Enhanced catalytic performance of Pd‐Ga bimetallic catalysts for 2‐ethylanthraquinone hydrogenation

A series of Pd and Pd‐Ga bimetallic catalysts were prepared by a co‐impregnation method for 2‐ethylanthraquinone (EAQ) hydrogenation to produce hydrogen peroxide. Compared with 0.6Pd catalyst, the hydrogenation efficiency of 0.6Pd1.2Ga catalyst (11.9 g L^?1) increases by 32.2%, and the stability of 0.6Pd1.2Ga catalyst is also higher than that of 0.6Pd catalyst. The structures of the samples were determined by N₂ adsorption–desorption, ICP, XRD, CO chemisorption, TEM, H₂‐TPR, in situ CO‐DRIFTS and XPS. The results suggest that incorporation of Ga species improves Pd dispersion and generates a strong interaction between Ga₂O₃ and Pd interface or between Pd and support. DFT calculation results indicate that the strong adsorption of carbonyl group on Ga₂O₃/Pd interface facilitates the activation of EAQ and promotes the hydrogenation efficiency.     

Understanding the Influence of Donor-Acceptor Diazo Compounds on the Catalyst Efficiency of B(C6F5)3 Towards Carbene Formation

Diazo compounds have been largely used as carbene precursors for carbene transfer reactions in a variety of functionalization reactions. However, the ease of carbene generation from the corresponding diazo compounds depends upon the electron donating/withdrawing substituents either side of the diazo functionality. These groups strongly impact the ease of N₂ release. Recently, tris(pentafluorophenyl)borane [B(C₆F₅)₃] has been shown to be an alternative transition metal-free catalyst for carbene transfer reactions. Herein, a density functional theory (DFT) study on the generation of carbene species from α-aryl α-diazocarbonyl compounds using catalytic amounts of B(C₆F₅)₃ is reported. The significant finding is that the efficiency of the catalyst depends directly on the nature of the substituents on both the aryl ring and the carbonyl group of the substrate. In some cases, the boron catalyst has negligible effect on the ease of the carbene formation, while in other cases there is a dramatic reduction in the activation energy of the reaction. This direct dependence is not commonly observed in catalysis and this finding opens the way for intelligent design of this and other similar catalytic reactions.     

Density function studies on the PtCl2‐catalyzed asymmetric cycloisomerization reaction of hydroxylated enynes

The PtCl₂‐catalyzed asymmetric cycloisomerization reaction of hydroxylated enynes was studied using density functional theory (DFT). All structures have been optimized completely at the B3LYP/6‐311G(d,p) level. As shown, the cycloisomerization reaction is exothermic. The cycloisomerization reaction mainly undergoes the formation of catalyst‐hydroxylated enzyme coordination, the asymmetric cyclopropyl platinum carbene, catalyst–cyclopropyl enol coordination, and catalyst–cyclopropyl ketone coordination. The chirality‐limiting step for the asymmetric cycloisomerization reaction is the formation of the asymmetric cyclopropyl platinum carbene, and the rate‐determining step for this reaction is the formation of the catalyst–cyclopropyl ketone coordination. The dominant products predicted theoretically are (R,S) ‐syn_5a, in agreement with the experiment. © 2006 Wiley Periodicals, Inc. Int J Quantum Chem, 2006     

Chemoselective Synthesis of Substituted Imines,Secondary Amines,and β‐Amino Carbonyl Compounds from Nitroaromatics through Cascade Reactions on Gold Catalysts

Substituted imines, α,β‐unsaturated imines, substituted secondary amines, and β‐amino carbonyl compounds have been synthesized by means of new cascade reactions with mono‐ or bifunctional gold‐based solid catalysts under mild reaction conditions. The related synthetic route involves the hydrogenation of a nitroaromatic compound in the presence of a second reactant such as an aldehyde, α,β‐unsaturated carbonyl compound, or alkyne, which circumvents an ex situ reduction process for producing the aromatic amine. The process is shown to be highly selective towards other competing groups, such as double bonds, carbonyls, halogens, nitriles, or cinnamates, and thereby allows the synthesis of different substituted nitrogenated compounds. For the preparation of imines, substituted anilines are formed and condensed in situ with aldehydes to provide the final product through two tandem reactions. High chemoselectivity is observed, for instance, when double bonds or halides are present within the reactants. In addition, we show that the Au/TiO₂ system is also able to catalyze the chemoselective hydrogenation of imines, so that secondary amines can be prepared directly through a three‐step cascade reaction by starting from nitroaromatic compounds and aldehydes. On the other hand, Au/TiO₂ can also be used as a bifunctional catalyst to obtain substituted β‐amino carbonyl compounds from nitroaromatics and α,β‐unsaturated carbonyl compounds. Whereas gold sites promote the in situ formation of anilines, the intrinsic acidity of Ti species on the support surface accelerates the subsequent Michael addition. Finally, two gold‐catalyzed reactions, that is, the hydrogenation of nitro groups and a hydroamination, have been coupled to synthesize additional substituted imines from nitroaromatic compounds and alkynes.     

Silver‐Catalyzed Allylation of Ketones and Intramolecular Cyclization through Carbene Intermediates from Cyclopropenes Under Ambient Conditions

Tandem C?C bond formation was achieved through silver‐catalyzed ring‐opening of cyclopropenes via carbene intermediates. The reaction of cyclopropenes in the presence of a silver catalyst gave indene derivatives under ambient conditions. In contrast, the insertion of organozinc reagents to silver carbene or allylic cation intermediates afforded allylmetal intermediates for the tandem allylation of carbonyl compounds.     

Chemoselective Hydrazine‐mediated Transfer Hydrogenation of Nitroarenes by Co3O4 Nanoparticles Immobilized on an Al/Si‐mixed Oxide Support

Cobalt oxide nanoparticles (size 2 to 3.5 nm) were successfully impregnated on an alumina–silica (mixed oxide) support through an experimentally viable and easily reproducible protocol. The prepared material was well characterized by XRD, HR‐TEM, BET surface area, EDX and XPS analyses. Porous alumina–silica having a high surface area served as a protective heterogeneous support on which the well‐dispersed Co₃O₄ nanoparticles served as an active catalytic species for the hydrazine‐mediated transfer hydrogenation of nitroarenes. About 2 mol % of the active catalyst in ethanol at 60 °C was adequate for a successful conversion. Moreover, transfer hydrogenation of nitroarenes by the catalyst was found to take place chemoselectively in the presence of other labile functional groups such as halide, alkene, nitrile, carbonyl, and ester. This inexpensive catalyst was also able to catalyze the reaction on a gram scale reaction and found to be robust and recyclable up to eight runs.     

Kinetic Modeling of Secondary Methane Formation and 1‐Olefin Hydrogenation in Fischer–Tropsch Synthesis over a Cobalt Catalyst

A detailed kinetic model of Fischer–Tropsch synthesis (FTS) product formation, including secondary methane formation and 1‐olefin hydrogenation, has been developed. Methane formation in FTS over the cobalt‐based catalyst is well known to be higher‐than‐expected compared to other n‐paraffin products under typical reaction conditions. A novel model proposes secondary methane formation on a different type of active site, which is not active in forming C₂₊ products, to explain this anomalous methane behavior. In addition, a model of secondary 1‐olefin hydrogenation has also been developed. Secondary 1‐olefin hydrogenation is related to secondary methane formation with both reactions happening on the same type of active sites. The model parameters were estimated from experimental data obtained with Co/Re/γ‐Al₂O₃ catalyst in a slurry‐phase stirred tank reactor over a range of conditions (T = 478, 493, and 503 K, P = 1.5 and 2.5 MPa, H₂/CO feed ratio = 1.4 and 2.1, and X _CO = 16–62%). The proposed model including secondary methane formation and 1‐olefin hydrogenation is shown to provide an improved quantitative and qualitative prediction of experimentally observed behavior compared to the detailed model with only primary reactions.     

Unexpected Direct Hydride Transfer Mechanism for the Hydrogenation of Ethyl Acetate to Ethanol Catalyzed by SNS Pincer Ruthenium Complexes

The hydrogenation of ethyl acetate to ethanol catalyzed by SNS pincer ruthenium complexes was computationally investigated by using DFT. Different from a previously proposed mechanism with fac‐[(SNS)Ru(PPh₃)(H)₂] ( 5′ ) as the catalyst, an unexpected direct hydride transfer mechanism with a mer‐SNS ruthenium complex as the catalyst, and two cascade catalytic cycles for hydrogenations of ethyl acetate to aldehyde and aldehyde to ethanol, is proposed base on our calculations. The new mechanism features ethanol‐assisted proton transfer for H₂ cleavage, direct hydride transfer from ruthenium to the carbonyl carbon, and C?OEt bond cleavage. Calculation results indicate that the rate‐determining step in the whole catalytic reaction is the transfer of a hydride from ruthenium to the carbonyl carbon of ethyl acetate, with a total free energy barrier of only 26.9 kcal mol^?1, which is consistent with experimental observations and significantly lower than the relative free energy of an intermediate in a previously postulated mechanism with 5′ as the catalyst.     

Ruthenium(II) Complexes Containing Lutidine‐Derived Pincer CNC Ligands: Synthesis,Structure, and Catalytic Hydrogenation of CN bonds

A series of Ru complexes containing lutidine‐derived pincer CNC ligands have been prepared by transmetalation with the corresponding silver‐carbene derivatives. Characterization of these derivatives shows both mer and fac coordination of the CNC ligands depending on the wingtips of the N‐heterocyclic carbene fragments. In the presence of tBuOK, the Ru‐CNC complexes are active in the hydrogenation of a series of imines. In addition, these complexes catalyze the reversible hydrogenation of phenantridine. Detailed NMR spectroscopic studies have shown the capability of the CNC ligand to be deprotonated and get involved in ligand‐assisted activation of dihydrogen. More interestingly, upon deprotonation, the Ru‐CNC complex 5 e (BF₄) is able to add aldimines to the metal–ligand framework to yield an amido complex. Finally, investigation of the mechanism of the hydrogenation of imines has been carried out by means of DFT calculations. The calculated mechanism involves outer‐sphere stepwise hydrogen transfer to the C?N bond assisted either by the pincer ligand or a second coordinated H₂ molecule.     

Acceptorless Dehydrogenative Oxidation of Secondary Alcohols Catalysed by Cp*IrIII–NHC Complexes

A series of new Ir^III complexes with carbene ligands that contain a range of benzyl wingtip groups have been prepared and fully characterised by NMR spectroscopy, HRMS, elemental analysis and X‐ray diffraction. All the complexes were active in the acceptorless dehydrogenation of alcohol substrates in 2,2,2‐trifluoroethanol to give the corresponding carbonyl compounds. The most active complex bore an electron‐rich carbene ligand; this complex was used to catalyse the highly efficient and chemoselective dehydrogenation of a wide range of secondary alcohols to their respective ketones, with turnover numbers up to 1660. Mechanistic studies suggested that the turnover of the dehydrogenation reaction is limited by the H₂‐formation step.     

N‐Heterocyclic carbene/phosphite synergistically assisted Pd/C‐catalyzed Suzuki coupling of aryl chlorides

An N‐heterocyclic carbene and phosphite synergistically enhanced Pd/C catalyst system has been developed for Suzuki coupling of aryl chlorides and aryl boronic acids from commercially available Pd/C with sterically demanding N,N′‐bis(2,6‐diisopropylphenyl)imidazolylidene and trimethylphosphite. A remarkable increase in catalytic activity of Pd/C was observed when used along with 1 equiv. N,N′‐bis(2,6‐diisopropylphenyl)imidazolium chloride and 2 equiv. phosphite with respect to palladium in appropriate solvents that were found to play a crucial role in Pd/C‐NHC‐P(OR)₃‐catalyzed Suzuki coupling. A dramatic ortho‐substitution effect of carbonyl and nitrile groups in aryl chlorides was observed and explained by a modified quasi‐heterogeneous catalysis mechanism. The Pd/C catalyst could be easily recovered from reaction mixtures by simple filtration and only low palladium contamination was detected in the biparyl products. A practical process for the synthesis of 4‐biphenylcarbonitrile has therefore been developed using the N‐heterocyclic carbene/phosphite‐assisted Pd/C‐catalyzed Suzuki coupling. Copyright © 2013 John Wiley & Sons, Ltd.     

Magnetically recoverable supported ruthenium catalyst for hydrogenation of alkynes and transfer hydrogenation of carbonyl compounds

A ruthenium (Ru) catalyst supported on magnetic nanoparticles (NiFe₂O₄) has been successfully synthesized and used for hydrogenation of alkynes at room temperature as well as transfer hydrogenation of a number of carbonyl compounds under microwave irradiation conditions. The catalyst shows excellent selectivity toward the desired products with very high yield even after five repeated uses.     

Mechanism of the Asymmetric Hydrogenation of Exocyclic α,β‐Unsaturated Carbonyl Compounds with an Iridium/BiphPhox Catalyst: NMR and DFT Studies

The mechanism of the asymmetric hydrogenation of exocyclic α,β‐unsaturated carbonyl compounds with the (aS)‐Ir/iPr‐BiphPhox catalyst was studied by NMR experiments and DFT computational analyses. Computed optical yields of the asymmetric hydrogenation proceeding by an iridium(I)/iridium(III) mechanism involving a transition state stabilized through two intramolecular hydrogen bonds are in good accordance with the experimental ee values.     

Ambient-pressure hydrogenation of ketones and aldehydes by a metal-ligand bifunctional catalyst [Cp*Ir(2,2′-bpyO)(H2O)] without using base

An efficient catalytic system for hydrogenation of ketones and aldehydes using a Cp*Ir complex [Cp*Ir(2,2′-bpyO)(H₂O)] bearing a bipyridine-based functional ligand as catalyst has been developed. A wide variety of secondary and primary alcohols were synthesized by the catalyzed hydrogenation of ketones and aldehydes under facile atmospheric-pressure without a base. The catalyst also displays an excellent chemoselectivity towards other carbonyl functionalities and unsaturated motifs. This catalytic system exhibits high activity for hydrogenation of ketones and aldehydes with H₂ gas.     

Production of Primary Amines by Reductive Amination of Biomass‐Derived Aldehydes/Ketones

Transformation of biomass into valuable nitrogen‐containing compounds is highly desired, yet limited success has been achieved. Here we report an efficient catalyst system, partially reduced Ru/ZrO₂, which could catalyze the reductive amination of a variety of biomass‐derived aldehydes/ketones in aqueous ammonia. With this approach, a spectrum of renewable primary amines was produced in good to excellent yields. Moreover, we have demonstrated a two‐step approach for production of ethanolamine, a large‐market nitrogen‐containing chemical, from lignocellulose in an overall yield of 10 %. Extensive characterizations showed that Ru/ZrO₂‐containing multivalence Ru association species worked as a bifunctional catalyst, with RuO₂ as acidic promoter to facilitate the activation of carbonyl groups and Ru as active sites for the subsequent imine hydrogenation.     

CC Coupling of Ketones with Methanol Catalyzed by a N‐Heterocyclic Carbene–Phosphine Iridium Complex

An N‐heterocyclic carbene–phosphine iridium complex system was found to be a very efficient catalyst for the methylation of ketone via a hydrogen transfer reaction. Mild conditions together with low catalyst loading (1 mol %) were used for a tandem process which involves the dehydrogenation of methanol, C?C bond formation with a ketone, and hydrogenation of the new generated double bond by iridium hydride to give the alkylated product. Using this iridium catalyst system, a number of branched ketones were synthesized with good to excellent conversions and yields.     

High performance of nitrogen-doped carbon-supported cobalt catalyst for the mild and selective synthesis of primary amines

A nitrogen-doped carbon-supported Co catalyst (Co/N-C-800) was discovered to be highly active for the reductive amination of carbonyl compounds with NH₃ and the hydrogenation of nitriles into primary amines using H₂ as the hydrogen source. Structurally diverse carbonyl compounds were selectively transformed into primary amines with good to excellent yields (82.8–99.6%) under mild conditions. The Co/N-C-800 catalyst showed comparable or better catalytic performance than the reported noble metal catalysts. The Co/N-C-800 catalyst also showed high activity for the hydrogenation of nitriles, affording the corresponding primary amines with high yields (81.7–99.0%). An overall reaction mechanism is proposed for the reductive amination of benzaldehyde and the hydrogenation of benzonitrile, which involves the same intermediates of phenylmethanimine and N-benzylidenebenzylamine.     

DFT study on RuII‐catalyzed cyclization of terminal alkynals to cycloalkenes

The reaction mechanism of [CpRu(MeCN)₃]PF₆‐catalyzed cyclization of terminal alkynals 1 to cycloalkenes 2 was investigated by means of density functional methods combined with polarizable continuum models. Calculations indicate that the coordination of the cationic catalyst [CpRu(CH₃CN)₂]⁺ to the carbon–carbon triple bond of the substrate 1 enhances the electrophilicity of alkynyl group, and the subsequent nucleophilic attack of the carbonyl oxygen to the electron‐deficient alkyne forms ate complex IM2 , which would further isomerize into 2H‐oxete complex IM3 . Then a replacement of MeCN by AcOH occurs, followed by two proton‐migrations, which leads to a Fischer‐type carbene complex IM6 . Finally, a decarbonylation takes place leading to cycloalkene 2 . The terminal alkynal is activated by its combination with ruthenium, which leads to a decrease in the natural bond orbital energy of π*_(C1?C2). The four‐membered ring formation is the rate‐controlling step. However, AcOH helps proton shift through coordination with metal center and decreases the reaction energy barriers. Throughout the reactions, all the Ru^II complexes obey the 18‐electron‐rule. © 2008 Wiley Periodicals, Inc. Int J Quantum Chem, 2009     

Synthesis,characterization, and thermal properties of ring‐opening metathesis polynorbornenes and their hydrogenated derivatives bearing various ester and cyano groups

Doubly functionalized polar norbornenes 3a – 3g substituted by both a variety of ester and cyano groups were polymerized by ring‐opening metathesis polymerization (ROMP) with a Ru carbene complex 2 bearing 3‐bromopyridine as a ligand (third generation Grubbs' catalyst) in a living manner. The successive hydrogenation of the main‐chain double bond in the synthesized living ROMP polymers 4a – 4g with a hydridoruthenium complex was exploited. The comparison of thermal properties of a series of ring‐opening metathesis polymers 4a – 4g with those of their hydrogenated derivatives 5a – 5g revealed the decrease of glass transition temperatures (T_g) but little change of the 5% decomposition temperature (T_d⁵). In all cases examined in this study, a decrease of T_g by hydrogenation was around 20–40 °C, regardless of the ester substitutents. In the presence of the additional PCy₃, triethylamine, and methanol after complete consumption of monomer 3a under the living ROMP condition, the tandem ROMP‐hydrogenation of the resulting polymer 4a generated in situ was attained under a H₂ (9.8 MPa) atmosphere at 80 °C to afford the hydrogenated polymer 5a , retaining the narrow polydispersity of 1.03. © 2008 Wiley Periodicals, Inc. J Polym Sci Part A: Polym Chem 46: 3314–3325 2008     

Quantitative Description of Structural Effects on the Stability of Gold(I) Carbenes

The gas‐phase bond‐dissociation energies of a SO₂–imidazolylidene leaving group of three gold(I) benzyl imidazolium sulfone complexes are reported (E₀=46.6±1.7, 49.6±1.7, and 48.9±2.1 kcal mol^?1). Although these energies are similar to each other, they are reproducibly distinguishable. The energy‐resolved collision‐induced dissociation experiments of the three [L]–gold(I) (L=ligand) carbene precursor complexes were performed by using a modified tandem mass spectrometer. The measurements quantitatively describe the structural and electronic effects a p‐methoxy substituent on the benzyl fragment, and trans [NHC] and [P] gold ligands, have towards gold carbene formation. Evidence for the formation of the electrophilic gold carbene in solution was obtained through the stoichiometric and catalytic cyclopropanation of olefins under thermal conditions. The observed cyclopropane yields are dependent on the rate of gold carbene formation, which in turn is influenced by the ligand and substituent. The donation of electron density to the carbene carbon by the p‐methoxy benzyl substituent and [NHC] ligand stabilizes the gold carbene intermediate and lowers the dissociation barrier. Through the careful comparison of gas‐phase and solution chemistry, the results suggest that even gas‐phase leaving‐group bond‐dissociation energy differences of 2–3 kcal mol^?1 enormously affect the rate of gold carbene formation in solution, especially when there are competing reactions. The thermal decay of the gold carbene precursor complex was observed to follow first‐order kinetics, whereas cyclopropanation was found to follow pseudo‐first‐order kinetics. Density‐functional‐theory calculations at the M06‐L and BP86‐D3 levels of theory were used to confirm the observed gas‐phase reactivity and model the measured bond‐dissociation energies.