bluuebunny/crossref_metadata_2025_split huggingface dataset:
Stereochemistry of Electrophilic Reactions of 4‐t‐Butyl‐1‐phenylcyclohexyllithium, ‐sodium, ‐potassium and ‐cesium
Authors: Keys Boyd A. , Eliel Ernest L. , Juaristi Eusebio 
Published: November 2013

AbstractThe first three title compounds were prepared by treating the diastereomeric 4‐t‐butyl‐1‐phenyl‐1‐phenylthiocyclohexanes 8 or 9 (Scheme 3) with lithium, sodium, or potassium naphthalenide, respectively. The potassium derivative was also obtained by treatment of 9 with sodium/potassium alloy and the cesium derivative was similarly obtained by reaction with cesium/potassium/sodium alloy. Quenching of any of these salts with D2O gave cis‐ and trans‐4‐t‐butyl‐1‐phenylcyclohexane‐1‐d(cis‐ and trans‐7‐d) in an approximately 1:1 ratio. The reaction was stereoconvergent and the product ratio independent of the metal; the result is explained in terms of a diffusion controlled reaction of a planar or near‐planar carbanion. In contrast, methylation with methyl iodide gave the product of axial methylation 10 in an approximately 2:1 predominance over diastereomer 11. This result is explained in terms of Curtin‐Hammett kinetics involving a stereoelectronically preferred axial approach to the electrophile. 13C NMR spectral studies of the lithium, potassium, and cesium salts indicate extensive delocalization of the anionic charge into the ring—somewhat more so with the K and Cs than with the Li salt. This finding supports the hypothesis that at least the K and Cs salts involve a planar carbanion moiety, whereas the Li salt may be slightly nonplanar. Both spectrally and stereochemically, the title compounds differ substantially from the earlier studied 2‐phenyl‐1,3‐dithianes.
2,2′,7,7′‐Tetrahydroxy‐1,1′‐Binaphthyl: A Versatile Chiral Spacer for Monotopic and Ditopic Cyclophane Hosts with Apolar Binding Sites
Authors: Hester Matthew R. , Uyeki Mike A. , Diederich François 
Published: November 2013

AbstractDerivatives of 2,2′,7,7′‐tetrahydroxy‐1,1′‐binaphthyl (5) are synthetically readily available chiral shapes with a major and a minor groove. Previous work by Cram et al. has shown that efficient chiral crown ether binding sites can be constructed at the binaphthyl minor groove. The major groove is almost twice as wide and possesses suitable dimensions to shape flat apolar binding sites in chiral cyclophane hosts. The eight‐step syntheses of the two novel cyclophanes 6 and 7 are described. The apolar cavities of these hosts are formed by bridging with two linear four‐carbon chains the 7,7′‐oxygens at the major groove of 5 to the two phenolic oxygens of a 4,4‐bis(4‐hydroxy‐3,5‐dimethylphenyl)piperidine unit. 1H NMR complexation studies show that 1:1 complexes of 6 and 7 with 6‐methoxy‐2‐naphthonitrile and other naphthalene derivatives in aqueous methanol are as stable as the previously reported complexes of bis(diphenylmethane)‐hosts. The different solvation characteristics of cation and apolar binding are demonstrated with the ditopic host 7. In addition to the apolar binding site at the major groove, 7 also possesses a crown ether binding site at the minor groove of the binaphthyl spacer. By changing the methanol content of a water‐methanol mixture, this host can be switched from an efficient binder of neutral naphthalene guests to a good host for potassium cations. The activities of the two binding sites in 7 are almost entirely independent of each other; consequently, the binding of a naphthalene guest in the 1:1:1 (host‐naphthalene‐K+) complex is nearly as strong as in the 1:1 complex. The methyl ester of (S)‐naproxen forms stable complexes with the racemic hosts 6 and 7. The 1H NMR spectra of these complexes show considerably different shifts for the aromatic resonances of the host enantiomers. These differences indicate the formation of diastereomeric complexes of different geometries or stabilities.
Design and Synthesis of Functionalized Oxaziridines as Topological Analogs of Penicillins
Authors: Marchand‐Brynaert Jacqueline , Bounkhala‐Khrouz Zoubida , van Keulen Berend Jan , Vanlierde Huguette , Ghosez LÉOn 
Published: November 2013

AbstractOxaziridines were examined as potential alkylating inhibitors of bacterial serine D,D‐peptidases. Structures were designed to mimic known β‐lactam antibiotics taking into account stereoelectronic requirements for a nucleophilic attack on a three‐membered ring. Novel oxaziridines equipped with functionalized side chains were prepared. They showed poor antibacterial activities in vitro, perhaps as a result of their low chemical stability.
Quantum Effects in Three‐Dimensional H + D2 Reaction at High Energies
Authors: Last Isidore , Ron Shlomo , Baer Michael 
Published: November 2013

AbstractThe H + D2 reactive system was studied in the energy range 0.84 < Etr < 2.70 eV employing the ordinary Quasi‐Classical Trajectory Approach (QCTA), the Classical and the Quantum Reactive Infinite Order Sudden Approximations (CRIOSA and QRIOSA), and the Angle‐Oriented Transition State Theory (AOTST). From a detailed analysis of the various integral cross‐sections and opacity functions, it was concluded that pronounced quantum effects ( > 10%) are expected in the high‐energy region (≤ 2 eV).
Negligible Nuclear Spin Relaxation Concurrent with Extensive Rotational Cooling in Jet Expansions of Methyl Halides, from Analysis of Hexapole Focusing
Authors: Choi Seung E. , Bernstein Richard B. 
Published: November 2013

AbstractExperimental data of the UCLA group (S.R. Gandhi et al. Chem. Phys. Lett., 1986, 132: 6; J. Phys. Chem., 1987, 91: 5437) on focusing and rotational state selection of supersonic, seeded beams of methyl halide molecules via the electrostatic hexapole technique are analyzed to ascertain the extent of spin conversion (i.e., proton spin orientational relaxation) accompanying rotational cooling. Just as for the ortho‐para hydrogen system, the rate of spin conversion (here, between the two different symmetry species, A and E, of the methyl halide molecules) is found to be negligible compared to the extensive collision‐induced rotational cooling (down to rotational temperatures as low as 2 K).
Dynamics and Potential Surfaces of Small Clusters
Authors: Berry R. Stephen , Braier Paul , Hinde R.J. , Cheng Hai‐Ping 
Published: November 2013

AbstractSome of the tantalizing puzzles regarding clusters are described. Some specific results concerning the dynamical behavior of clusters of particular sizes and having various potentials are described, to give some insight into the relationship between the form of a potential and the kind of behavior it yields. Solid‐liquid, solid‐soft solid, surface melting, and a remarkable single “subsurface layer mobility” all can be found in particular instances.
Correlation of Zeno (Z = 1) Line for Supercritical Fluids with Vapor‐Liquid Rectilinear Diameters
Authors: Ben‐Amotz Dor , Herschbach Dudley R. 
Published: November 2013

AbstractFor a wide range of substances, extending well beyond the regime of corresponding states behavior, the contour in the temperature‐density plane along which the compressibility factor Z = P/ρkT is the same as for an ideal gas is nearly linear. This Z = 1 contour, termed the Zeno line, begins deep in the liquid region and ascends as the density decreases to the Boyle point of the supercritical fluid, specified by the temperature TB for which (dZ/dρ)T = 0 as ρ → 0; equivalently, at TB the second virial coefficient vanishes. The slope of the Z = 1 line is — B3/(dB2/dT), in terms of the third virial coefficient and the derivative of the second, evaluated at TB. Previous work has examined the Zeno line as a means to extend corresponding states and to enhance other practical approximations. Here we call attention to another striking aspect, a strong correlation with the line of rectilinear diameters defined by the average of the subcritical vapor and liquid densities. This correlation is obeyed well by empirical data for many substances and computer simulations for a Lennard‐Jones potential; the ratios of the intercepts and slopes for the Zeno and rectilinear diameter lines are remarkably close to those predicted by the van der Waals equation, 8/9 and 16/9, respectively. Properties of the slightly imperfect fluid far above the critical point thus implicitly determine the diameter of the vapor‐liquid coexistence curve below the critical point.
Foam Fractionation and Ion Flotation of Simple and Complex Anions with Cationic Surfactants
Authors: Grieves Robert B. 
Published: November 2013

AbstractExperimental data are presented and interpreted on the foam fractionation of an extensive series of simple and complex anions from dilute (of the order 10−6−10−4 molar) aqueous solutions, utilizing a quaternary ammonium surfactant with which the anions form soluble ion pairs in competition with the surfactant's counterion. Selectivity coefficients, based on a bubble‐interface, ion exchange model, are established in a single‐equilibrium‐stage, continuous‐flow, foam fractionation unit for each of a series of 13 anions and oxyanions versus the surfactant's bromide counterion. Three additional series of batch experiments with multi‐metal solutions establish the separation and concentration in the foam of the oxyanions of Re(VII), Mo(VI), Cr(VI), W(VI), and V(V); of the cyanide complex anions of Zn(II), Cd(II), Hg(II), and Au(III); and of the chloride complex anions of Zn(II), Cd(II), Hg(II), and Au(III). The metal oxyanions and metal cyanide and chloride complex anions can be separated from each other and from competing chloride, cyanide, or nitrate.Ion flotation of anionic species with a cationic surfactant involves an entirely different mechanism, in which a precipitation reaction occurs and particle flotation follows. Hexavalent chromium (primarily HCrO−4) is ion‐floated with each of a series of variable chain length quaternary ammonium surfactants, elucidating the effects of temperature, the surfactant/Cr(VI) feed ratio, mixing time, and surfactant chain length in terms of the roles of the surfactant as precipitant, dispersant, collector, and frother.
Metal Induced Decarboxylation of Aliphatic Free Radicals. I. Kinetics of the Reactions of Copper(I) and Copper(II) Ions with the 2‐Methyl‐2‐Carboxylicacid‐Propyl Free Radical in Aqueous Solutions. A Pulse Radiolysis Study
Authors: Masarwa Mohamed , Cohen Haim , Saar Jacob , Meyerstein Dan 
Published: November 2013

AbstractCH2C(CH3)2CO2H free radicals react with Cu2+aq to form CuIII‐CH2C(CH3)2CO2H2+aq, which probably is rapidly transformed into intermediate I. This intermediate decomposes into Cu+aq + CO2 + CH2=C(CH3)2. This is the first system in which a metal ion induced β‐carboxyl elimination reaction was observed. ·CH2C(CH3)2CO2H free radicals react with CuIII‐CH2C(CH3)2CO2H2+aq to form Cu2+aq and a mixture of (‐CH2C(CH3)2CO2H)2, (CH3)3CCO2H, and HOCH2C(CH3)2CO2H. CH2C(CH3)2CO2H reacts with Cu+aq to form CuII‐CH2C(CH3)2CO2H+aq. The latter intermediate decomposes both via homolysis and by a reaction with Cu2+aq to form 2Cu2+aq + (CH3)3CCO2H. The spectra of the intermediates and the kinetics of reaction are reported.
Dinuclear Rhodium Complexes Containing the Diyne 1,4‐C6H4(C≡CH)2 and the Isomeric Bisvinylidene 1,4‐C6H4(CH=C:)2 as Bridging Units
Authors: Werner Helmut , Rappert Thomas , Wolf Justin 
Published: November 2013

AbstractThe reaction of [RhCl(PiPr3)2] (1) with 1,4‐C6H4(C≡CH)2 at 0°C leads almost quantitatively to the formation of the bis(alkyne) complex [(PiPr3)2ClRh‐(HC≡C6H4‐C≡CH)RhCl(PiPr3)2] (2). At elevated temperatures (THF, 60°C) it rearranges to give the isomeric bis(vinylidene) complex [(PiPr3)2ClRh‐(=C=CH‐C6H4‐CH=C=)RhCl(PiPr3)2] (3). A one‐pot synthesis of 3 is also described. Treatment of either 2 or 3 with pyridine affords the bis(alkynyl)dihydrido compound [(PiPr3)2(py)Cl(H)Rh(‐C≡C‐C6H4‐C≡C‐)Rh(H)Cl(py)(PiPr3)2] (4) in which both metal centers are octahedrally coordinated. Whereas the reaction of 2 with NaC5H5 produces the complex CsH5(PiPr3)Rh(HC≡C‐C6H4‐C≡CH)Rh(PiPr3)C5H5 (7), the bis(vinyl‐idene) isomer C5H5(PiPr3)Rh(=C=CH‐C6H4‐CH=C=)Rh(PiPr3)C5Hs (8) is obtained from 4 and NaC5H5. Electrophiles preferably attack the Rh=C bonds of 8 and thus on protonation with CF3CO2H the bis(vinyl) complex C5H5(PiPr3)(CF3CO2)‐Rh(Z,Z‐CH=CH‐C6H4‐CH=CH)Rh(O2CCF3)(PiPr3)C5Hs (Z‐9) is formed. In acetone solution, it rearranges to give the E isomer. Reaction of 8 with sulfur affords the bis(thioketene) complex C5H5(PiPr3)Rh(≡2‐C,S; η2‐C,S‐S=C=CH‐C6H4‐CH=C=S)‐Rh(PiPr3)C5H5 (12), for which only one diastereomer is observed. All attempts to prepare mononuclear rhodium compounds containing the diyne HC≡C‐C6H4‐G≡CH or the isomeric vinylidene: C=CH‐C6H4‐G≡CH as ligand failed.
Hydrogen‐Atom Transfer Reactions of Transition‐Metal Hydrides
Authors: Eisenberg David C. , Norton Jack R. 
Published: November 2013

AbstractMany reactions of transition‐metal hydrides involve H* transfer. With olefins such transfer gives a radical cage, from which escape and collapse lead to product formation. Inverse isotope effects, second‐order kinetics independent of ligand concentration, and CIDNP are diagnostic for this mechanism. Many other reactions of transition‐metal hydrides occur by radical chain mechanisms, in which H* is abstracted by carbon‐centered radicals or by metal radicals.Reasonably accurate values are now available for the M‐H bond strengths of most of the common hydrides, and these values help rationalize the known H* transfer reactions of these hydrides. While the rates of certain H* transfer reactions have been measured by radical clock methods, the measurement of H* transfer rates to a substituted trityl radical has provided the first general comparison of the H* donor abilities of the various hydrides. These relative H* transfer rates are significantly affected by steric factors.
Interface Structure in a Polymer Blend in the Presence of Selective Solvents
Authors: Winnik Mitchell A. , Pekcan ÖNder , Disanayaka Bimsara , Croucher Melvin D. 
Published: November 2013

AbstractEnergy transfer experiments were carried out on a material composed of poly(methyl methacrylate) (PMMA, 98.5 monomer mol%) and polyisobutylene (PIB, 1.5 monomer mol%), with PMMA labeled with phenanthrene, the donor, and PIB labeled with pyrene, the acceptor. In this material, prepared by nonaqueous dispersion polymerization, all of the PIB is present as a graft copolymer with PMMA. Here we demonstrate that the extent of energy transfer increases enormously when the particles are dispersed in aliphatic hydrocarbon liquids. These swell the PIB phase leading to formation of a diffuse interface where substantial mixing of the PMMA and PIB components occurs. The amount of energy transfer, and hence the amount of swelling, varies significantly among the various solvents.
A New Method for the Stereoselective Synthesis of Nucleosides by Means of Sodium Borohydride Mediated Reductive C‐C or C‐N Bond Cleavage Reaction
Authors: Kaneko Chikara , Katagiri Nobuya , Nomura Masahiro , Sato Hiroshi 
Published: November 2013

AbstractNew dienophiles, dimethyl acetoxymethylenemalonate and its analogues, have been synthesized. These dienophiles reacted with cyclopentadiene to give Diels‐Alder adducts, which served as versatile synthetic precursors for carbocyclic C‐nucleosides. The method has the following three characteristics: (1) the C‐C bond in the adduct originated from the dienophile can be cleaved stereoselectively by the reductive retrograde aldol (RRA) reaction, (2) the CC bond in the adduct serves as a latent functionality of 2′‐ and 3′‐ positions of the nucleoside, and (3) the malonate unit can be transformed to a variety of heterocycles. The high pressure mediated Diels‐Alder reaction of furan with the same dienophiles also gave the cycloadducts, which served as synthons for C‐nucleosides. Use of chiral dienophiles, such as di‐1‐menthyl acetoxymethylenemalonate, for the above Diels‐Alder reactions has offered a new route to the enantioselective synthesis of C‐nucleosides and their carbocyclic analogues. Essentially the same methodology has also been applied to the synthesis of carbocyclic nucleosides from 2‐azabicyclo[2.2.1]hept‐5‐en‐3‐one, in which a reductive amido bond cleavage reaction similar to (1) is involved. Synthesis of carbocyclic oxetanocin and its analogues is also described.
Chiral Molecular Clefts For Dicarboxylic Acid Complexation
Authors: Alcázar Victoria , Morán joaquín R. , Diederich François 
Published: November 2013

AbstractThree efficient cleft‐type receptors, 3,5, and 6 have been prepared by attachment of two amidopyridine units as hydrogen bonding centers to either the 2,2′‐positions of 9, 9′‐spirobifluorene or the 6,6′‐positions of 1,1′‐binaphthyl spacers. The easy availability of these compounds in short synthetic routes make them attractive complexing agents for aliphatic and aromatic dicarboxylic acids which undergo bidentate binding in chloroform solution. 1H NMR binding studies show that substrates of different size can be accommodated into the clefts and form 1:1 complexes that are predominantly stabilized by multiple host‐guesthydrogen bonds. The flexible aliphatic substrates diethylmalonic, 2,2‐diphenylsuccinic, glutaric, and pimelic acid form complexes with association constants Ka ranging from 103 to 104 L mol−1. Significantly more stable complexes (Ka > 105 L mol−1) are obtained with the more rigid, preorganized substrate 5‐dodecyloxyisophthalic acid.
Calixarenes 30. Calixquinones
Authors: Reddy P. Amruta , Kashyap Ram P. , Watson William H. , Gutsche C. David 
Published: November 2013

AbstractChlorine dioxide and thallium trifluoroacetate are shown to be useful reagents for the preparation of calixquinones. p‐H‐Calix[4]arene(1a),p‐H‐calix[5]arene (1b), and p‐H‐calix[6]arene (1c) are oxidized in modest yields by ClO2 to the fully quinonoid compounds calix[4]tetraquinone (2a), calix[5]pentaquinone (2b), and calix[6]hexaquinone (2c), respectively. Although Tl(OCOF3)3 is less effective for the oxidation of 1a‐c, it proves to be the reagent of choice for converting partially etherified or esterified calixarenes carrying p‐tert‐butyl groups directly to partially quinonoid calixarenes. Thus, monosubstituted calix[4]arenes yield triquinones; disubstituted calix[4]arenes yield diquinones; trisubstituted calix[4]arenes yield monquinones; and tetrasubstituted calix[6]arenes yield diquinones. The structures of the calixquinones have been established by elemental analysis, 1H NMR spectroscopy, mass spectroscopy, and in the case of 2c by X‐ray crystallography. Since the starting materials are readily accessible, the calixquinones become easily available compounds for further study.
Correlation of Isotropic and Anisotropic Chemical Shifts in Solids by Two‐Dimensional Variable‐Angle‐Spinning NMR
Authors: Frydman Lucio , Chingas Gerard C. , Lee Young K. , Grandinetti Philip J. , Eastman Margaret A. , Barrall Geoffrey A. , Pines Alexander 
Published: November 2013

AbstractWe describe a new solid‐state nuclear magnetic resonance (NMR) technique for correlating anisotropic and isotropic chemical shifts in powdered samples. Two‐dimensional (2D) NMR spectra are obtained by processing signals acquired in independent experiments for different angles between the sample spinning axis and the Zeeman magnetic field. This 2D NMR approach can therefore resolve individual static anisotropic lineshapes according to their isotropic chemical shift frequencies, without use of sudden mechanical motions or multiple‐pulse irradiation schemes. Applications of the technique are illustrated with an analysis of the chemical shift anisotropy for the eight distinct 13C sites in tyrosine.
A Simple Theory of 13C Nuclear Spin Diffusion in Organic Solids
Authors: Tycko R. , Dabbagh G. 
Published: November 2013

AbstractWe derive an expression for the rate of spin diffusion between 13C nuclei based on a simple model appropriate for organic solids. In this model, couplings between carbons and protons and among protons are included only as a source of fluctuating local magnetic fields at the carbon nuclei. Our results are essentially equivalent to the results of previous calculations in which the surrounding protons are treated quantum mechanically using memory function formalisms.
63Cu NQR Nutation Spectroscopy and Spin Counting in Copper Oxides
Authors: Vega Alexander J. 
Published: November 2013

AbstractNutation spectroscopy of 63Cu nuclei at zero static magnetic field is explored as a method for the determination of the quadrupole asymmetry parameters η of Cu sites in copper oxides, with special emphasis on high‐Tc superconductors. Since the nutation spectra reveal the actual radio frequency (rf) intensities experienced by the 63Cu nuclei, the method also provides a verification of a quantitative Cu spin counting procedure based on integration of nuclear quadrupole resonance (NQR) echo‐signal intensities over wide frequency ranges. The paper presents numerical simulations of spectra and signal intensities that were obtained with a density‐matrix formalism whereby the spin dynamics is described as precessions in a 3D operator space. The quantitative effects of the echo refocusing pulse, rf amplitude fluctuations, and variations in η are examined. Examples of 63Cu NQR nutation spectra are shown for randomly oriented powders of Cu2O and YBa2Cu3O7.
The Transition Amplitudes of Centerband and Sidebands in NMR Spectra of Rotating Solids
Authors: Schmidt Asher , Vega Shimon 
Published: November 2013

AbstractExpressions for the transition amplitudes of the centerband and sidebands of magic angle spinning (MAS) NMR spectra are derived by applying the Floquet theory. Signal amplitudes are defined in terms of transition amplitude operators, which are linear combinations of the Floquet operators. Two examples of the utilization of the Floquet approach for the evaluation of MAS signals are presented. In the first, the REDOR experiment on heteronuclear spin systems is discussed. The effects of finite pulse lengths on the REDOR signals are also derived. The second example deals with the MAS spectrum of a spin coupled to a heteronuclear spin that is irradiated by an rf field. Recoupled spectra are examined with the help of the Floquet theory and decoupled spectra are evaluated. In all cases the methodology of the Floquet approach is emphasized.
Fine Structure in NMR Correlation Spectroscopy
Authors: Freeman Ray , Mcintyre Lisa 
Published: November 2013

AbstractA simplified treatment of two‐dimensional homonuclear correlation spectroscopy (COSY) is proposed, based on a vector model and a consideration of the disturbance of energy level populations caused by the pulse sequence. It predicts the properties of diagonal and crosspeaks in the same manner as the more rigorous product operator treatment. Methods of studying the fine structure of COSY crosspeaks are discussed with a particular emphasis on the extraction of coupling constants when the splittings are poorly resolved. Three new schemes are described for the accurate measurement of couplings; they are called “J‐extension”, “J‐deconvolution”, and “J‐doubling”. All three rely on processing the experimental time‐domain signals with simple trigonometrical functions, followed by Fourier transformation. The J‐deconvolution and J‐doubling methods lend themselves to automated analysis by computer. These procedures can be applied to either active or passive couplings and yield an accuracy of ±0.04 Hz in the cases studied. J‐deconvolution may be implemented repetitively in order to reduce a COSY crosspeak to a single absorption peak at the chemical shift coordinates.