Interaction between super-reduced cobalamin and selenite

The kinetics of the reaction between the two-electron reduced form of cobalamin (super-reduced cobalamin, cob(I)alamin, or Cbl(I)) and sodium selenite in an alkaline medium is studied spectrophotometrically. It is shown that the selenite rapidly oxidizes Cbl(I) to cob(II)alamin (Cbl(II)). It is established that the active form of the oxidant is the protonated selenite anion (HSeO₃^-), which receives six electrons during the reaction and transforms into HSe^–. The reactions of cob(I)alamin oxidation by selenite and sulfite are compared.     

Highly stereoselective TiCl4-mediated aldol reactions from (S)-2-benzyloxy-3-pentanone

Stereoselectivity of TiCl4-mediated aldol reactions from (S)-2-benzyloxy-3-pentanone is dramatically improved when the reaction is carried out in the presence of 1.1 equiv of tetrahydrofuran (THF) or 1,2-dimethoxyethane (DME). The resultant 2,4-syn-4,5-syn adducts are then obtained in diastereomeric ratios up to 97:3, which proves that the appropriate choice of the Lewis acid (TiCl4-THF or DME vs Ti(i-PrO)Cl3) engaged in the process permits access to both syn-aldol adducts.     

Lanthanide compounds with fluorinated OC6F5 ligands: homo- and heterovalent complexes of Eu(II) and Eu(III)

The fluorinated phenoxide OC6F5 forms the stable Eu(II) and Eu(III) derivatives (DME)2Eu(mu-OC6F5)3Eu(mu-OC6F5)3Eu(DME)2 and (DME)2Eu(OC6F5)3, as well as the heterovalent product (DME)2Eu(mu-OC6F5)3Eu(DME)(OC6F5)2, in redox reactions of Eu with HOC6F5 or in proton-transfer reactions of HOC6F5 with Eu(SPh)2. The divalent complex crystallizes as a trimer with three bridging phenoxides bridging each pair of metals, with the terminal metals coordinating DME and the central metal ion encapsulated totally by O(C6F5) and dative fluoride interactions. The trivalent compound is monomeric with terminal phenoxide ligands and no Eu-F interactions. The heterovalent compound has clearly localized metal valence states and coordination features that mimic the homovalent species with the terminal OC6F5 bound to the Eu(III) ion, three bridging OR ligands spanning the Eu(II) and Eu(III) ions, and dative Eu(II)-F bonds. At elevated temperatures, these compounds decompose to give a mixture of solid-state fluoride phases.     

ZnVSe2O7 and Cd6V2Se5O21: new d10 transition-metal selenites with V(IV) or V(V) cations

Two new metal selenites with a combination of vanadium(IV) or vanadium(V) cations, namely, ZnVSe 2O 7 and Cd 6V 2Se 5O 21, have been synthesized by hydrothermal and high-temperature solid-state reactions, respectively. The structure of ZnVSe 2O 7 features a 3D network of vanadium(IV) selenite with 1D tunnels occupied by zinc(II) ions. The 3D network of vanadium(IV) selenite is formed by corner-sharing V (IV)O 6 octahedral chains bridged by selenite groups. In Cd 6V 2Se 5O 21, the interconnection of cadmium(II) ions by bridging and chelating selenite groups led to a 3D framework with large tunnels along the b axis, and the 1D chains of corner-sharing V (V)O 4 tetrahedra are inserted in the above large tunnels and are bonded to the cadmium selenite framework via Cd-O-V bridges. Both compounds exhibit broad emission bands in the blue-light region. Results of magnetic property measurements show there is significant antiferromagnetic interaction between V (4+) centers in ZnVSe 2O 7. The electronic structure calculations for both compounds have been also performed.     

Synthesis of Dimethyl Ether from CO Hydrogenation： a Thermodynamic Analysis of the Influence of Water Gas Shift Reaction

Three reactions involved in dimethyl ether (DME) synthesis from CO hydrogenation: methanol synthesis reaction (MSR), methanol dehydration reaction (MDR) and water gas shift reaction (WGSR) are studied by thermodynamic calculation. For demonstrating this process in detail, three models, MSR,MSR MDR, MSR MDR WGSR, are used. Their basic characteristics can be obtained by varying widely the ratios of H2 to CO in the feed (no CO2). Through thermodynamic analysis a chemical synergic effect obviously exists in the second and third models. By comparison between two models it is found that WGSR plays a special role in dimethyl ether synthesis. It is possible for the two models to shift one to the other by regulating CO2 concentration in feed. For Model 2, the selectivity for DME in oxygenates (DME methanol) does not change with the ratio of H2 to CO.     

Methane formation route in the conversion of methanol to hydrocarbons

The influence factors and paths of methane formation during methanol to hydrocarbons （MTH） reaction were studied experimentally and thermodynamically. The fixed-bed reaction results show that the formation of methane was favored by not only high temperature, but also high feed velocity, low pressure, as well as weak acid sites dominated on deactivated catalyst. The thermodynamic analysis results indicate that methane would be formed via the decomposition reactions of methanol and DME, and the hydrogenolysis reactions of methanol and DME. The decomposition reactions are thermal chemistry processes and easily occurred at high temperature. However, they are influenced by catalyst and reaction conditions through DME intermediate. By contrast, the hydrogenolysis reactions belong to catalytic processes. Parallel experiments suggest that, in real MTH reactions, the hydrogenolysis reactions should be mainly enabled by surface active H atom which might come from hydrogen transfer reactions such as aromatization. But H2 will be involved if the catalyst has active components like NiO.     

Surface reactions of dimethyl ether on γ-Al2O3

The surface reactions of dimethyl ether (DME) on industrial alumina (γ-Al₂O₃) were studied by chromatographic analysis of the products at the outlet of the flow reactor and (independently) by diffuse reflectance IR spectroscopy. The major products of the reactions at 250°С were found to be methanol formed in the reaction of DME with hydroxyl groups (the 3720 and 3674 cm^–1 bands in the diffuse reflectance spectrum) and various methoxy groups (the 1121, 1070, 695, and 670 cm^–1 bands in the differential spectra). The presence of molecularly adsorbed methanol was confirmed by experiments with methanol fed in a high-temperature IR cell. The interaction of the resulting methanol molecule with the hydroxyl group led to the formation of a water molecule in the gas phase and a methoxy group on the oxide surface. Strong adsorption of molecular DME was revealed, which was favored by an increase in the temperature of the preliminary calcination of oxide from 250 to 450–500°С; treatment of alumina with water vapor after its preliminary contact with DME led to a recovery of the hydroxyl coating and a replacement of molecularly adsorbed DME with hydroxyl. The thermal effect recorded in a flow reactor was positive during the adsorption of DME and negative during the desorption of weakly bonded DME. Schemes of formation of methoxy groups in the interaction of DME and methanol with surface hydroxyls were suggested.     

Pulsed-field ionization electron spectroscopy and binding energies of alkali metal-dimethyl ether and -dimethoxyethane complexes

Lithium and sodium complexes of dimethyl ether (DME) and dimethoxyethane (DXE) were produced by reactions of laser-vaporized metal atoms with organic vapors in a pulsed nozzle cluster source. The mono-ligand complexes were studied by photoionization and pulsed field ionization zero electron kinetic energy (ZEKE) spectroscopy. Vibrationally resolved ZEKE spectra were obtained for Li(DME), Na(DME) and Li(DXE) and a photoionization efficiency spectrum for Na(DXE). The ZEKE spectra were analyzed by comparing with the spectra of other metal-ether complexes and with electronic structure calculations and spectral simulations. Major vibrations measured for the M(DME) (M=Li,Na) ions were M-O and C-O stretches and M-O-C and C-O-C bends. These vibrations and additional O-Li-O and O-C-C-O bends were observed for the Li(DXE) ion. The M(DME) complexes were in C2v symmetry with the metal atom binding to oxygen, whereas Li(DXE) was in a C2 ring configuration with the Li atom attaching to both oxygen atoms. Moreover, the ionization energies of these complexes were measured from the ZEKE or photoionization spectra and bond dissociation energies were derived from a thermodynamic cycle.     

Ni(3)(Mo(2)O(8))(XO(3)) (X = Se, Te): the first nickel selenite- and tellurite-containing Mo4 clusters

Two new nickel(II) molybdenum(VI) selenium(IV) and tellurium(IV) oxides generally formulated as Ni3(Mo2O8)(XO3) (X = Se, Te) have been synthesized by solid-state reactions of NiO, MoO3, and SeO2 (or TeO2). Both compounds feature 3D network structures built of [Mo4O16]8- tetranuclear cluster units and 2D nickel(II) selenite or tellurite layers. The nickel(II) selenite layer in Ni3(Mo2O8)(SeO3) is formed by [Ni6O22]32- hexanuclear clusters interconnected by selenite groups whereas the thick nickel(II) tellurite layer in Ni3(Mo2O8)(TeO3) is constructed by corrugated nickel(II) oxide chains bridged by the tellurite groups. The results of magnetic property measurements indicate that there are considerable ferromagnetic interactions between nickel(II) centers in both compounds. Their optical properties and band structures have been also studied.     

Methylene substitution reactions in a quadrupole ion trap selectivity of ethylene,ethylene oxide,and dimethyl ether reactive ions

To elucidate the selectivity of methylene substitution reactions of monosubstituted and disubstituted oxyaromatic compounds in a low pressure quadrupole ion trap environment, the relative abundances of covalently bound and loosely bound adducts formed by ion/molecule reactions with ethylene (ET), ethylene oxide (ETOX), and dimethyl ether (DME) were compared. Adduct ions of all three reagent gases were formed in both a conventional ion source and a quadrupole ion trap and characterized by collisionally activated dissociation. For DME and ET, the covalently bound adducts formed at (M + 45)⁺ and (M + 41)⁺, respectively, are direct precursors to the methylene substitution product ions at (M + 13)⁺. ETOX and ET do not demonstrate the same functional group selectivity for methylene substitution as previously observed for DME. This is attributed to differences in reaction exothermicities and competing reactions.     

Cobalt(II)-catalyzed cross-coupling between polyfunctional arylcopper reagents and aryl fluorides or tosylates

[reaction: see text] Organocopper compounds prepared by the transmetalation of functionalized arylmagnesium halides with CuCN.2LiCl undergo smooth cross-coupling reactions with aryl fluorides and tosylates bearing a carbonyl function in the ortho position in the presence of Co(acac)(2) (7.5 mol %), Bu(4)NI (1 equiv), and 4-fluorostyrene (20 mol %) as promoters in DME/THF/DMPU leading to polyfunctional aromatics or heterocycles.     

Reactivity of UH3 with mild oxidants

Addition of 2 equiv of I2 to a stirring suspension of UH3 in Et2O results in vigorous gas evolution and the formation of UI4(OEt2)2 (1), which can be isolated in good yields as an air- and moisture-sensitive brick-red powder. Addition of 3 equiv of AgBr to UH3 in DME produces UBr3(DME)2 (2), while addition of 4 equiv of AgX to UH3 in DME-CH2Cl2 provides UX4(DME)2 (X = Br, 3; Cl, 4). Similarly, the reaction of 4 equiv of AgOTf with UH3 in neat DME generates U(OTf)4(DME)2 (5). Each of these reactions proceeds with the evolution of hydrogen. Complex can also be generated by reaction of 4 equiv of Me3SiI with UCl4 in Et2O. All complexes were fully characterized, including analysis by X-ray crystallography.     

Monte Carlo predictions of phase equilibria and structure for dimethyl ether + sulfur dioxide and dimethyl ether + carbon dioxide

A new force field for dimethyl ether (DME) based on the Lennard-Jones (LJ) 12-6 plus point charge functional form is presented in this work. This force field reproduces experimental saturated liquid and vapor densities, vapor pressures, heats of vaporization, and critical properties to within the statistical uncertainty of the combined experimental and simulation measurements for temperatures between the normal boiling and critical point. Critical parameters and normal boiling point are predicted to within 0.1% of experiment. This force field is used in grand canonical histogram reweighting Monte Carlo simulations to predict the pressure composition diagrams for the binary mixtures DME + SO(2) at 363.15 K and DME + CO(2) at 335.15 and 308.15 K. For the DME + SO(2) mixture, simulation is able to qualitatively reproduce the minimum pressure azeotropy observed experimentally for this mixture, but quantitative errors exist, suggesting that multibody effects may be important in this system. For the DME + CO(2) mixture, simulation is able to predict the pressure-composition behavior within 1% of experimental data. Simulations in the isobaric-isothermal ensemble are used to determine the microstructure of DME + SO(2) and DME + CO(2) mixtures. The DME + SO(2) shows weak pairing between DME and SO(2) molecules, while no specific pairing or aggregation is observed for mixtures of DME + CO(2).     

改性H-MOR沸石催化甲醇转化(英文)

The conversion of methanol was carried out over various mordenite zeolite catalysts to evaluate their catalytic performance. A post-preparation treatment of the H-MOR catalyst by halogenation with NH4Cl or NH4F and by hydrohalogenation with HCl or HF was carried out and its effect on methanol conversion reactions at 100–300 °C in a continuous flow reactor was investigated. The as-synthesized Hmordenite (H-MOR) is generally more active during dimethyl ether (DME) production than the NH4-MOR and Na-MOR. Fluor...     

Mechanisms and kinetics of noncatalytic ether reaction in supercritical water. 2. Proton-transferred fragmentation of dimethyl ether to formaldehyde in competition with hydrolysis

Noncatalytic reaction pathways and rates of dimethyl ether (DME) in supercritical water are determined in a tube reactor made of quartz according to liquid- and gas-phase 1H and 13C NMR observations. The reaction is studied at two concentrations (0.1 and 0.5 M) in supercritical water at 400 degrees C and over a water-density range of 0.1-0.6 g/cm3. The supercritical water reaction is compared with the neat one (in the absence of solvent) at 0.1 M and 400 degrees C. DME is found to decompose through (i) the proton-transferred fragmentation to methane and formaldehyde and (ii) the hydrolysis to methanol. Formaldehyde from reaction (i) is consecutively subjected to four types of redox reactions. Two of them proceed even without solvent: (iii) the unimolecular proton-transferred decarbonylation forming hydrogen and carbon monoxide and (iv) the bimolecular self-disproportionation generating methanol and carbon monoxide. When the solvent water is present, two additional paths are open: (v) the bimolecular self-disproportionation of formaldehyde with reactant water, producing methanol and formic acid, and (vi) the bimolecular cross-disproportionation between formaldehyde and formic acid, yielding methanol and carbonic acid. Methanol is produced through the three types of disproportionations (iv)-(vi) as well as the hydrolysis (ii). The presence of solvent water decelerates the proton-transferred fragmentation of DME; the rate constant is reduced by 40% at 0.5 g/cm3. This is caused by the suppression of low-frequency concerted motion corresponding to the reaction coordinate for the simultaneous C-O bond scission and proton transfer from one methyl carbon to the other. In contrast to the proton-transferred fragmentation, the hydrolysis of DME is markedly accelerated by increasing the water density. The latter becomes more important than the former in supercritical water at densities greater than 0.5 g/cm3.     

Niobium and tantalum tris(methimazolyl)borate complexes [M(=NC6H3iPr2-2,6)Cl2{HB(mt)3}] (M = Nb, Ta; mt = methimazolyl)

The first early transition metal tris(methimazolyl)borate com-plexes [M(=NR)Cl2{HB(mt)3}] (M = Nb, Ta; R = C6H3(i)Pr(2)-2,6; mt = methimazolyl) have been obtained from the reactions of [Nb(=NR)Cl3(DME)] or [Ta(=NR)Cl3(THF)2] (DME = dimethyl ether; THF = tetrahydrofuran) with Na[HB(mt)3] and structurally characterized, illustrating that the HB(mt)3 ligand can indeed be compatible with "hard" metals in high oxidation states.     

Palladium-catalyzed cross-alkynylation of aryl bromides by sodium tetraalkynylaluminates

Sodium tetraalkynylaluminates (1-4), prepared from NaAlH4 and terminal alkynes, cross-couple with aryl bromides in the presence of Pd(0) and Pd(II) catalysts. The reactions take place in boiling THF or DME. The process is applicable to both homo- and heterocyclic aryl bromides and can be used for conversion of polybromo compounds into polyalkynes. The reactions are high yielding and selective, free of undesired homocoupling and hydrogenolysis processes. The reagents selectively react with the ring-bound bromine atoms but do not affect chloro, cyano, triflate, or ester functions.     

Highly enantioselective synthesis of gamma-hydroxy-alpha,beta-acetylenic esters catalyzed by a beta-sulfonamide alcohol

This work concerns the asymmetric addition of methyl propiolate to aldehydes with 1,2-dimethoxyethane (DME) as additive and beta-sulfonamide alcohol titanium complex as a catalyst. The reactions proceeded under mild conditions and gave the highly functionalized chiral propargylic alcohols with high ee values and good yields. Differences between three types of ligands have also been discussed.     

浆态床二甲醚合成中V2O5、Sm2O3对脱水组分γ-Al2O3的修饰作用

采用等容浸渍法制备改性脱水催化剂,通过H2-TPR、Pyridine-IR、还原态NH3-TPD、XRD等表征手段,以及目标反应浆态床CO+H2合成二甲醚,研究了催化剂的还原性能以及酸中心分布与反应性能之间的关系。H2-TPR结果表明,在脱水催化剂γ-Al2O3、V2O5/γ-Al2O3和Sm2O3/γ-Al2O3上不出现还原峰,V2O5、Sm2O3的加入改善了复合催化剂中Cu的还原性能,促进了甲醇催化剂的还原。Pyridine-IR表明,V2O5和Sm2O3的加入对L酸、B酸的量影响不大。还原态NH3-TPD说明V2O5和Sm2O3的加入改变了酸中心的分布,增加了弱酸中心的比率。XRD结果发现,V2O5和Sm2O3均匀分散在γ-Al2O3上,没有新的物种生成。二甲醚合成目标反应的结果表明,改性后催化剂的反应活性增强,合成反应中CO转化率、二甲醚的选择性都得到提高。V2O5和Sm2O3的添加增加了弱酸中心数量,促进了脱水活性,从而提高了复合催化剂合成二甲醚的活性和选择性。     

Supercritical fluid extraction of polycyclic aromatic hydrocarbons (PAH) from native soil and fly ash with pure and modified carbon dioxide and dimethylether

Polycyclic aromatic hydrocarbons (PAH) and alkyl-PAH in the lower mg/kg range were extracted from soil and fly ash. Extraction yields were measured for toluene Soxhlet, supercritical carbon dioxide, toluene modified carbon dioxide, and toluene modified dimethylether (DME) extractions. Pure DME similarly to toluene Soxhlet extraction enhances extraction yields (32 mg/kg) as compared to pure carbon dioxide (21 mg/kg). In particular, higher molecular weight PAH are extracted with pure DME. 5% Toluene modified carbon dioxide (43 mg/kg) and toluene modified DME (50 mg/kg) yield much better extraction yields than Soxhlet (32 mg/kg) or pure supercritical fluid extractions. Received: 19 July 1996 / Revised: 23 September 1996 / Accepted: 22 October 1996