Synthesis and characterization of cationic tungsten(V) methylidynes

Cationic tungsten(V) methylidynes [L4W(X)[triple bond]CH]+[B(C6F5)4]- [L = PMe3, 0.5dmpe (dmpe = Me2PCH2CH2PMe2), X = Cl, OSO2CF3] have been prepared in high yield by a one-electron oxidation of the neutral tungsten(IV) methylidynes L4W(X)[triple bond]CH with [Ph3C]+[B(C6F5)4]-. The ease and reversibility of the one-electron oxidation of L4W(X)[triple bond]CH were demonstrated by cyclic voltammetry in tetrahydrofuran (E1/2 is approximately -0.68 to -0.91 V vs Fc). The paramagnetic d1 (S = 1/2) complexes were characterized in solution by electron spin resonance (g = 2.023-2.048, quintets due to coupling to 31P) and NMR spectroscopy and Evans magnetic susceptibility measurements (mu = 2.0-2.1 muB). Single-crystal X-ray diffraction showed that the cationic methylidynes are structurally similar to the neutral precursor methylidynes. In addition, the neutral (PMe3)4W(Cl)[triple bond]CH was deprotonated with a strong base at the trimethylphosphine ligand to afford (PMe3)3(Me2PCH2)W[triple bond]CH, a tungsten(IV) methylidyne complex that features a (dimethylphosphino)methyl ligand.     

Microwave-assisted asymmetric organocatalysis. A probe for nonthermal microwave effects and the concept of simultaneous cooling

A series of five known asymmetric organocatalytic reactions was re-evaluated at elevated temperatures applying both microwave dielectric heating and conventional thermal heating in order to probe the existence of specific or nonthermal microwave effects. All transformations were conducted in a dedicated reactor setup that allowed accurate internal reaction temperature measurements using fiber-optic probes. In addition, the concept of simultaneous external cooling while irradiating with microwave power was also applied in all of the studied cases. This method allows a higher level of microwave power to be administered to the reaction mixture and, therefore, enhances any potential microwave effects while continuously removing heat. For all of the five studied (S)-proline-catalyzed asymmetric Mannich- and aldol-type reactions, the observed rate enhancements were a consequence of the increased temperatures attained by microwave dielectric heating and were not related to the presence of the microwave field. In all cases, in contrast to previous literature reports, the results obtained either with microwave irradiation or with microwave irradiation with simultaneous cooling could be reproduced by conventional heating at the same reaction temperature and time in an oil bath. No evidence for specific or nonthermal microwave effects was obtained.     

Aza-Morita-Baylis-Hillman reactions and cyclizations of conjugated dienes activated by sulfone, ester, and keto groups

The aza-Morita-Baylis-Hillman reactions of aldimines 2 with several activated conjugated dienes were found to proceed smoothly in DMF in the presence of 3-hydroxyquinuclidine (HQD). Imines 2 reacted with 1-(p-toluenesulfonyl)-1,3-butadiene (3), methyl 2,4-pentadienoate (6), hexa-3,5-dien-2-one (7), and 1-phenylpenta-2,4-dien-1-one (8) to afford adducts 4, 13, 14, and 15, respectively. While products 4, 13, and 15 were formed as E,Z mixtures, adducts 14 were obtained as essentially pure E-isomers. Cyclization of the E-isomers of the products derived from the dienyl sulfone 3 and the dienoate ester 6 occurred via intramolecular conjugate addition under base-catalyzed conditions to afford functionalized piperidines 5 and 16, respectively. The aza-Morita-Baylis-Hillman reaction and subsequent cyclization of the imine 2a with 3 were also carried out as a one-pot reaction, while the reaction mixture was simultaneously irradiated at 300 nm to effect the photoisomerization of the unreactive Z-adduct of the corresponding 4 to the more reactive E-isomer.     

Two-dimensional drift solitary structures in inhomogeneous magnetized O?H ion plasmas with generalized (r,q) distributed electrons

In this paper, the effect of generalized (r, q) distributed electrons on the linear and nonlinear coupling of drift and ion acoustic waves in a nonuniform plasma containing Hydrogen and Oxygen ions is investigated. In the linear regime, it is observed that increasing the percentage of flat-topped (i.e. r > 0) electrons enhances the frequency of the coupled drift-ion acoustic waves, whereas the increasing values of the spectral index q mitigates it. In the nonlinear regime, one- and two-dimensional Korteweg de Vries-like and Kadomtsev-Petviashvili-like equations are derived and their solutions are plotted for different ratios of ion number densities and for different values of double spectral indices r and q of the generalized distribution of electrons. It is found that only rarefactive structures exist for two-dimensional solitons, however, both rarefactive and compressive structures are observed for the one-dimensional case. The limiting cases of kappa and Maxwellian distributions are also discussed and their comparison with the generalized (r, q) distribution is thoroughly investigated. Spatial scales for the formation of rarefactive and compressive solitary structures are also discussed with reference to the changing electron distribution functions. The possible applications of the present study are also spelled out with special reference to space plasmas.     

Management of radioactive waste from molybdenum-99 production using low enriched uranium foil target and modified CINTICHEM process

Technetium-99m is the short-lived daughter product of the parent molybdenum-99, which is mainly produced by the fission of uranium-235. Management of radioactive waste is an integral part of fission ⁹⁹Mo production and has high priority during the planning, design, construction and operational phase of a molybdenum-99 production facility. Within the licensing procedure the strategy of waste management, including the route for the spent targets and long lived fission products has to be described and approved by the responsible authorities. Waste will be generated as solids, liquids, and gases, and will include material in the low, intermediate and even highly radioactive categories. Initial treatment of waste streams is usually required at the production site, prior to short or long term storage. This technology is established and generally available in countries with an existing nuclear industry, such as Pakistan. In some instances storage facilities may need to be constructed. On site, treatment of gaseous waste will be carried out in the production facility while off-site treatment will be performed for solids and liquid radioactive waste.     

18‐De­oxy‐13β,14‐dihydrolycoctam: the lycoctamone rearrangement confirmed

The structure of the title compound, C₂₃H₃₅NO₄, contains a unique penta­cyclic ring system wherein one cyclo­hexyl ring adopts a chair conformation, two cyclo­hexyl rings are in boat conformations, and a six‐membered heterocyclic ring and a cyclo­pentyl ring are in envelope conformations. The structures of the lycoctamones, α,β‐unsaturated aldehydes produced by acid‐catalyzed degradation of lactams of lycoctonine‐type alkaloids, previously deduced from the results of extensive chemical investigations have been proven to be correct by the determination of the crystal structure of this compound.     

Crystallographic report: Polymeric [3‐(triphenylgermyl)‐3‐o‐methoxyphenylpropionato]trimethyltin(IV)

The polymeric chains of [Sn(CH₃)₃(C₂₈H₂₅O₃Ge)]_n contain trimethyltin moieties bridging two neighboring 3‐(triphenylgermyl)‐3‐o‐methoxypropionate ligands via carboxyl groups. The germanium atom has a distorted tetrahedral geometry and the tin atom has a distorted trigonal‐bipyramidal geometry, the latter with three methyl groups in the equatorial plane and oxygen atoms defining the axial positions. Copyright © 2005 John Wiley & Sons, Ltd.     

Silica Gel Coated with Schiff's Base: Synthesis and Application as an Adsorbent for Cadmium, Copper, Zinc, and Nickel Determination after Preconcentration by Flame Atomic Absorption Spectrometry

A method for the preconcentration of Cd(II), Cu(II), Zn(II), and Ni(II) is proposed using a minicolumn filled with silica gel modified by Schiff's base. The retained analytes on the ligand-coated silica gel were recovered with a small volume of HNO₃ (nitric acid). The metal ions in the eluent were determined by flame atomic absorption spectrometer (FAAS). Different factors, including the pH of the sample solution, the sample volume, the amount of silica gel, eluent volume and matrix effects for preconcentration were examined. The recoveries for the analytes under optimum working conditions were higher than 95%. The relative standard deviations of the determination were 1.50, 2.10, 2.40, 3.43% for Cu(II), Cd(II), Zn(II) and Ni(II). The limits of detection (3s, n = 10) for Cd(II), Cu(II), Zn(II), and Ni(II) were found to be 3, 5, 5 and 4.7 ngL^–1. The proposed method was applied to the analysis of some natural water samples and standard reference aluminum alloy material (NBS SRM 85b).