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Turnover Frequency (turnover + frequency)

Distribution by Scientific Domains

Chemistry	96%

Distribution within Chemistry

Organic Chemistry	44%
General & Introductory Chemistry	23%

Selected Abstracts

Photooxidation of Benzyl Alcohols with Immobilized Flavins

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 1-2 2009
Harald Schmaderer
Abstract Benzyl alcohols are oxidized cleanly and efficiently to the corresponding aldehydes under irradiation using flavin photocatalysts and aerial oxygen as the terminal oxidant in homogeneous aqueous solution. Turnover frequencies (TOF) of more than 800,h,1 and turnover numbers (TON) of up to 68 were obtained. Several flavin photocatalysts with fluorinated or hydrophobic aliphatic chains were immobilized on solid supports like fluorous silica gel, reversed phase silica gel or entrapped in polyethylene pellets. The catalytic efficiency of the heterogeneous photocatalysts was studied for the oxidation of different benzyl alcohols in water and compared to the analogous homogeneous reactions. Removal of the heterogeneous photocatalyst stops the reaction conversion immediately, which shows that the immobilized flavin is the catalytically active species. The immobilized catalysts are stable, retain their reactivity if compared to the corresponding homogeneous systems and are easily removed from the reaction mixture and reused. TOF of up to 26,h,1, TON of 280 and up to 3 reaction cycles without loss of activity are possible with the heterogeneous flavin photocatalysts. [source]

Olefin Epoxidation with a New Class of Ansa -Molybdenum Catalysts in Ionic Liquids

CHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 5 2010
Daniel Betz
Is that a finalansa? Different room temperature ionic liquids (RTILs) are examined as solvents in the biphasic epoxidation of selected olefins with the ansa compounds 1 and 2 as catalyst precursors. Turnover frequencies of up to 44,000,h,1 are reached and the catalyst can easily be separated and recycled. [source]

Biphasic oligomerization of ethylene with nickel complexes immobilized in organochloroaluminate ionic liquids

APPLIED ORGANOMETALLIC CHEMISTRY, Issue 11 2009
Lixia Pei
Abstract Ethylene was selectively oligomerized by nickel complexes such as (PPh3)2NiBr2 and (PPh3)2NiCl2 immobilized in chloroaluminate ionic liquid in biphasic catalytic reactions. The influence of reaction parameters such as reaction media, reaction temperature and Et2AlCl:Ni molar ratio was also evaluated. Turnover frequency up to 24000 mol C2H4/(mol Ni h) was achieved under mild reaction conditions (0.5 atm and 40 °C). GC-MS analyses showed that the obtained oligomers completely consist of C4 and C6. The olefinic products can be easily separated from the catalytic ionic liquid phase by simple decantation, and the nickel catalyst can be reused without a significant decrease in turnover frequency and change of the distribution of the olefinic products. Copyright © 2009 John Wiley & Sons, Ltd. [source]

Palladium-Catalyzed Aerobic Oxidation of Naturally Occurring Allylbenzenes as a Route to Valuable Fragrance and Pharmaceutical Compounds

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2010
Luciana
Abstract A palladium-catalyzed, aerobic oxidation of naturally occurring allylbenzenes, i.e., eugenol, methyleugenol, safrole, and estragole, in dimethylacetamide/water solutions under mild conditions has been developed, in which palladium(II) chloride is used in the absence of co-catalysts or special stabilizing ligands as the sole and recyclable catalyst. Methyl ketones that are important for the flavour and pharmaceutical industries have been obtained in good to excellent yields with low catalyst loadings (1,2,mol%) and high average turnover frequencies. This simple catalytic method represents an ecologically benign and economically attractive route to industrially valuable compounds starting from renewable substrates easily available from essential oils. [source]

Micellar Effects in Olefin Hydroformylation Catalysed by Neutral, Calix[4]arene-Diphosphite Rhodium Complexes

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 10 2009
Laure Monnereau
Abstract The combination of calixarene-derived surfactants and neutral rhodium complexes containing a hemispherical "1,3-calix-diphosphite" ligand led to efficient catalysts for the hydroformylation of octene and other olefins in water. While the surfactants allowed the formation of micelles that dissolve both the catalyst and the alkene, thereby resulting in high olefin:rhodium ratios, the diphosphite provided a tight envelope about the catalytic centre able to drive the reaction towards the linear aldehydes. Best results in the hydroformylation of 1-octene were obtained when using [tetra(p -sulfonato)]-(tetra- n -butoxy)-calix[4]arene as surfactant. With this additive remarkable linear to branched aldehyde ratios of up to 62 were obtained, the corresponding activities being higher than those observed when operating in an organic solvent [turnover frequencies (TOFs) up to 630 mol(converted 1-octene), mol(Rh),1,h,1]. [source]

Highly Regioselective and Rapid Hydroformylation of Alkyl Acrylates Catalyzed by a Rhodium Complex with a Tetraphosphorus Ligand

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 4 2009
Shichao Yu
Abstract Alkyl acrylates have been hydroformylated to the linear aldehydes with high regioselectivity (linear/branch>99/1) and extraordinarily high average turnover frequencies (up to 5400,h,1) by using a rhodium complex with a tetraphosphorus ligand. This protocol is in sharp contrast to the most of other processes that favor production of the branched aldehyde (typically>95% branched for most Rh-catalyzed reaction systems). The high turnover number achieved by this new catalytic system is also remarkable considering the less reactive character of alkyl acrylates to the hydroformylation reaction conditions. [source]

The Immobilization of Rhodium-4-(diphenylphosphino)-2- (diphenylphosphinomethyl)-pyrrolidine (Rh-PPM) Complexes: A Systematic Study

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 12-13 2006
Benoît Pugin
Abstract A modular toolbox for the immobilization of homogeneous catalysts to various supports is described. It consists of functionalized chiral diphosphines and three different linkers based on isocyanate chemistry and it is used to attach the 4-(diphenylphosphino)-2-(diphenylphosphinomethyl)-pyrrolidine (PPM) ligand to a large variety of soluble, swellable and non-swellable solid organic polymers and to silica gels. As model reaction the hydrogenation of acetamidocinnamic acid derivatives, catalyzed with high enantioselectivity was chosen. Besides information on the usefulness of a particular type of support for synthetic applications, the experiments were also designed to address the question how parameters such as solubility, swellability, cage or pore size and solvent affect the rate and enantioselectivity of an immobilized catalyst. Rhodium complexes of ligands attached to soluble polymers and inorganic supports achieved ees up to 95,% and turnover frequencies between 700 and 1400,h,1, very close to the values of the homogeneous Rh catalyst (ee 95,%, TOF 1320,h,1). Insoluble or strongly cross-linked organic polymers led to catalysts with lower enantioselectivity and activity. PPM ligands attached to water soluble dendrimer fragments allowed hydrogenation in water solution with ees up to 94,%, albeit with much lower activity compared to reactions in methanol with the homogeneous catalyst. [source]

Diphosphines of dppf-Type Incorporating Electron-Withdrawing Furyl Moieties Substantially Improve the Palladium-Catalysed Amination of Allyl Acetates

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2005
Aziz Fihri
Abstract Highly active Pd/diphosphine catalytic systems incorporating new, air-stable ferrocenyl-furylphosphines allow nucleophilic allylic amination at room temperature with unprecedented turnover frequencies. For instance, in the presence of 0.01,mol,% catalyst the coupling of aniline to allyl acetate occurs at a TOF of more than 10,000,h,1; even the addition of the less nucleophile morpholine to allyl acetate is observed with a TOF of 4250,h,1. The amination of the sterically demanding geranyl acetate, a monoterpene derivative of interest in the flavour industry, at low catalyst loadings demonstrates the scope of this methodology, which provides in addition noticeable advantages in terms of economical (resource- and energy-saving) and sustainable chemistry (high selectivity, no additive, low metal content, and thus easier purification). [source]

Conversion of carbon dioxide to cyclic carbonates using diimine Ru(II) complexes as catalysts

APPLIED ORGANOMETALLIC CHEMISTRY, Issue 2 2009
Mahmut Ulusoy
Abstract Cationic diimine Ru(II) complexes were synthesized and tested as catalysts for the formation of cyclic organic carbonates from CO2 and liquid epoxides (propylene oxide, epichlorohydrine, 1,2-epoxybutane and styrene oxide) which served as both reactant and solvent. The reaction rates not only depended on the type of ligand, but also on reaction conditions such as temperature, pressure, base, the epoxide substrates and the use of an additional solvent. Reaction rates in terms of turnover frequencies up to 4050 molproduct molcat.,1 h,1 at 99% selectivity were achieved by optimizing the diimine ligand as well as the reaction temperature and CO2 pressure. Consistent with CV measurements, the electron donating group on the p -position of the aryl ring accelerated the reaction rate. Copyright © 2008 John Wiley & Sons, Ltd. [source]

The Role of Surface Oxides in NOx Storage Reduction Catalysts

CHEMCATCHEM, Issue 6 2010
Jelena Jelic Dr.
Monte Carlo or bust: First-principles kinetic Monte Carlo simulations are used to examine NO oxidation over Pd(101)/Pd(100). Under typical conditions for NO oxidation in a NOx storage reduction system (600,K, 105,Pa,O2, 100,Pa,NO), turnover frequencies are comparable to those of Pt(111) surfaces, implying that the surface oxide is similar in reactivity to an oxygen-covered metal surface. [source]

Efficient Intramolecular Hydroalkoxylation of Unactivated Alkenols Mediated by Recyclable Lanthanide Triflate Ionic Liquids: Scope and Mechanism

CHEMISTRY - A EUROPEAN JOURNAL, Issue 11 2010
Alma Dzudza Dr.
Abstract Lanthanide triflate complexes of the type [Ln(OTf)3] (Ln=La, Sm, Nd, Yb, Lu) serve as effective, recyclable catalysts for the rapid intramolecular hydroalkoxylation (HO)/cyclization of primary/secondary and aliphatic/aromatic hydroxyalkenes in imidazolium-based room-temperature ionic liquids (RTILs) to yield the corresponding furan, pyran, spirobicyclic furan, spirobicyclic furan/pyran, benzofuran, and isochroman derivatives. Products are straightforwardly isolated from the catalytic solution, conversions exhibit Markovnikov regioselectivity, and turnover frequencies are as high as 47,h,1 at 120,°C. The ring-size rate dependence of the primary alkenol cyclizations is 5>6, consistent with a sterically controlled transition state. The hydroalkoxylation/cyclization rates of terminal alkenols are slightly more rapid than those of internal alkenols, which suggests modest steric demands in the cyclic transition state. Cyclization rates of aryl-functionalized hydroxyalkenes are more rapid than those of the linear alkenols, whereas five- and five/six-membered spirobicyclic skeletons are also regioselectively closed. In cyclization of primary, sterically encumbered alkenols, turnover-frequency dependence on metal-ionic radius decreases by approximately 80-fold on going from La3+ (1.160,Å) to Lu3+ (0.977,Å), presumably reflecting steric impediments along the reaction coordinate. The overall rate law for alkenol hydroalkoxylation/cyclization is v,k[catalyst]1[alkenol]1. An observed ROH/ROD kinetic isotope effect of 2.48 (9) is suggestive of a catalytic pathway that involves kinetically significant intramolecular proton transfer. The present activation parameters,enthalpy (,H,)=18.2 (9),kcal,mol,1, entropy (,S,)=,17.0 (1.4),eu, and energy (Ea)=18.2 (8),kcal,mol,1,suggest a highly organized transition state. Proton scavenging and coordinative probing results suggest that the lanthanide triflates are not simply precursors of free triflic acid. Based on the kinetic and mechanistic evidence, the proposed catalytic pathway invokes hydroxyl and olefin activation by the electron-deficient Ln3+ center, and intramolecular H+ transfer, followed by alkoxide nucleophilic attack with ring closure. [source]

Metal-Free and PdII -Promoted [2+3] Cycloadditions of a Cyclic Nitrone to Phthalonitriles: Syntheses of Oxadiazolines as well as Phthalamide,PdII and Dihydropyrrolyl-iminoisoindolinone,PdII Complexes with High Catalytic Activity in Suzuki,Miyaura Cross-Coupling Reactions

CHEMISTRY - A EUROPEAN JOURNAL, Issue 30 2008
Jamal Lasri Dr.
Abstract The previously unknown reactions between phthalonitriles, 1,2-(CN)2(C6)R1R2R3R41 (1,a, R1=R2=R3=R4=H; 1,b, R1=R2=R4=H, R3=CH3; 1,c, R1=R4=H, R2=R3=Cl; 1,d, R1=R2=R3=R4=Cl; 1,e, R1=R2=R3=R4=F), and a cyclic nitrone, ,O+NCHCH2CH2CMe22, proceed under heating in a sealed tube to give phthalimides 3, 2-oxadiazolyl-benzonitriles 4 or ortho -bis(oxadiazolyl)tetrafluorobenzene 4,e,. In the presence of palladium(II) chloride, phthalonitriles 1 react with 2 at room temperature, to give bis(pyrrolidin-2-ylidene)phthalamide PdII complexes 5 via metal-promoted rupture of the NO bond of the oxadiazoline ring. The ketoimine ligands thus generated can be liberated from the metal by displacement with a diphosphine. Although the first [2+3] cycloaddition of 2 to 1 can occur in the absence of the metal to give the mono-cycloadducts 4, the second [2+3] coupling at the still-unreacted cyano group requires its activation by coordination to PdII, affording complexes 6 containing two ligated oxadiazolyl-benzonitriles. These ligands undergo either i) further cycloaddition with 2 to afford ultimately (upon rearrangement) the bis(pyrrolidinylidene)phthalamide complexes 5 or ii) NO bond cleavage in the oxadiazoline ring with intramolecular attack of the imine nitrogen on the cyano carbon and bridging to a second PdII center to afford dimeric palladium(II) complexes 7, with chloride bridges, that bear a dihydropyrrolyl-iminoisoindolinone, a new type of ligand. The compounds were characterized by IR, 1H, and 13C,NMR spectroscopy, ESI MS or FAB+ MS, elemental analyses and, in the case of 4,c, 5,a, 5,c, and 7,c, also by X-ray diffraction analysis. Complexes 5,a and 7,c show high catalytic activity for the Suzuki,Miyaura cross-coupling reaction of bromobenzene and phenylboronic acid and give biphenyl in high yields with turnover frequencies (TOFs) of up to 9.0×105,h,1. [source]

Olefins as Steering Ligands for Homogeneously Catalyzed Hydrogenations

CHEMISTRY - A EUROPEAN JOURNAL, Issue 17 2004
Pascal Maire
Abstract Iridium(I) complexes containing a (5H -dibenzo[a,d]cyclohepten-5-yl)-phosphane (troppR; R = phosphorus-bound substituent = Ph, Cyc) as a rigid, concave-shaped, mixed phosphane olefin ligand were prepared and tested as catalyst precursors in the hydrogenation of imines. With the complex [Ir(troppCyc)(cod)]OTf, turnover frequencies (TOFs) of >6000 h,1 were reached in the hydrogenation of N -phenyl-benzylidenamine, PhNCHPh. Lower activities (TOF>80 h,1) are observed with N -phenyl-(1-phenylethylidene)amine, PhNCMePh. Chiral tropp-type ligands were prepared in few simple steps. Monosubstitution of the olefinic unit in the dibenzo[a,d]cycloheptenyl moiety with (R)- or (S)-mentholate gave mixtures of diastereomers that could be separated and isolated in enantiomerically pure form. Iridium(I) complexes with these ligands are rare examples of side-on bonded enolether complexes. In catalytic imine hydrogenations, complete conversion (>98,%) was reached in all cases (conditions: p[H2] = 50 bar, T = 50,°C, t = 2 h, substrate/catalyst 100:1). The best enantiomeric excess (ee = 86,% S isomer) was reached with PhNCMePh as substrate and the R,R form of the (10-menthyloxy-5H -dibenzo[a,d]cyclohepten-5-yl)diphenylphosphane ligand. The iridium(I) complex containing the same phosphane gave a 60,% ee (S isomer) with the enamide N-(1-phenylvinyl)acetamide as substrate (conditions: p[H2] = 4 bar, T = 50,°C, t = 18 h, substrate/catalyst = 50:1). These reactions constitute the first examples in which chiral olefins have been used as steering ligands in catalytic enantioselective hydrogenations. [source]

Facile and Efficient Reduction of Ketones in the Presence of Zinc Catalysts Modified by Phenol Ligands

CHEMISTRY - AN ASIAN JOURNAL, Issue 9 2010
Stephan Enthaler Dr.
Abstract In the present study, the zinc-catalyzed hydrosilylation of various ketones to give their corresponding alcohols has been examined in detail. Diethyl zinc that can be modified by easily accessible phenol ligands allows the efficient reduction of various aryl and alkyl ketones. By using a practical in situ catalyst, excellent turnover frequencies up to 1000,h,1 and a broad functional group tolerance were achieved. Im Rahmen dieser Arbeit wird die Zink-katalysierte Hydrosilylierung von Ketonen zu den entsprechenden Silylethern mit anschließender Hydrolyse zu deren Alkoholen vorgestellt. Wobei exzellente Ausbeuten und Umsätze durch die Modifikation des katalytisch eingesetzten Diethylzink mit Phenolen erreicht werden konnten. Nach genauer Untersuchung verschiedenster Reaktionsparameter konnten turnover frequencies (TOF) von ca. 1000,h,1 erzielt werden. Die hervorragenden Eigenschaften des Katalysatorsystems konnte weiterhin in der Hydrosilylierung verschiedenster Ketone erfolgreich gezeigt werden. Zum besseren Verständnis der Reaktion wurden verschiedene mechanistische Experimente durchgeführt. [source]

Copper-Catalyzed Highly Efficient Aerobic Oxidation of Alcohols under Ambient Conditions

CHEMSUSCHEM CHEMISTRY AND SUSTAINABILITY, ENERGY & MATERIALS, Issue 10 2008
Nan Jiang Dr.
Up the TEMPO: A novel, three-component catalyst system consisting of acetamido-TEMPO, copper bromide, and 4-pyrrolidinopyridine not only gives the highest reported turnover frequencies (up to 200 turnovers per hour) for solvent-free aerobic oxidation of primary alcohols to aldehydes at ambient temperature and pressure, but also displays exceptionally high selectivity toward benzylic and allylic primary alcohols. [source]

Water-Soluble Arene Ruthenium Complexes Containing a trans -1,2-Diaminocyclohexane Ligand as Enantioselective Transfer Hydrogenation Catalysts in Aqueous Solution

EUROPEAN JOURNAL OF INORGANIC CHEMISTRY, Issue 22 2005
Jérôme Canivet
Abstract The cationic chloro complexes [(arene)Ru(H2N,NH2)Cl]+ (1: arene = C6H6; 2: arene = p -MeC6H4iPr; 3: arene = C6Me6) have been synthesised from the corresponding arene ruthenium dichloride dimers and enantiopure (R,R or S,S) trans -1,2-diaminocyclohexane (H2N,NH2) and isolated as the chloride salts. The compounds are all water-soluble and, in the case of the hexamethylbenzene derivative 3, the aqua complex formed upon hydrolysis [(C6Me6)Ru(H2N,NH2)OH2]2+ (4) could be isolated as the tetrafluoroborate salt. The molecular structures of 3 and 4 have been determined by single-crystal X-ray diffraction analyses of [(C6Me6)Ru(H2N,NH2)Cl]Cl and [(C6Me6)Ru(H2N,NH2)OH2][BF4]2. Treatment of [Ru2(arene)2Cl4] with the monotosylated trans -1,2-diaminocyclohexane derivative (TsHN,NH2) does not yield the expected cationic complexes, analogous to 1,3 but the neutral deprotonated complexes [(arene)Ru(TsN,NH2)Cl] (5: arene = C6H6; 6: arene = p -MeC6H4iPr; 7: arene = C6Me6; 8: arene = C6H5COOMe). Hydrolysis of the chloro complex 7 in aqueous solution gave, upon precipitation of silver chloride, the corresponding monocationic aqua complex [(C6Me6)Ru(TsHN,NH2)(OH2)]+ (9) which was isolated and characterised as its tetrafluoroborate salt. The enantiopure complexes 1,9 have been employed as catalysts for the transfer hydrogenation of acetophenone in aqueous solution using sodium formate and water as a hydrogen source. The best results were obtained (60 °C) with 7, giving a catalytic turnover frequency of 43 h,1 and an enantiomeric excess of 93,%. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2005) [source]

Kinetics of (Porphyrin)manganese(III)-Catalyzed Olefin Epoxidation with a Soluble Iodosylbenzene Derivative

EUROPEAN JOURNAL OF ORGANIC CHEMISTRY, Issue 12 2006
James P. Collman
Abstract We examined the kinetics of a well-behaved system for homogeneous porphyrin-catalyzed olefin epoxidation with a soluble iodosylbenzene derivative 1 as the terminal oxidant and Mn(TPFPP)Cl (2) as the catalyst. The epoxidation rates were measured by using the initial rate method, and the epoxidation products were determined by gas chromatography. The epoxidation rate was found to be first order with respect to the porphyrin catalyst and zero order on the terminal oxidant. In addition, we found the rate law to be sensitive to the nature and concentration of olefin substrates. Saturation kinetics were observed with all olefin substrates at high olefin concentrations, and the kinetic data are consistent with a Michaelis,Menten kinetic model. According to the observed saturation kinetic results, we propose that there is a complexation between the active oxidant and the substrate, and the rate-determining step is thought to be the breakdown of this putative substrate,oxidant complex that generates the epoxidation products and the resting state porphyrin catalyst. Competitive epoxidations further indicate a reversible complexation of the active oxidant and the olefin substrate. The activation parameters ,H, and ,S, for the oxygen-transfer process (k2) in the cis -cyclooctene epoxidation were determined to be 12.3,±,0.9 kcal,mol,1 and,15.6,±,3.2 cal,mol,1,K,1, respectively. In addition, the Hammett constant ,+ was measured for the epoxidation of para -substituted styrenes, and the value of ,0.27,±,0.04 is too low to be consistent with the involvement of a discrete carbocation in the transition state. We also prepared a (porphyrin)manganese catalyst immobilized on silica support, and found the epoxidation of cis -cyclooctene catalyzed by this heterogeneous catalyst proceeds at virtually the same turnover frequency as by the homogeneous porphyrin catalyst. (© Wiley-VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2006) [source]

Highly Enantioselective Hydrogenation of Quinoline and Pyridine Derivatives with Iridium-(P-Phos) Catalyst

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 6 2010
Wei-Jun Tang
Abstract The use of a chiral iridium catalyst generated in situ from the (cyclooctadiene)iridium chloride dimer, [Ir(COD)Cl]2, the P-Phos ligand [4,4,-bis(diphenylphosphino)-2,2,,6,6,-tetramethoxy-3,3,-bipyridine] and iodine (I2) for the asymmetric hydrogenation of 2,6-substituted quinolines and trisubstituted pyridines [2-substituted 7,8-dihydroquinolin-5(6H)-one derivatives] is reported. The catalyst worked efficiently to hydrogenate a series of quinoline derivatives to provide chiral 1,2,3,4-tetrahydroquinolines in high yields and up to 96% ee. The hydrogenation was carried out at high S/C (substrate to catalyst) ratios of 2000,50000, reaching up to 4000,h,1 TOF (turnover frequency) and up to 43000 TON (turnover number). The catalytic activity is found to be additive-controlled. At low catalyst loadings, decreasing the amount of additive I2 was necessary to maintain the good conversion. The same catalyst system could also enantioselectively hydrogenate trisubstituted pyridines, affording the chiral hexahydroquinolinone derivatives in nearly quantitative yields and up to 99% ee. Interestingly, increasing the amount of I2 favored high reactivity and enantioselectivity in this case. The high efficacy and enantioselectivity enable the present catalyst system of high practical potential. [source]

Continuous Hydrogen Generation from Formic Acid: Highly Active and Stable Ruthenium Catalysts

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 14-15 2009
Albert Boddien
Abstract The ruthenium-catalyzed decomposition of formic acid was investigated with respect to continuous hydrogen generation and long-term stability of the catalytic systems. A highly active and stable system is presented, which was studied in batch and continuous modes for up to two months. The optimized catalyst system containing N,N -dimethyl- n -hexylamine with an in situ generated catalyst from (benzene)ruthenium dichloride dimer [RuCl2(benzene)]2 and 6 equivalents of 1,2-bis(diphenylphosphino)ethane (dppe) reached at room temperature a total turnover number (TON) of approximatly 260,000 with average turnover frequency (TOF) of about 900,h,1. Only hydrogen and carbon dioxide were detected in the produced gas mixture which makes this system applicable for direct use in fuel cells. [source]

Efficient Aerobic Oxidation of Alcohols using a Hydrotalcite-Supported Gold Nanoparticle Catalyst

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 11-12 2009
Takato Mitsudome
Abstract Hydrotalcite-supported gold nanoparticles (Au/HT) were found to be a highly efficient heterogeneous catalyst for the aerobic oxidation of alcohols under mild reaction conditions (40,°C, in air). This catalyst system does not require any additives and is applicable to a wide range of alcohols, including less reactive cyclohexanol derivatives. This Au/HT catalyst could also function in the oxidation of 1-phenylethanol under neat conditions; the turnover number (TON) and turnover frequency (TOF) reached 200,000 and 8,300,h,1, respectively. These values are among the highest values compared to those of other reported catalyst systems at high conversion. Moreover, the Au/HT can be recovered by simple filtration and reused without any loss of its activity and selectivity. [source]

Simple and Fast Synthesis of New Axially Chiral Bipyridine N,N, -Dioxides for Highly Enantioselective Allylation of Aldehydes

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2009
Aneta Kadl, íková
Abstract Unsymmetrically 3,3,-substituted axially chiral bis(tetrahydroisoquinoline) N,N,- dioxides can be prepared in just three steps. They exhibit unique catalytic activity (turnover frequency, enantioselectivity, substrate scope) in the asymmetric allylation of aromatic aldehydes (up to 96% ee). The product of the enantioselective allylation of benzaldehyde served as a building block for the preparation of an intermediate useful in the enantioselective synthesis of diospongines. [source]

Efficient Hydroformylation in Dense Carbon Dioxide using Phosphorus Ligands without Perfluoroalkyl Substituents

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 9 2009
Ard C.
Abstract Rhodium catalysts modified with triphenylphosphine, triphenyl phosphite, and tris(2,4-di- tert -butylphenyl) phosphite have been evaluated for their performance in the hydroformylation of 1-octene using carbon dioxide as the solvent. It is demonstrated that these catalysts are very efficient for the hydroformylation in carbon dioxide, although they are not designed for use in this medium. In particular, the catalyst prepared in situ from dicarbonyl(2,4-pentanedione)rhodium(I) and tris(2,4-di- tert -butyl-phenyl) phosphite gave rise to an initial turnover frequency in excess of 3×104,molaldehyde,molRh,h,1. Such a reaction rate is unprecedented for hydroformylation in supercritical carbon dioxide-rich reaction mixtures. [source]

Rhodium Complexed C2 -PAMAM Dendrimers Supported on Large Pore Davisil Silica as Catalysts for the Hydroformylation of Olefins

ADVANCED SYNTHESIS & CATALYSIS (PREVIOUSLY: JOURNAL FUER PRAKTISCHE CHEMIE), Issue 10 2005

Abstract Polyamidoamine (PAMAM) dendrimers up to the third generation were grown for the first time on the surface of a large-pore (18,nm) Davisil silica support. The supported dendrimers of generations 0, 1, 2 and 3 were phosphinomethylated and complexed with rhodium. All the generations were found to be very active for the hydroformylation of olefins. The hydroformylation of 1-octene was accomplished with a turnover frequency of 1700,h,1 at 70,°C. The G(1) material was found to be the most active when the different generations were compared at 50% conversion at 70,°C [source]

Spectral index of the H2O-maser-emitting planetary nebula IRAS 17347 , 3139

MONTHLY NOTICES OF THE ROYAL ASTRONOMICAL SOCIETY, Issue 2 2005
J. F. Gómez
ABSTRACT We present radio-continuum observations of the planetary nebula (PN) IRAS 17347 , 3139 (one of the only two known to harbour water maser emission), made to derive its spectral index and the turnover frequency of the emission. The spectrum of the source rises in the whole frequency range sampled, from 2.4 to 24.9 GHz, although the spectral index seems to decrease at the highest frequencies (0.79 ± 0.04 between 4.3 and 8.9 GHz, and 0.64 ± 0.06 between 16.1 and 24.9 GHz). This suggests a turnover frequency of around 20 GHz (which is unusual among PNe, whose radio emission usually becomes optically thin at frequencies <10 GHz), and a relatively high emission measure (1.5 × 109 cm,6 pc). The radio-continuum emission has increased by a factor of ,1.26 at 8.4 GHz in 13 yr, which can be explained as expansion of the ionized region by a factor of ,1.12 in radius with a dynamical age of ,120 yr and at an expansion velocity of ,5,40 km s ,1. These radio-continuum characteristics, together with the presence of water maser emission and a strong optical extinction, suggest that IRAS 17347 , 3139 is one of the youngest PNe known, with a relatively massive progenitor star. [source]

Polymer-supported palladium complexes with C,N-ligands as efficient recoverable catalysts for the Heck reaction

APPLIED ORGANOMETALLIC CHEMISTRY, Issue 9 2010
Yu-xia Liu
Abstract A series of new polymer-supported palladium complexes with C,N-ligands (1a,e and 2a,c) were easily synthesized. The synthesized catalysts could be applied as efficient heterogeneous catalysts for the Heck coupling reaction (turnover frequency up to 12 600 h,1). Additionally, the catalysts could be recovered by a simple filtration progress and could be reused for at least five times with a slow progressive decrease in activity. Copyright © 2010 John Wiley & Sons, Ltd. [source]

Biphasic oligomerization of ethylene with nickel complexes immobilized in organochloroaluminate ionic liquids

X-Ray Photoelectron Spectroscopy and Reactivity Studies of a Series of Ruthenium Catalysts

CHEMCATCHEM, Issue 1 2009
Katarzyna Jarzembska
Abstract X-Ray photoelectron spectroscopy (XPS) was applied to six selected ruthenium precatalysts. The XPS data obtained were compared against reactivity and structural results. The XPS data confirmed some dependencies such as the electron-donor properties of the substituents at the ruthenium center. Additionally, the data combined with structural and reactivity results explain the differences between the character of Grubbs and Hoveyda catalysts. It was found that changing the PCy3 ligand to OiPr (PCy3=tricyclohexylphosphane, OiPr=isopropoxy) has a major influence on relative electron-donating properties of the N-heterocyclic carbene ligand (NHC) and PCy3 groups, which was supported by the relative charges on the Ru center for the examined compounds. Moreover, the turnover frequency (TOF) of a selected example reaction decreased when introducing a NHC group in the case of Grubbs catalysts, but increased in the case of Hoveyda-type catalysts. The XPS data also explained the relative activity values of some catalysts (higher reactivity of nitro-Hoveyda than Hoveyda second-generation catalysts). However, the binding energies do not predict TOFs. Sole examination of the XPS data does not provide a base for reaching unambiguous and binding conclusions as to the relative reactivity of all the investigated systems. [source]

Quinaphos and Dihydro-Quinaphos Phosphine,Phosphoramidite Ligands for Asymmetric Hydrogenation

CHEMISTRY - A EUROPEAN JOURNAL, Issue 25 2010
Thomas Pullmann Dr.
Abstract New derivatives of the Quinaphos ligands and the related Dihydro-Quinaphos ligands based on the more flexible 1,2,3,4-tetrahydroquinoline backbone have been prepared and fully characterised. A general and straightforward separation protocol was devised, which allowed for the gram-scale isolation of the Ra,Sc and Sa,Rc diastereomers. These new phosphine,phosphoramidite ligands have been applied in the Rh-catalysed asymmetric hydrogenation of functionalised olefins with the achievement of excellent enantioselectivities (,99,%) in most cases and turnover frequency (TOF) values of up to ,20,000,h,1. These results substantiate the practical utility of readily accessible Quinaphos-type ligands, which belong to the most active and selective category of ligands for Rh-catalysed hydrogenation known to date. [source]

Palladium-Catalyzed Cyclopropanation of Alkenyl Silanes by Diazoalkanes: Evidence for a Pd0 Mechanism

CHEMISTRY - A EUROPEAN JOURNAL, Issue 12 2009
Guillaume Berthon-Gelloz Dr.
Abstract Pd0does the trick! Alkenyl silanes are efficiently cyclopropanated by diazoalkanes at low Pd loadings (see scheme). Clear evidence for the involvement of a Pd0 resting state for this reaction is given. Alkenyl silanes are efficiently converted to the corresponding silyl cyclopropanes in the presence of a slight excess of diazomethane (2,4,equiv) and a low loading of Pd(OAc)2 (<0.5,mol,%). Diazoethane and diazobutane can also be employed and yield silyl cyclopropanes with diastereoselectivities of up to 10:1 for the trans isomer. When conducted on a 4,g scale, the reaction only required a catalyst loading of 5×10,3,mol,%, which corresponds to a turnover frequency of 40,000,h,1. Competition experiments revealed that vinyl silanes can be selectively cyclopropanated in the presence of an aliphatic terminal alkene and styrene. The complex [Pd02(DVTMS)3] (38, DVTMS=divinyltetramethyldisiloxane) proved to be an exceptionally active catalyst for the cyclopropanation reaction, giving complete conversion at ,35,°C in 1,min. Intermolecular and intramolecular competition experiments with DVTMS (36), both with Pd(OAc)2 and 38, provided strong evidence for a Pd0(alkenyl silane)3 resting state. Detailed density functional calculations on the reaction pathways for the cyclopropanation of trimethylvinylsilane and DVTMS by diazomethane with Pd0 corroborated the experimental observations. [source]

Synthetic Scope and Mechanistic Studies of Ru(OH)x/Al2O3 -Catalyzed Heterogeneous Hydrogen-Transfer Reactions

CHEMISTRY - A EUROPEAN JOURNAL, Issue 22 2005
Kazuya Yamaguchi Dr.
Abstract Three kinds of hydrogen-transfer reactions, namely racemization of chiral secondary alcohols, reduction of carbonyl compounds to alcohols using 2-propanol as a hydrogen donor, and isomerization of allylic alcohols to saturated ketones, are efficiently promoted by the easily prepared and inexpensive supported ruthenium catalyst Ru(OH)x/Al2O3. A wide variety of substrates, such as aromatic, aliphatic, and heterocyclic alcohols or carbonyl compounds, can be converted into the desired products, under anaerobic conditions, in moderate to excellent yields and without the need for additives such as bases. A larger scale, solvent-free reaction is also demonstrated: the isomerization of 1-octen-3-ol with a substrate/catalyst ratio of 20,000/1 shows a very high turnover frequency (TOF) of 18,400 h,1, with a turnover number (TON) that reaches 17,200. The catalysis for these reactions is intrinsically heterogeneous in nature, and the Ru(OH)x/Al2O3 recovered after the reactions can be reused without appreciable loss of catalytic performance. The reaction mechanism of the present Ru(OH)x/Al2O3 -catalyzed hydrogen-transfer reactions were examined with monodeuterated substrates. After the racemization of (S)-1-deuterio-1-phenylethanol in the presence of acetophenone was complete, the deuterium content at the ,-position of the corresponding racemic alcohol was 91,%, whereas no deuterium was incorporated into the ,-position during the racemization of (S)-1-phenylethanol-OD. These results show that direct carbon-to-carbon hydrogen transfer occurs via a metal monohydride for the racemization of chiral secondary alcohols and reduction of carbonyl compounds to alcohols. For the isomerization, the ,-deuterium of 3-deuterio-1-octen-3-ol was selectively relocated at the ,-position of the corresponding ketones (99,% D at the ,-position), suggesting the involvement of a 1,4-addition of ruthenium monohydride species to the ,,,-unsaturated ketone intermediate. The ruthenium monohydride species and the ,,,-unsaturated ketone would be formed through alcoholate formation/,-elimination. Kinetic studies and kinetic isotope effects show that the RuH bond cleavage (hydride transfer) is included in the rate-determining step. [source]