Catalysts For Transfer Hydrogenation Research Articles

Vitamin B1-based thiazol-2-ylidene-Ru(II) complexes: recyclable transfer hydrogenation catalysts in water.

The [(p-cymene)RuCl(κ2C,N-{CNHC-NH})]+X- (CNHC = thiazol-2-ylidene) complexes with a bidentate ligand (2: X = Cl and 3: X = PF6) were prepared by a one-pot reaction of vitamin B1 (VB1, 1), Ag2O and [(p-cymene)RuCl2]2. In the complexes, VB1 coordinates through C2 and the exocyclic N in the imino form with the κ2-(C,N) coordination mode. The complexes 2 and 3 are stable in the solid state, but slowly release p-cymene in solution. Furthermore, upon heating in polar solvents, 2 or 3 can be converted by ligand exchange reactions to produce [(L)3RuCl(κ2C,N-{CNHC-NH})]+PF6- (4: L = py).Robustness was improved remarkably for 4. The complex 4 is stable in the solid state and in solution. The complexes 2-4 have been identified by 1H and 13C{1H} 2D NMR spectroscopy and 2 and 4 were studied by X-ray crystallography. In an effort to develop a recyclable catalyst in water, 2-4 were evaluated for TH of ketones and aldehydes with an azeotropic mixture of HCOOH/Et3N in water. The complexes 3 and 4 exhibited very good catalytic activity and 4 could be reused nine times without significant loss of activity, giving a high turnover frequency (TOF50%(h-1) = 1286).

Evaluation of indole-based organometallics as transfer hydrogenation catalysts with anticancer activity

Half-sandwich complexes containing transition metals offer a wide range of applications owing to their interactions with a variety of targets/molecules that are not accessible for some organic scaffolds. The use of such complexes is therefore a way to provide new mechanisms of drug action. Here, we report a series of neutral metal complexes of the type [(arene)M(O-cyclohexyl-1H-indole-2-carbothiolate)Cl] that have the ability to reduce the coenzyme NAD+ at physiological conditions in the presence of sodium formate as an hydride donor. The influence of the metal ion (Ru, Os, Rh, Ir) is studied to establish some structure-activity relationships. The [(η6-p-cym)Ru(O-cyclohexyl-1H-indole-2-carbothioate)Cl] is the most active catalyst of this family of complexes with a turnover frequency (TOF) of 14.79 h-1 for the transfer hydrogenation reaction of NAD+ to give 1,4-NADH in the presence of formate. Further in vitro studies were carried out to determine a relationship between catalytic activity and potency as an anticancer drug, with the rhodium complex promisingly showing 7x more selectivity towards cancer cells (A2780) than normal cells (PNT2). Preliminary in vivo studies demonstrated the importance of co-treatment with formate to enhance activity. This work gives insights into the mechanism of action of this family of indole-based organometallic complexes.

Poly(divinylbenzene-maleic acid) Hollow Nanospheres Coordinated with Zirconium: An Effective Catalyst for the Catalytic Transfer Hydrogenation of Furfural to Furfuryl Alcohol

Poly(divinylbenzene-maleic acid) Hollow Nanospheres Coordinated with Zirconium: An Effective Catalyst for the Catalytic Transfer Hydrogenation of Furfural to Furfuryl Alcohol

Primary alcohols as killers of Ni-based catalysts in transfer hydrogenation

The effect of different types of alcohols used as hydrogen donors on the activity of a metal Ni-based catalyst in the hydrogenation of benzofuran as a model substrate under transfer hydrogenation conditions has been studied. The hydrogenation process of benzofuran using 2-PrOH as a hydrogen donor leads sequentially to dearomatization and then to deoxygenation of the substrates. At the same time, the use of primary alcohols such as MeOH, EtOH and 1-PrOH as hydrogen donors leads to irreversible deactivation of the Ni-containing catalyst. A clear mechanism of deactivation of Ni-based metal catalysts by primary alcohols has been established. The interaction of the catalyst with primary alcohols at a temperature of 250 °C leads to the formation of an inactive Ni3C carbide phase, as well as sintering and segregation of metal particles on the surface of the alumina support.

Facile Transfer Hydrogenation of N-Heteroarenes and Nitroarenes Using Magnetically Recoverable Pd@SPIONs Catalyst.

Catalysts with active, selective, and reusable features are desirable for sustainable development. The present investigation involved the synthesis and characterization of bear-surfaced ultrasmall Pd particles (<1 nm) loaded onto the surface of magnetic nanoparticles (8-10 nm). The amount of Pd loading onto the surface of magnetite is recorded as 2.8 wt %. The characterization process covered the utilization of scanning electron microscopy (SEM), energy-dispersive spectroscopy (EDS), transmission electron microscopy (TEM), inductively coupled plasma (ICP), and X-ray photoelectron spectroscopy (XPS) methods. The Pd@Fe3O4 catalyst has shown remarkable efficacy in the hydrogenation of quinoline, resulting in the production of >99% N-ring hydrogenated (py-THQ) product. Additionally, the catalyst facilitated the conversion of nitroarenes into their corresponding aniline derivatives, where hydrogen was achieved by H2O molecules with the aid of tetrahydroxydiboron (THDB) as an equilibrium supportive at 80 °C in 1 h. The high efficiency of a transfer hydrogenation catalyst is closely related to the metal-support synergistic effect. The broader scope of functional group tolerance is evaluated. The potential mechanism underlying the hydrogenation process has been elucidated through the utilization of isotopic labeling investigations. The application of the heterocyclic compound hydrogenation reaction is extended to formulate the medicinally important tubular polymerization inhibitor drug synthesis. The investigation of the recyclability of Pd@Fe3O4 has been conducted.

Isomorphic Insertion of Ce(III)/Ce(IV) Centers into Layered Double Hydroxide as a Heterogeneous Multifunctional Catalyst for Efficient Meerwein-Ponndorf-Verley Reduction.

The development of highly active acid-base catalysts for transfer hydrogenations of biomass derived carbonyl compounds is a pressing challenge. Solid frustrated Lewis pairs (FLP) catalysis is possibly a solution, but the development of this concept is still at a very early stage. Herein, stable, phase-pure, crystalline hydrotalcite-like compounds were synthesized by incorporating cerium cations into layered double hydroxide (MgAlCe-LDH). Besides the insertion of well-isolated cerium centers surrounded by hydroxyl groups, the formation of hydroxyl vacancies near the aluminum centers, which were formed by the insertion of cerium centers into the layered double hydroxides (LDH) lattice, was also identified. Depending on the initial cerium concentration, LDHs with different Ce(III)/Ce(IV) ratios were produced, which had Lewis acidic and basic characters, respectively. However, the acid-base character of these LDHs was related to the actual Ce(III)/Ce(IV) molar ratios, resulting in significant differences in their catalytic performance. The as-prepared structures enabled varying degrees of transfer hydrogenation (Meerwein-Ponndorf-Verley MPV reduction) of biomass-derived carbonyl compounds to the corresponding alcohols without the collapse of the original lamellar structure of the LDH. The catalytic markers through the test reactions were changed as a function of the amount of Ce(III) centers, indicating the active role of Ce(III)-OH units. However, the cooperative interplay between the active sites of Ce(III)-containing specimens and the hydroxyl vacancies was necessary to maximize catalytic efficiency, pointing out that Ce-containing LDH is a potentially commercial solid FLP catalysts. Furthermore, the crucial role of the surface hydroxyl groups in the MPV reactions and the negative impact of the interlamellar water molecules on the catalytic activity of MgAlCe-LDH were demonstrated. These solid FLP-like catalysts exhibited excellent catalytic performance (cyclohexanol yield of 45%; furfuryl alcohol yield of 51%), which is competitive to the benchmark Sn- and Zr-containing zeolite catalysts, under mild reaction conditions, especially at low temperature (T = 65 °C).

Influence of Charge Delocalization on Manganese Catalyzed Transfer Hydrogenation

AbstractHydrogen transfer reactions promoted by Mn(I)‐based catalysts are a recently emerged field of research. Herein, we demonstrate the crucial effect of charge delocalization on the catalytic activity in transfer hydrogenation. Therefore, we synthesized pyridyl‐pyrazole ligands and a pyridyl‐pyridazinone ligand which exhibit resonance structures upon deprotonation with varying capability to transfer charge to the manganese center. The corresponding Mn(I) complexes were characterized and their activity as catalyst in transfer hydrogenation of acetophenone was determined using in‐situ IR techniques. Kinetic data and DFT calculations reveal that the ligands are deprotonated throughout the catalytic cycle and that the catalysts in which resonance structures point towards more negative charge at the metal center are also the more active ones. Thus, we deliver ample evidence that for effective hydrogen transfer the manganese catalyst requires a ligand with a deprotonable moiety allowing efficient delocalization to the metal while geometrical access of the acidic proton to the substrate is not essential. Our findings highlight the importance of an anionic hydrido intermediate, where mesomerism allows an effective charge transfer to the metal. This may support the ongoing development of new manganese‐based catalysts.

Bidentate Ru(II)‐NC Complexes as Catalysts for Transfer Hydrogenation of Ketones with Ethanol

AbstractIn this study, we developed a bidentate Ru(II)‐NC‐catalyzed transfer hydrogen reaction of ketones using ethanol (EtOH) as the hydrogen source under weak base conditions. The control experiments suggested that the reaction proceeded via a Ru‐H mechanism, yielding ethyl acetate as the only byproduct. The deuterium‐labeling experiments showed that hydrogen evolved from the CH2 and OH groups. These results provide a new weak base system for the transfer hydrogen reaction of ketones using EtOH as the hydrogen donor.

Effect of Ligand Substituents on Spectroscopic and Catalytic Properties of Water-Compatible Cp*Ir-(pyridinylmethyl)sulfonamide-Based Transfer Hydrogenation Catalysts.

Transition-metal-based hydrogenation catalysts have applications ranging from high-value chemical synthesis to medicinal chemistry. A series of (pyridinylmethyl)sulfonamide ligands substituted with electron-withdrawing and -donating groups were synthesized to study the influence of the electronic contribution of the bidentate ligand in Cp*Ir piano-stool complexes. A variable-temperature NMR investigation revealed a strong correlation between the electron-donating ability of the substituent and the rate of stereoinversion of the complexes. This correlation was partially reflected in the catalytic activity of the corresponding catalysts. Complexes with electron-withdrawing substituents followed the trend observed in the variable-temperature NMR study, thereby confirming the rate-determining step to be donation of the hydride ligand. Strongly electron-donating groups, on the other hand, caused a change in the rate-determining step in the formation of the iridium-hydride species. These results demonstrate that the activity of these catalysts can be tuned systematically via changes in the electronic contribution of the bidentate (pyridinylmethyl)sulfonamide ligands.

Asymmetric Reductive Amination of α-Keto Acids Using Ir-Based Hydrogen Transfer Catalysts: An Access to Unprotected Unnatural α-Amino Acids.

A direct asymmetric reductive amination of α-keto acids catalyzed by Cp*Ir complexes bearing a chiral N-(2-picolyl)sulfonamidato ligand is described. The combined use of optically active 2-phenyglycinol as an aminating agent is effective for the chemo- and stereoselective transfer hydrogenation using formic acid. The subsequent elimination of the hydroxyethyl moiety by orthoperiodic acid can afford various unprotected α-amino acids in satisfactory isolated yields (20 examples) with excellent optical purities (up to >99% ee).

Air‐Stable Manganese NNS Pincer Complexes Enable Ketone Reduction at Room Temperature

AbstractA series of air‐stable Mn complexes bearing NNS pincer ligands have been synthesized and fully characterized. The activity of the complexes as (transfer) hydrogenation catalysts was investigated in detail, and a variety of ketones (&gt;20 examples) were converted to the corresponding secondary alcohols at room temperature. The synthetic applicability of this class of catalysts is demonstrated by the tolerance of several functional groups including halides, primary amines, nitriles, esters, and nitro‐groups. Moreover, the optimal catalytic system 5c displayed a high chemoselectivity in the presence of other reducible groups such as nitriles, esters, and alkynes. Competitive catalytic experiments with the analogous bidentate NN complex 8 indicated decoordination of the hemilabile thioether‐moiety during formation of the catalytically active hydrido species.

BICAAC-Derived Covalent and Cationic Ir(I) Complexes: Application of Ir(BICAAC)Cl(COD) Complexes as Catalysts for Transfer Hydrogenation and Hydrosilylation Reactions.

The ambiphilic bicyclic (alkyl)(amino)carbenes (Me/iPrBICAAC) upon reaction with [IrCl(COD)]2 smoothly afford mononuclear Ir(I) complexes that have been spectroscopically and structurally characterized. These complexes exhibit good catalytic activity for transfer hydrogenation (TH) of 4-chlorobenzaldehyde using isopropyl alcohol (iPrOH), with turnover frequency values ranging between 6269 and 8093 h-1. Choosing the covalent complex Ir(MeBICAAC)Cl(COD) as a catalyst, a wide array of carbonyls and imines functionalized with electron-withdrawing and electron-donating substituents have been surveyed and afforded their reduced products in moderate-to-good yields. No detachment of the BICAAC unit from the Ir center was observed upon prolonged heating of Ir(MeBICAAC)Cl(COD) in toluene-d8 or isopropyl alcohol-d8, which evidenced good thermal stability of the catalyst. Complex Ir(MeBICAAC)Cl(COD) was also found to be catalytically active for the hydrosilylation of a variety of aldehydes using triethylsilane (Et3SiH).

Synthesis of an MXene supported Co nanoparticle catalyst for efficient catalytic transfer hydrogenation of nitro compounds with formic acid

The developed 2.6% Co/MXene exhibited excellent catalytic performance for the catalytic transfer hydrogenation of readily available nitroarenes employing HCOOH, leading to the specific generation of essential and versatile formamides.

Recent progress of heterogeneous catalysts for transfer hydrogenation under the background of carbon neutrality.

Under the background of carbon neutrality, the direct conversion of greenhouse CO2 to high value added fuels and chemicals is becoming an important and promising technology. Among them, the generation of liquid C1 products (formic acid and methanol) has made great progress; nevertheless, it encounters the problem of how to use it efficiently to solve the overcapacity issue. In this review, we suggest that the catalytic transfer hydrogenation using formic acid and methanol as the hydrogen sources is a critical and potential route for the substitution for the fossil fuel-derived H2 to generate essential bulk and fine chemicals. We mainly focus on summarizing the recent progress of heterogeneous catalysts in such reactions, including thermal- and photo-catalytic processes. Finally, we also propose some challenges and opportunities for this development.

Construction of Cu–Ru bimetallic catalyst for the selective catalytic transfer hydrogenation of carbonyl (C[dbnd]O) in biomass-derived compounds

The catalytic transfer hydrogenation (CTH) of levulinate ester to γ-valerolactone is one of the important reactions for production of renewable fuels from biomass. An efficient and robust bimetallic Cu–Ru catalyst were developed for the hydrogenation of biomass-derived ethyl levulinate to γ-valerolactone. A series of bimetallic catalysts were prepared and characterized in detail. Subsequently, the as-prepared catalysts were evaluated in the hydrogenation system of ethyl levulinate. The results showed that Cu–Ru/ZrO2–SO3–SiO2-450 catalyst provided 100 % GVL yield in 12 h at 180 °C. Meanwhile, it also exhibited high catalytic hydrogenation activity for the various biomass-derived carbonyl compounds. The structure-reactivity relationship of catalysts was investigated in detail, and the results show the interaction between support and metal sites played an important role in the hydrogenation activity. The sulfur species could tune the electronic states of metallic sites to gain optimal activity. Besides, the Cu–Ru/ZrO2–SO3–SiO2-450 catalyst performed excellent recyclability in five reaction runs without an obvious loss of activity.

Heteropolyacids promoted MOF-derived high-performance Co(H4)@C-HPW0.25 catalysts for catalytic transfer hydrogenation of vanillin in mild condition

Catalytic transfer hydrogenation (CTH) was recognized as a promising technique for converting various lignin derivatives to fuels and high-value compounds in heterogeneous systems. Herein, Co@C catalysts with diverse organic ligands, Keggin-type heteropolyacids, acid addition quantities, and metal/organic ligand molar ratios were synthesized via one-pot hydrothermal processes and carbonization methods. The optimized catalyst demonstrated highly efficient active hydrogen generation from isopropanol and the best conversion (100 %) for vanillin (VAN) hydrodeoxygenation (HDO) with 90 % 2-methoxy-4-methylphenol (MMP) yield at 180 °C for one hour under mild conditions of 0 MPa N2 (hydrogen-free circumstance), which were strongly connected to the higher specific surface area (154.45 m2/g), acidic site density (4.67 mmol/g), and stronger metal-P interactions of the Co(H4)@C-HPW0.25 catalyst. The findings in this work provided an advanced thinking for designing high-performance non-noble-metal catalysts that could be applied in the catalytic HDO transformation of various biomass derivatives into green fuel and chemicals under mild circumstances.

Ruthenium Complexes with N-Bound 2-Pyridonato Ligand as O-Donors: A New Synthetic Approach and the Effect on Reactivity.

In this study, Ru complexes [(η6-p-cymene)RuX(2-{(2,6-iPr2-C6H3)N=CH}-C5H3N-6-(O))] (3: X=Cl; 4: X=I) were prepared with N-bound 2-pyridonato ligand by thermal base-free MeX elimination from ionic N,N-chelated Ru complexes [(η6-p-cymene)RuX(κ2-L1)](X) (1: X=Cl; 2: X=I; L1={2-[(2,6-iPr2-C6H3)N=CH]-6-(OMe)C5H3N}). The Ru complex 3 was used as O-donor for Lewis (LA) or Brönsted acids. The reactions of 3 with SnCl2, Ph3SnCl, ZnCl2 or HCl provided [(η6-p-cymene)Ru(SnCl3)(2-{(2,6-iPr2-C6H3)N=CH}-C5H3N-6-(O→SnCl2)] (6), [(η6-p-cymene)RuCl(2-{(2,6-iPr2-C6H3)N=CH}-C5H3N-6-(O→SnPh3Cl)] (7), and [(η6-p-cymene)RuCl(2-{(2,6-iPr2-C6H3)N=CH}-C5H3N-6-(O→)]2(μ-ZnCl2) (8) and [(η6-p-cymene)RuCl(2-{(2,6-iPr2-C6H3)N=CH}-C5H3N-6-(OH)}](Cl) (9). The easy conversion of the 2-pyridonato ligand in 3 to its 2-hydroxypyridine in 9 evoked testing of 3 and 4 as potential catalysts in base-free transfer-hydrogenation of ketones.

Ruthenium(III) acyl thiourea complex: A catalyst for transfer hydrogenation of nitroarenes

The complex of ruthenium (III) ([RuCl2(PPh3)2BTU], 1) was prepared by reacting the acylthiourea ligand with a ruthenium salt ([RuCl2(PPh3)3]), and has been thoroughly characterised using various analytical techniques such as elemental analysis, magnetic susceptibility, UV–Vis, 1H NMR and single-crystal X-ray diffraction. The crystal packing of 1 has been found to be stabilised by intramolecular and intermolecular hydrogen bonding interactions, as well as CH⋅⋅⋅π contacts, namely C9-H9A⋅⋅⋅Cg(5), C12-H12A⋅⋅⋅Cg(4) and C45-H45⋅⋅⋅Cg(3), which play a crucial role in determining the overall supramolecular crystal structure of the compound. Hirshfeld surface analysis and 2D fingerprint analysis of 1 revealed that the compound was stabilised by various intermolecular interactions, including CH⋯C and CH⋯Cl interactions, with intermolecular H⋅⋅⋅H (54.9 %), C⋅⋅⋅H/H⋅⋅⋅C (23.6 %) and Cl⋅⋅⋅H/H⋅⋅⋅Cl (19.0 %) interactions being more prominent than other types of interactions. Furthermore, the conversion of nitroarenes to their corresponding aniline compounds was studied at different time intervals in ethanol at 50 °C in the presence of NaBH4. Examination of different nitro substrates revealed that 1 was a remarkable catalyst for the catalytic reduction of nitroarenes (0.01 mol of catalyst, >99 yield).

Highly active, ultra-low loading single-atom iron catalysts for catalytic transfer hydrogenation

Highly effective and selective noble metal-free catalysts attract significant attention. Here, a single-atom iron catalyst is fabricated by saturated adsorption of trace iron onto zeolitic imidazolate framework-8 (ZIF-8) followed by pyrolysis. Its performance toward catalytic transfer hydrogenation of furfural is comparable to state-of-the-art catalysts and up to four orders higher than other Fe catalysts. Isotopic labeling experiments demonstrate an intermolecular hydride transfer mechanism. First principles simulations, spectroscopic calculations and experiments, and kinetic correlations reveal that the synthesis creates pyrrolic Fe(II)-plN3 as the active center whose flexibility manifested by being pulled out of the plane, enabled by defects, is crucial for collocating the reagents and allowing the chemistry to proceed. The catalyst catalyzes chemoselectively several substrates and possesses a unique trait whereby the chemistry is hindered for more acidic substrates than the hydrogen donors. This work paves the way toward noble-metal free single-atom catalysts for important chemical reactions.

Polymeric Micelles with Chiral Diamine-Ru(II) Catalysts for Asymmetric Transfer Hydrogenation of Ketones in Water

Polymeric Micelles with Chiral Diamine-Ru(II) Catalysts for Asymmetric Transfer Hydrogenation of Ketones in Water