl)-10b,10c- dimethyldihydropyrene (7). Molecular mechanics calculations were carried out to correlate the lowest energy conformers of 6 with the experimental X-ray structure. High field NMR spectra of 6 were fully assigned and correlated with the lower energy conformers, which in solution equilibrate such that the annulene ring can freely spin. The molecule, which is necklace shaped, has the protons H(9/6), and not those at the centre of the chain of the necklace [H(8/7)], closest to the centre of the annulene ring, and most shielded by the ring current. Shielding of various protons of the chain approximately correlates with the distances of their appropriate carbons, averaged over the 10 lowest energy conformers, from the centre of the ring current.Key words: paracyclophane, bridged annulene, molecular mechanics, ring current.
Carbon-carbon coupling of 4,6-dinitrobenzofuroxan with 3-aminothiophenes: a kinetic and structural study
Authors: Terrier François , Pouet Marie-José , Gzouli Khalid , Hallé Jean-Claude , Outurquin Francis , Paulmier Claude
Published: April 2011

Potentiometric measurements carried out in a 50-50 (v/v) H2O-Me2SO mixture together with 1H and 13C NMR studies carried out in pure Me2SO-d6 reveal that 3-aminothiophene (4a), 3-N-(methylamino)thiophene (4b), and 3-N,N-dimethylaminothiophene (4c) undergo protonation exclusively at the amino groups in dilute acid solutions. The pKa values of 4a, 4b, and 4c are equal to 3.38, 3.65, and 3.53, respectively, at 25°C, comparing well with those for related anilines in the same aqueous Me2SO mixtures. However, despite this similarity of the nitrogen basicities, compounds 4a, 4b, and 4c are found to behave differently from anilines upon treatment with super-electrophilic 4,6-dinitrobenzofuroxan (DNBF). While anilines react to give first an N-bonded sigma -adduct that subsequently decomposes to afford the thermodynamically more stable C-bonded adduct, the 3-aminothiophenes react only as carbon nucleophiles, giving rise directly to the corresponding C-adducts. Based on 1H and 13C NMR data as well as measurements of the rate constants (k1DNBF) for DNBF addition to 4a, 4b, and 4c, it is demonstrated that 3-aminothiophenes possess such high negative charge density at C-2 that they can be regarded as strongly enaminic in nature. Estimates of the pKa values for protonation at C-2 of 4a, 4b, and 4c could be made: pKa4a approx -0.4; pKa4b approx 0.2; pKa4c approx 0.8. Thus, these compounds are more susceptible to electrophilic addition at this position than most of the five-membered ring heterocycles studied so far. Altogether, the results demonstrate the importance of the hyper-ortho relationship in the thiophene systems, accounting for the ease of both electrophilic and nucleophilic reactivity of 3-X-thiophenes in σ -complex formation processes.Key words: aminothiophenes, enamines, superelectrophiles, dinitrobenzofuroxan, hyper-ortho relationship.
Phenanthroimidazole dimers: structure and free radical reactivity; piezochromism, thermochromism, and photochromism
Authors: Zhou Yuehui , Baker W E , Kazmaier P M , Buncel E
Published: April 2011

A series of 2-phenyl phenanthroimidazole dimers was synthesized by means of a two-phase preparative method, which allowed their isolation in pure form. Definitive proof of structure was obtained by means of 1H and 13C NMR spectroscopy; five of the six possible structural isomers of the dimer could thus be eliminated. The dimers exhibited thermochromic and piezochromic behavior, but not photochromic. The colored species were shown to be free radicals formed on dissociation of the dimers; this was confirmed through ESR measurements. The electronic absorption spectra of the radicals were influenced by substituents on the phenyl ring. The UV-VIS measurements allowed the evaluation of the dimer-radical equilibrium constants, which were also found to be dependent on substituents on the phenyl ring.Key words: phenanthroimidazole dimers, free radical equilibrium, piezochromism, thermochromism, photochromism.
Mechanisms for the oxidation of secondary alcohols by dioxoruthenium(VI) complexes
Authors: Wang Zhao , Chandler W David , Lee Donald G
Published: April 2011

Possible mechanisms for the oxidation of alcohols by dioxoruthenium(VI) complexes are critically evaluated. Rate constants for the reduction of trans-[(TMC)RuVI(O)2]++ (TMC = 1,4,8,11-tetramethyl-1,4,8,11-tetraazacyclotetradecane) by substituted benzhydrols are correlated more satisfactorily with Hammett σ substituent constants ( rho = -1.44 ± 0.08, r2 = 0.98) than with σ + substituent constants ( rho = -0.72 ± 0.11, r2 = 0.83). Similar observations for the oxidation of substituted benzyl alcohols have recently been reported, confirming that the transition state for these reactions is not carbocation-like. Primary deuterium isotope effects indicate that cleavage of the α -C-H bond is rate-limiting. The lack of an observable O-D isotope effect and the ease of oxidation of ethers indicates that the presence of a hydroxyl is not essential. The previously reported observation that cyclobutanol is quantitatively converted into cyclobutanone by dioxoruthenium(VI) complexes eliminates free-radical intermediates from consideration as part of the mechanism, and negative entroπes of activation (-Δ Sdouble dagger = 96-137 J mol-1 K-1) suggest a structured transition state. Only two of eight possible reaction mechanisms considered were found to be consistent with the available data. A critical analysis of the available data indicates that a 2 + 2 (C-H + Ru font 35137 roman T O) addition and a reaction initiated by ligand formation through the interaction of the reductant's HOMO with the oxidant's LUMO are the most likely reaction mechanisms.Key words: oxidation, alcohols, ruthenium(VI), mechanism, substituent effects.
Ion channels based on bis-macrocyclic bolaamphiphiles: effects of hydrophobic substitutions
Authors: Fyles T M , Loock D , Zhou X
Published: April 2011

Four new bis-macrocyclic bolaamphiphiles were prepared to explore the effects of hydrophobic substitutions on ion transport. In bilayer vesicles the new compounds were remarkably similar to more hydrophilic derivatives prepared previously. Planar bilayer conductance experiments showed the new compounds induced an unique current-time signal consisting of a rapid rise time, followed by a slower decay time. Signal shape was cation dependent and was related to a modest selectivity between cations. Cation-anion selectivity was very high, approaching an ideal cation selectivity. One compound also showed voltage dependence of the signal shape and duration. Qualitative changes in signal shape, duration, and voltage dependence were provoked by variation in the electrolyte pH and by masking the head-group electrostatic interactions with low levels of barium ions. A model for the signal shape is proposed, involving a rapid current rise due to aggregate restructuring, followed by slower decay due to development of the local Donnan potential that results from the high cation-anion selectivity.Key words: ion channel, synthesis, bilayer membrane, bilayer clamp, mechanism.
Effect of the hydrogen bond network in carbonic anhydrase II zinc binding site. A theoretical study
Authors: Álvarez-Santos Silvia , González-Lafont Àngels , Lluch José M
Published: April 2011

The hydrogen bond network influence on the carbonic anhydrase II (CAII) zinc binding site has been studied theoretically by using the semiempirical AM1 method. To this aim, quantum mechanical reduced models of wild-type CAII and several CAII variants have been constructed. We have shown that, when a direct metal ligand donates a hydrogen bond to an indirect metal ligand, the first-shell residues enhance their electrostatic interaction with the zinc cation. Thus, the hydrogen-bond network is able to modulate the zinc binding affinity and the zinc-water pKa.Key words: hydrogen bond network, carbonic anhydrase II, Zn2+ metalloenzyme ligands.
Conformational isomers of 1,2,5,6-tetrathiocins and the photoisomerization of a 1,2,5,6-tetrathiocin into a 1,2,3,6-tetrathiocin: X-ray structures of (C6X4S2)2 (X = F, Cl) and C6F4SSSC6F4S
Authors: Chivers Tristram , Parvez Masood , Vargas-Baca Ignacio , Schatte Gabriele
Published: April 2011

The 1,2,5,6-tetrathiocins (C6X4S2)2 (3a, X = F; 3b, X = Cl) are obtained in high yields by the oxidation of the dithiols 1,2-C6X4(SH)2 (X = F, Cl) with I2 or SO2Cl2, respectively. In the solid state 3a has the C2h (chair) conformation and crystallizes in two different phases: alpha-(C6F4S2)2, monoclinic, P21/a, a = 9.351(2), b = 6.465(2), and c = 11.546(2) Å, ß = 95.60(1)°, V = 694.6(2) Å3, Z = 2; and ß-(C6F4S2)2, monoclinic, P21/c, a = 4.825(2), b = 11.302(2), and c = 12.453(2) Å, ß = 91.45(3)°, V = 678.8(3) Å3, Z = 2. By contrast, 3b displays a D2 (twist-boat) structure and crystallizes in the C2/c space group with a = 15.243(3), b = 8.703(2), and c = 27.010(14) Å, ß = 92.81(4)°, V = 3578(1) Å3, and Z = 8. The derivative 3a exists as an equilibrium mixture of two conformational isomers in toluene solution. The VT 19F NMR data afford the thermodynamic parameters H° = 9.9 ± 0.4 kJ mol-1 and S° = 14 ± 1 J K-1 mol-1. Density functional theory calculations for 3a indicate that the D2 conformation is lower in energy than the C2h isomer by 4.6 kJ mol-1. The photolysis of 3a in benzene promotes a transannular sulfur migration to give the 1,2,3,6-tetrathiocin, C6F4SSSC6F4S (6), which exists in a chair conformation with respect to antipodal sulfur atoms. Crystal structure of 6: orthorhombic, space group Pnma, a = 8.652(6), b = 19.084(4), and c = 8.301(6) Å, V = 1370.6(14) Å3, and Z = 4. The isomers 3a and 6 were also characterized by EI mass spectrometry, 19F NMR, FTIR, and Raman spectroscopies.Key words: tetrathiocins, conformational isomers, photoisomerization.
Crystal structure of the spirophosphorane (OCMe2C(O)O)2PH
Authors: Montgomery Craig D , Rettig Steven J , Shurmer Bryn
Published: April 2011

The crystal structure of the spirobicyclic phosphorane (OCMe2C(O)O)2PH, 1, has been determined. Crystals of C8H13O6P, 1, are orthorhombic, a = 10.515(2), b = 10.623(2), c = 20.552(2) Å, Z = 8, space group Pca21. The structure was solved by direct methods and refined by full-matrix least-squares procedures to R = 0.037 (Rw = 0.033) for 1616 reflections with I > 3sigma(I). The structure consists of two independent molecules each displaying a distorted trigonal bipyramidal geometry; the distortion follows closely the Berry pseudorotation coordinate.Key words: crystal structure, phosphorane, Berry pseudorotation, trigonal bipyramid.
Article
Authors: Loss Troy AE , Rogers Cerrie W , Wolf Michael O
Published: April 2011

Composite thin films of poly(p-phenylenevinylene) and the polyelectrolyte Nafion have been prepared by the polymerization of 1,1prime-[(p-phenylene)bis(methylene)bis[tetrahydrothiophenium] dichloride electrostatically bound to Nafion. The composite films are strongly fluorescent. The absorption and photoluminescence spectra of the films have been measured and are similar to those of poly(p-phenylenevinylene). X-ray microanalysis and weight gain experiments performed on the films suggest that the dicationic monomer occupies less than 15% of the cation exchanging sites in the Nafion, leaving sites available for binding of additional small cations.Key words: thin films, conducting polymers, poly(p-phenylenevinylene), polyelectrolyte, photoluminescence.
Importance de la présence de magnésie dans le stockage de la chaux: carbonatation de l'oxyde et de l'hydroxyde de magnésium
Authors: Dheilly Rose-Marie , Sebaibi Yahya , Tudo Joseph , Queneudec Michèle
Published: April 2011

The influence of relative air moisture, temperature, and concentration of carbon dioxide on the carbonation of magnesium oxide and hydroxide have been studied in normal climatic conditions. The MgO and Mg(OH)2 used for this original work are pure granular materials. The different species that appear during the process of carbonation and the most stable form in usual conditions of storage were identified. Moreover, the study of the different climatic factors showed that the carbonation is never complete even after a prolonged exposure.Key words: carbonation, magnesium oxide, magnesium hydroxide, storage, climatic conditions.
Article
Authors: Myland Jan C , Oldham Keith B , Schiewe Jörg , Taylor Alicia L
Published: April 2011

Three methods are discussed by which the internal geometry of a porous insulator may be investigated electrochemically. By adopting a simple model, the pore geometry is characterized by two parameters: the tortuosity and the porosity. In each method, a cylindrical sample of the porous medium is soaked with an aqueous electrolyte solution and its lower face is brought into contact with a mercury pool electrode. The other electrode is also a mercury pool, but in the first technique it is remote, whereas in the second and third methods it floods the upper surface of the cylindrical sample. The first technique employs a d.c. potential step, whereby the concentration of a probe ion is depleted at the lower electrode. The attempt of diffusion to replenish this surface concentration leads to a faradaic current whose time dependence is analyzed to provide estimates of the tortuosity and porosity parameters. The second method measures the frequency-dependent cell impedance. The pore structure parameters are estimated from the equivalent circuit of the cell. No faradaic process is involved in the third method, which measures the frequency-dependent conductance of the cell as a means of characterizing the pore structure. These methods are applied successfully to sandstone and fritted glass samples.Key words: porous media, Fourier-transform methods, impedance.
Article
Authors: Renzo M Di , Feninat F El , Jiménez-Esquivel B , Poulin S , Yavari M , Xu J , Ellis T H , Kruus E , Sacher E , Stangel I
Published: April 2011

Human dentin consists of protein and mineral phases in the form of a collagen network reinforced with apatite. The bonding of materials to dentin, an important goal of dental materials research, can be achieved by the reaction of adhesion promoters with either of these phases. Our group is concerned with such bonding, and has used surface-sensitive instrumentation to elucidate the nature of a variety of surface treatments of dentin. Here, we discuss some of our findings, with particular emphasis on the kinetics of surface demineralization and deproteination, the reaction of formaldehyde with collagen, and the reaction of organophosphate with dentin apatite. The ramifications of our findings will be examined.Key words: dentin, adhesion, vibrational spectroscopy, atomic force microscopy.
A solid state 31P NMR study of the synthesis of phosphorus sulfides from PCl3 and H2S in microporous materials
Authors: Lee Garry SH , Ratcliffe Christopher I , Ripmeester John A
Published: April 2011

The interaction and reaction of PCl3 and PCl3-H2S mixtures with the microporous materials Silicalite, ALPO-5, NaY, NaX, and NaA have been investigated, with the intention of producing phosphorus sulfide clusters in the pores. A rich chemistry was observed and monitored by solid state 31P NMR. The presence of P4S3, α-P4S5, β-P4S6, and P4S7 inside the NaY α-cages was demonstrated, as well as a new species that is possibly a third geometric isomer of P4S4 with C3v symmetry. 129Xe NMR showed the exclusion of Xe from the micropores by the phosphorus sulfides. Sulfides lower than P4S7 are small enough that they undergo rapid pseudo-isotropic reorientation inside the NaY α-cage. Key words: phosphorus sulfides, intrazeolitic cluster synthesis.
The first reagent-controlled asymmetric halolactonizations. Dihydroquinidine-halogen complexes as chiral sources of positive halogen ion
Authors: Grossman Robert B , Trupp Robert J
Published: April 2011

The first reagent-controlled asymmetric halolactonizations are described. Complexes of I+ with O-acyl- and O-aryldihydroquinidines are used to promote the asymmetric halolactonization of prochiral θ, δ-unsaturated carboxylic acids with low but measurable and reproducible enantioselectivity. Experimental factors affecting the ee's are described. Key words: halocyclization, halogenation, halonium ions, chiral, stereoselective.
FTIR analysis of the phase content in low-density polyethylene
Authors: Bernazzani P , Bich V T , Phuong-Nguyen H , Haine A , Chapados C , Dao Lê H , Delmas G
Published: April 2011

The phase content of a low-density polyethylene was studied by analysis of the CH2 rocking vibrations in non-oriented films prepared from the press (P-films) or from solution (S-films). Spectral simulations of the transmission spectra give the mass fractions of the orthorhombic phase αortho and of two noncrystalline phases (monoclinic-like and amorphous). The values of αortho (IR) are compared to αortho (i) where (i) stands for the X-ray diffraction, density, and DSC techniques. New results are obtained concerning the orthorhombic order and the change of phase content with aging. A two-phase analysis is justified for non-aged films containing a small amount of the monoclinic-like phase. The values of αortho (IR) are larger than αortho (i), the difference ranging between 0.12 and 0.43. The difference is a measure of the short-range order. αortho (IR) can reach 0.73 for the S-films. The stability of the short-range order phase is investigated. The sample is also analyzed using the trace of slow calorimetry. The difference between αortho by DSC and by slow calorimetry is a measure of strainable order. During aging, the variation in the phase content is large for the noncrystalline phases (in content and frequency) and small for the orthorhombic. The increase of the monoclinic-like phase during aging suggests that it is a precursor of the more stable orthorhombic organization. The quantification of two noncrystalline phases on fresh and aged films clarifies some ambiguity found in the literature about the monoclinic-like phase and the localization of bands in the rocking region for sample characterization. Analysis of other regions of the spectrum is needed to confirm the present results. Key words: low-density PE, phase content, FTIR, network phase, slow calorimetry.
Article
Authors: Halls Mathew D , Aroca Ricardo
Published: April 2011

The vibrational spectra of one of the most commonly used light-emitting materials for organic electroluminescence (EL) devices, tris(8-hydroxyquinoline)aluminum (Alq3), is presented. The geometry and normal modes of fac-Alq3 isomer in the ground state have been computed for the first time using ab initio methods (HF/6-31G*) to aid in discussion of normal-mode assignment. Normal inelastic scattering excited at 633 nm and low-temperature FT-Raman in addition to the infrared spectra of this material are reported. The vibrational characterization of thin solid films of the material was attempted using transmission and reflection-absorption FT-IR spectroscopy (RAIRS) on smooth silver films.Key words: tris(8-hydroxyquinoline)aluminum, Alq3, ab initio, assignments, infrared, Raman, thin films.