bluuebunny/crossref_metadata_2025 huggingface dataset:
Specific interactions between amino acid side chains — a partial molar volume study
Authors: Rao Mandapati V. R. , Atreyi Mahalakshmi , Rajeswari Moganty R.
Published: May 2006

Partial molar volumes of the following mixtures, [Formula: see text](mix) were determined in water at 20 °C: (1) glutamic acid: glycine; (2) arginine:glycine; (3) arginine:glutamic acid; (4) arginine: aspartic acid; (5) lysine:glutamic acid, and (6) lysine: aspartic acid. Partial molar volumes of the mixtures calculated using [Formula: see text] of various ionic species of the amino acids agreed with the experimental values for systems 1 and 2, but were less than the experimental values for systems 3 to 6. The positive deviations, [Formula: see text] (mix), of the experimental values from the calculated values are discussed in terms of "specific interactions" between ionogenic side chains. The extent of interaction is greater in systems containing aspartic acid and/or arginine.
Simulations of O2 adsorbed on graphite at 45 K: the monolayer to bilayer transition
Authors: Bhethanabotla Venkat , Steele William
Published: May 2006

Molecular dynamics computer simulations are reported for O2 physisorbed on the graphite basal plane at 45 K. Surface coverages ranged from the low density monolayer through the compressed monolayer up to roughly two layers. Thermodynamic properties, orientational distribution functions, and two-dimensional structure factors are presented. The time-dependent translational motion is characterized both by computer-generated trajectory plots and by evaluation of the mean square in-plane displacements as a function of time. It is concluded that the O2 orientations shift from predominantly surface-parallel to surface-perpendicular as the first layer density increases, in agreement with deductions from experiment. The structure factors indicate that some translational ordering is present in the fluid phases and that the bilayer is a composite made up of a solid monolayer plus a fluid overlayer; these observations are in agreement with recent experimental studies of this system.
Intermolecular hydrogen tunneling in solids. Comparison between diabatic and adiabatic rate expressions
Authors: Pacey Philip D. , Siebrand Willem
Published: May 2006

Rate expressions are derived for hydrogen transfer between two molecules in a solid, typical examples being hydrogen abstraction by methyl radicals in solid methane and in glassy methanol. These expressions are based on two-dimensional potential-energy surfaces describing the motion of the hydrogen atom along with that of the atoms between which it is transferred. A diabatic rate expression, based on the Golden Rule, is compared with an adiabatic rate expression, based on transition-state theory with a tunneling correction. In both cases, the two degrees of freedom must be treated quantum-mechanically rather than classically. For the adiabatic case, this leads to a new expression in which the barrier permeability is averaged over the wavefunction of the slow motion. The result differs from the Golden-Rule expression but yields similar rate constants. Numerical estimates are presented to illustrate the temperature and isotope dependence of these rate constants. The concept of a tunneling path is shown to break down at low temperature, so that the conventional one-dimensional tunneling approach becomes invalid.
Energies of the phases of ice at low temperature and pressure relative to ice Ih
Authors: Handa Y. Paul , Klug D. D. , Whalley Edward
Published: May 2006

The enthalpy of transformation to ice Ic of ice II, IX, V, VI, and VIII that have been recovered at 77 K and ambient pressure, and of the transformation of ice Ic to Ih, has been measured in a heat-flow calorimeter. The enthalpy of transformation of ice Ic to Ih depends on the high-pressure phase used to make the ice Ic. The thermodynamics of the transformation of ice IX, which was made by cooling ice III at 0.3–0.7 K s−1, to ice III shows that the ice IX was fully orientationally ordered to the precision of the experiments.
An efficient algorithm for estimating dimensionalities
Authors: Somorjai R. L. , Ali M. K.
Published: May 2006

We present an algorithm for estimating dimensionality. It is based on a multivariate k nearest neighbor (k-NN) density estimator and shows explicitly that the dependence of k-NN distances on k does not follow pure power law behavior. We achieve very high efficiency (≤ 2000 points for up to 20 dimensions) by abandoning the distribution free feature of our algorithm; it is calibrated for two distributions and tested on a number of strange attractors, the discretized Mackay–Glass system, and trajectories of 3, 10, and 20 coupled harmonic oscillators. The danger of embedding in higher dimensional spaces is demonstrated.
Mechanism of electrodeposition of cadmium selenide from solution
Authors: Szabo Jeffrey P. , Cocivera Michael
Published: May 2006

The mechanism of the electrodeposition of CdSe from selenosulfite and cadmium–NTA complex has been studied by analysis of the Tafel regions of linear sweep voltammograms. The Tafel plots were linear for up to 20% of the peak height, and were used to calculate the dependence of the reduction rate on the concentration of each of the reactants. The electron transfer process was found to be first order with respect to [Formula: see text] and zero order with respect to each of the other reactants. These results are consistent with a two-step reduction of [Formula: see text]to Se2−, followed by precipitation of CdSe. The reduction of Cd(NTA)− to cadmium metal appears to be blocked or inhibited in the presence of [Formula: see text], resulting in CdSe films of nearly perfect stoichiometry to be deposited.
The structure, stereochemistry, and biosynthetic origin of a diterpenoid fungal metabolite, traversianal, established by 1H, 2H, and 13C magnetic resonance
Authors: Stoessl Albert , Rock G. L. , Stothers J. B. , Zimmer R. C.
Published: May 2006

The structure of traversianal, a tricyclic diterpenoid fungal metabolite of Cercospora traversiana, has been elucidated through detailed 1H and 13C magnetic resonance studies, including homo- and heteronuclear correlation spectra of the natural product and examination of 13C-labelled material obtained by [1,2-13C2]acetate incorporation experiments. Its stereochemistry was established from a series of nuclear Overhauser effect difference spectra. The tricyclic carbon skeleton of traversianal is that of the fusicoccin/cotylenin and ophiobolane terpenes although the oxygenation pattern closely resembles the latter. Incorporation experiments with [2,2,2-2H3, 1-13C1]acetate revealed that traversianal arises by a sequence that differs substantially from that established for the fusicoccanes but rather resembles that previously shown for the ophiobolanes, in the retention of hydride at C-2, -10, and -14. However, the opposite configuration of the methyl group at C-3 suggests that the route to traversianal involves a terminal trans-geranylgeranyl unit instead of the cis unit implicated in ophiobolin generation.
Diffusion in associating nonelectrolyte mixtures: stepwise aggregation and micelle formation
Authors: Leaist Derek G.
Published: May 2006

Equations have been developed to predict diffusion coefficients and Onsager coefficients for associating nonelectrolyte solutes in binary or multicomponent solutions with any number of association equilibria. The equations are used to interpret previously reported data for binary diffusion with stepwise association of ethanol and N-methylacetamide in carbon tetrachloride solutions. Diffusion of Triton X-100, a non-ionic micelle-forming surfactant, is also analyzed. In contrast to the gradual decrease in the diffusivity caused by stepwise association, formation of micellar aggregates produces a sharp drop in the diffusivity at the critical micelle concentration. The relation between interdiffusion coefficients and intradiffusion coefficients for associating nonelectrolyte solutes is discussed.
A single-crystal electron spin resonance study of the cyclic 1,2,4,3,5-trithiadiazolyl radical, S3N2+•
Authors: Preston K. F. , Charland J.-P. , Sutcliffe L. H.
Published: May 2006

An anisotropic ESR signal detected in single crystals of 5-methyl-1,3,2,4-dithiazolylium hexafluoroarsenate [Formula: see text] is shown to be that of 1,2,4,3,5-trithiadiazolyl, S3N2+•. The g- and 14N-hyperfine tensors for this radical were established in the monoclinic abc* crystal-axis system from spectral measurements at room temperature on aligned single crystals. Agreement of the principal values of the tensors with those obtained in our earlier studies by simulation of powder spectra is excellent. The g-tensor (2.0021, 2.0072, 2.0248) and the two 14N-tensors (25.4, 0.8, 0.8 MHz) are collinear within experimental error and lie with minimum g and maximum hyperfine perpendicular to the plane of the host cation. The electronic structure of the radical deduced from our earlier studies (2A2 in C2V) is confirmed, and the impurity is shown to substitute for the host cation in the anticipated manner.
Phenyl group as a probe in the 1H nuclear magnetic resonance study of the conformations of quaternary heteroaromatic carbapenem antibiotics. Identification of the site of quaternization in -SCH2- substituted nitrogen-containing heteroaromatic derivatives
Authors: Rao Vanga S. , Daris Jean-Paul , Menard Marcel
Published: May 2006

The phenyl group is shown as a useful probe in sensing the environment in the 1H nuclear magnetic resonance conformational study of a variety of quaternary heteroaromatic carbapenem derivatives. The influence of the -SCH2- substituent in determining the site of quaternization in nitrogen-containing heteroaromatic compounds is demonstrated through the acidity measurements of these methylene protons.
The photochemistry of electron donor–acceptor (EDA) complexes in micellar solutions. I. The stilbene–methylviologen system
Authors: Hamity M. , Lema R. H.
Published: May 2006

The association constants for the electron donor–acceptor (EDA) complexes formed between both cis-stilbene (cS) and trans-stilbene (tS) as donors and methylviologen (MV+2) as acceptor were determined in ethanol and micellar sodium dodecyl sulfate (SDS) solutions in the range of SDS concentration 0.05–0.1 M. The values obtained in micellar solutions were much higher than those in ethanol and were heavily dependent upon SDS concentration. This effect is due to an increase in the local concentration of the reactants in the micellar pseudophase. The tS fluorescence quenching by MV+2 was also studied in the same solvent media. In ethanol, the Stern–Volmer plot was found to be linear, with quenching constant (KSV) similar to the association constant determined by the absorption method. In micellar solutions, although upward curvature of the Stern–Volmer plots was observed, a reaction scheme based on static quenching via ground state EDA complex is proposed, which explains the experimental results. Irradiation in the absorption band of the EDA complexes formed by tS or cS and MV+2 was carried out in ethanol and SDS solutions, in the absence of oxygen. Only cis–trans isomerization of cS in SDS solution was observed, with a quantum yield value of Φcis = 0.012.
The reactions of simple dimethylallylamines with dimethyl acetylenedicarboxylate. Formation of 1-dimethylamino-2-allylmaleates via formal allyl transfer
Authors: Schwan Adrian L. , Warkentin John
Published: May 2006

Tertiary amines bearing two methyl groups and an allylic substituent (X) react with dimethyl acetylenedicarboxylate (DMAD) to afford the corresponding 1-dimethylamino-2-X′ maleates, in which X′ is the allylic isomer of X. The mechanism postulated involves reversible formation of a zwitterion by attack of the amine at an sp-carbon of DMAD. The zwitterion then undergoes intramolecular allyl transfer, through a 6-membered transition state. Evidence for a zwitterionic intermediate (quaternary ammonium allenolate) includes capture of the allenolate centre by intramolecular addition to a carbonyl group and by proton transfer from chloroform.
MP2 calculation of nuclear magnetic shielding constants involving gauge-invariant bases
Authors: Vauthier Edouard C. , Comeau Michel , Odiot Simone , Fliszar Sandor
Published: May 2006

Dia- and paramagnetic shielding constants of HF, NH3, N2, H2O, and CO were calculated in the 4-31G, 4-31G*, and 6-31G** bases, using gauge-invariant atomic orbitals. Correlation effects were evaluated by 2nd-order perturbation theory: while the diamagnetic contributions are virtually unaffected by correlation, the paramagnetic parts (σρ) are modified by ~10 ppm in the 6-31G** basis. With or without correlation, calculated σρ values differ significantly from results obtained from measured spin-rotation constants. However, improvements in the basis set descriptions and inclusion of correlation yield chemical shift values for 15N and 17O converging toward their experimental counterparts obtained from conventional nuclear magnetic resonance spectroscopy.
Structural studies of organoboron compounds. XXXI. (3-Benzoyl-(+)-camphorato)diphenylboron
Authors: Cullen William R. , Rettig Steven J. , Trotter James , Wickenheiser Eugene B.
Published: May 2006

Details of the synthesis and physical properties of the title compound are reported along with a new preparation of 3-benzoyl-(+)-camphor. Crystals of (3-benzoyl-(+)-camphorato)diphenylboron are triclinic, a = 9.0362(3), b = 9.4679(3), c = 14.2731(7) Å, α = 101.920(3), β = 100.086(3), γ = 93.251(3)°, Z = 2, space group P1. The structure was solved by non-routine application of direct methods and was refined by full-matrix least-squares procedures to R = 0.036 and Rw = 0.039 for 4094 reflections with I ≥ 3σ(I). The unit-cell contains two crystallographically independent molecules, related to one another in the crystal lattice by a pseudo-inversion centre, and having the same configuration but different conformations. The structure analysis shows that the chiral, anionic, 1,3-diketonate ligand derived from 3-benzoyl-(+)-camphor is not symmetrically delocalized like most related ligands, resulting in significantly different O—B bond lengths. Bond lengths (corrected for libration) include: O—B = 1.514(4), 1.522(4); 1.568(4) and 1.567(4) Å; C—B = 1.594(5)−1.616(5) Å.
Ab initio calculations on tetramethoxymethane
Authors: McEachern R. J. , Weil J. A. , Mezey P. G.
Published: May 2006

Minimal basis set ab initio SCF-MO calculations were performed on the 21-atom system of tetramethoxymethane (tetramethyl orthocarbonate). The geometric configuration of this model was optimized in two conformations, one having quasi-S4 symmetry and the other D2d symmetry. The S4 conformation was found to be 8 kJ mol−1 lower in energy than the D2d conformation, at the STO-3G level. The calculated energy difference is consistent with the recently measured geometric configuration of crystalline tetrabenzyl orthocarbonate. The calculated values of the bond lengths and angles were compared to the results of an electron diffraction study of the methyl species, and agree well with experiment. The theoretical electric dipole moment was calculated to be 0.01 D.
Catalytic decarbonylation of aldehydes using iron(II) porphyrin complexes, and the crystal structure of (5,10,15,20-tetraphenylporphinato)bis(tri-n-butylphosphine)iron(II)
Authors: Belani Ramesh M. , James Brian R. , Dolphin David , Rettig Steven J.
Published: May 2006

The complex (5, 10, 15, 20-tetraphenylporphinato)bis(tri-n-butylphosphine)iron(II), 1, has been isolated in a triclinic crystal form, a = 12.499(3), b = 12.528(2), c = 12.039(2) Å, α = 116.39(1), β = 109.79(1), γ = 98.13(1)°, Z = 1, space group [Formula: see text]. The structure was solved by conventional heavy atom methods and was refined by full-matrix least-squares procedures to R = 0.060 and Rw = 0.070 for 3551 reflections with I ≥ 3σ(I). The molecule, which has crystallographic [Formula: see text], symmetry, displays a relatively undistorted pseudo-octahedral coordination geometry with Fe—P = 2.346(1) and Fe—N = 1.998(3) and 1.993(3) Å. In CH2Cl2 solution, 1 reacts with CO and aldehydes to generate the carbonyl(phosphine) derivative, and decarbonylation of phenylacetaldehyde to toluene becomes catalytic under an Ar stream. The aldehyde decarbonylations involve radical pathways via carbonyl loss from [Formula: see text]. The hydrogen abstraction/initiation reaction probably utilizes trace O2 (and possibly trace (water); speculative mechanisms are discussed.
The direct electrochemical synthesis of thiolato complexes of cobalt and nickel, and the crystal structure of bis(phenylthiolato)bis(1,10-phenanthroline)cobalt(III) perchlorate
Authors: Chadha Raj K. , Kumar Rajesh , Lopez-Grado Jaime Romero , Tuck Dennis G.
Published: May 2006

Cobalt(II) and nickel(II) thiolates, M(SR)2, can be prepared in high yield by the electrochemical oxidation of a metal anode in an acetonitrile or acetone solution of RSH (R = C6H5, o-CH3C6H4, 2-C10H7, 2,3,4,5-C6F4H; not all combinations). When 2,2-bipyridine or 1,10-phenanthroline (=L) is added to the electrolyte phase, the products are the adducts M(SR)2L2. In the case of Co(SC6H5)2(phen)2, aerial oxidation leads to the formation of the cobalt(III) cation [Co(SC6H5)2(phen)2]+, isolated as the perchlorate salt. X-ray crystallographic analysis showed that this cation has a cis-CoS2(N2)2 kernel.
Lithum ion complexation effects on stereochemical dynamics of 14-crown-4 ethers in solution as studied by 13C nuclear magnetic resonance
Authors: Buchanan G. W. , Kirby R. A. , Bourque K. , Morat C.
Published: May 2006

The barrier to ring inversion for the cis–syn–cis isomer of dicyclohexano-14-crown-4 has been determined via 13C nuclear magnetic resonance coalescence temperature measurements to be 42.3 ± 0.8 kJ/mol. For the LiNCS complex of this isomer, the inversion barrier increases by ca. 5 kJ/mol. For the Li+ complex of the cis–anti–cis isomer, evidence for two dynamic conformational processes in solution is presented, with Li complexation inducing an increase in the barrier to degenerate cyclohexane ring inversion similar to that found in the cis–syn–cis isomer. 13C T1 measurements indicate a substantial decrease in the mobilities of both isomers in the presence of Li+ ions in solution.
Air-stable binuclear alkoxytungsten complexes as catalyst precursors for olefin metathesis
Authors: Savard Jacques , Alper Howard
Published: May 2006

The use of the air-stable complexes tetrachlorobis(μ-methoxy)bis(methanolo)dimethoxyditungsten (1) and tetrachlorobis(μ-ethoxy) tetraethoxyditungsten (2) as catalyst precursors for the metathesis of 1-octene is described. Using mild conditions, the latter was converted to 7-tetradecene (4) in fair yields when diethylaluminum chloride (3) was used as a cocatalyst. Product stereoselectivity was 4/1 in favor of the trans isomer. The reaction was also shown to be efficient for the production of 2-nonenes.
The characterization of primary, secondary, and tertiary vanadate alkyl esters by 51V nuclear magnetic resonance spectroscopy
Authors: Tracey Alan S. , Gresser Michael J.
Published: May 2006

A variety of alkyl vanadates has been studied by 51V nuclear magnetic resonance spectroscopy. It was found that the equilibrium constant for condensation of vanadate with alcohols is insensitive to whether the hydroxyl group is primary, secondary, or tertiary. These products, however, have characteristic vanadium chemical shifts that allow assignment of nmr signals to the appropriate ester. It was also found that chemical shifts are additive in the sense that the chemical shifts of the esters ROVO3H− are one half the chemical shift of the diesters (RO)2VO2− when those shifts are given relative to −559 ppm. This effect is independent of whether the signals are to high or low field of −559 ppm and the additivity extends to mixed ligand systems. This value of −559 ppm is close but not equal to the chemical shift of the vanadate monoanion, H2VO41−, which is at −561 ppm. These results are at variance with arguments concerning the effects of ligand bulkiness on chemical shifts of vanadium(V) complexes.
A reinvestigation of the interaction between triplet states of cyclohexenones and amines
Authors: Weir D. , Scaiano J. C. , Schuster D. I.
Published: May 2006

Laser flash photolysis studies lead to the conclusion that the short-lived triplet states of cyclohexenones are readily quenched by amines. For example, in the case of 2-cyclohexen-1-one (1) its triplet state (τT = 40 ns in acetonitrile) is quenched by triethylamine with a rate constant of (9.0 ± 0.8) × 107 M−1 s−1. Cyclohexenone triplets are also quenched efficiently by DABCO and by triphenylamine leading to the formation of the corresponding amine radical cations. The new evidence reported rules out the involvement of long-lived detectable exciplexes.
Transfer of divalent metal ions into the organic phase of the systems water–chloroform–ZnCl2–CdCl2–HgCl2–NaOH–(NaCl)–n-alkyltri(oxyethylene)carboxylic acid
Authors: Strzelbicki Jerzy , Charewicz Witold , Beger Jorg , Hinz Lutz
Published: May 2006

The influence of the length of an n-alkyl chain incorporated into the molecule of an n-alkyltri(oxyethylene) carboxylic acid of a general formula CnH(2n+1)(OCH2CH2)3OCH2CO2H(n = 6, 8, 10, and 12, respectively) on transfer of the zinc group element ions into chloroform was investigated. Metal ions were transferred from the aqueous solutions of ZnCl2, CdCl2, and HgCl2 at initial concentrations of 0.835 mM and 1.67 mM with, and without, added NaCl. From the aqueous solutions to which no NaCl was added, the selectivity of transfer of divalent metal ions into the organic phase was low.The selectivity of transfer of the divalent metal ions improved remarkably when NaCl was added to the aqueous phase due to the chemical equilibria involving complexation of the divalent metal ions by Cl−, OH−, and n-alkyltri(oxyethylene) carboxylic acid. In most systems (with NaCl added), Zn(II) was separated from Cd(II) and Hg(II). Less lipophilic n-alkyltri(oxyethylene) carboxylic acids (with a shorter n-alkyl chain incorporated) exhibited higher selectivity and efficiency as divalent metal ion carriers.
Substituent effects on the rate of carbene formation in the pyrolysis of indenyl diazo compounds
Authors: Kapur S. , Mathur N. C. , McManus K. , Pincock A. L. , Pincock J. A.
Published: May 2006

Rate constants for the pyrolysis of the indenyl diazo compounds, 5a–5e, in methanol have been determined and correlated with σ+, giving a ρ+ = −0.76. The product of this pyrolysis reaction is the methyl ether resulting from trapping at the carbenic centre. The mechanism proposed for this process involves rate-determining unimolecular loss of nitrogen by the allowed nonlinear pathway to form the singlet σ2 carbene. Evidence presented implies that this singlet state is more stable than the p2 state despite the difference in aromatic stabilization of the two.
A study of thermodynamic acidities of carbon acids by AM1
Authors: Werstiuk Nick Henry
Published: May 2006

Monocyclic carbon acids 1–8 have been studied by the semiempirical computational method AM1 in an attempt to develop a methodology for estimating pKs. The results indicate that there is a good correlation between [ΔHf(R−) − ΔHf(R-H)] obtained computationally and the pKs measured in DMSO. The pKs of 9a, 9b, 10, 11 (tetronic acid), and the dioxolane dione 12 are predicted on the basis of this correlation.
Nature of the oil/water interface and equilibrium surfactant aggregates in systems exhibiting low tensions
Authors: Aveyard Robert , Binks Bernard P. , Lawless Thomas A. , Mead Jeremy
Published: May 2006

Oil/water interfacial tensions are reported for systems containing pure alkane, aqueous sodium chloride, and a pure anionic surfactant, either Aerosol OT or p-dihexylbenzene sodium sulphonate (DHBS). Evidence is produced to support the claim that monolayer adsorption at the oil/water interface can produce ultralow tensions (~ 1 µN m−1), and that the presence at the interface of a third, surfactant-rich phase is not necessary. The aggregation of DHBS and its distribution between oil and aqueous phases of various salinities have been investigated. It has been confirmed that the behaviour of DHBS in these respects is similar to that of Aerosol OT, as might be expected from its molecular structure. The sizes of microemulsion droplets in equilibrium with planar adsorbed monolayers have been determined, and related to the tensions of the plane oil/aqueous phase interfaces using simple existing theory.
Isotope effects in nucleophilic substitution reactions. VII. The effect of ion pairing on the substituent effects on SN2 transition state structure
Authors: Lai Zhu-Gen , Westaway Kenneth Charles
Published: May 2006

The secondary α-deuterium kinetic isotope effects and substituent effect found in the SN2 reactions between a series of para-substituted sodium thiophenoxides and benzyldimethylphenylammonium ion are significantly larger when the reacting nucleophile is a free ion than when it is a solvent-separated ion pair complex. Tighter transition states are found when a poorer nucleophile is used in both the free ion and ion pair reactions. Also, the transition states for all but one substituent are tighter for the reactions with the solvent-separated ion pair complex than with the free ion. Hammett ρ values found by changing the substituent on the nucleophile do not appear to be useful for determining the length of the sulphur–α-carbon bond in the ion pair and free ion transition states. Keywords: Isotope effects, ion pairing, nucleophilic substitution, SN2 reactions, transition states.
Nucleophilic addition and substitution reactions between the cyanide ion and cyclopentadienyliron complexes of chlorobenzenes. Syntheses of benzonitriles and phthalonitriles
Authors: Sutherland Ronald G. , Zhang Chun-Hao , Piórko Adam , Lee Choi Chuck
Published: May 2006

Reaction of cyclopentadienyliron (CpFe) complexes of chlorobenzenes with NaCN in DMF could give rise to products resulting from the addition of the cyanide ion or from substitution and addition reactions with the cyanide ion. Demetallation–oxidation of such products by treatment with DDQ would lead to the synthesis of benzonitriles and phthalonitriles. With the CpFe complex of chlorobenzene, reaction with NaCN in DMF for 3 min, 30 min, or 3 h gave rise to an approximately 90:10 mixture of (1–5-η5-1-chloro-exo-6-cyanocyclohexadienyl)(η5-cyclopentadienyl)iron (9) and (1–5-η5-1,exo-6-dicyanocyclohexa-dienyl)(η5-cyclopentadienyl)iron (10), a 40:60 mixture of 9 and 10 or pure 10, respectively. When the 3-h reaction was worked up in the presence of an added aqueous solution of NH4PF6, only the substitution product, the (η6-benzonitrile)(η5-cyclopentadienyl)iron cation (11), was obtained as its hexafluorophosphate. Apparently, under the acidic conditions of the added aqueous NH4PF6, 10 could readily revert to 11 which then gave rise to its hexafluorophosphate. DDQ treatment of the 3-min or 3-h reaction product gave, respectively, o-chlorobenzonitrile or phthalonitrile. Similarly reaction of CpFe complexes of o-substituted chlorobenzenes could give rise to CpFe-complexed cyclohexadienyl systems with two cyano groups at the 1- and 6-positions and the original o-substituent at the 2-position. DDQ treatment of such products would lead to the formation of 3-substituted phthalonitriles, and in the present work, 3-methyl, 3-methoxy-, and 3-phenoxyphthalonitriles were prepared in this way. In another set of reactions, depending on the experimental conditions employed, the CpFe complex of 2,6-dichlorotoluene (20) could give products from cyanide addition only, the DDQ treatment of which led to the formation of 2,4-dichloro-3-methylbenzonitrile, or products from substitution and addition and subsequent DDQ treatment of which gave rise to 4-chloro-3-methylphthalonitrile. Keywords: η6-chlorobenzene- η5-cyclopentadienyliron cation, complexed chlorobenzenes, nucleophilic addition of cyanide ion, nucleophilic substitution with cyanide ion, synthesis of benzonitriles and phthalonitriles.
Photocyclization of aryl enaminones. An efficient route to indole alkaloid synthons
Authors: Gardette Daniel , Gramain Jean-Claude , Lepage Marie-Eve , Troin Yves
Published: May 2006

The photocyclization of enaminones was extended to aryl enaminones bearing a substituent on the aromatic moiety. This reaction was studied in order to achieve the synthesis of indole alkaloid synthons. Trials of regioselectivity control were made by using groups with enhanced steric hindrance. The reactivity of secondary enaminones was tested, and the ratio of C-alkylation to N-alkylation was shown to be dependent on the nature of the aromatic substituent. During this work, new hexahydrocarbazolones were synthesized, with substituents on the A ring or the modified C ring. Keywords: photocyclization, aryl enaminones, indole alkaloids, hexahydrocarbazolones-4, cyclopenta[b]indoles.
Counterion micellar effects upon the reaction of low-spin diimine iron(II) complexes
Authors: Ortega Francisco , Rodenas Elvira
Published: May 2006

The rate of reaction of tris(1,10-phenanthroline)iron(II) ion (1a), tris(3,4,7,8-tetramethyl-1,10-phenanthroline)iron(II) ion (1b), and tris(4,7-diphenhyl-1, 10-phenanthroline)iron(II) ion (1c) with hydroxide ion, in cationic micelles, is strongly affected by the concentration of micellar counterion in solution. The reaction of la in CTACl is modestly speeded up by the addition of added KCl, while the reactions of 1b and 1c are strongly inhibited by the addition of large amounts of KCl and KBr to micellar solutions of CTACl and CTABr, respectively. These rate effects fit the pseudophase-ion exchange model, assuming the binding of the substrates to the micelles depends upon the counterion concentration. Keywords: counterion micellar effects, low-spin diimine iron(II) complexes.
The chemistry of thujone. XIII. Synthetic studies in the digitoxigenin series
Authors: Kutney James P. , Piotrowska Krystyna , Somerville John , Huang Shyh-Pyng , Rettig Steven J.
Published: May 2006

The thujone-derived chiral synthon 1 is converted, via selenium chmistry, to the important cross-conjugated dienone 2 and the latter is then convertible to the unsaturated cardenolide analogue 3, which through known methodology (3 → 26 → 27 → 4), completes a formal synthesis of digitoxigenin (4). Extensive studies to afford 2 from androstenedione (5) are also described and a new approach to elaborate the essential butenolide ring system, characteristic of the cardiac active steroids, is developed. Keywords: thujone, steroid synthesis, digitoxigenin.
Synthesis, structural and magnetic properties of macrocyclic aza-amido binuclear copper(II) complexes
Authors: Mandal Sanat K. , Thompson Laurence K. , Newlands Michael J. , Biswas Amal K. , Adhikary Bibhutosh , Nag Kamalaksha , Gabe Eric J. , Lee Florence L.
Published: May 2006

Binuclear, antiferromagnetically coupled, macrocyclic copper(II) complexes, [Cu2(C28H32N4O4)]•H2O (II) and [Cu2(C36H32N4O4)]•CH3CN•H2O (III), involving asymmetric ligands with two deprotonated amide, two azomethine nitrogen, and two phenoxide donors at the binuclear centre, have been synthesized and characterized by single-crystal X-ray diffraction and variable temperature magnetic studies. Complex II crystallizes in the monoclinic system, space group P21/n, with a = 16.4854(9) Å, b = 7.6005(13) Å, c = 21.1617(11) Å, β = 104.090(5)°, Z = 4, Rf = 0.068 for 2062 significant reflections. The two copper(II) centres have square planar N2O2 donor sets with two phenoxide oxygen atoms bridging the copper centres with a copper–copper separation of 2.898(2) Å. A long copper–oxygen (amide) contact (2.808(10) Å) forms a weak dimer association. Complex III crystallizes in the triclinic system, space group [Formula: see text], with a = 8.7771(9) Å, b = 12.3983(16) Å, c = 15.7299(16) Å, α = 85.003(11)°, β = 84.574(8)°, γ = 76.838(10)°, Z = 2, Rf = 0.041 for 2966 significant reflections. The two copper(II) centres have distorted square-pyramidal geometry involving an N2O2 in plane donor set and two phenoxide oxygen bridges with a copper–copper separation of 3.018(1) Å. The fifth coordination site at each copper centre involves an amide oxygen from a neighbouring molecule (Cu(1)—O 2.371(4), Cu(2)—O 2.413(3) Å) in a staggered intermolecular array. Very strong antiferromagnetic exchange is observed in both cases (−2J = 689 ± 7 cm−1 (II), −2J = 816 ± 8 cm−1 (III)). Keywords: macrocycles, binuclear copper(II) complexes.
Behaviour of 2′-O-p-toluenesulfonyl uridine towards excess of a Grignard reagent
Authors: Grouiller Annie , Essadiq Hassan
Published: May 2006

Two C-methylated base nucleosides were obtained when 2′-O-tosyl-5′-O-trityluridine was reacted with 30 equivalents of methylmagnesium iodide. Keywords: nucleoside, Grignard reaction, C-methyl pyrimidinone, anhydro derivative, arabinofuranosyl.
Coordination compounds of indium. Part 45. Indium(I) derivatives of aromatic diols
Authors: Mabrouk Hassan E. , Tuck Dennis G.
Published: May 2006

The electrochemical oxidation of anodic indium in acetonitrile solutions of the aromatic diols R(OH)2 (11,2-dihydroxybenzene; 2,3-dihydroxynaphthalene; 2,2′-dihydroxybiphenyl; 1,2-dihydroxytetrabromobenzene) yields the new compounds In[OR(OH)]. These unusual indium(I) species can be oxidized by iodine to InIIII2[OR(OH)], and by the ortho-quinone Br4C6O2 to InIII[OR(OH)](O2C6Br4). The hydroxyl–oxo ligand can be deprotonated by Et3N to give the salts Et3NH[In(O2R)]. The nature of these reactions, and the structure of the initial products, are confirmed by infrared and 1H nmr spectroscopy. Keywords: indium, electrochemistry, oxidation, aromatic diols.
Model compounds of aromatic nylons. 3. The conformation of the 4T Nylon, poly(tetramethylene terephthalamide), using X-ray diffraction, solid-state 13C nmr spectroscopy and ir spectroscopy
Authors: Brisson Josée , Gagné Johanne , Brisse François
Published: May 2006

Three model compounds of poly(tetramethylene terephthalamide) or 4T Nylon have been synthesized and characterized using single crystal X-ray diffractometry, infrared spectroscopy, and 13CCP/MAS nmr spectroscopy. The model compounds are the para-substituted N,N′-tetramethylene dibenzamides, where the substituents are the—OMe, —tBu, —CN, and —CH3 groups. The crystal structure determinations reveal three distinct conformations for the CO—NH—(CH2)4—NH—CO sequence of atoms. The conformation is all trans for OMe, tgtttg−t for the tBu substituent, and tsttts−t for the CN substituent ([Formula: see text], [Formula: see text], and [Formula: see text]). In all three derivatives, the dihedral angle between the aromatic ring and the amide plane is around 30°. The OMe and tBu para-substituted molecules are interconnected by nearly linear hydrogen bonds of normal N … O distances. However, for the CN derivative the N … O distance is exceptionally short, 2.402(4) Å. On the basis of its infrared and solid state nmr spectra, it is proposed that N,N′-tetramethylene di-para-methyl benzamide has a crystal structure comparable to that of its unsubstituted analog. The methylenic sequence of the parent polyamide, 4T Nylon, has the tgtttg−t conformation. Furthermore, the polyamide chains form sheets within which the chains, parallel to one another, are connected through hydrogen bonds. Keywords: polyamide, crystal structure, solid-state nmr.
Complexes binaires et ternaires du cuivre(II) avec la 1-canavanine et la 1-arginine examinés par potentiométrie, spectrophotométrie visible et dichroïsme circulaire
Authors: Albourine Abdallah , Petit-Ramel Michelle , Thomas-David. Germaine , Vallon Jean-Jacques
Published: May 2006

The protonation constants of 1 -canavanine (Can) and its structural analogue 1-arginine (Arg) have been calculated at 25 °C and at ionic strength μ = 0.1 (NaNO3). Under the same conditions, the stability constants of the binary and ternary complexes with Cu(II) have been determined by potentiometry, spectrophotometry, and by circular dichroïsm in the visible region. A comparative study of these two amino acids allows us to differentiate the order of protonation of the three functional groups (carboxylic, amino, and guanidyl), mainly by the different modes of complexation with copper(II), which is found in an octahedral coordination: arginine behaves like a bidentate ligand (glycine type) whereas canavanine behaves like a tridentate ligand. Keywords: 1-arginine, 1-canavanine, circular dichroïsm, copper ternary coordination, potentiometry.
Asymmetric induction in Diels–Alder reactions of α-alkoxyorthoquinodimethanes
Authors: Charlton James L. , Plourde Guy L. , Penner Glenn H.
Published: May 2006

It has been shown that dienophiles cycloadd selectively to one face of o-quinodimethanes (o-QDMs) bearing chiral α-alkoxy groups. The face selectivity (diastereoselectivity) increases for the series of chiral groups -OCH(Ph)CH3, -OCH(Ph)CH(CH3)2, and -OCH(Ph)C(CH3)3. A similar effect on the face selectivity of the Diels–Alder reactions of chiral alkoxy vinyl ethers for the same series of chiral groups has been noted previously by others. A mechanism has been proposed to explain the face selectivity in the cycloaddition reactions of the alkoxy o-QDMs. Abinitio molecular orbital calculations with geometry optimization on vinyl 1-phenylethyl ether to determine its lowest energy conformations support the proposed mechanism. The absolute stereochemistries of the o-QDM cycloadducts have been determined to verify the predictions of the model. Keywords: o-quinodimethanes, asymmetric, Diels–Alder, cycloaddition.
An evaluation of the substrate specificity and asymmetric synthesis potential of the cloned L-lactate dehydrogenase from Bacillusstearothermophilus
Authors: Bur Daniel , Luyten Marcel A. , Wynn Hla , Provencher Louis R. , Jones J. Bryan , Gold Marvin , Friesen James D. , Clarke Anthony R. , Holbrook J. John
Published: May 2006

The potential utility of the L-lactate dehydrogenase of Bacillusstearothermophilus (BSLDH) for stereospecific, preparative-scale reductions of α-keto acids to (S)-α-hydroxy acids of > 99% ee has been demonstrated. BSLDH is a stable, thermophilic, enzyme whose gene has been cloned into a high-expression vector to assure its plentiful supply. Its specificity for keto acid substrates possessing straight- and branched-chain alkyl, cyclopropyl, or phenyl groups has been evaluated in preparative and kinetic terms, and compared with that of the mammalian pig heart enzyme (PHLDH). The specificities of BSLDH and PHLDH are similar, with branched alkyl-chain keto acids being poor substrates for both enzymes. Keywords: enzymes in organic syntheses, lactate dehydrogenase, asymmetric synthesis.
Intramolecular rhodium carbenoid insertions into aromatic C—H bonds. Preparation of 1,3-dihydrothiophene 2,2-dioxides fused onto aromatic rings
Authors: Babu Suresh D. , Hrytsak Michael D. , Durst Tony
Published: May 2006

The preparation of 1-carboalkoxy-1,3-dihydrobenzo[b]thiophene 2,2-dioxides via rhodium acetate or rhodium trifiuoro-acetate catalyzed decomposition of α-diazo-β-arylmethanesulfonyl esters is described. The reaction has been extended to yield 1,3-dihydrothiophene 2,2-dioxides fused to the 2,3 position of thiophene and indole, but not of furans. In the latter case products derived from the opening of the furan ring were obtained. Keywords: synthesis, 1-carboalkoxy-1,3-dihydrobenzo[b]thiophene 2,2-dioxides, intramolecular carbenoid insertions, rhodium acetate catalysis.
Thermal instability of germacrone: implications for gas chromatographic analysis of thermally unstable analytes
Authors: Reichardt Paul B. , Anderson Barbara J. , Clausen Thomas P. , Hoskins L. Claron
Published: May 2006

The rearrangement of germacrone (1) to β-elemenone (2) is common during gas chromatographic (GC) analysis, with the conversion taking place in the injector port at temperatures above 250 °C as well as on the column. While production of 2 in the injector is evident from the gas chromatographic trace, 2 produced from 1 during its migration down the column is difficult to detect because of the diffuse nature of the chromatographic peak produced by this process. A mathematical model and kinetic parameters (A = (1.35 ± 0.03) × 10−3s−1; Ea = 137 ± 4 kJ mol−1) determined from the reaction in solution can be used to estimate the degree of conversion of 1 to 2 for chromatographic runs (including temperature-programmed analyses). The results of these calculations verify that substantial amounts of 1 are converted to 2 under conditions that are commonly used for the analysis. The method for modeling thermally induced rearrangements in nonisothermal processes represents the first approach to quantitatively assessing their importance in GC analyses of thermally labile substances. Keywords: kinetics, gas chromatography, thermal rearrangement, germacrone, β-elemenone.
Electronic spectrum of bibenzimidazole homologue: effects of solvents and acid concentration
Authors: Kumar Awadesh , Sinha Hemant K. , Dogra Sneh K.
Published: May 2006

The absorption and fluorescence spectra of bibenzimidazole (BBI), N, N′-dimethylbibenzimidazole (MBBI), and methylene, 2,2′-bibenzimidazole (MtBBI) have been recorded in six solvents of different characteristics and at various acid concentrations. This study indicates the presence of intramolecular hydrogen bonding in BBI in both S0 and S1 states, leading to large conjugation of two benzimidazole (BI) rings. The geometry of BBI is the same in the S0 and S1 states. The spectral characteristics of BBI are insensitive to the solvents. The interaction of solvent molecule attached to the nitrogen lone pair and methyl group rotates the benzimidazolyl ring in MBBI around the single bond. This leads to a blue shift in the long wavelength absorption band maximum and a decrease in the intensity of this band. The two MBI rings are coplanar in the S1 state. Presence of methylene group inhibits the direct interaction between the BI groups and these rings behave almost independently, i.e., the spectral characteristics of this molecule nearly resemble those of benzimidazole. Keywords: absorption spectrum, fluorescence spectrum, pKa, bibenzimidazoles, excited state pKa.
Synthesis and reaction chemistry of the heterobimetallic FeIr complexes (CO)3(PPh3)Fe(μ-PCy2)Ir(PPh3)(CO)2 and (CO)4Fe(μ-PCy2)Ir(1,5-COD)
Authors: Jenkins Hilary A. , Loeb Stephen J.
Published: May 2006

Reactions of Li[Fe(CO)4(PCy2)] with trans-IrCl(CO)(PPh3)2 and [IrCl(1,5-COD)]2 (COD = cyclooctadiene) results in the formation of the μ-PCy2 bridged complexes (CO)3(PPh3)Fe(μ-PCy2)Ir(PPh3)(CO)2, 1, and (CO)4Fe(μ-PCy2)Ir(1,5-COD), 2, respectively. 1 is coordinatively saturated with an FeIr dative bond and does not form an adduct with CO or undergo oxidative addition with H2, HCl, or CH3I. The protonation of 1 with HBF4 gives the Ir hydride compound [(CO)3(PPh3)Fe(μ-PCy2)Ir(H)(PPh3)(CO)][BF4]. 1 undergoes phosphine substitution reactions in which PPh3 is replaced by PR3. Substitution is at Ir for PEt3, PnBu3, PiPr3, PBz3, and PCy3, at Fe for PMe2Ph, and at both metal centers for PPh2Me. The substitution pattern is selective and appears to be sterically controlled. There is also evidence for cooperativity between metal centers in the formation of the PPh2Me adduct since only the disubstituted product can be detected. Coordinative unsaturation was generated at the Ir centre by thermally induced CO loss from (CO)3(PPh3)Fe(μ-PCy2)Ir(PCy3)(CO)2 to give (CO)3(PPh3)Fe(μ-PCy2)Ir(PCy3)(CO). The reaction of 2 with excess CO(g) resulted in the formation of the carbonyl species (CO)4Fe(μ-PCy2)Ir(CO)3. 2 also reacts with 2 PEt3 or DPPE (DPPE = 1,2-bis(diphenylphosphino)ethane) under CO(g) to give the disubstituted products (CO)3(PEt3)Fe(μ-PCy2)Ir(PEt3)(CO)2 and (CO)3Fe(μ-PCy2)(μ-DPPE)Ir(CO)2 with loss of 1,5-COD. Keywords: heterobimetallic FeIr complexes, cooperativity, phosphido-bridge.
Kinetic study of the oxidation mechanism of glutathione by hydrogen peroxide in neutral aqueous medium
Authors: Abedinzadeh Z. , Gardes-Albert M. , Ferradini C.
Published: May 2006

The oxidation kinetics of glutathione (GSH) by hydrogen peroxide has been studied at neutral pH for different concentration ratios [GSH]0/[H2O2]0 between 0.2 and 2 (5 × 10−4 M ≤ [H2O2]0 ≤ 2.5 × 10−3M; 4 × 10−4 M ≤ [GSH]0 ≤ 2.5 × 10−3 M). In all cases studied, glutathione disulfide GSSG is the main product formed via two different oxidation ways, each of them contributing respectively to 80–85% and 10–15%.Our kinetic data indicate that an important fraction of hydrogen peroxide disappears without oxidizing the thiol function. This can be attributed to a combination between GSH and H2O2 protecting the sulfide group. Chemical evidences of the existence of a peroxide bond with GSSG are described. In our experimental conditions, the overall oxidation equation is 2mol GSH reacting with 2mol H2O2 giving 1mol GSSG. It is very different from the usually accepted stoichiometric reaction:[Formula: see text]A kinetic scheme is proposed and the corresponding rate constants are determined. Keywords: glutathione, hydrogen peroxide, kinetic mechanism, glutathione disulfide.
An NMR study of steric and hyperconjugative barriers in benzyl X(CH3)3, X = C, Si, Ge, Sn, Pb
Authors: Schaefer Ted , Penner Glenn H. , Takeuchi Craig S. , Beaulieu Christian
Published: May 2006

The 1H nuclear magnetic resonance spectra and their analyses are reported for benzyltrimethyl X (X = C, Si, Ge, Sn, Pb) and for the 2,6-dichloro derivatives where X = Si, Ge, Sn, Pb. The steric barrier to rotation about the Csp2—Csp3bond for X = C, based on the long-range proton–proton coupling constant over six bonds, 6J(H,CH2), must be greater than 20 kJ/mol. Unexpectedly, 6J(H,CH2) implies that the barriers for X = Si, Ge, Sn, Pb have relatively minor steric components. If these barriers are dominated by hyperconjugative interactions of the C—X bond with the aromatic π system, then they are probably mainly twofold in nature. Furthermore, their magnitude is likely to be near 9.5 kJ/mol for X = Si, Ge, Sn and may be as high as 12 kJ/mol for X = Pb. Keywords: NMR, benzyl X(CH3)3, X = C, Si, Ge, Sn, Pb; NMR, conformations; NMR, hyperconjugation.
The isolation, structure, and stereochemistry of traversiadiene. The precursor hydrocarbon of traversianal biosynthesis
Authors: Stoessl Albert , Rock G. L. , Stothers J. B.
Published: May 2006

A tricyclic diene, traversiadiene, isolated from cultures of Cercosporatraversiana has been shown to have the structure and stereochemistry of the previously postulated hydrocarbon intermediate on the biosynthetic pathway to traversianal (1). Detailed:1H and 13C magnetic resonance studies, including homo- and heteronuclear correlation spectra, led to the gross structure, and the stereochemistry was established through a series of nuclear Overhauser effect difference spectra. Keywords: diterpene, traversiadiene, 1H and 13C magnetic resonance spectra.
Direct synthesis of indenones from alkenyliron complexes
Authors: Butler Ian R. , Elliott Joanne E. , Houde Jr. Jean
Published: May 2006

Alkenyliron complexes have been prepared by the metathesis of dicarbonyl(η5-cyclopentadienyl)iodoiron with aryl-substituted alkenyllithiums. Thermolysis of the product alkenyliron complexes leads to the formation of indenones in good yields. Keywords: alkenyliron, carbonyl, indenone, insertion, metallation, synthesis.