Metal-catalyzed Hydrogenation Research Articles

Microwave-Assisted Organic Reduction Reactions

The present review covers the topic of microwave-assisted reduction of organic compounds including catalytic (metalcatalyzed hydrogenation, catalytic transfer hydrogenation) and stochiometric reactions (ionic and transfer hydrogenation). The article is subcategorized with respect to the different functionalities which so far have been subjected to reduction: C-C multiple bonds, carbonyls, N-containing functional groups, aromatics and heteroaromatics, reductive deprotection strategies, and miscellaneous reactions like dehalogenation or hydrogen-deuterium exchange.

Organocatalytic asymmetric transfer hydrogenation of imines

The asymmetric organocatalytic transfer hydrogenation of imines can be accomplished in good yields with high enantioselectivities through the use of BINOL-derived phosphoric acids as catalysts and Hantzsch esters or benzothiazoles as the hydride source. The same method can also be applied to the enantioselective reduction of benzo-fused heterocycles, such as quinolines, benzoxazines, benzothiazines, benzoxazinones, quinoxalines, quinoxalinones and a limited number of pyridines containing electron-withdrawing groups. Cascade reactions involving multiple reductions and rearrangements have been reported as well as combinations of metal-catalysed reactions, such as gold-catalyzed hydroaminations of alkynes combined with the reduction of the ensuing enamine. Although turnover frequencies of the organocatalytic imine hydrogenations are still lower than those of metal-catalyzed hydrogenations, there are several advantages. Mild and environmentally friendly conditions as well as excellent selectivity make this method a valuable approach to enantiopure amine building blocks.

ChemInform Abstract: An Efficient Synthesis of Pyrimidines from β-Amino Alcohols.

ChemInform is a weekly Abstracting Service, delivering concise information at a glance that was extracted from about 100 leading journals. To access a ChemInform Abstract of an article which was published elsewhere, please select a “Full Text” option. The original article is trackable via the “References” option.

A green pathway for hydrogenations on ionic liquid-stabilized nanoparticles

Ionic liquid and supercritical carbon dioxide have been employed to facilitate a green pathway for solventless noble metal-catalyzed hydrogenations. The catalytic system consisted of in situ-synthesized palladium and rhodium nanoparticles stabilized by various ionic liquids ([BMIm][PF 6], [BMIm][OTf], and N(C 6H 13) 4Br). Supercritical CO 2 extraction was employed for the removal of the metal precursor ligands in the catalyst synthesis step as well as for the separation of the hydrogenation products. All steps, including catalysts synthesis, reaction, and product separation, were followed by in situ infrared spectroscopy. The potential of this concept is shown using the example of the palladium- and rhodium-catalyzed hydrogenation of acetophenone. The most efficient catalyst, Pd-[BMIm][PF 6], exhibited good activity and excellent selectivity and showed no deactivation during multiple reaction cycles.

Heterogeneously Catalyzed Asymmetric Hydrogenation of C═C Bonds Directed by Surface-Tethered Chiral Modifiers

Asymmetric hydrogenation of C=C bonds is of the highest importance in organic synthesis, and such reactions are currently carried out with organometallic homogeneous catalysts. Achieving heterogeneous metal-catalyzed hydrogenation, a highly desirable goal, necessitates forcing the crucial enantiodifferentiating step to take place at the metal surface. By synthesis and application of six chiral sulfide ligands that anchor robustly to Pd nanoparticles and resist displacement, we have for the first time accomplished heterogeneous enantioselective catalytic hydrogenation of isophorone. High resolution XPS data established that ligand adsorption from solution occurred exclusively on the Pd nanoparticles and not on the carbon support. All ligands contained a pyrrolidine nitrogen to enable their interaction with the isophorone substrate while the sulfide functionality provided the required interaction with the Pd surface. Enantioselective turnover numbers of up to approximately 100 product molecules per ligand molecule were found with a very large variation in asymmetric induction between ligands: observed enantiomeric excesses increased with increasing size of the alkyl group in the sulfide. This likely reflects varying degrees of ligand dispersion on the surface: bulky substituent groups hinder close approach of ligand molecules to each other, inhibiting close-packed island formation, favoring dispersion as separate molecules, and leading to effective asymmetric induction. Conversely, small substituents favor island formation leading to very low asymmetric induction. Enantioselective reaction most likely involves initial formation of an enamine or iminium species, confirmed by use of an analogous tertiary amine, which leads to racemic product. Ligand rigidity and resistance to self-assembled monolayer formation are important attributes that should be designed into improved chiral modifiers.

Experimental and Theoretical Analysis of Asymmetric Induction in Heterogeneous Catalysis: Diastereoselective Hydrogenation of Chiral α-Hydroxyketones over Pt Catalyst

Assessing the origin of asymmetric induction in heterogeneously catalyzed hydrogenation is a challenging task. In this work, hydrogenation of a chiral compound, (R)-1-hydroxy-1-phenyl-2-propanone [(R)-PAC], in toluene over cinchonidine modified and unmodified Pt/Al(2)O(3) was studied. To reveal the detailed reaction mechanism and the origin of stereoselectivity in the Pt-catalyzed hydrogenation of the CO double bond, the structures and energies of several adsorption modes of (R)-PAC as well as whole reaction paths for hydrogenation were investigated on Pt(111) by density functional theory (DFT). In agreement with experimental results, the theoretically obtained potential energy profiles for the studied hydrogenation mechanisms implied that (1R,2S)-1-phenyl-1,2-propanediol is formed in excess with respect to the other diastereomeric product diol, (1R,2R)-1-phenyl-1,2-propanediol. Generally, if the elementary hydrogen addition step was thermodynamically more favorable on one of the two diastereotopic faces, it was also kinetically preferred on the same face, and vice versa. Pairwise addition of hydrogen was the most energetically favorable mechanism. Adsorption and hydrogenation of other structurally similar chiral alpha-hydroxyketones, (R)-3-hydroxy-2-butanone and (R)-2-hydroxy-1-cyclohexanone, were also studied computationally on Pt(111). The results showed that cluster model DFT calculations can be used to assess (dia)stereoselectivity in metal-catalyzed hydrogenation of even such complex organic molecules as studied here.

Simultaneous Hydroconversion of n-Hexane and Benzene over Pt/WO3−ZrO2 in the Presence of Sulfur Impurities

The properties of a commercial Pt/WO3−ZrO2 for the simultaneous hydro-isomerization of n-hexane and benzene have been assessed at varying values of total system pressure, fixed reaction temperature (300 °C), and molar ratio H2/HC = 6. A total pressure of 20 atm was satisfactory for performing both the hydrogenation of benzene and the isomerization of C6 at convenient reaction rates. Greater pressures increased the cracking and formation of light gases. Also, the total conversion was decreased at higher pressures. Benzene conversion proceeded by metal-catalyzed hydrogenation and subsequent acid-catalyzed ring contraction with no occurrence of ring opening. The results show that the presence of a small amount of benzene (3%, vol/vol) is beneficial for the hydro-isomerization of n-hexane over Pt/WO3−ZrO2. Hydrocracking is largely suppressed, and both the liquid C5+ yield and the selectivity to branched isoparaffins are enhanced. Sulfur impurities in the feed decrease the n-hexane and benzene conversion by po...

Reduction of Carbon−Carbon Double Bonds Using Organocatalytically Generated Diimide

An efficient method has been developed for the reduction of carbon-carbon double bonds with diimide, catalytically generated in situ from hydrazine hydrate. The employed catalyst is prepared in one step from riboflavin (vitamin B(2)). Reactions are carried out in air and are a valuable alternative when metal-catalyzed hydrogenations are problematic.

Stereoselective cascade hydrogenation of 4- tert-butylphenol and p-cresol over Zr-zeolite beta-supported rhodium

The hydrogenation of 4- tert-butylphenol and p-cresol was investigated over Zr-beta-supported rhodium catalysts. By designing a suitable bifunctional catalyst, the intermediate, 4-alkylcyclohexanone, formed by metal-catalyzed hydrogenation of 4-alkylphenol, could be reduced via the highly stereoselective Meerwein–Ponndorf–Verley reduction over zirconium Lewis acid sites. Thus, in the presence of 2-propanol as solvent and MPV reductant, a high stereoselectivity to cis-4-alkylcyclohexanol was observed. Over 0.5% Rh/Zr-beta, 4- tert-butylphenol, and p-cresol were hydrogenated to the cis-alcohols with 95 and 89% stereoselectivity, respectively. A higher metal loading or the use of solvents such as hexane or tert-butanol led to a lower stereoselectivity, as metal-catalyzed hydrogenation predominated. Similarly, the cis: trans alcohol ratio was lower for rhodium supported on zirconia or Al-beta. Compared with rhodium, palladium was less active in the hydrogenation of the 4-alkylphenols, requiring a higher hydrogen pressure and temperature. A two-step cascade reaction mechanism is proposed for the conversion of 4-alkylphenols to cis-4-alkylcyclohexanols.

Mechanistic aspects of transition metal-catalyzed hydrogen transfer reactions

In this tutorial review recent mechanistic studies on transition metal-catalyzed hydrogen transfer reactions are discussed. A common feature of these reactions is that they involve metal hydrides, which may be monohydrides or dihydrides. An important question is whether the substrate coordinates to the metal (inner-sphere hydrogen transfer) or if there is a direct concerted transfer of hydrogen from the metal to substrate (outer-sphere hydrogen transfer). Both experimental and theoretical studies are reviewed.

Effect of sulfur and temperature on ruthenium-catalyzed glycerol hydrogenolysis to glycols

The hydrogenolysis of glycerol to ethylene glycol and propylene glycol represents an initial system for the development of an improved mechanistic understanding of the conversion of the more complex higher polyhydric alcohols. Previous work in our laboratory demonstrated the effects that pH, product degradation, and competitive adsorption have on the reaction system. This work is an extension that studies the effects of temperature and sulfur loading. Batch reactor studies with ruthenium on carbon catalysts were performed at a temperature range of 205–240 °C and a sulfur loading range of 0–1.0 mol S/Ru. Previous Langmuir–Hinshelwood-type models were extended to include the data. Apparent activation energies for the glycerol reaction and the glycol degradation were determined. Previous studies for this system have focused on the aqueous-phase scission reactions and metal-catalyzed (de)hydrogenation reactions. Analysis of the reaction flux map with and without sulfur, however, leads to the conclusion that catalytic dehydration must also be occurring on the solid catalyst.

Hydrogenation of Citral over Activated Carbon Cloth Catalyst

Catalytic hydrogenation of an α,β-unsaturated aldehyde, citral, over different Ni and Pt fiber catalysts as well as a commercial Pt powder catalyst was performed in a batch reactor under pressurized conditions. Active carbon was used as a support material. The influence of the important variables, such as temperature and pressure, was investigated in the batch reactor. The metal loading in the catalyst was varied as well. The catalysts were characterized with scanning electron microscopy, atomic force microscopy, N2-physisorption, temperature-programmed hydrogen desorption, and inductively coupled plasma mass spectroscopy. Four consecutive experiments were carried out with each fiber catalyst in the batch reactor to elucidate eventual catalyst deactivation. The decrease in activity was fairly notable. The results of the kinetic experiments demonstrated the feasibility of utilizing woven active carbon cloths as supports in three-phase metal-catalyzed hydrogenation of citral. The Ni on active carbon cloth (...

Synthesis of tritiated Cyclosporin A and FK‐506 by metal‐catalyzed hydrogen isotope exchange

AbstractTritium‐labeled Cyclosporin A (I) and FK‐506 (II) have been prepared using a metal‐catalysed hydrogen isotope exchange procedure and high specific activity tritiated water. Specific activities of the labeled compounds were 0.15 and 0.59 TBq/mmol, respectively. Copyright © 2004 John Wiley & Sons, Ltd.

Transition metal hydrides as active intermediates in hydrogen transfer reactions

Catalytic hydrogen transfer reactions involving transfer hydrogenation of ketones, imines as well as Oppenauer-type oxidation of alcohols occur via active transition metal hydride intermediates. In the RuCl 2(PPh 3) 3-catalyzed hydrogen transfer reaction a dramatic rate enhancement by base was observed. It was found that the role of base is to generate a highly active dihydride catalyst RuH 2(PPh 3) 3. The mechanism of Ru-, Rh-, and Ir-catalyzed hydrogen transfer was probed by using α-deuterated alcohol as hydrogen donor and measuring the amount of deuterium transferred to the keto carbon of the hydrogen acceptor. Two different mechanisms are proposed for the transition metal-catalyzed hydrogen transfer, one via a monohydride (giving a high D-content) and another via a dihydride (giving about half of D-content).

Preparation of tetrahydroagathic acid: a serum metabolite of isocupressic acid, a cattle abortifacient in ponderosa pine.

Isocupressic acid (1) was used to synthetically prepare a mixture of (8S,13R,S)-labda-15,19-dioic acid (tetrahydroagathic acid) (5) via a two-step oxidation procedure followed by hydrogenation of the double bonds at C13 and C8. Reduction of the C8,17 double bond was stereospecific producing only the 8S isomer and confirmed by the nOe interaction between the resulting C17 and C20 methyl groups. The 13R and 13S isomers of 5 were separated and analyzed by HPLC/MS, and (13S)-tetrahydroagathic acid was isolated and identified by comparison to a standard prepared by hydrogenation of naturally occurring (13S)-dihydroagathic acid (4). (13R,S)-dihydroagathic acid was prepared by selective sodium metal-catalyzed hydrogenation of the C13,14 allylic double bond of agathic acid (3). The prepared compounds were then used as standards to confirm the presence of 4 and 5 and their respective 13R and 13S isomers in bovine serum samples. Tetrahydroagathic acid was shown to be the only metabolite detected in serum samples taken from a suspected cattle abortion case submitted for diagnosis; and, thus, 5 could be a valuable diagnostic marker for pine needle-induced abortions.

Studies on the mechanism of metal-catalyzed hydrogen transfer from alcohols to ketones.

The mechanism of metal-catalyzed hydrogen transfer from alcohols to ketones has been studied. Hydrogen transfer (H-transfer) from (S)-alpha-deutero-alpha-phenylethanol ((S)-1) to acetophenone was used as a probe to distinguish between selective carbon-to-carbon H-transfer and nonselective transfer involving both oxygen-to-carbon and carbon-to-carbon H-transfer. The progress of the reaction was monitored by the decreasing enantiomeric excess of (S)-1. After complete racemization, the alcohol was analyzed for its deuterium content in the alpha-position, which is a measure of the degree of selectivity in the H-transfer. A number of different rhodium, iridium, and ruthenium complexes (in total 21 complexes) were investigated by using this probe. For all rhodium complexes a high degree of retention of deuterium at alpha-carbon (95-98%) was observed. Also most iridium complexes showed a high degree of retention of deuterium. However, the results for the ruthenium complexes show that there are two types of catalysts: one that gives a high degree of deuterium retention at alpha-carbon and another that gives about half of the deuterium content at alpha-carbon (37-40%). Two different mechanisms are proposed for transition-metal-catalyzed hydrogen transfer, one via a monohydride (giving a high D content) and another via a dihydride (giving about half of D content). As comparison non-transition-metal-catalyzed hydrogen transfer was studied with the same probe. Aluminum- and samarium-catalyzed racemization of (S)-1 gave 75-80% retention of deuterium in the alpha-position of the alcohol, and involvement of an electron transfer pathway was suggested to account for the loss of deuterium.

Aqueous-phase hydrogenation of lactic acid to propylene glycol

The metal-catalyzed hydrogenation of lactic acid to propylene glycol (PG) in aqueous solution is investigated in a laboratory-scale stirred batch reactor. Ruthenium on activated carbon is identified as an active catalyst for the reaction, with nearly complete conversion achieved at reaction temperatures of 100–170°C and hydrogen pressures of 7–14 MPa. Selectivity to PG in excess of 90% at 95% lactic acid conversion has been obtained at optimal reaction conditions. Potassium and calcium lactate salts cannot be directly converted to PG, but simple addition of sulfuric acid suffices to convert the salt to free acid, which is then hydrogenated. The ready availability of low-cost lactic acid, produced via fermentation of corn-derived carbohydrates, makes this biomass-based route an attractive alternative to the traditional petroleum-based pathways to PG.

Use of TOF-SIMS for the analysis of surface metals in H2O2-bleached lignocellulosic fibers

Abstract Oxidation of cellulose, caused by metal-catalyzed hydrogen peroxide decomposition, is one of the main potential reactions that can occur during the alkaline peroxide bleaching of pulp. This oxidation decreases the individual fiber strength and thus the sheet strength. The bleaching reaction should be controlled to limit cellulose degradation. The goal of this project was to characterize the fiber surface metal distribution using time-of-flight secondary ion mass spectrometry (TOF-SIMS) on laboratory-generated hydrogen peroxide-bleached pulps. For this study, a single black spruce was chosen and kraft pulped. Peroxide bleaching was conducted via benchtop polyethylene bag bleaching in a temperature-controlled waterbath. The overall metals content was determined using inductively coupled plasma (ICP), whereas the surface metals were determined with TOF-SIMS. Many fundamental approaches to study metal identity and content in fibers have been undertaken, including ICP, electron spectroscopy for chemical analysis (ESCA), X-ray fluorescence, and UV­vis. None of these methods can provide the localized density distribution and metal-mapping ability that is available through TOF-SIMS. It has spectacular imaging capabilities that were exploited for evaluating metals on the fiber sheets used in this work. The samples were subjected to other sheet and fiber testing. Tests conducted on select samples included brightness and viscosity. Factors in this experiment included metal addition, chelation, and peroxide charge.

Compensation Phenomena in Heterogeneous Catalysis: General Principles and a Possible Explanation

The compensation phenomenon in heterogeneous catalysis takes the form of a sympathetic linear correlation between the observed parameters of the Arrhenius equation E app and In A app for a series of related reactions or catalysts: Exact obedience to this Cremer-Constable relation requires all Arrhenius plots in the set to intersect at the isokinetic temperature (Ti ). When this is established with statistical rigor (which is rare), the term “isokinetic relationship” (IKR) is used, the term “compensation” being reserved for cases in which this has not been done. Early work suggested that Ti equated to that at which the catalysts were prepared, but this observation has not been generally confirmed. Experimental error in the Arrhenius plots and other causes, such as a change in mechanism or the onset of diffusion limitation, can give rise to false “apparent” compensation which is of no real significance. The scope of this review is strictly limited to heterogeneously catalyzed reactions; thermal desorption is excluded. Explanations advanced include [1] distribution of active-site energy and [2] an enthalpy–entropy relation originating either in the thermodynamics of chemisorption, or in the activation parameters of the Transition State Theory, or in the process of energy transfer between initial and final states. The distinction between apparent (E app and ln A app) and true (Et and In At ) Arrhenius parameters is clearly drawn: The latter are observed only where the catalyst exhibits a high affinity for the adsorbate(s) and where, therefore, the coverage is high and independent of temperature within the range of measurement; zero-order kinetics then apply. Structure insensitivity is often observed under these conditions. Compensation is only seen with apparent Arrhenius parameters, which occur when chemisorption is weaker: The resulting lower coverages are temperature dependent and sensitive to many variables such as catalyst composition and the nature of the reactant(s). Reaction orders are then greater than zero, and under these conditions, structure sensitivity is often found. Values of Et then exceed those of E app by the appropriate adsorption enthalpy terms. Model calculations based on Langmuir–Hinshelwood bimolecular kinetics show that compensation may occur within a single system, due simply to a change of coverage with reactant pressure: Consequential changes in the exponential term exp(—E app/RT) are greater than changes in rate, so that compensation is inevitable. These concepts are illustrated by reference to three systems: [1] acid-catalyzed hydrocarbon transformations, where independent measurements of adsorption enthalpies are available; [2] metal-catalyzed hydrogenation of benzene and its homologs, where mathematical modeling extracts values for Et and adsorption enthalpies; and [3] metal-catalyzed hydrogenolysis of alkanes, where the endothermic initial C—H bond splitting causes the value of E app to be larger than Et , thus explaining inter alia the variation of E app with chain length. If, in a set of reaction systems, conditions for measuring the temperature dependence of rate are chosen such that adsorbed reactant concentrations vary to an appreciable degree, values of E app will necessarily differ, and compensation will appear if accompanying changes in rate are small (i.e., if Ti lies within or close to the temperature range used).

The selectivity and activity determining roles of carbonaceous species and metal–metal oxide interface in metal-catalyzed hydrogenation and isomerization reactions

The activity and/or selectivity determining roles of carbonaceous deposits of various kinds and metal–metal oxide interfaces are described in metal-catalyzed hydrogenation and isomerization reactions. The catalysts used were silica-supported Pt, Pd, Rh, Ni and Cu, while a wide range of probe molecules (cyclopropane, 1,1-dimethylcyclopropane, propylcyclobutane, methyloxirane, 2-methyl-2-butene, cyclohexene) were tested, in most cases, in the presence of H 2 or D 2. The properties of catalytic surfaces were studied by infrared spectroscopy and kinetic methods in static or flow reactors. Six treatment or reaction types were applied for learning about surface transformations during or after the reaction. Carbonaceous deposits of various kinds were identified and their reaction influencing properties are analyzed. Metal–metal oxide interfaces are found to be crucial in the hydrogenative ring opening of methyloxirane and the roles of the metal and the metal oxide are elucidated as well.