Distribution by Scientific Domains
Distribution within Chemistry

Kinds of Catalyzes

  • complex catalyze
  • enzyme catalyze
  • synthase catalyze

  • Selected Abstracts


    JOURNAL OF PHYCOLOGY, Issue 1 2009
    Leonardo Tavares Salgado
    The interaction between phenolic substances (PS) and alginates (ALG) has been suggested to play a role in the structure of the cell walls of brown seaweeds. However, no clear evidence for this interaction was reported. Vanadium bromoperoxidase (VBPO) has been proposed as a possible catalyst for the binding of PS to ALG. In this work, we studied the interaction between PS and ALG from brown algae using size exclusion chromatography (SEC) and optical tweezers microscopy. The analysis by SEC revealed that ALG forms a high-molecular-weight complex with PS. To study the formation of this molecular complex, we investigated the in vitro interaction of purified ALG from Fucus vesiculosus L. with purified PS from Padina gymnospora (Kütz.) Sond., in the presence or absence of VBPO. The interaction between PS and ALG only occurred when VBPO was added, indicating that the enzyme is essential for the binding process. The interaction of these molecules led to a reduction in ALG viscosity. We propose that VBPO promotes the binding of PS molecules to the ALG uronic acids residues, and we also suggest that PS are components of the brown algal cell walls. [source]

    Palladium,NHC Complexes Do Catalyze the Diboration of Alkenes: Mechanistic Insights.

    CHEMINFORM, Issue 50 2007
    Vanesa Lillo
    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]

    Laccase from Basidiomycetous Fungus Catalyzes the Synthesis of Substituted 5-Deaza-10-oxaflavins via a Domino Reaction

    Mazaahir Kidwai
    Abstract The present investigation provides a simple and convenient route for the synthesis of substituted 5-deaza-10-oxaflavins owing to their importance as probable redox coenzymes. The reaction of ,,,-unsaturated derivatives of barbituric acid and dimedone with catechol or 1,4-hydroquinones was catalyzed using laccase in aqueous medium. Quinones, generated in situ by the oxidation of the corresponding catechol or 1,4-hydroquinones, underwent a domino reaction with chalcones to produce 5-deaza-10-oxaflavins and tetrahydroxanthen-1-ones. [source]

    ChemInform Abstract: Unmodified Nano-Powder Magnetite Catalyzes a Four-Component Aza-Sakurai Reaction.

    CHEMINFORM, Issue 42 2008
    Ricardo Martinez
    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 of an article which was published elsewhere, please select a "Full Text" option. The original article is trackable via the "References" option. [source]

    Heterogeneous Catalyst: Amberlyst-15 Catalyzes the Synthesis of 14-Substituted-14H-dibenzo[a,j]xanthenes under Solvent-Free Conditions.

    CHEMINFORM, Issue 14 2007
    Shengkai Ko
    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]

    [Ru(DMSO)4]Cl2 Catalyzes the ,-Alkylation of Ketones by Alcohols.

    CHEMINFORM, Issue 37 2005
    Ricardo Martinez
    Abstract For Abstract see ChemInform Abstract in Full Text. [source]

    Rhodium-Catalyzed Asymmetric 1,4-Addition of Alkenylsilanes Generated by Hydrosilylation of Alkynes: A One-Pot Procedure Where a Rhodium/(S)-Binap Complex Catalyzes the Two Successive Reactions.

    CHEMINFORM, Issue 3 2005
    Yusuke Otomaru
    Abstract For Abstract see ChemInform Abstract in Full Text. [source]

    Palladium(II) Chloride Catalyzes the Cross-Coupling Reaction of 2,5-Bis-(butyltelluro)-furan and 1-Alkynes.

    CHEMINFORM, Issue 19 2003
    Gilson Zeni
    Abstract For Abstract see ChemInform Abstract in Full Text. [source]

    Combinatorial expression patterns of heparan sulfate sulfotransferases in zebrafish: II.

    The 6- O -sulfotransferase family
    Abstract Heparan sulfate (HS) is an unbranched chain of repetitive disaccharides, which specifically binds ligands when attached to the cell surface or secreted extracellularly. HS chains contain sulfated domains termed the HS fine structure, which gives HS specific binding affinities for extracellular ligands. HS 6- O -sulfotransferases (6-OST) catalyze the transfer of sulfate groups to the 6- O position of glucosamine residues of HS. We report here the characterization and developmental expression analysis of the 6-OST gene family in the zebrafish. The zebrafish 6-OST gene family consists of four conserved vertebrate orthologues, including a gene duplication specific to zebrafish. We examined the mRNA expression patterns in several tissues/organs throughout early zebrafish development, including early cleavage stages, eyes, somites, brain, internal organ primordial, and pectoral fin development. Members of the 6-OST gene family have spatially and temporally distinct restricted expression, suggesting in vivo functional differences exist between members of this family. Developmental Dynamics 235:3432,3437, 2006. © 2006 Wiley-Liss, Inc. [source]

    Amperometric L -Lactate Biosensor Based on Gold Nanoparticles

    ELECTROANALYSIS, Issue 7-8 2007
    Bikash, Kumar Jena
    Abstract A novel amperometric biosensor for the sensing of L -lactate is developed using L -lactate dehydrogenase (LDH) and hydroxylamine enlarged gold nanoparticles (GNPs). LDH and GNPs have been integrated with the sol,gel 3-D silicate network derived from 3-(mercaptopropyl)trimethoxysilane (MPTS). The biosensing of L -lactate is based on the electrocatalytic determination of enzymatically generated NADH by GNPs of the integrated assembly. The GNPs on the network efficiently catalyze the oxidation of NADH at ,0.065,V, which is ca. 915,mV less positive than on the bulk Au electrode. The biosensor was characterized in terms of the effects of enzyme loading, solution pH, and cofactor concentration. The integrated biosensor was successfully utilized for the amperometric sensing of L -lactate and it shows excellent sensitivity with a detection limit of 100,nM. The common interfering electroactive compounds in the biological system do not interfere the amperometric measurement of L -lactate. This biosensor linearly responds to L -lactate in the range of 0,0.8,mM and the sensitivity of the electrode was 0.446,nA/nM. Excellent reproducibility, long time storage and operational stability have been achieved. [source]

    Electrocatalytic Oxidation and Voltammetric Determination of Hydrazine on the Tetrabromo- p -Benzoquinone Modified Carbon Paste Electrode

    ELECTROANALYSIS, Issue 5 2007
    Jahan-Bakhsh Raoof
    Abstract The electrochemical properties of hydrazine studied at the surface of a carbon paste electrode spiked with p -bromanil (tetrabromo- p -benzoquinone) using cyclic voltammetry (CV), double potential-step chronoamperometry and differential pulse voltammetry (DPV) in aqueous media. The results show this quinone derivative modified carbon paste electrode, can catalyze the hydrazine oxidation in an aqueous buffered solution. It has been found that under the optimum conditions (pH,10.00), the oxidation of hydrazine at the surface of this carbon paste modified electrode occurs at a potential of about 550,mV less positive than that of a bar carbon paste electrode. The electrocatalytic oxidation peak current of hydrazine showed a linear dependent on the hydrazine concentrations and linear analytical curves were obtained in the ranges of 6.00×10,5 M,8.00×10,3 M and 7.00×10,6 M,8.00×10,4 M of hydrazine concentration with CV and differential pulse voltammetry (DPV) methods, respectively. The detection limits (3,) were determined as 3.6×10,5 M and 5.2×10,6 M by CV and DPV methods. This method was also used for the determination of hydrazine in the real sample (waste water of the Mazandaran wood and paper factory) by standard addition method. [source]

    Multilayer Assemblies Consisting of Tri-Vanadium-Substituted Heteropolyanions and Its Electrocatalytic Properties

    ELECTROANALYSIS, Issue 14 2003
    Shengyong Zhai
    Abstract We describe the controlled fabrication of ultrathin multilayer films consisting of tri-vanadium- substituted heteropolytungstate anions (denoted as P2W15V3) and a cationic polymer of quaternized poly (4-vinylpyridine) partially complexed with osmium bis(2,2,-bipyridine) (denoted as QPVP-Os) on the 4-aminobenzoic acid (4-ABA) modified glassy carbon electrode (GCE) surface based on layer-by-layer assembly. Cyclic voltammetry and UV-vis absorption spectrometry have been used to easily monitor the thickness and uniformity of thus-formed multilayer films. The V-centered redox reaction of P2W15V3 in the multilayer films can effectively catalyze the reduction of BrO and NO. The resulting P2W15V3/QPVP-Os multilayer film modified electrode behaves as a much promising electrochemical sensor because of the low overpotential for the catalytic reduction of BrO and NO, and the catalytic oxidation of ascorbic acid. [source]

    Gold nanoparticle-enhanced capillary electrophoresis-chemiluminescence assay of trace uric acid

    ELECTROPHORESIS, Issue 15 2009
    Shulin Zhao
    Abstract A sensitive method based on gold nanoparticle-enhanced CE-chemiluminescence (CL) detection was developed for quantifying uric acid (UA) in serum. In this work, gold nanoparticles were added into the running buffer of CE to catalyze the post-column CL reaction between luminol and hydrogen peroxide, achieving highly efficient CL emission. Negative peaks were produced due to the inhibitory effects on CL emission from UA eluted from the electrophoretic capillary. The decrease in CL intensity was proportional to the concentration of UA in the range of 2.5×10,7,1.0×10,5,M. Detection limit was 4.6×10,8,M UA. Ten human serum samples were analyzed by the presented method. Serum level of UA was found to be in the range from 204 to 324,,M for healthy subjects (n=5), and from 464 to 497,,M for diabetic patients (n=5). The two groups were significantly different (p<0.05). The results suggested a potential application of the proposed assay in rapid primary diagnosis of diseases such as diabetes. [source]

    Potential Applications of Oxidoreductases for the Re-oxidation of Leuco Vat or Sulfur Dyes in Textile Dyeing

    F. Xu
    Abstract Conventional textile dyeing by vat and sulfur dyes includes reduction and re-oxidation steps (with chemical reductants and oxidants), so that the insoluble dyes can be solubilized in the dyeing solution, adsorbed by the fabric, and fixed onto the dyed fabric. The treatments often involve hazardous chemicals, expensive catalysts, or conditions that are suboptimally effective, energy-intensive, caustic, or polluting. Improving these steps with enzyme technology could be of significant interest in terms of better dyeing, handling of hazardous chemicals, disposal of waste, or production economy. The idea of an enzymatic re-oxidation step for vat and sulfur dyeings was tested under simplified laboratory conditions. Selected vat and sulfur dyes, including Vat Blue,43, Vat Orange,7, Vat Green,3, Vat Orange,2, Vat Red,13, Vat Yellow,2, and Sulfur Black,1, were first chemically reduced. The reduced (leuco) dyes were then re-oxidized by aerated buffer solutions or H2O2, in the presence or absence of an oxidoreductase, selected from seven laccases from Myceliophthora thermophila, Scytalidium thermophilum, Coprinus cinereus, Trametes villosa, Rhizoctonia solani, Pycnoporus cinnabarinus, Botrytis cinerea, a bilirubin oxidase from Myrothecium verrucaria, and a heme peroxidase from Coprinus cineresu. It was shown that the enzymes were able to catalyze and accelerate the re-oxidation of the reduced dyes, even when they were adsorbed on cotton fabric, by dissolved air (O2) or H2O2. Small redox-active mediators could facilitate the enzymatic re-oxidation. For Sulfur Black,1, a higher conversion of the leuco dye was achieved with laccase-catalyzed re-oxidation. The further development of this potential enzyme application is discussed. [source]

    Complexes of glutathione with heavy metal ions as a new biochemical marker of aquatic environment pollution,

    Jiri Baloun
    Abstract Reduced glutathione (GSH) plays a number of key roles in many biochemical pathways. This peptide is highly reactive and forms conjugates with other molecules via its sulfhydryl moiety. The interactions of the common heavy metal pollutant Cd(II) with GSH were determined by using the Brdicka reaction to evaluate whether this technique would be suitable as a biomarker. After GSH interaction with Cd(II) ions, two characteristic changes in the measured voltammogram were observed: Cat2 signal height decreased, and a new signal called P1 was found. The observed signal probably relates to the formation of a GSH,heavy metal ion complex adsorbed on the surface of the working electrode. When the interaction of GSH with cisplatin was studied, the same characteristic changes in the voltammogram were observed, which confirmed our hypothesis. Moreover, changes in the height of P1 and Cat2 signals with increasing time of GSH interaction with Cd(II) ions and/or cisplatin were also investigated. Cat2 peak height decreased proportionally with increasing time of interaction. This decrease can be explained by shielding of free sulfhydryl moiety by heavy metal ions, so it cannot catalyze the evolution of hydrogen from the supporting electrolyte. In addition, we found that, with increasing time of the interaction, the P1 signal was enhanced and shifted to more positive potentials for both Cd(II) ions and cisplatin. Environ. Toxicol. Chem. 2010;29:497,500. © 2009 SETAC [source]

    Metalloporphyrin solubility: A trigger for catalyzing reductive dechlorination of tetrachloroethylene

    Ishai Dror
    Abstract Metalloporphyrins are well known for their electron-transfer roles in many natural redox systems. In addition, several metalloporphyrins and related tetrapyrrole macrocycles complexed with various core metals have been shown to catalyze the reductive dechlorination of certain organic compounds, thus demonstrating the potential for using naturally occurring metalloporphyrins to attenuate toxic and persistent chlorinated organic pollutants in the environment. However, despite the great interest in reductive dechlorination reactions and the wide variety of natural and synthetic porphyrins currently available, only soluble porphyrins, which comprise a small fraction of this particular family of organic macrocycles, have been used as electron-transfer shuttles in these reactions. Results from the present study clearly demonstrate that metalloporphyrin solubility is a key factor in their ability to catalyze the reductive dechlorination of tetrachloroethylene and its daughter compounds. Additionally, we show that certain insoluble and nonreactive metalloporphyrins can be activated as catalysts merely by changing solution conditions to bring about their dissolution. Furthermore, once a metalloporphyrin is fully dissolved and activated, tetrachloroethylene transformation proceeds rapidly, giving nonchlorinated and less toxic alkenes as the major reaction products. Results from the present study suggest that if the right environmental conditions exist or can be created, specific metalloporphyrins may provide a solution for cleaning up sites that are contaminated with chlorinated organic pollutants. [source]

    16-Electron (Arene)ruthenium Complexes with Superbasic Bis(imidazolin-2-imine) Ligands and Their Use in Catalytic Transfer Hydrogenation

    Thomas Glöge
    Abstract The ligands N,N, -bis(1,3,4,5-tetramethylimidazolin-2-ylidene)-1,2-ethanediamine (BLMe) and N,N, -bis(1,3-diisopropyl-4,5-dimethylimidazolin-2-ylidene)-1,2-ethanediamine(BLiPr) react with [(,5 -C5Me5)RuCl]4 to afford cationic 16-electron half-sandwich complexes [(,5 -C5Me5)Ru(BLR)]+ (R = Me, 3; R = iPr, 4), which resist coordination of the chloride counterion because of the strong electron-donating ability of the diimine ligands. Upon reaction with [(,6 -C6H6)RuCl2]2 or [(,6 -C10H14)RuCl2]2, these ligands stabilize dicationic 16-electron benzene and cymene complexes of the type [(,6 -C6H6)Ru(BLR)]2+ (R = Me, 5; R = iPr, 6) and [(,6 -C10H14)Ru(BLR)]2+ (R = Me, 7; R = iPr, 8). The X-ray crystal structure of [5]Cl2 reveals the absence of any direct Ru,Cl interaction, whereas a long Ru,Cl bond, supported by two CH···Cl hydrogen bonds, is observed for [(6)Cl]Cl in the solid state. Treatment of the dichlorides of 6 and 8 with NaBF4 affords [6](BF4)2 and [8](BF4)2, which are composed of individual dications and tetrafluoroborate ions with no direct Ru,F interaction. All complexes catalyze the transfer hydrogenation of acetophenone in boiling 2-propanol. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

    Fluoride-Free Hiyama and Copper- and Amine-Free Sonogashira Coupling in Air in a Mixed Aqueous Medium by a Series of PEPPSI-Themed Precatalysts,

    Chandrakanta Dash
    Abstract A new series of robust, user-friendly, and highly active PEPPSI-themed (pyridine-enhanced precatalyst preparation, stabilization and initiation) (NHC)PdX2(pyridine)-type (X = Cl, Br) precatalysts of C4,C5 saturated imidazole- (1,4) and triazole-based (5 and 6) N-heterocyclic carbenes for the Hiyama and Sonogashira couplings under amenable conditions are reported. Specifically 1,6 efficiently catalyze the fluoride-free Hiyama coupling of aryl halides with PhSi(OMe)3 and CH2=CHSi(OMe)3 in air in the presence of NaOH as a base in a mixed aqueous medium (dioxane/H2O, 2:1 v/v). Along the same lines, these 1,6 precatalysts also promote the Cu-free and amine-free Sonogashira coupling of aryl bromides and iodides with phenylacetylene in air and in a mixed aqueous medium (DMF/H2O, 3/1 v/v). The complexes 1,6 were synthesized by the direct reaction of the respective imidazolinium and triazolium halide salts with PdCl2 in pyridine in the presence of K2CO3 as a base. DFT studies on the catalytically relevant palladium(0) (NHC)Pd(pyridine) precursors 1a,6a reveal significant donation from the N-heterocyclic carbene lone pair onto the unfilled ,* orbital of the trans Pd,pyridine bond. This weakens the Pd-bound "throwaway" pyridine ligand, and its dissociation marks the initiation of the catalytic cycle.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2009) [source]

    A Polymer-Bound Oxidovanadium(IV) Complex Prepared from an L -Cysteine-Derived Ligand for the Oxidative Amination of Styrene

    Mannar R. Maurya
    Abstract The ligand H2sal-cys (I) derived from salicylaldehyde and L -cysteine has been covalently bonded to chloromethylated polystyrene cross-linked with 5,% divinylbenzene. Upon treatment with [VO(acac)2] in dimethylformamide (DMF) the polystyrene-bound ligand PS-H2sal-cys (II) gave the oxidovanadium(IV) complex, PS-[VO(sal-cys)·DMF] (1). The corresponding neat complex, [VO(sal-eta)]2 (2), has also been prepared similarly in methanol. These complexes have been characterised by IR, electronic, EPR spectroscopic studies, magnetic susceptibility measurements and thermal as well as scanning electron micrographs studies. Complex [VO(sal-eta)]2 exhibits a medium intensity band at 980 cm,1 in the IR spectrum due to ,(V=O) stretch. Broad features of the EPR spectrum for the neat complex along with magnetic susceptibility studies suggest the presence of antiferromagnetic exchange interaction between two vanadium centers in close proximity. Both complexes catalyze the oxidative amination of styrene, in mild basic conditions, with secondary amines (diethylamine, imidazole, and benzimidazole) and gave a mixture of two aminated products in good yields. Amongst the two aminated products, the anti-Markovnikov product is favored over the Markovnikov one due to the steric hindrance posed by the secondary amines. The polymer-anchored heterogeneous catalyst is free from leaching during catalytic action and recyclable.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

    Pyridine Carboxylate Complexes of MoII as Active Catalysts in Homogeneous and Heterogeneous Polymerization

    Maria Vasconcellos-Dias
    Abstract New lamellar materials intercalated with molybdenum(II) complexes with potential catalytic properties were prepared by a stepwise procedure. The lamellar material was first calcined at 823 K for four hours to eliminate all the carbonate ions; the layered structure was reconstructed after treatment with a solution of either pycH (pyridine-2-carboxylic acid) or pydcH2 (pyridine-2,6-dicarboxylic acid) in a KOH solution of dmf at 343 K. Impregnation with a solution of the organometallic precursor [Mo(CO)3I2(NCCH3)2] led to substitution of the nitrile groups by two pyridine ligands. All the materials were characterized by powder X-ray diffraction, FTIR, and 13C CP MAS and 27Al MAS solid-state NMR spectroscopy. Similar MoII complexes were also prepared by using pycH or pydcH2 and characterized by elemental analysis as well as FTIR and 1H and 13C solution NMR spectroscopy. These new materials and the complexes of pyc or pydc ligands containing 4.54 wt.-% and 6.33 wt.-% of Mo respectively, catalyze the ring-opening-metathesis polymerization of norbornene and the polymerization of styrene at 333 K, their performance increasing upon the addition of methylalumoxane (MAO) as cocatalyst.(© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

    Synthesis and Characterization of MoOI2(PMe3)3 and Use of MoOX2(PMe3)3 (X = Cl, I) in Controlled Radical Polymerization

    José A. Mata
    Abstract Complex MoOCl2(PMe3)3 smoothly reacts with NaI in acetone to produce MoOI2(PMe3)3 in good yields. The geometry of the compound is mer - cis octahedral, that is, identical to that of the dichloride precursor, as shown by NMR spectroscopy and by an X-ray crystallographic study. Electrochemical investigations of MoOX2(PMe3)3 show irreversible oxidation waves at Ep,a = +0.18 and +0.39 V for X = Cl and I, respectively. A study of the halide exchange between MoOCl2(PMe3)3 and NaI, or between MoOI2(PMe3)3 and Bu4NCl, shows two equilibrated isomers for the mixed halide intermediate MoOICl(PMe3)3. The diiodide complex rapidly exchanges the iodo ligands with chloride upon dissolution in chloroform at room temperature, and with bromide from (1-bromoethyl)benzene (BEB) under more forcing conditions. The equilibrium favors the softer halide (I) on C and the harder one (Cl or Br) on MoIV. Both oxido compounds catalyze the atom transfer radical polymerization (ATRP) of styrene in combination with the BEB initiator, yielding polymers with quite narrow molecular weight distributions (down to 1.11). The apparent polymerization rate constant is approximately doubled in the presence of 1 equiv. of the Al(OiPr)3 cocatalyst. On the other hand, the system is not capable of efficiently controlling the radical chain growth for methyl acrylate polymerization. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

    Cobalt(II) Complexes with Substituted Salen-Type Ligands and Their Dioxygen Affinity in N,N -Dimethylformamide at Various Temperatures

    Andreas Huber
    Abstract Several unsymmetrically substituted salen-type cobalt(II) Schiff-base complexes CoL [H2L = 1,6-bis(2-hydroxyphenyl)-3,3-dimethyl-2,5-diaza-1,5-hexadiene (1); 1,6-bis(2-hydroxyphenyl)-3,3-dimethyl-2,5-diaza-1,5-heptadiene (2); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-6-(2-hydroxyphen-yl)-3,3-dimethyl-2,5-diaza-1,5-heptadiene (3); 1-(2-hydroxyphenyl)-6-methyl-2,5-diaza-1,5-nonadien-8-one (4); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-6-methyl-2,5-di-aza-1,5-nonadien-8-one (5); 1-(2-hydroxyphenyl)-3,3,6-trimethyl-2,5-diaza-1,5-nonadien-8-one (6); 1-(3- tert -butyl-2-hydroxy-5-methylphenyl)-3,3,6-trimethyl-2,5-diaza-1,5-nonadien-8-one (7)] were prepared and characterized by their UV/Vis absorption spectra, magnetic moments, and oxidation potentials. Except for complex 4 (irreversible oxidation with t½ , 3 h), complexes 1,3 and 5,7 are remarkably resistant against irreversible auto-oxidation in air-saturated N,N -dimethylformamide (DMF) at ambient temperature. To characterize the Lewis acidity of the cobalt center in 1,7, the equilibrium constant Kpy was determined for monoadduct formation with pyridine (CoL + pyCoL·py). An O2 -sensitive optode was used to determine the Henry constant, KH, for the system O2/DMF in the temperature range 298,228 K. The formation of 1:1 adducts of complexes 1,7 with O2 in DMF, as characterized by the equilibrium constant K, was followed spectrophotometrically in the temperature range 298,228 K. The parameters ,Ho, ,So, and K are reported. At 298 K, K ranges from 21.9 M,1 (5) to 155 M,1 (7). The overall spectroscopic information, including EPR spectra obtained with frozen solutions of 3 and 7 in O2 -saturated DMF, confirm that the 1:1 adducts CoL·O2 are cobalt(III) superoxo compounds. The symmetrically substituted salen complex8 [H2L = 1,6-bis(3- tert -butyl-2-hydroxy-5-methylphenyl)-3,3,4,4-tetramethyl-2,5-diaza-1,5-hexadiene in 8] is shown to catalyze the oxidation of triphenylphosphane and 2,6-di- tert -butylphenol by O2 in DMF at ambient temperature. The correlation of the data obtained for K, Kpy, and the oxidation potential E½ is discussed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

    Enantioselective CpRu-Catalyzed Carroll Rearrangement , Ligand and Metal Source Importance

    David Linder
    Abstract The addition of unstabilized carbonyl nucleophiles to unsymmetrical allyl-metal fragments still represents a challenge to generate stereogenic centers enantio- and regioselectively. In this context, the decarboxylative Carroll rearrangement of allyl ,-keto esters is particularly interesting, since chiral ,,,-unsaturated ketones are obtained. Herein, we show that CpRu half-sandwich complexes can, with selected enantiopure pyridine-monooxazoline ligands, catalyze this transformation and afford complete conversions along with good levels of regioselectivity and enantioselectivity. Even more challenging (electron-poor) substrates react (up to 86,% ee, branched/linear ratio , 97:03). In addition, the use of an air-stable metal precursor, namely [CpRu(,6 -naphthalene)][PF6], allows the reaction to be carried out reproducibly evenin non-anhydrous THF with a catalyst loading as low as2 mol-%. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

    Neutral Group-IV Metal Catalysts for the Intramolecular Hydroamination of Alkenes

    Carsten Müller
    Abstract A detailed comparison of the group-IV metal catalysts Ti(NMe2)4, Ind2TiMe2, Ind2ZrMe2 and Ind2HfMe2 in the intramolecular hydroamination of amino alkenes is presented. Among these catalysts, the benchmark catalyst Ti(NMe2)4 is the most active in the formation of pyrrolidines. A comparison between Ind2TiMe2, Ind2ZrMe2 and Ind2HfMe2 suggests that in the synthesis of pyrrolidines, Zr complexes show the highest catalytic activity of the group-IV metal catalysts. Although Ind2TiMe2 - and the Ind2ZrMe2 -catalyzed formation of a pyrrolidine is first-order in the concentration of the substrate, the corresponding Ti(NMe2)4 -catalyzed cyclization is second-order in the concentration of the substrate. The results obtained for the formation of piperidines catalyzed by Ti(NMe2)4, Ind2TiMe2, Ind2ZrMe2 and Ind2HfMe2 suggest that for these reactions, Ti catalysts show increased catalytic activity compared with the corresponding Zr catalysts. Unfortunately, the formation of aminocyclopentane side-products by C,H activation processes is a severe drawback of the Ti catalysts. The corresponding side-products are not formed in Ind2ZrMe2 - and Ind2HfMe2 -catalyzed reactions. However, the former catalyst gives better yields of the desired piperidine products. In contrast to the results obtained for the synthesis of pyrrolidines, the formation of a piperidine is zero-order in the concentration of the substrate for the indenyl catalysts Ind2TiMe2 and Ind2ZrMe2, and first-order for the homoleptic catalyst Ti(NMe2)4. Interestingly, Ind2TiMe2 is able to catalyze a slow hydroamination of an N -methylated amino alkene, whereas the homoleptic complex Ti(NMe2)4 as well as Ind2ZrMe2 and Ind2HfMe2 do not catalyze the same reaction. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

    Ruthenium-Catalyzed Allenyl Carbamate Formation from Propargyl Alcohols and Isocyanates

    Edgar Haak
    Abstract Ruthenium complexes of redox-coupled cyclopentadienone ligands catalyze the formation of allenyl carbamates from propargyl alcohols and isocyanates. This efficient and atom-economical process represents the first catalytic access to allenyl carbamates, compounds of high synthetic potential. The reaction needs an acidic co-catalyst and can be performed at room temperature. In addition, new (cyclopentadienone)iron and -ruthenium complexes were synthesized, and mechanistic aspects regarding catalytic transformations of propargyl systems with ruthenium catalysts are obtained. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2008) [source]

    The Ritter Reaction under Truly Catalytic Brønsted Acid Conditions

    Roberto Sanz
    Abstract Simple organic acids like 2,4-dinitrobenzenesulfonic acid (DNBSA) catalyze the Ritter reaction of secondary benzylic alcohols giving rise to the corresponding N -benzylacetamides in usually high yields. Reactions can be conducted without exclusion of oxygen and without the need of dry solvents. With tertiary ,,,-dimethylbenzylic alcohols a different pathway involving a formal dimerization reaction takes place under the acid-catalytic conditions used. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007) [source]

    Thermus thermophilus Glycosynthases for the Efficient Synthesis of Galactosyl and Glucosyl ,-(1,3) - Glycosides

    Jullien Drone
    Abstract Inverting mutant glycosynthases were designed according to the Withers strategy, starting from wild-type Thermus thermophilus retaining Tt-,-Gly glycosidase. Directed mutagenesis of catalytic nucleophile glutamate 338 by alanine, serine, and glycine afforded the E338A, E338S, and E338G mutant enzymes, respectively. As was to be expected, the mutants were unable to catalyze the hydrolysis of the transglycosidation products. In agreement with previous results, the E338S and E338G catalysts were much more efficient than E338A. Moreover, our results showed that these enzymes were inactive in the hydrolysis of the ,- D -glycopyranosyl fluorides used as donors, and so suitable experimental conditions, under which the rate of spontaneous hydrolysis of the donor was considerably lower than that of enzymatic transglycosidation, provided galactosyl and glucosyl ,-(1,3) - glycosides in yields of up to 90,%. The structure of native Tt-,-Gly available in the Protein Data Bank offers a good basis for interpretation of our results by means of molecular modeling. Thus, in the case of the E338S mutant, a lower energy of the system was obtained when the donor and the acceptor were in the right position to form the ,-(1,3) - glycosidic bond. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

    Simple Derivatives of Natural Amino Acids as Chiral Ligands in the Catalytic Asymmetric Addition of Phenylacetylene to Aldehydes

    Zhi-jian Han
    Abstract Optically active propargylic alcohols are important chiral building blocks in asymmetric synthesis, and asymmetric addition of terminal alkynes to aldehydes is one of the most important and interesting procedures by which to prepare these chiral building blocks. In this work we have identified some simple derivatives of (S)-proline and other natural amino acids as chiral ligands that can be combined with Ti(OiPr)4 and then used to catalyze the asymmetric addition of zinc acetylide, produced in situ by the reaction of phenylacetylene with diethylzinc, to aldehydes. The ee value was as high as 90,%. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

    Oxidation of ,4 - and ,5 -Steroids with Hydrogen Peroxide Catalyzed by Porphyrin Complexes of MnIII and FeIII

    Susana L. H. Rebelo
    Abstract In this paper we describe a new environmentally friendly method to promote the stereoselective epoxidation of ,4 - and ,5 -steroids. Metalloporphyrins efficiently catalyze the epoxidation reactions of 17,-acetoxy-4-androstene (1), 4-cholestene (2) and 3,-acetoxy-5-cholestene (3) in the presence of H2O2 as oxygen donor. Modeling the molecular structure of the porphyrin as well as the central metal allows the control of the preferential formation of ,- or ,-epoxides. Porphyrins with bulky, electron-withdrawing groups in the ortho positions of the meso phenyls and with MnIII as the central metal ion, such as [Mn(TDCPP)Cl], gave preferentially the ,-epoxide of ,4 - and ,5 -steroids. [Fe(TPFPP)Cl] catalyzes preferentially the ,-epoxidation of ,4 -steroids and also increases the stereoselectivity for the ,-epoxide in ,5 -steroids, similar to the results obtained with m -CPBA (m -chloroperbenzoic acid) as oxidant. The substrate structure strongly influences the chemoselectivity of the reactions. The X-ray structures of two main products were determined, and two-dimensional NMR techniques allowed the full assignment of 1H and 13C NMR resonances as well as the stereochemistry of these products. A mechanistic proposal involving oxo species for the ,-approach and peroxy species for the ,-approach is proposed. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]

    Activity of Adenosine Deaminase (ADA) and Adenylate Deaminase (AMPDA) Towards 6-Chloropurine Nucleosides Modified in the Ribose Moiety

    Pierangela Ciuffreda
    Abstract The enzymes adenosine deaminase (ADA) and adenylate deaminase (AMPDA) are able to catalyze the hydrolytic dechlorination of 6-chloropurine riboside and the corresponding 2,,3,- O -isopropylidene derivative, but show no activity towards the 3,4- O -isopropylidene-1-methylriboside of 6-chloropurine and adenine. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2004) [source]