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Two-step Reaction (two-step + reaction)
Selected AbstractsPalladium-Catalyzed Phenyl-Selenylation with n-Bu3SnSePh in One-Pot Two-Step Reactions.CHEMINFORM, Issue 35 2006Mariana Bonaterra Abstract ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 200 leading journals. To access a ChemInform Abstract, please click on HTML or PDF. [source] Study of the Complexation, Adsorption and Electrode Reaction Mechanisms of Chromium(VI) and (III) with DTPA Under Adsorptive Stripping Voltammetric ConditionsELECTROANALYSIS, Issue 19 2003Sylvia Sander Abstract The complexation of Cr(III) and Cr(VI) with diethylenetriaminepentaacetic acid (DTPA), the redox behavior of these complexes and their adsorption on the mercury electrode surface were investigated by a combination of electrochemical techniques and UV/vis spectroscopy. A homogenous two-step reaction was observed when mixing Cr(III), present as hexaquo complex, with DTPA. The first reaction product, the electroactive 1,:,1 complex, turns into an electroinactive form in the second step. The results indicate that the second reaction product is presumably a 1,:,2 Cr(III)/DTPA complex. The electroreduction of the DTPA-Cr(III) complex to Cr(II) was found to be diffusion rather than adsorption controlled. The Cr(III) ion, generated in-situ from Cr(VI) at the mercury electrode at about ,50,mV (vs. Ag|AgCl) (3,mol,L,1 KCl), was found to form instantly an electroactive and adsorbable complex with DTPA. By means of electrocapillary measurements its surface activity was shown to be 30 times higher than that of the complex built by homogenous reaction of DTPA with the hydrated Cr(III). Both components, DTPA and the in-situ built complex Cr(III) ion were found to adsorb on the mercury electrode. The effect of nitrate, used as catalytic oxidant in the voltammetric determination method, on the complexation reaction and on the adsorption processes was found to be negligible. The proposed complex structures and an overall reaction scheme are shown. [source] Nickel(II) and palladium(II) complexes with ,-dioxime ligands as catalysts for the vinyl polymerization of norbornene in combination with methylaluminoxane, tris(pentafluorophenyl)borane, or triethylaluminum cocatalyst systems,JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 21 2002Bernd Berchtold Abstract Nickel(II) and palladium(II) complexes with ,-dioxime ligands dimethylglyoxime, diphenylglyoxime, and 1,2-cyclohexanedionedioxime represent six new precatalysts for the polymerization of norbornene that can be activated with methylaluminoxane (MAO), the organo-Lewis acid tris(pentafluorophenyl)borane [B(C6F5)3], and triethylaluminum (TEA) AlEt3. The palladium but not the nickel precatalysts could also be activated by B(C6F5)3 alone, whereas two of the three nickel precatalysts but none of the palladium systems are somewhat active with only TEA as a cocatalyst. It was possible to achieve very high polymerization activities up to 3.2 · 107 gpolymer/molmetal · h. With the system B(C6F5)3/AlEt3, the activation process can be formulated as the following two-step reaction: (1) B(C6F5)3 and TEA lead to an aryl/alkyl group exchange and result in the formation of Al(C6F5)nEt3,n and B(C6F5)3,nEtn; and (2) Al(C6F5)nEt3,n will then react with the precatalysts to form the active species for the polymerization of norbornene. Variation of the B:Al ratio shows that Al(C6F5)Et2 is sufficient for high activation. Gel permeation chromatography indicated that it was possible to control the molar mass of poly(norbornene)s by TEA or 1-dodecene as chain-transfer agents; the molar mass can be varied in the number-average molecular weight range from 2 · 103 to 9 · 105 g · mol,1. © 2002 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 40: 3604,3614, 2002 [source] Hydrothermal Synthesis and Formation Mechanisms of Lanthanum Tin Pyrochlore OxideJOURNAL OF THE AMERICAN CERAMIC SOCIETY, Issue 11 2001Jooho Moon Well-defined La2Sn2O7 with a phase-pure pyrochlore structure was produced by hydrothermal synthesis at temperatures as low as 200°C. Production of phase-pure La2Sn2O7 requires a pH above 10, and higher pH accelerates the crystallization process. The synthesis produced spherical particles of average particle size ,0.59 ,m (±0.12) and surface area ,14.1 m2/g. SEM and TEM observation for morphologic evolution and kinetic analysis during crystallization indicated that La2Sn2O7 formation probably proceeds via a two-step reaction. First a transient dissolution,precipitation occurs. Then the primary crystallites aggregate because of their colloidal instability, and heterocoagulation with the lanthanum hydrous oxide precursor particles also occurs. The sluggish reaction rate at the later stage of reaction is characterized by an insitu transformation, where the soluble tin species is diffused through the porous La2Sn2O7 aggregates to react with entrapped lanthanum precursors. [source] Structure of the d -alanylgriseoluteic acid biosynthetic protein EhpF, an atypical member of the ANL superfamily of adenylating enzymesACTA CRYSTALLOGRAPHICA SECTION D, Issue 6 2010Asim K. Bera The structure of EhpF, a 41,kDa protein that functions in the biosynthetic pathway leading to the broad-spectrum antimicrobial compound d -alanylgriseoluteic acid (AGA), is reported. A cluster of approximately 16 genes, including ehpF, located on a 200,kbp plasmid native to certain strains of Pantoea agglomerans encodes the proteins that are required for the conversion of chorismic acid to AGA. Phenazine-1,6-dicarboxylate has been identified as an intermediate in AGA biosynthesis and deletion of ehpF results in accumulation of this compound in vivo. The crystallographic data presented here reveal that EhpF is an atypical member of the acyl-CoA synthase or ANL superfamily of adenylating enzymes. These enzymes typically catalyze two-step reactions involving adenylation of a carboxylate substrate followed by transfer of the substrate from AMP to coenzyme A or another phosphopantetheine. EhpF is distinguished by the absence of the C-terminal domain that is characteristic of enzymes from this family and is involved in phosphopantetheine binding and in the second half of the canonical two-step reaction that is typically observed. Based on the structure of EhpF and a bioinformatic analysis, it is proposed that EhpF and EhpG convert phenazine-1,6-dicarboxylate to 6-formylphenazine-1-carboxylate via an adenylyl intermediate. [source] Structure of spinach acetohydroxyacid isomeroreductase complexed with its reaction product dihydroxymethylvalerate, manganese and (phospho)-ADP-riboseACTA CRYSTALLOGRAPHICA SECTION D, Issue 4 2000Karine Thomazeau Acetohydroxyacid isomeroreductase catalyses a two-step reaction composed of an alkyl migration followed by an NADPH-dependent reduction. Both steps require a divalent cation and the first step has a strong preference for magnesium. Manganese ions are highly unfavourable to the reaction: only 3% residual activity is observed in the presence of this cation. Acetohydroxyacid isomeroreductase has been crystallized with its substrate, 2-aceto-2-hydroxybutyrate (AHB), Mn2+ and NADPH. The 1.6,Å resolution electron-density map showed the reaction product (2,3-dihydroxy-3-methylvalerate, DHMV) and a density corresponding to (phospho)-ADP-ribose instead of the whole NADP+. This is one of the few structures of an enzyme complexed with its reaction product. The structure of this complex was refined to an R factor of 19.3% and an Rfree of 22.5%. The overall structure of the enzyme is very similar to that of the complex with the reaction-intermediate analogue IpOHA [N -hydroxy- N -isopropyloxamate; Biou et al. (1997), EMBO J.16, 3405,3415]. However, the active site shows some differences: the nicotinamide is cleaved and the surrounding amino acids have rearranged accordingly. Comparison between the structures corresponding to the reaction intermediate and to the end of the reaction allowed the proposal of a reaction scheme. Taking this result into account, the enzyme was crystallized with Ni2+ and Zn2+, for which only 0.02% residual activity were measured; however, the crystals of AHB/Zn/NADPH and of AHB/Ni/NADPH also contain the reaction product. Moreover, mass-spectrometry measurements confirmed the cleavage of nicotinamide. [source] The first structure of a cold-adapted superoxide dismutase (SOD): biochemical and structural characterization of iron SOD from Aliivibrio salmonicidaACTA CRYSTALLOGRAPHICA SECTION F (ELECTRONIC), Issue 2 2009Hege Lynum Pedersen Superoxide dismutases (SODs) are metalloenzymes that catalyse the dismutation of the superoxide radical anion into O2 and H2O2 in a two-step reaction. The crystal structure of the iron superoxide dismutase from the cold-adapted and fish-pathogenic bacterium Aliivibrio salmonicida (asFeSOD) has been determined and refined to 1.7,Å resolution. The protein has been characterized and compared with the closely related homologous iron superoxide dismutase from the mesophilic Escherichia coli (ecFeSOD) in an attempt to rationalize its environmental adaptation. ecFeSOD shares 75% identity with asFeSOD. Compared with the mesophilic FeSOD, the psychrophilic FeSOD has distinct temperature differences in residual activity and thermostability that do not seem to be related to structural differences such as intramolecular or intermolecular ion bonds, hydrogen bonds or cavity sizes. However, an increased net negative charge on the surface of asFeSOD may explain its lower thermostability compared with ecFeSOD. Activity measurements and differential scanning calorimetry measurements revealed that the psychrophilic asFeSOD had a thermostability that was significantly higher than the optimal growth temperature of the host organism. [source] Synthesis, properties, and sulfonation of novel dendritic multiblock copoly(ether-sulfone)JOURNAL OF POLYMER SCIENCE (IN TWO SECTIONS), Issue 19 2008Sumiko Matsumura Abstract Multiblock copoly(ether-sulfone)s (PESs) bearing anchor units for the construction of dendritic blocks were synthesized by two-step reactions: (1) synthesis of PES block with both phenoxide end-groups; (2) chain extension and end-capping of the block by use of excess novel hexafunctional agent, hexakis(4-(4-fluorophenylsulfonyl)phenyl)benzene. The optimum average block length (n) and amount (x) of the hexafunctional agent used for the synthesis of high-molecular-weight PES without crosslinking were n = 26 and x = 2.6 equiv, respectively. The dendritic blocks in the PES were constructed by the aromatic nucleophilic substitution reaction of the activated aromatic fluoride groups on the anchor units using 4-tritylbenzenethiol. The clean substitution of the fluoride groups in the PES was confirmed by 1H NMR and 19F NMR. Three sulfonic acid groups were introduced on the pendant phenyl rings of the trityl groups in the PES by the reaction with chlorosulfonic acid. This is the first example of a dendritic PES bearing clusters of sulfonic acid groups only on the dendritic blocks. Cast films of presulfonated dendritic PES were strong and flexible, however, the membranes of sulfonated dendritic PES were brittle so that the conductivity measurements were not performed. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 6365,6375, 2008 [source] Synthesis and thermal properties of ester-type crosslinked epoxy resins derived from lignosulfonate and glycerolPOLYMER INTERNATIONAL, Issue 2 2010Tuan Noor Maznee Tuan Ismail Abstract Among various biomass-based components, both lignin and glycerol are important, since they are abundantly produced as by-products in industrial processes. Accordingly, in the present study, new types of crosslinked epoxy resins were synthesized from lignin and glycerol. Polymers derived from two types of lignin-based crosslinked epoxy resins were prepared through two-step reactions, ester-carboxylic acid derivative preparation followed by crosslinked epoxy resin preparation, in order to establish a crosslinked epoxy resin system in which glycerol units were included. The resins obtained were labeled as follows: series 1, lignosulfonate-glycerol polyacid (Ser1LSGLYPA); and series 2, glycerol diglycidyl ether (Ser2GLYDGE). The functional groups of the resins were analyzed using Fourier transform infrared spectrometry. The thermal properties of the resins were analyzed using differential scanning calorimetry and thermogravimetry. The glass transition temperature of the crosslinked epoxy resins increased with increasing LSGLYPA and GLYDGE contents for Ser1LSGLYPA and Ser2GLYDGE, respectively. The thermal degradation temperature for Ser1LSGLYPA and Ser2GLYDGE did not show significant change, suggesting that the crosslinked epoxy resins were thermally stable. The mass residue at 500 °C was not affected by the changes of LSGLYPA and GLYDGE contents. Copyright © 2009 Society of Chemical Industry [source] Structure of the d -alanylgriseoluteic acid biosynthetic protein EhpF, an atypical member of the ANL superfamily of adenylating enzymesACTA CRYSTALLOGRAPHICA SECTION D, Issue 6 2010Asim K. Bera The structure of EhpF, a 41,kDa protein that functions in the biosynthetic pathway leading to the broad-spectrum antimicrobial compound d -alanylgriseoluteic acid (AGA), is reported. A cluster of approximately 16 genes, including ehpF, located on a 200,kbp plasmid native to certain strains of Pantoea agglomerans encodes the proteins that are required for the conversion of chorismic acid to AGA. Phenazine-1,6-dicarboxylate has been identified as an intermediate in AGA biosynthesis and deletion of ehpF results in accumulation of this compound in vivo. The crystallographic data presented here reveal that EhpF is an atypical member of the acyl-CoA synthase or ANL superfamily of adenylating enzymes. These enzymes typically catalyze two-step reactions involving adenylation of a carboxylate substrate followed by transfer of the substrate from AMP to coenzyme A or another phosphopantetheine. EhpF is distinguished by the absence of the C-terminal domain that is characteristic of enzymes from this family and is involved in phosphopantetheine binding and in the second half of the canonical two-step reaction that is typically observed. Based on the structure of EhpF and a bioinformatic analysis, it is proposed that EhpF and EhpG convert phenazine-1,6-dicarboxylate to 6-formylphenazine-1-carboxylate via an adenylyl intermediate. [source] | |||||||||||||