Hydrogenation Of Organic Substrates Research Articles

Visible-Light-Driven, Iridium-Catalyzed Hydrogen Atom Transfer: Mechanistic Studies, Identification of Intermediates, and Catalyst Improvements.

The harvesting of visible light is a powerful strategy for the synthesis of weak chemical bonds involving hydrogen that are below the thermodynamic threshold for spontaneous H2 evolution. Piano-stool iridium hydride complexes are effective for the blue-light-driven hydrogenation of organic substrates and contra-thermodynamic dearomative isomerization. In this work, a combination of spectroscopic measurements, isotopic labeling, structure–reactivity relationships, and computational studies has been used to explore the mechanism of these stoichiometric and catalytic reactions. Photophysical measurements on the iridium hydride catalysts demonstrated the generation of long-lived excited states with principally metal-to-ligand charge transfer (MLCT) character. Transient absorption spectroscopic studies with a representative substrate, anthracene revealed a diffusion-controlled dynamic quenching of the MLCT state. The triplet state of anthracene was detected immediately after the quenching events, suggesting that triplet–triplet energy transfer initiated the photocatalytic process. The key role of triplet anthracene on the post-energy transfer step was further demonstrated by employing photocatalytic hydrogenation with a triplet photosensitizer and a HAT agent, hydroquinone. DFT calculations support a concerted hydrogen atom transfer mechanism in lieu of stepwise electron/proton or proton/electron transfer pathways. Kinetic monitoring of the deactivation channel established an inverse kinetic isotope effect, supporting reversible C(sp2)–H reductive coupling followed by rate-limiting ligand dissociation. Mechanistic insights enabled design of a piano-stool iridium hydride catalyst with a rationally modified supporting ligand that exhibited improved photostability under blue light irradiation. The complex also provided improved catalytic performance toward photoinduced hydrogenation with H2 and contra-thermodynamic isomerization.

Photocatalytic hydrogen evolution using a Ru(ii)-bound heteroaromatic ligand as a reactive site.

A RuII complex, [RuII(tpphz)(bpy)2]2+ (1) (tpphz = tetrapyridophenazine, bpy = 2,2'-bipyridine), whose tpphz ligand has a pyrazine moiety, is converted efficiently to [RuII(tpphz-HH)(bpy)2]2+ (2) having a dihydropyrazine moiety upon photoirradiation of a water-methanol mixed solvent solution of 1 in the presence of an electron donor. In this reaction, the triplet metal-to-ligand charge-transfer excited state (3MLCT*) of 1 is firstly formed upon photoirradiation and the 3MLCT* state is reductively quenched with an electron donor to afford [RuII(tpphz˙-)(bpy)2]+, which is converted to 2 without the observation of detectable reduced intermediates by nano-second laser flash photolysis. The inverse kinetic isotope effect (KIE) was observed to be 0.63 in the N-H bond formation of 2 at the dihydropyrazine moiety. White-light (380-670 nm) irradiation of a solution of 1 in a protic solvent, in the presence of an electron donor under an inert atmosphere, led to photocatalytic H2 evolution and the hydrogenation of organic substrates. In the reactions, complex 2 is required to be excited to form its 3MLCT* state to react with a proton and aldehydes. In photocatalytic H2 evolution, the H-H bond formation between photoexcited 2 and a proton is involved in the rate-determining step with normal KIE being 5.2 on H2 evolving rates. Density functional theory (DFT) and time-dependent DFT (TD-DFT) calculations on the reaction mechanism of H2 evolution from the ground and photo-excited states of 2 were performed to have a better understanding of the photocatalytic processes.

Paired Electrocatalytic Oxygenation and Hydrogenation of Organic Substrates with Water as the Oxygen and Hydrogen Source

AbstractThe use of water as an oxygen and hydrogen source for the paired oxygenation and hydrogenation of organic substrates to produce valuable chemicals is of utmost importance as a means of establishing green chemical syntheses. Inspired by the active Ni3+ intermediates involved in electrocatalytic water oxidation by nickel‐based materials, we prepared NiBx as a catalyst and used water as the oxygen source for the oxygenation of various organic compounds. NiBx was further employed as both an anode and a cathode in a paired electrosynthesis cell for the respective oxygenation and hydrogenation of organic compounds, with water as both the oxygen and hydrogen source. Conversion efficiency and selectivity of ≥99 % were observed during the oxygenation of 5‐hydroxymethylfurfural to 2,5‐furandicarboxylic acid and the simultaneous hydrogenation of p‐nitrophenol to p‐aminophenol. This paired electrosynthesis cell has also been coupled to a solar cell as a stand‐alone reactor in response to sunlight.

Supported Heterogeneous Gold Nanoparticles As a Catalyst for the 4-Nitrophenol Hydrogenation Reaction

Bulk gold cannot oxidize in air and is known for its minimal reactive ability with many chemical reagents. Because of this, bulk gold is known to be chemically inert, and this low activity is due to the particle sizes.1 However, gold nanoparticles (AuNPs) are the most highly active of catalyst due to the lack of a stable oxide (unlike most elements).2–4 As the Fermi potential of metal nanoparticles becomes more negative, they exhibit a substantial decrease in potential. The result of this unique property allows them to behave as catalysts in electron transfer processes, such as hydrogenation reactions. Electrical, optical, and catalytic properties are frequently seen in small nanomaterials and gold nanoparticle catalysts show excellent potential for the selective oxidation or hydrogenation of organic substrates.1,5 It is believed that gold nanoparticles will be key building blocks for many disciplines in the 21st century. For example, their individual particle size behaviors, and the electronic, magnetic and optical properties (quantum size effect), show great promises to the catalysis, nanotechnology, and biological field.2–4 Four different sizes of nanoparticles were synthesized and then suspended on four different economically favorable supports. The catalytic properties were investigated in this study with the reduction of 4-nitrophenol, a common hydrogenation reaction. Kinetics of the catalysis was monitored with UV-Vis spectrophotometry and samples were characterized with both transmission electron microscopy (TEM) and X-ray fluorescence (XRF). It was found that all reactions followed first order kinetics. Activated carbon did not behave as a catalytic surface, but was found to absorb the nitrophenol for all nanoparticle sizes. Glass wool and the largest AuNP (53.5nm) proved to be the best support by providing the fastest rate constants. This heterogeneous catalyst was also found to be recyclable. REFERENCES 1) Abdel-Fattah, T. M. & Wixtrom, A. Catalytic reduction of 4-nitrophenol using gold nanoparticles supported on carbon nanotubes. ECS J. Solid State Sci. Technol. 3, M18–M20 (2014). 2) Seoudi, R. & Said, D. A. Studies on the Effect of the Capping Materials on the Spherical Gold Nanoparticles Catalytic Activity. World J. Nano Sci. Eng. 01, 51–61 (2011). 3) Daniel, M.-C. & Astruc, D. Gold nanoparticles: assembly, supramolecular chemistry, quantum-size-related properties, and applications toward biology, catalysis, and nanotechnology. Chem. Rev. 104, 293–346 (2004). 4) Heddle, J. Gold Nanoparticle-Biological Molecule Interactions and Catalysis. Catalysts 3, 683–708 (2013). 5) Huang, J. et al. Ag Dendrite-Based Au/Ag Bimetallic Nanostructures with Strongly Enhanced Catalytic Activity. Langmuir 25, 11890–11896 (2009).

Palladium-catalyzed transfer hydrogenation of organic substrates by hypophosphite in water containing a nonionic surfactant

Palladium-catalyzed transfer hydrogenation of some organic substrates in water containing a nonionic surfactant was examined using sodium hypophosphite as a hydrogen source. Hydrogenation of organohalides such as polychloroarenes, alkenes, alkynes, nitro compounds, aromatic aldehydes, and O-benzyl and N-benzyl derivatives efficiently proceeded to give the corresponding reduction products. The addition of a nonionic surfactant, such as Tween 20, proved to be effective in obtaining satisfactory results in most cases.

Design of Novel Structured Gold Nanocatalysts

Small gold nanoparticles dispersed on certain oxide supports exhibit unprecedented catalytic activities in low-temperature CO oxidation, and gold catalysts show a great potential for selective oxidation or hydrogenation of organic substrates. Nevertheless, most gold catalysts (e.g., Au/TiO2, Au/Al2O3, Au/Fe2O3, Au/SiO2, Au/CeO2) have been prepared by loading gold on unmodified or modified solid supports through traditional synthesis methodologies (e.g., deposition precipitation, wet impregnation), therefore having simple metal-on-support structures and metal–support interactions. The current Perspective highlights some recent progress in the design of novel structured gold nanocatalysts, including unsupported or supported core–shell or yolk–shell structures, gold nanoparticles encapsulated in an inorganic matrix, postmodified gold catalysts, gold-based alloy catalysts, and gold catalysts with additional interfacial sites (or metal oxide components) carried to supports or formed in situ on supports. The objective of most of these studies was to demonstrate synthetic protocols by testing the catalytic performance of the prepared catalysts in simple probe reactions, and the focus was more on materials synthesis than on catalytic reactions or reaction mechanisms. These novel structured gold catalysts will certainly bring new opportunities for studying their performance in various catalytic reactions, the nature of active sites, reaction mechanisms, and correlations between structure and catalytic properties.

Transition‐Metal‐Catalyzed Dehydrocoupling: A Convenient Route to Bonds Between Main‐Group Elements.

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Palladium hydrogenation catalysts modified with aluminum-and phosphorus-containing compounds and with alcohols: Effect of modifiers

The catalytic properties of nanosize catalysts derived from Pd(acac)2 and triethylaluminum in hydrogenation of organic substrates were studied, and the optimal conditions for the catalyst preparation were found. The maximum observed on the plot of the catalyst specific activity vs. Al/Pd ratio was explained by experiments.

Hydrogenation of single and multiple N–N or N–O bonds by Ru(II) catalysts in homogeneous phase

The ruthenium(II) complexes RuH 2(CO) 2(P n Bu 3) 2, RuH 2(CO) 2(PPh 3) 2, and RuH 2(PPh 3) 4 are catalytically active in the hydrogenation of organic substrates containing a N N, N(O) N or NO 2 group. The reduction of the first two groups leads to hydrazine as intermediate and amine as the final product, while reducing a NO 2 group the corresponding amine is selectively formed. A complete conversion was reached, depending on temperature, catalyst and substrate concentration. The catalysts are also active in the hydrogenolysis of an N–N group giving the corresponding amine with a 97.3% conversion using RuH 2(PPh 3) 4 as catalyst. A first-order reaction rate with respect to substrate, catalyst or hydrogen pressure was detected in all cases. Finally, the activation parameters and the kinetic constants of these reactions were calculated. In the hydrogenation of azobenzene, the rate determining step involves an associative or a dissociative step depending on the catalyst employed while in the hydrogenation of all other substrates an associative rate determining step is always involved. A catalytic cycle is suggested for the hydrogenation of azobenzene, taking into account the intermediate complexes identified in the reaction medium.

Electrocatalytic hydrogenation of organic compounds using a nickel sacrificial anode

An electrochemical method, aimed at the hydrogenation of organic substrates, was developed, using a nickel sacrificial anode. The electrochemical system included an iron cathode with deposited nickel and presented good electrochemical efficiency. Some different parameters such as co-solvent, current density, supporting electrolyte and pH were studied to obtain the maximum efficiency for the process. An elevated cell voltage was observed (2.3 V) when 0.2 M NH 4Ac was used as supporting electrolyte, whereas in NH 4Cl experiments could be carried out at a low potential (1.3 V). Electrocatalytic hydrogenation (ECH) of organic substrates showed that non-conjugated olefins (cyclohexene and geraniol) were not reactive, while conjugated substrates (2-cycloexen-1-one, benzaldehyde, acetophenone, styrene, trans-trans-2,4-hexadien-1-ol, 1,3-cyclohexadiene, citral and linalool) showed good reactivity and selectivity in some cases. A direct correlation was observed between the electrochemical efficiency, substrate concentration and current density.

Hydrogenation of organic substrates by an heterogenized catalyst based on a bis(diphenylphosphino)methane polymer-bound palladium(II) complex

Pd(OAc) 2 undergoes easy and quantitative coordination to a cross-linked styrene/divinylbenzene resin functionalized with bis(diphenylphosphino)methane ligands to yield a supported palladium complex which behaves as a versatile and recyclable heterogenized catalyst for the hydrogenation of nitrocompounds and α,β-unsaturated aldehydes under mild conditions. Whereas nitrobenzene is converted only into aniline, cinnamaldehyde is selectively converted into hydrocinnamaldehyde.

Modelling of a trickle-bed reactor for a catalytic hydrogenation in supercritical CO2

The hydrogenation of organic substrates in a fixed-bed catalytic reactor is considered, using supercritical CO2 as a solvent to increase the solubility of hydrogen in the liquid reactants. Reaction rates can be adjusted by changing operating conditions, allowing the reaction volume to be much smaller than in conventional processes. Although it is possible to perform the reaction in a homogenous phase, to avoid interphase mass-transfer limitations, extremely high temperature and pressure may be needed to reach such a condition; therefore the reaction is carried out at conditions where both a gas and a liquid phase are present. Taking into account the solid catalyst, a three-phase reaction system has to be delat with.A trickle-bed high-pressure reactor was simulated using a pseudo-homogenous two-dimensional model, where both mass and energy balance are considered. The PDE system was solved using anorthogonal collocation method. The model does not include any adjustable parameters, and was developed as an user-written subroutine to be linked with a widely used process simulator (Aspen Plus), thus allowing accurate and flexible thermophysical and VLE calculations.Experiments were carried out in a pilot-scale unit running at Hoffmann-La Roche facilities, so that simulated and experimental values could be compared to state the accuracy of the model.

Hydrogenation of organic substrates by isocyanide polymer-bound Rh(PPh 3) 3Cl

Rh(PPh 3) 3Cl undergoes easy and quantitative coordination to hydrophilic macromolecular isocyanides to give a supported rhodium complex which, activated at 50°C under 1 atm dihydrogen behaves as a versatile and recyclable catalyst for the hydrogenation of alkenes, alkynes, nitriles and nitrocompounds under mild conditions. Unsaturated aldehydes are converted into saturated aldehydes whereas nitriles are selectively converted into the corresponding imines or secondary amines depending on the reaction conditions.

Bulk alloy formation during Ge adsorption onto a Pd-powder catalyst

The modification of Pd or Pt by a second, inactive metal component is a well-known method in catalysis by which to improve the selectivity at the expense of the activity in the hydrogenation of organic substrates. The promotion of Pd with Ge is one of the few exceptions where the bimetallic system can exhibit a higher activity than that of the noble metal alone [l-4]. However, the structure of the catalysts and the role of Ge is far from being understood. This prompted us to undertake a systematic study of the Ge + Pd system. The adsorption and electrocatalytic activity of Ge on Pt has been thoroughly investigated [5-91 but there are only few data available on the Ge + Pd bimetallic system [lo]. We have reported in a previous paper that the oxidation of Ge adatoms adsorbed on a palladized Pt electrode results in two peaks, i.e. for the ionization of weakly (about 0.5 V) and strongly (about 0.65 V) bound forms [lo]. Moreover, indirect evidence for bulk alloy formation during Ge adsorption has also been found. The hydrogen adsorption is suppressed by Ge deposition and a hindering effect on the hydrogen transport from the bulk of Pd has been observed at higher coverages. In general, only the outer two or three atomic layers take part in the catalytic and electrocatalytic processes. In the bulk processes, such as the hydrogen dissolution in the p phase or Ge + Pd alloy formation, the outer surface layer behaves like a ‘gate’, which has a strong effect on the rate of the process. It was supposed that a relatively high surface-to-volume ratio (high dispersion of Pd) is necessary for the study of Ge dissolution into the lattice of Pd. The electrodes usually contain several thousands of atomic layers, which corresponds to a dispersion of 0.001 or less. The dispersion of commercial carbon-supported Pd catalysts is