Homogeneous Hydrogenation Catalysts Research Articles

Selection of systems for catalyst recovery by micellar enhanced ultrafiltration

The most discussed problem of homogeneous catalysis, the recovery of expensive catalyst, may be solved with the application of a technology, already well established in other fields: micellar enhanced ultrafiltration (MEUF). Catalyst embedded in surfactant micelles is easy to recover from the reaction medium by an ultrafiltration process. In this contribution the selection of surfactants and membranes suitable for performing homogeneously catalyzed reactions in combination with a catalyst recovery by MEUF is described. For a group of representative anionic, cationic and non-ionic surfactants that are compatible with a homogeneous rhodium-based hydrogenation catalyst, membranes were selected that allow high catalyst recovery. For ionic surfactant micelles best performance is achieved with polyethersulfone membranes, while non-ionic systems are separated most efficiently using regenerated cellulose membranes.

Green chemistry and catalysis

Green chemistry and catalysis

An Iron Catalyst for Ketone Hydrogenations under Mild Conditions

Casey and Guan reported a homogeneous catalyst for ketone hydrogenation that does not require a precious metal, but instead is based on iron. Excellent yields and chemoselectivity for hydrogenation are found under mild conditions (25 °C, 3 atm H2). An ionic hydrogenation mechanism allows the delivery of a proton from the OH and a hydride from the metal. RMB gratefully acknowledges funding from the Division of Chemical Sciences, Office of Basic Energy Sciences, US Department of Energy, and from a grant from the Laboratory Directed Research and Development Program. Pacific Northwest National Laboratory is operated by Battelle for the U.S. Department of Energy.

Selective Catalysis and Nanoscience: An Inseparable Pair

Selective catalysts can be considered as nanomachines designed to perform the synthesis of molecules with high reaction activity and high selectivity. These properties arise from a precise control of the structure of the active sites, of the three-dimensional environment and of their relationship. In both homogeneous and heterogeneous catalysts the active site three-dimensional environment ensemble is always a complex structure resembling the tuneable structure of enzymes, which are the most efficient catalysts optimized by nature over billions of years. To illustrate this concept the structure of a few homogeneous and heterogeneous catalysts for alkenes hydrogenation and for olefin polymerization are chosen and discussed as examples.

Heterogeneization of a new Ru(II) homogeneous asymmetric hydrogenation catalyst containing BINAP and the N-tridentate bpea ligand, through covalent attachment on amorphous AlPO4 support

Following a new methodology to obtain hybrid organic–inorganic solids, the homogeneous complex [RuII(bpea) {(S)(−)(BINAP)}Cl](BF4), (bpea = N,N-bis(2-pyridylmethyl)ethylamine and BINAP = 2,2′-bis(diphenylphosphino)-1,1′-binaphthyl) has been covalently immobilized on an AlPO4 amorphous solid. It was tested in the successive liquid phase enantioselective hydrogenation of several prochiral substrates such us dimethyl itaconate, methyl 2-acetamidoacrylate and methyl 2-acetamidocinnamate, in CH2Cl2 as solvent, ([subs]/[cat] = 0.016/0.000352 = 45.4), hydrogen pressure 6.8 atm and 50–70 °C reaction temperatures. Results obtained indicate that the catalytic activity with a high enantioselectivity (99%) was maintained throughout 10 successive reactions. Furthermore, the immobilized catalyst was easily recovered when repeatedly used and handled for several days, under the same experimental conditions as those in a conventional heterogeneous supported Ru catalyst.

Biomimetic transfer hydrogenation of ketones with iron porphyrin catalysts

For the first time in situ generated iron porphyrins have been applied as homogeneous catalysts for the transfer hydrogenation of ketones. Using 2-propanol as hydrogen source various ketones are reduced to the corresponding alcohols in good to excellent yield and selectivity. Under optimized reaction conditions high catalyst turnover frequencies up to 642 h −1 are achieved.

Redox-Active Silica Nanoparticles. Part 1. Electrochemistry and Catalytic Activity of Spherical, Nonporous Silica Particles with Nanometric Diameters and Covalently Bound Redox-active Modifications,

Nonporous spherical silica nanoparticles resulting from a controlled Stöber process are covalently surface modified with redox-active molecules. Ferrocene, a ruthenium(II) complex with an N2P2Cl2 ligand set, and a sterically hindered biphenylamine are used as modifiers. The resulting materials are characterized by physical, spectroscopic, electrochemical, and chemical methods. The cyclic voltammetric behavior is studied in detail and reveals effects of charge transport by electron hopping along the surface of particles adsorbed on a Pt electrode. The ruthenium(II) complex remains catalytically active with respect to hydrogenation upon immobilization on the particles. Thus, the respective material provides a heterogenized homogeneous hydrogenation catalyst on a solid support.

Polymer‐Bound Cationic Rh(I) Phosphine Catalyst for Homogeneous Asymmetric Hydrogenation.

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Molybdenum Carbonyl Complexes in the Solvent-Free Catalytic Hydrogenation of Ketones

The heterodifunctional ligand Li[η5-C5H4(CH2)2PR2] (R = Ph, Cy, and tBu) reacts with Mo(CO)3(diglyme) to give the molybdenum anion complex Li{Mo(CO)3[η5-C5H4(CH2)2PR2]}. Protonation with HOAc gives the metal hydride complexes HMo(CO)2[η5:η1-C5H4(CH2)2PR2], in which the phosphine and cyclopentadienyl ligands are linked by a two-carbon bridge. Crystal structures of HMo(CO)2[η5:η1-C5H4(CH2)2PR2] with all three R groups (R = Ph, Cy, and tBu) are reported. Syntheses of the C3-bridged complex, HMo(CO)2[η5:η1-C4H5(CH2)3PPh2], and a W analogue, HW(CO)3[η5-C5H4(CH2)2PtBu2], were carried out by analogous routes. Hydride transfer to Ph3C+BAr‘4- [Ar‘ = 3,5-bis(trifluoromethyl)phenyl] from the catalyst precursors HMo(CO)2[η5:η1-C5H4(CH2)2PR2] leads to homogeneous catalysts for hydrogenation of ketones, with the best performance being found for R = Cy. Protonation of HMo(CO)2[η5:η1-C5H4(CH2)2PR2] by HOTf leads to metal triflate complexes (TfO)Mo(CO)2[η5:η1-C5H4(CH2)2PR2], which are used in ketone hydrogenation. Compared to the previously prepared complexes that did not have the phosphine and Cp linked together, these new complexes provide catalysts that have much longer lifetimes (up to about 500 turnovers) and higher thermal stability. Solvent-free ketone hydrogenation can be carried out with these complexes at catalyst loadings as low as 0.1 mol %.

Combinatorial micro electrochemistry. Part 4: Cyclic voltammetric redox screening of homogeneous ruthenium(II) hydrogenation catalysts

Organometallic ruthenium(II) complexes, which act as homogeneous hydrogenation catalysts, are characterized electrochemically with respect to their redox properties by means of a new screening technique (“redox screening”). Samples of the complexes are dissolved in an electrolyte and placed in the wells of microtiter plates. Electrode bundles are moved under computer control between these wells, and cyclic voltammograms are automatically recorded. Analysis of the current/potential curves shows a relation between the voltammogram shape or position and the catalytic activity of the complexes. Thus, the technique proves well suited as an electrochemistry-based high-throughput method.

The preparation of silica entrapped homogeneous hydrogenation catalysts by conventional and ionic liquid mediated sol–gel routes

[RhCl(PPh 3) 3] has been entrapped inside silica matrices by two methods: a conventional sol–gel synthesis in ethanol/water and a new route performed in an ionic liquid. The activity of these heterogenised catalysts has been tested for the hydrogenation of styrene. The catalyst prepared in an ionic liquid was found to be more active and have low Rh leaching.

Searching for molecular arene hydrogenation catalysis in ionic liquids

Arene hydrogenation by homogeneous catalysts is a highly controversial area of research, with many of the mononuclear complexes shown to catalyse the reaction, being found to be pre-catalysts to nanoparticles, on closer examination. The solvent properties of ionic liquids, i.e., low nucleophilicity and high polarity, make them ideal, at least in principal, for homogeneous arene hydrogenation catalysts. In this paper, we described our attempts to prepare and study such systems, using either simple metal halides or ruthenium complexes including trinuclear ruthenium clusters as catalyst precursors.

Enhancing the enantioselectivity of novel homogeneous organometallic hydrogenation catalysts.

By designing well-defined, single-site asymmetric organometallic complexes anchored to a concave siliceous support (see picture) high ee values may be obtained in heterogeneous catalytic hydrogenations. O red, Si orange, Rh yellow, C grey, N blue, F green, B pink, H white.

Enhancing the Enantioselectivity of Novel Homogeneous Organometallic Hydrogenation Catalysts

Ein am konkaven Silicatträger verankerter wohldefinierter asymmetrischer Single-Site-Komplex (siehe Bild) ermöglicht hohe Enantiomerenüberschüsse in heterogenen Hydrierungen. O rot, Si orange, Rh gelb, C grau, N blau, F grün, B pink, H weiß. Supporting information for this article is available on the WWW under http://www.wiley-vch.de/contents/jc_2001/2003/z50861_s.pdf or from the author. Please note: The publisher is not responsible for the content or functionality of any supporting information supplied by the authors. Any queries (other than missing content) should be directed to the corresponding author for the article.

A Concise Synthesis of a New Xylyl‐Biaryl Diphosphine Ligand for Asymmetric Hydrogenation of Ketones.

AbstractFor Abstract see ChemInform Abstract in Full Text.

A concise synthesis of a new xylyl-biaryl diphosphine ligand for asymmetric hydrogenation of ketones

A concise synthesis of a symmetrical biaryl diphosphine ligand bearing 3,5-dimethylphenyl substituents at phosphorus is described. The ruthenium catalysts [diphosphine RuCl 2 diamine] containing the new ligand Xyl-TetraPHEMP were found to be as active and as selective as the state-of-the-art catalysts for homogeneous asymmetric ketone hydrogenation.

New nickel-containing homogeneous hydrogenation catalysts

The didentate phosphane ligands 1,3-bis(di(ortho-methoxyphenyl)phosphanyl)-2-propanol (o-MeO-dpppol) and 1,2bis(dicyclohexylphosphanyl)ethane (dcpe) have been used in a study towards catalytic hydrogenation using homogeneous nickel catalysts. The nickel halide complexes [Ni(o-MeO-dpppol)X2] and [Ni(dcpe)X2 ]( X= Cl, Br or I) have been synthesized and were characterized using electronic absorption and IR spectroscopy. The structures of the complexes [Ni(o-MeO-dpppol)Cl2] and [Ni(dcpe)Cl2] have been determined by X-ray crystallography. The nickel ions in these complexes are in (distorted) square–planar geometries with NiP2Cl2 chromophores. The synthesized halide complexes and in situ mixtures of nickel acetate and the ligands were tested on catalytic activity in homogeneous hydrogenation. The ligand dcpe yields very active catalysts; turnover numbers up to 3000 in 1 h have been obtained. Generally, it appeared that the catalytic activities are higher in methanol than in methanol/dichloromethane mixtures. It was found that the catalytic activities observed for complexes containing the ligand o-MeO-dpppol are less reproducible than those of catalysts with the ligand dcpe. This lower reproducibility is probably related to the fact that the former ligand is more readily oxidized. © 2003 Elsevier Science B.V. All rights reserved.

Iridium‐Catalyzed Enantioselective Hydrogenation of Olefins

AbstractCationic iridium complexes with chiral P,N‐ligands and tetrakis[3,5‐(trifluoromethyl)phenyl]borate (BArF) as the counterion are efficient homogeneous catalysts for the enantioselective hydrogenation of olefins. The complexes are readily prepared, air‐stable, and easy to handle. In contrast to chiral rhodium‐ and ruthenium‐phosphine catalysts, they do not require the presence of a polar coordinating group near the CC bond. In the hydrogenation of unfunctionalized arylolefins, high enantioselectivities of >95% ee with turnover numbers of up to 5000 and turnover frequencies of >5000 h−1 have been achieved.

Recycling of homogeneous hydrogenation catalysts by dialysis coupled catalysis.

Although transition-metal complexes are very attractive as homogeneous catalysts in fine chemistry, their high prices often limit their applications. A means to recycle those catalysts would solve this problem and would simultaneously facilitate the downstream purification of the product. This is now realized in a new concept in which homogeneous catalysis is coupled to dialysis. The advantages of homogeneous catalysis (off-the-shelf catalysts, high activities and selectivities) are thus combined with those of heterogeneous catalysis (easy catalyst separation from product solution, reuse of catalyst, and possibility for continuous operation). Since the heart of the process is the membrane, self-prepared membranes were preferred as they allow a better control and understanding of the separation characteristics. Rhodamine B was used as a probe molecule to define the working conditions of the membrane. The concept is proven to work for two relevant chiral reactions: a hydrogenation with Ru-BINAP and a hydrogen transfer reaction with Ru-TsDPEN [BINAP=(1,1'-binaphthalene)-2,2'-diylbis(diphenylphosphine); TsDPEN= tosyl-N,N'-diphenyl-1,2-ethanediamine].

A highly selective arene hydrogenation catalyst that operates in ionic liquid.

The synthesis and structural characterization of [Ru(eta(6)-p-cymene)(eta(2)-TRIPHOS)Cl][PF(6)] is described. The complex is a highly active, homogeneous arene hydrogenation catalyst that is selective toward the hydrogenation of aromatic rings in preference to alkenes, as demonstrated by the hydrogenation of allylbenzene to allylcyclohexane. The catalyst operates in both dichloromethane and ionic liquids and undergoes no decomposition in the latter solvent.