Transfer Hydrogenation Of Aryl Ketones Research Articles

RuCl2(DPEphos)(N-N)] (DPEphos = bis[(2-diphenylphosphino)phenyl]ether; N-N = 2-aminomethylpyridine or ethylenediamine): synthesis, characterization and transfer hydrogenation of aryl-ketones

The complexes trans/cis-[RuCl2(DPEphos)(ampy)] (trans – 1a; cis – 1b) and trans/cis-[RuCl2(DPEphos)(en)] (trans – 2a; cis – 2b) were obtained from the reactions of mer-[RuCl2(DPEphos)(PPh3)] (PPh3 = triphenylphosphine) with the corresponding amine in CH2Cl2. The isomer 1a was observed in situ when the reaction was carried out at r.t.; isolation leads to mixtures with 1b, while the latter was isolated when the reaction was performed under reflux. On the other hand, trans-[RuCl2(DPEphos)(en)] (2a) was obtained at r.t., in good yield and the cis-isomer 2b was observed when the reaction was carried out under reflux or when a CH2Cl2 solution of 2a is exposed to ambient light, but 2b was not isolated in pure form even under reflux for one week. Cyclic voltammetry studies revealed the presence of one reversible process with E1/2 0.57 and 0.40 V for 1b and 2a, respectively. Both 1b and 2a were used as pre-catalysts for reduction of acetophenone and p-substituted acetophenones (F–, Me– and MeO–) in transfer-hydrogenation conditions (isopropanol, 82 °C, and pre-catalyst:KOH:acetophenones in molar ratio 1:20:500 and/or 1:20:1000) showing conversions of 93–99% and TOF values up to 4.2 104 h−1.

A Ruthenium Protic N-Heterocyclic Carbene Complex as a Precatalyst for the Efficient Transfer Hydrogenation of Aryl Ketones

A neutral azole precursor to a protic N-heterocyclic carbene (pNHC) ligand, 6-((4,5-diphenyl-1H-imidazol-1-yl)methyl)-2,2′-bipyridine (3), was prepared from 6-(bromomethyl)-2,2′-bipyridine (2) and 4,5-diphenylimidazole. Complex [RuCl(pNHC-bpy)(PPh3)2](PF6) (4) bearing a protic, bipyridine-tethered NHC ligand was prepared by refluxing 3 with RuCl2(PPh3)3 and KPF6 in methanol and was characterized by nuclear magnetic resonance (NMR) spectroscopy, mass spectrometry, elemental analysis, and a single-crystal X-ray diffraction study. The hydrido complex [RuH(pNHC-bpy)(PPh3)2](PF6) (5) was prepared by reaction of 4 with NaBH4 in ethanol and characterized by NMR and Fourier transform infrared spectroscopy. Complex 5 was used as the catalyst (0.1 mol % loading) in the transfer hydrogenation of a range of alkyl/aryl ketones in basic iso-propanol at 60 °C. Bulky alkyl groups or ortho-substituted aryl groups at the ketones slowed down or inhibited the catalytic transformation. The addition of an excess of triphenylphosphine also slowed the catalysis, providing an indication for a mechanism involving phosphine dissociation, while the addition of an excess of elemental mercury had only a small effect on the conversion. The importance of potassium cations in the mechanism is consistent with the observation of reduced catalytic conversion when [2,2,2]-cryptand was present or when 1,8-diazabicyclo[5.4.0]undec-7-ene was used as the base. A plausible homogeneous catalysis mechanism involving the innersphere addition of hydride to the substrate in the transition state TS1 is supported by density functional theory calculations where the potassium ion has replaced the hydrogen atom of the N–H group in a protic NHC.

Tetrahydroxydiboron‐Mediated Palladium‐Catalyzed Deoxygenative Transfer Hydrogenation of Aryl Ketones

AbstractWe describe a palladium‐catalyzed deoxygenative transfer hydrogenation of ketones using B2(OH)4 as the sole additive. Deuterium labeling experiments using B2(OD)4 show that the incorporated protons originate exclusively from the diboron reagent. Spectroscopic evidence implicates the intermediacy of a borate ester. A variety of aromatic ketones can be deoxygenated using this approach.magnified image

Design of Iridium N-Heterocyclic Carbene Amino Acid Catalysts for Asymmetric Transfer Hydrogenation of Aryl Ketones

A series of chiral complexes of the form Ir(NHC)2(aa)(H)(X) (NHC = N-heterocyclic carbene, aa = chelated amino acid, X = halide) was synthesized by oxidative addition of α-amino acids to iridium(I) bis-NHC compounds and screened for asymmetric transfer hydrogenation of ketones. Following optimization of the reaction conditions, NHC, and amino acid ligands, high enantioselectivity was achieved when employing the Ir(IMe)2(l-Pro)(H)(I) catalyst (IMe = 1,3-dimethylimidazol-2-ylidene), which asymmetrically reduces a range of acetophenone derivatives in up to 95% enantiomeric excess.

Heteroleptic 1,4‐Diazabutadiene Complexes of Ruthenium: Synthesis, Characterization and Utilization in Catalytic Transfer Hydrogenation

Reaction of [Ru(trpy)Cl3] with 1,4‐diazabutadienes (p‐RC6H4N=C(H)‐(H)C=NC6H4R‐p; R = OCH3, CH3, H and Cl; abbreviated as L‐R) in refluxing ethanol in the presence of triethylamine has afforded a family of complexes, isolated as perchlorate salts, of type [Ru(trpy)(L‐R)Cl]ClO4 [depicted as complexes 1 (R = OCH3), 2 (R = CH3), 3 (R = H) and 4 (R = Cl)]. Crystal structures of complexes 1, 2 and 4 have been determined, and structure of complex 3 has been optimized by DFT method. The 1,4‐diazabutadiene ligand in each complex is bound to ruthenium as a N,N‐donor forming five‐membered chelate. Complexes 1–4 catalyze transfer hydrogenation of aryl aldehydes to the corresponding alcohols with high (ca. 106) TON. They are also found to catalyze transfer hydrogenation of aryl ketones to corresponding secondary alcohols, but with much less efficiency. Catalytic transfer hydrogenation of nitroarenes to the corresponding amines has also been achieved.

Electrochemical response of a Ru(II) benzothiazolyl-2-pyridinecarbothioamide pincer towards carbon dioxide and transfer hydrogenation of aryl ketones in air

A ruthenium(II) complex of 6-(4,7-dimethoxy-2-benzothiazolyl)-N-(2,5-dimethoxyphenyl)-2-pyridinecarbothioamide (pbcta), of the formula [Ru(pbcta)Cl2(dmf)] (1, where DMF = dimethyl formamide) was prepared from RuCl3•xH2O and pbcta in DMF at reflux under argon atmosphere. The identity of 1 was confirmed from its elemental analysis, ESI MS, and a series of spectroscopic measurements. Voltammetric measurements on 1 in DMF and DFT studies on the structure optimized in the gas phase revealed predominantly ligand based electron transfer processes under argon. In the presence of a proton source, proton coupled electron transfer to the ligand occurs. Under a carbon dioxide atmosphere, voltammetric studies revealed that 1 is inactive for CO2 reduction, and the redox responses observed in the presence of the proton source and/or CO2 are ligand based leading to reactions with the coordinated pbcta. Transfer hydrogenation (TH) of aryl ketones was efficiently carried out in 2-propanol using 1 at reflux. TH of the aryl ketone substrates proceeded in air with almost quantitative conversions at 0.2–1.0 mol% catalyst.

Arene-Ruthenium(II) Complexes Containing 11H-Indeno[1,2-b]quinoxalin-11-one Derivatives and Tryptanthrin-6-oxime: Synthesis, Characterization, Cytotoxicity, and Catalytic Transfer Hydrogenation of Aryl Ketones.

A series of novel mono- and binuclear arene–ruthenium(II) complexes [(p-cym)Ru(L)Cl] containing 11H-indeno[1,2-b]quinoxalin-11-one derivatives or tryptanthrin-6-oxime were synthesized and characterized by X-ray crystallography, IR, NMR spectroscopy, cyclic voltammetry, and elemental analysis. Theoretical calculations invoking singlet state geometry optimization, solvation effects, and noncovalent interactions were done using density functional theory (DFT). DFT calculations were also applied to evaluate the electronic properties, and time-dependent DFT was applied to clarify experimental UV–vis results. Cytotoxicity for cancerous and noncancerous human cell lines was evaluated with cell viability MTT assay. Complexes demonstrated a moderate cytotoxic effect toward cancerous human cell line PANC-1. The catalytic activity of the complexes was evaluated in transfer hydrogenation of aryl ketones. All complexes exhibited good catalytic activity and functional group tolerance.

Transfer Hydrogenation of Aryl Ketones on Mesoporous Silica-Supported Nickel

Transfer Hydrogenation of Aryl Ketones on Mesoporous Silica-Supported Nickel

Ruthenium-cymene containing pyridine-derived aldiimine ligands: Synthesis, characterization and application in the transfer hydrogenation of aryl ketones and kinetics studies

Five electron-rich NN′−pyridylimine ligands, which form five-membered chelate ring when coordinate to a metal center, have been synthesized by the easily accessible condensation between 2-pyridinecarboxyaldhehyde and the corresponding substituted aniline in diethyl ether solution, in the presence of catalytic amount of p-toluenesulfonic acid. The reaction of NN′–pyridylimine ligands with [RuCl2(p-cym)]2(1) in the presence of excess of KPF6 led to the formation of mononuclear adducts with general formula [RuCl(p-cym)(NN′)]PF6, where NN′ = N-phenyl-1-(pyridin-2-yl)methanimine (Amp) (2), N-(4-chlorophenyl)-1-(pyridin-2-yl)methanimine (Clmp) (3), N-(4-methylphenyl)-1-(pyridin-2-yl)methanimine (Memp) (4), N-(4-tert-butylphenyl)-1-(pyridin-2-yl)methanimine (Tbump) (5), N-(2,6-diethylphenyl)-1-(pyridin-2-yl)methanimine (Diemp) and N-[2,6-bis(propan-2-yl)phenyl]-1-(pyridin-2-yl)methanimine (Diipmp) (7). Additionally, the reaction of 2-acetylbenzenolate (OO′) with (1) produces a less bulky neutral congener [RuCl(p-cym)(OO′)] (8). All complexes were isolated and fully characterized by molar conductivity, elemental analysis, cyclic voltammetry and spectroscopic methods, including single crystal X-ray diffraction of (4), (7) and (8). The structures (2)–(7) were also optimized with the Density Functional Theory (DFT) and theoretical absorption spectra were obtained with TD-DFT formalism. The complexes (2)–(8) were applied as pre-catalyst in the transfer hydrogenation of the acetophenone and 4-methylacetophenone, showing good catalytic activity, with productivity up to 90%, and TOF up to 302 h−1 within 3 h of reaction; except for (8), which has no activity for the catalytic reduction performed in this work. A kinetic investigation using (3) and (7) as pre-catalysts showed that the time dependence for the production of 1-phenylethanol has a sigmoidal shape, with an induction time of 20 min. The catalytic activity of (3) is more sensitive to the temperature variation and its rate constant (k) increases faster than for (7). An Arrhenius plot reveals a smaller activation barrier (Ea) for the rate-determining step for (7) than for (3), Ea = 14.75 and 49.38 kJ mol−1, respectively. The ΔS≠ values obtained from an Eyring plot suggest an associative mechanism in the transition state of the catalytic reactions.

Highly dispersed Ni nanoparticles on mesoporous silica nanospheres by melt infiltration for transfer hydrogenation of aryl ketones.

Nickel-based catalysts have been applied to the catalytic reactions for transfer hydrogenation of carbonyl compounds. In the present work, highly dispersed nickel particles located at the pores of mesoporous silica spheres (Ni@mSiO2) were prepared via an optimized melt infiltration route. The nickel nanoparticles of 10 wt% in the Ni@mSiO2 catalyst could be uniformly loaded with high dispersion of 36.3%, resulting excellent performance for catalytic transfer hydrogenation of aryl ketones.

Nickel complexes of primary amido-functionalized N-heterocyclic carbene ligands: Synthesis, characterization, and base-free transfer hydrogenation

Three new nickel complexes featuring primary amido-functionalized N-heterocyclic carbene ligands have been synthesized and characterized using NMR spectroscopy, combustion analysis, and, in one case, single crystal x-ray diffraction. Each complex serves as a pre-catalyst for base-free transfer hydrogenation of aryl ketones in air using 2-propanol or 2-butanol as the solvent and hydrogen donor. Furthermore, in methanol-d4 solutions of the nickel complexes, hydrogen-deuterium exchange is observed on the ligands at moderate temperatures. These nickel complexes represent rare examples of transfer hydrogenation catalysts that operate under base-free and aerobic conditions.

Proteins as Macromolecular Ligands for Metal‐Catalysed Asymmetric Transfer Hydrogenation of Ketones in Aqueous Medium

Biohybrid catalysts resulting from the dative anchoring of half‐sandwich organometallic complexes [M(arene)(H2O)x(Cl)y]n+ (M = RuII, arene = η6‐benzene, p‐cymene or mesitylene; M = IrIII, RhIII, arene = η5‐Cp*; x = 1–3, y = 0–2, n = 0–2) to bovine beta‐lactoglobulin (βLG) or hen egg white lysozyme showed unprecedented behaviour. These constructs were shown to catalyse the asymmetric transfer hydrogenation of aryl ketones in water with sodium formate as hydrogen donor at a much faster rate than the complexes alone. Full conversion of the benchmark substrate 2,2,2‐trifluoroacetophenone was reached with an ee of 86 % for the most selective biohybrid. Surprisingly, even the crude biohybrid gave a good ee despite the presence of non‐protein‐bound metal species in the reaction medium. Other aryl ketones were reduced in the same way, and the highest ee was obtained for ortho‐substituted acetophenone derivatives. Furthermore, treatment of βLG with dimethyl pyrocarbonate resulted in a noticeable decrease of the activity and selectivity of the biohybrid, indicating that the sole accessible histidine residue (His146) was probably involved in the coordination and activation of Ru(benzene). This work underscores that protein scaffolds are efficient chiral ligands for asymmetric catalysis. The use of sodium formate instead of dihydrogen makes this approach safe, inexpensive and environmentally friendly.

Ionic [Ru] complex with recyclability by electro-adsorption for efficient catalytic transfer hydrogenation of aryl ketones

Abstract The efficient reuse of homogeneous catalyst is important. Cation complex of [Ru(η6-p-cymene)(PPh3)(CH3CN) Cl]PF6 with different ligands was synthesized and characterized by infrared spectroscopy (IR), 1H-, 13C- and 31P-nuclear magnetic resonance spectroscopy (1H-, 13C- and 31P-NMR), element analysis (EA), and high resolution mass spectrometry (HR-MS). The complex was used as a catalyst for the hydrogen transfer reduction of carbonyl for the first time, presenting an excellent catalytic performance of 89%–98% conversion of acetophenone and its derivatives. The catalyst may be efficiently reused by the electro-adsorption of 10 times to one catalyst recovery. The cation [Ru] complex presented advantages of both homogeneous and heterogeneous catalysts.

Transfer hydrogenation of aryl ketones with homogeneous ruthenium catalysts containing diazafluorene ligands

Novel cationic ruthenium(II) complexes bearing a 4,5‐diazafluorene unit and p‐cymene as ligands have been synthesised. The complexes were characterised based on elemental analysis and Fourier transform infrared and nuclear magnetic resonance spectroscopies. The synthesised Ru(II) complexes were employed as pre‐catalysts for the transfer hydrogenation of aromatic ketones using 2‐propanol as both hydrogen source and solvent in the presence of NaOH. All complexes showed high catalytic activity as catalysts in the reduction of substituted acetophenones to corresponding secondary alcohols. The products of catalysis were obtained with conversion rates of between 80 and 99%. Among the seven new complexes investigated, the most efficient catalyst showed turnover frequencies in the range 255–291 h−1 corresponding to 85 to 97% conversion, respectively. Copyright © 2016 John Wiley & Sons, Ltd.

Piano-stool d6-rhodium(III) complexes of chelating pyridine-based ligands and their papain bioconjugates for the catalysis of transfer hydrogenation of aryl ketones in aqueous medium

Two half-sandwich d6-rhodium(III) complexes of the general formula [(η5-Cp*)Rh(N^N)Cl]Cl where N^N is a phenanthroline or a bispyridine methane derivative carrying a thiol-targeting maleimide or chloroacetamide function were synthesized and characterized. Both complexes were able to catalyse the transfer hydrogenation of 2,2,2-trifluoroacetophenone in aqueous medium using formate or phosphite as hydrogen donor. Covalent anchoring of these complexes to the cysteine endoproteinase papain yielded hybrid metalloproteins with transfer hydrogenase properties. Under optimized conditions of pH, hydrogen donor concentration and catalyst load, conversion of substrate was nearly quantitative within 24h at 40°C and the (S)-enantiomer was obtained preferably albeit with a modest enantiomeric excess of 7⿿10%. Covalent docking simulations complemented the experimental findings suggesting a molecular rationale for the observed low enantioselectivity. The harmonious use of experimental and theoretical approaches represents an unprecedented starting point for driving the rational design of artificial metalloenzymes built up from papain with higher catalytic efficiency.

ChemInform Abstract: Simple 1,3‐Diamines and Their Application as Ligands in Ruthenium(II) Catalysts for Asymmetric Transfer Hydrogenation of Aryl Ketones.

In this research work simple unsymmetrical 1,3-diamines were studied. The synthesis of the diamines started from non-commercial available compounds. S-5a and S,S-5c were obtained by biocatalysis with non conventional yeast, Rhodotorula rubra MIM 147, with excellent 99% e.e. and d.e. up to 90%. Different approaches of synthesis were applied to the same backbone to study both the steric and electronic effects of the ligands. The reactivity of the corresponding ruthenium complexes was evaluated in the asymmetric hydrogen transfer reduction of acetophenone as a standard substrate and of other different aryl ketones, highlighting the flexibility of the six membered chelating ring. A screening of the reaction conditions indicated aqueous media in the presence of HCOONa as a hydrogen donor to be the best system for overcoming the lack of stereocontrol thus allowing us to obtain 56% e.e. in the reduction of acetophenone with the complex in which the ligand was diamine 1, revealed as the best in terms of reactivity and stereoselectivity also in the reduction of the other different aryl ketones, in particular for α-tetralone, i (63% e.e.).

Novel ruthenium(II)–N-heterocyclic carbene complexes; synthesis, characterization and catalytic application

The synthesis of highly active ruthenium(II) catalyst based on N-heterocyclic carbene (NHC) bearing alkyl and aryl groups is reported. The complexes are easily handled in very good yields via synthesis of silver–NHC complexes and subsequent reaction with [RuCl2(p-cymene)]2 in dichloromethane (DCM) under N2 condition. All new compounds were characterized by LC-MSMS, elemental analysis, NMR spectroscopy and X-ray diffraction. With the development of a more efficient catalytic system for transfer hydrogenation of aryl ketones and aldehydes in mind, the catalytic performance of the Ru–NHC complexes for transfer hydrogenation was investigated.

ChemInform Abstract: Transfer Hydrogenation of Ketones to Alkanes Using Tributylamine under Heterogeneous Pd/C Catalysis.

AbstractThe Pd‐catalyzed transfer hydrogenation of aryl ketones with tributylamine leads to corresponding alkanes (II), whilst the similar reaction of dialkyl ketone (Ii) fails.

Simple 1,3-diamines and their application as ligands in ruthenium(ii) catalysts for asymmetric transfer hydrogenation of aryl ketones

A new efficient synthesis of 1,3-diamines was realized and their ruthenium(ii) complexes were studied in ATH in aqueous media.

Ru(II) impregnated Al2O3, Fe3O4, SiO2 and N-coordinate ruthenium(II) arene complexes: Multifunctional catalysts in the hydrogenation of nitroarenes and the transfer hydrogenation of aryl ketones

Six new [RuClL(p-cymene)]Cl (LN-arenesulfonyl-4,5-dichloro-o-phenylenediamines) complexes (6–10) were prepared from the starting complex [RuCl2(p-cymene)]2. Also, three novel Al2O3, Fe3O4, SiO2-[RuClL(p-cymene)]Cl materials (11–13) were synthesized by an impregnation method using (10) and were solid-supported. The structural elucidations of the complexes and materials were characterized by different methods such as 1H, 13C-NMR, FT-IR, elemental analysis, TG/DTA, nitrogen adsorption-desorption (BET), SEM and EDX. The 6–10 complexes and 11–13 materials were applied for the transfer hydrogenation (TH) of ketones in the presence of 2-propanol (as the hydrogen source). The TH experiments showed that all complexes are average catalytic precursors (6–10). In particular, {[N-4-chlorobenzenesulfonyl-4,5-dichloro-o-phenylenediamine]-(p-cymene)-chloro-ruthenium(II)}chloride (10) was found to be a good catalyst in comparison to the others giving the corresponding alcohols in a good turnover frequency value of 2192h−1. Additionally, the synthesized Al2O3, Fe3O4, SiO2-[RuClL(p-cymene)]Cl materials (11–13) obtained from the most active catalyst (10) were used as catalysts for the TH and their efficiency was screened in the hydrogenation of nitroarenes in aqueous media at ambient temperature in the presence of NaBH4 by UV–vis spectrophotometer. Furthermore, the Fe3O4-[RuClL(p-cymene)]Cl materials can be recovered by filtration and reused for four cycles for the reduction of 2-nitroaniline. This facile and environment-friendly approach provides a green way to effectively synthesize low cost solid-supported catalysts for the reduction of nitroarenes and is promising for the development of other useful materials.