Catalytic activity of dihydride ruthenium complexes in the hydrogenation of nitrogen containing heterocycles

The catalytic activity of the dihydride ruthenium complexes, RuH₂(CO)₂(PⁿBu₃)₂, RuH₂(CO)₂(PPh₃)₂ and RuH₂(PPh₃)₄, in the hydrogenation of nitrogen containing heterocycles has been tested by analyzing the influence of reaction parameters such as temperature, hydrogen pressure, catalyst concentration, on the rate and regioselectivity of the reaction.RuH₂(PPh₃)₄ shows a better catalytic activity with an 86.7% conversion of quinoline after 24 h at 100 °C under a hydrogen pressure of 25 bar, while RuH₂(CO)₂(PPh₃)₂ and RuH₂(CO)₂(PⁿBu₃)₂ in the same conditions give a conversion of 37.1% and 35.6%, respectively. These results are confirmed by the reaction rate of the hydrogenation of quinoline, since the K_c in the presence of RuH₂(PPh₃)₄ (1.46 × 10⁻⁵ s⁻¹) is higher than others (6.37 × 10⁻⁶ s⁻¹ for RuH₂(CO)₂(PPh₃)₂ and 6.36 × 10⁻⁶ s⁻¹ for RuH₂(CO)₂(PⁿBu₃)₂).Noteworthy is the selectivity of these catalytic systems in the hydrogenation of quinoline: in all tests the three catalysts lead to 1,2,3,4-tetrahydroquinoline as the major product, furthermore this compound is the only formed in the presence of RuH₂(CO)₂(PPh₃)₂. The selectivity is affected by the presence of an acid (CH₃COOH) or a base (NⁿBu₃) in the reaction media.The complex RuH₂(PPh₃)₄ is catalytically active, even if in a minor extent, in the hydrogenation of isoquinoline, pyridine and 2-methylpyridine.The basicity of the substrate and steric hindrance around the nitrogen atom show a great influence on the conversion.The results obtained suggest that the catalytic system activates a heterocyclic ring through the coordination of the heteroatom to the metal centre of the complexes.     

Copper(I)-imine complexes: Synthesis and catalytic activity in olefin cyclopropanation

Different imine-type ligands, prepared by the condensation of anilines or of α-methylbenzylamine with 2-pyridinecarboxaldehyde (pyim^1,2) or 2-quinolinecarboxaldehyde (quim^1,2) were prepared. These species act as N,N′-bidentate, chelating ligands upon coordination to Cu(I): treatment of [Cu(PPh₃)₃Cl] with an equimolar amount of the ligands resulted in the displacement of two molecules of PPh₃, giving rise to the formation of [Cu(pyim^1,2)(PPh₃)Cl] (1-2) and [Cu(quim^1,2)(PPh₃)Cl] (3-4), respectively. The copper derivatives 1-4 proved to be highly active catalysts in olefin cyclopropanation in the presence of ethyl diazoacetate, even using deactivated olefins (namely, 2-cyclohexen-1-one) as substrate. The X-ray structure of complex 2, [Cu(pyim²)(PPh₃)Cl], is also reported.     

Nanoheterogeneous catalytic hydrogenation of N-, O- or S-heteroaromatic compounds by re-usable aqueous colloidal suspensions of rhodium(0)

The hydrogenation of various nitrogen-, oxygen- or sulfur-heterocyclic aromatic compounds by various surfactant-stabilized aqueous rhodium(0) colloidal suspensions was investigated. The nanocatalysts in the size range of 2.1-2.4 nm have been synthesized by reducing RhCl₃ · 3H₂O with sodium borohydride and were stabilized by highly water soluble N,N-dimethyl-N-cetyl-N-(2-hydroxyethyl)ammonium bromide or chloride salts. The catalytic reactions were performed under mild reaction conditions, namely room temperature and under atmospheric hydrogen pressure. The influence of the bromide or chloride nature of the surfactant counter-ion on the recycling of the aqueous phase containing the Rh(0) particles was studied.     

Synthesis and characterization of Ru(II), Rh(III) and Ir(III) complexes of the “Daniphos” ligands and their application in the hydrogen transfer catalysis

The synthesis of arene-ruthenium(II) and C₅Me₅-rhodium(III) and -iridium(III) complexes of chiral arene-chromium-tricarbonyl-based P^∧P and P^∧N ligands is described. Three complexes were characterized in the solid state by X-ray structural analysis. The complexes were tested in the catalytic hydrogen transfer reactions as well as in the kinetic resolution of racemic alcohols, where some complexes showed good conversion, but low enantioselectivity.     

Synthesis, structural characterization and catalytic activity of a palladium(II) complex bearing a new ditopic thiophene-N-heterocyclic carbene ligand

A new thiophene-functionalized benzimidazolium salt (2) has been prepared by reacting N-methylbenzimidazole with 2-bromomethylthiophene (1), which in turn was obtained by bromination of 2-thiophenemethanol with PBr₃. Subsequent reaction of salt 2 with Pd(OAc)₂ afforded the cis-configured bis(carbene) Pd(II) complex (cis-3), which in solution exists as an inseparable mixture of cis-anti and cis-syn-rotamers in a 3.5:1 ratio. All new compounds have been fully characterized by spectroscopic and spectrometric methods. A preliminary catalytic study shows that cis-3 is highly active in the Suzuki-Miyaura coupling of aryl bromides with phenylboronic acid in/on water as environmentally benign reaction media.     

Increasing the catalytic efficiency of Candida rugosa lipase for the synthesis of tert-alkyl butyrates in low-water media

Abstract

Reactions involving tert-alcohols and their esters are generally not catalyzed by lipases. Candida rugosa lipase is one of the few lipases which shows at least limited catalytic activity towards tert-alcohols and their esters. Using transesterification of tributyrin with tertiary butyl and amyl alcohols as a model reaction, it is shown that precipitation of lipase by a tertiary alcohol in the presence of a buffer with optimum concentration enhances the catalytic activity 7 fold as compared to rates obtained with lyophilized powders. Optimization of the ratio of triglyceride to tert-alcohols and medium engineering gave an initial rate which was 41 times higher than that obtained with lyophilized powders. Hence, use of a simple enzyme formulation, coupled with optimization of reaction conditions led to Candida rugosa lipase becoming a useful catalyst for catalyzing transesterification involving tertiary alcohols.     

Studies on the catalytic ability of palladium wire, foil and sponge in the Suzuki-Miyaura cross-coupling

Different forms of metallic palladium (wire, foil and sponge) have been tested as potential catalysts in the Suzuki-Miyaura cross-coupling. All samples showed to be catalytically active for both electron-poor and electron-rich aryl bromides combined with a variety of arylboronic acids. Palladium wire has been recycled six times without decrease of activity. A series of poisoning experiments demonstrated that the true catalyst is a soluble form of palladium arising from a leaching process. Interestingly, metal leaching from palladium foil has been clearly evidenced by SEM.     

Rhodium(I) hydrogenation in water: Kinetic studies and the detection of an intermediate using C{H} PHIP NMR spectroscopy

The mechanism for hydrogenation of dimethylmaleate in water using cationic rhodium complexes with water-soluble bi-dentate phosphines has been investigated using kinetics and a novel method for the indirect detection of intermediates in catalytic hydrogenation reactions, whereby a late intermediate was detected. A mechanism is proposed involving fast, irreversible substrate binding followed by a rate-determining reaction with dihydrogen.     

The modulation of membrane fluidity by hydrogenation processes. III. The hydrogenation of biomembranes of spinach chloroplasts and a study of the effect of this on photosynthetic electron transport

A method is reported for the in situ modification of the lipids of isolated spinach chloroplast membranes. The technique is based on a direct hydrogenation of the lipid double bonds in the presence of the catalyst, chlorotris(triphenylphosphine)rhodium (I). The pattern of hydrogenation achieved suggests that the catalyst distributes amongst all of the membranes. The polyunsaturated lipids within the membranes are hydrogenated at a faster rate and at an earlier stage than are the monoenoic lipids.Whilst addition of the catalyst to the chloroplast causes an initial 10–20% decrease in Hill activity, saturation of up to 40% of the double bonds present can be accomplished without causing further significant alterations in photosynthetic electron transport processes or marked morphological changes of the chloroplast structure as observed in the electron microscope.     

Simultaneous butyrate oxidation by Syntrophomonas wolfei and catalytic olefin reduction in absence of interspecies hydrogen transfer

Methanogenesis by a Syntrophomonas wolfei/ Methanospirillum hungatei coculture was inhibited in presence of ethylene and the hydrogenation catalyst Pd-BaSO₄. However, butyrate oxidation by S. wolfei continued and ethylene was reduced to ethane. Per mol of butyrate oxidized, 2.4 mol acetate was produced and 0.8 mol ethylene was reduced. Acetylene, propylene and butene were less effective as H₂ acceptors than ethylene, and addition of bromoethanesulfonic acid was necessary to inhibit methanogenesis in the presence of the two longer-chain olefins. Other hydrogenation catalysts were less effective in the order Pd-charcoal < PE-asbestos < Pd-PEI beads < Pt-Al₂O₃, Pd-CaCO₃. Optimal ethylene hydrogenation was achieved with still incubation in presence of 7.2 mg Pd-BaSO₄ and 0.7 g sand per ml medium. The higher catabolic rate of S. wolfei in presence of the methanogen indicated that the biological H₂ removal mechanism was more efficient than the catalytic olefin reduction.Abbreviations BES bromoethane sulfonic acid - VFA volatile fatty acid     

A new type of electrochemical oxidation of alcohols mediated with a ruthenium-dioxolene-amine complex in neutral water

Electrochemical oxidation of [Ru^II(terpy)(sq)(NH₃)]⁺ in neutral water (pH 8.0) at +0.8 V (versus SCE) generated [Ru^II(terpy)(q)(NH₂)]²⁺ and/or [Ru^III(terpy)(sq)(NH₂)]²⁺ (terpy = 2,2′:6′,2′′-terpyridine, sq = 3,5-di-tert-butyl-1,2-semiquinonate, q = 3,5-di-tert-butyl-1,2-benzoquinone), which played roles in hydrogen abstraction and one-electron acceptor in the catalytic oxidation of methanol, ethanol, and 2-propanol affording formaldehyde, acetoaldehyde, and acetone, respectively, under the electrolysis conditions.     

Effect of solid-state buffers on the catalytic activity of papain in low water media

The catalytic activity of papain in the synthesis of Z-Gly-Phe-NH₂ in tert-butanol has been studied in the presence of solid-state acid–base buffers (acids and their sodium salts). All buffer pairs tested reduced the reaction rate compared with the control, particularly the most acidic and basic (assessed by either aqueous pK_a or the response of an organic phase indicator). The highest rates, close to the control, were found with glutamic acid/glutamate-Na, PIPES/PIPES-Na and NaH₂PO₄/Na₂HPO₄. However, these pairs were unable to erase the pH memory phenomenon, or to overcome the effect of spiking with acetic acid. Hence, at least these buffers do not seem to be able to affect the protonation state and catalytic activity of papain. In the last aqueous solution before drying, the presence of activating agents (cysteine plus EDTA) was more important than buffer ions.     

Iridium complexes of dehydroamino acids: The kinetic resolution of racemic diphosphines and their application in catalytic asymmetric hydrogenation

The reaction of chiral diphosphines with a configurationally pure cationic bis-enamide complex of iridium, bis(menthyl (Z)-α-benzamidocinnamate)-iridium tetrafluoroborate, is described. When the reactant ligand is racemic then kinetic resolution occurs with high specificity under the appropriate conditions. Since the iridium diphosphine complex is catalytically inactive in homogeneous hydrogenation, the residual enantiomer may be reacted with bis(norbornadiene)-rhodium tetrafluoroborate to produce an active catalyst. This effects the hydrogenation of methyl (Z)-α-acetamidocinnamate in optical yields comparable with those obtained separately with the enantiomerically pure ligand rhodium complex. The reaction of pure (+)- or (-)-enantiomer of bis(menthyl (Z)-α-benzamidocinnamate)-iridium tetrafluoroborate with enantiomerically pure diphosphines has been studied. Invariably one hand of the diphosphine reacts rapidly with a given enantiomer of the iridium complex to give a stable diphosphine iridium enamide complex in which the original configuration of the coordinated olefin is maintained. The other combination of enantiomers reacts much more slowly, in keeping with the kinetic resolution work, and produces an enamide complex which is unstable in solution, isomerising to a second diastereomer. Since the absolute configuration of the iridium bis-enamide complex has been established by X-ray crystallography, this experiment affords a method of determining the configuration of rhodium enamide complexes in asymmetric hydrogenation (assuming structural homology between Rh and Ir). In all cases the disfavoured enamide complex was the one involved in the catalytic pathway.     

Synthesis, crystal structure, solution behavior and catalytic activity of a palladium(II)-allyl complex containing a 2-pyridyl-1,2,3-triazole bidentate ligand

The synthesis and characterization of the cationic complex [Pd(η³-C₃H₅)(2-((4-phenyl-1H-1,2,3-triazol-1-yl)methyl)pyridine)](BF₄) (2) are reported. The solid-state structure of 2 has been unambiguously confirmed by single-crystal X-ray diffraction analysis. ¹H NMR spectroscopy reveals that in solution complex 2 is dynamic and that syn-syn, anti-anti exchange of the allyl protons occurs. Complex 2 exhibits good activity in the Suzuki-Miyaura coupling of aryl bromides with phenyl boronic acid.     

Relating catalytic activity and electrochemical properties: The case of arene-ruthenium phenanthroline complexes catalytically active in transfer hydrogenation

The electrochemical properties of cationic complexes [(η⁶-arene)Ru(N ∩ N)Cl]Cl (arene/N ∩ N = C₆H₆/1,10-phenanthroline (1), p-MeC₆H₄Prⁱ/1,10-phenanthroline (2), C₆Me₆/1,10-phenanthroline (3), C₆Me₆/5-NO₂-1,10-phenanthroline (4), and C₆Me₆/5-NH₂-1,10-phenanthroline (5)) were studied by cyclic voltammetry in order to rationalize catalytic activity in transfer hydrogenation of the respective aqua complexes [(η⁶-arene)Ru(N ∩ N)(OH₂)](BF₄)₂ (6-10). Complexes 1-5 were chosen because the ‘true’ catalysts 6-10 are unstable under the conditions of the measurement. The electrochemical behaviour of 1-5 in acetonitrile solution is rather complicated due to consecutive and parallel chemical reactions that accompany electron transfer processes. Nonetheless, interpretation of the electrochemical data allowed to assess the influence of the structure and substitution on the redox and catalytic properties: the catalytic ability correlates with the reduction potentials, indicating the decisive role of the η⁶-arene ring directly bonded to the catalytic centre (Ru).     

Use of hydrophobic bacterium Rhodococcus rhodochrous NBRC15564 expressed thermophilic alcohol dehydrogenases as whole-cell catalyst in solvent-free organic media

The hydrophobic bacterium Rhodococcus rhodochrous NBRC15564 was employed as a whole-cell biocatalyst to examine its potential for bioconversion in solvent-free organic media. The genes encoding two different thermostable alcohol dehydrogenases (ADH_Tt1 and ADH_Tt2) of Thermus thermophilus HB27 were expressed in R. rhodochrous cells. To inactivate indigenous mesophilic enzymes in R. rhodochrous, transformant cells were heated at 70 °C for 10 min. Heat-treated hydrophobic wet cells were used for the bioconversion of 2,2,2-trifluoroacetophenone (TFAP) to α-(trifluoromethyl) benzyl alcohol (TFMBA) as a model reaction with ADH_Tt1. NADH, which was supplied in aqueous solution, was regenerated by converting cyclohexanol to cyclohexanone by ADH_Tt2. All reactions were performed by suspending heat-treated cells in solvent-free organic media consisting of 3.7 M TFAP and 4.8 M cyclohexanol (1:1, v/v ratio) at 60 °C. When 800 mg heat-treated R. rhodochrous cells were dispersed in 2 mL of solvent-free organic media (400 mg cells/mL), the product concentration reached about 3.6 M TFMBA by 48 h with a total NADH turnover number of approximately 900. The overall productivity was 190 mol TFMBA/kg cells/h.     

Entrapping of Tyrosinase in a System of Reverse Micelles

The enzymatic activity of tyrosinase was studied both in aqueous and organic media. In the latter case tyrosinase was entrapped in a system of reverse micelles of Aerosol OT in octane. At hydration degree 25, when the inner cavity of the reverse micelles was comparable with the size of a tetrameric tyrosinase form known for aqueous solutions, an optimum level of catalytic activity was observed. Another peak of catalytic activity of tyrosinase was observed at hydration degree 12, when the size of the inner cavity of the reverse micelles was consistent with a monomeric form of tyrosinase. Thus, the system of reverse micelles can be exploited as a medium for the investigation of the monomeric form of tyrosinase, which is unstable in aqueous solution.     

Bis-N-heterocyclic carbene ruthenium(II) carbonyl complexes: Synthesis, structural characterization and catalytic activities in transfer hydrogenation of ketones

In this contribution, the synthesis and characterization of eight ruthenium(II) carbonyl complexes supported by chelating alkane-bridged bis-N-heterocyclic carbene ligands are reported. The products obtained are analyzed using infrared and NMR spectroscopies. The molecular structures of four metal complexes were determined by X-ray crystallography, which exhibit the six-coordinate octahedral geometry with two carbene carbon atoms from the bidentate Bi-NHCs, two carbonyl groups and two chlorine atoms in the trans(Cl)-cis(CO) configuration. All these complexes show catalytic activities in transfer hydrogenation of ketones.     

Evaluation of kappa carrageenan as a substitute for agar in microbiological media

Seventy-one samples of the colloid kappacarrageenan extracted from 12 seaweed species were subjected to a number of standard physical demands of solid bacteriological culture media. All samples had a lower melting temperature (less than 67° C) than agar and a gelling (setting) temperature between 16° C and 51° C, some the same and others lower or higher than agar. Temperature spreads were narrow (ca 10° C) to broad (ca 30° C), depending on the seaweed source, but none were as broad as that of agar (ca 40° C). The majority of commercially prepared samples held a slant when incubated at 37° C, but California seaweed colloids were best at 28° C in this test. The majority of samples released little to no water of syneresis in slant tests as well as in plates. Some plates prepared with the colloid were crystal clear as compared to agar plates. All test microorganisms grew as well on kappa-carrageenan media as on agar media. Some media responses could be attributable to the seaweed species, but others could be traced to chemical extraction methods and modification of the colloid.     

Asymmetric transfer hydrogenation of prochiral ketones in aqueous media with chiral water-soluble and heterogenized bifunctional catalysts of the RhCp*-type ligand

Asymmetric transfer hydrogenation (ATH) of prochiral aromatic ketones was carried out with a water-soluble complex of Rh(III)Cp* and mononitrobenzenesulfonamide bidentate ligand (1R,2R)-N-(2-aminocyclohexyl)-4-nitrobenzenesulfonamide 1 derived from chiral cyclohexane-1,2-diamine. Aqueous sodium formate was used as the hydride source. The reaction afforded the chiral alcohols in good enantioselectivities (79-93%) and yields (>99%). The modified monosulfonamide ligand was also covalently immobilized on solid phase such as silica, resin, and mesoporous SBA-15 silica and then explored as a catalyst with Rh(III)Cp* in the ATH of acetophenone.