Selective Hydrogenation of CO2 to Ethanol over Cobalt Catalysts

Methods for the hydrogenation of CO₂ into valuable chemicals are in great demand but their development is still challenging. Herein, we report the selective hydrogenation of CO₂ into ethanol over non‐noble cobalt catalysts (CoAlO_x), presenting a significant advance for the conversion of CO₂ into ethanol as the major product. By adjusting the composition of the catalysts through the use of different prereduction temperatures, the efficiency of CO₂ to ethanol hydrogenation was optimized; the catalyst reduced at 600 ° gave an ethanol selectivity of 92.1 % at 140 °C with an ethanol time yield of 0.444 mmol g^?1 h^?1. Operando FT‐IR spectroscopy revealed that the high ethanol selectivity over the CoAlO_x catalyst might be due to the formation of acetate from formate by insertion of *CH_x, a key intermediate in the production of ethanol by CO₂ hydrogenation.     

Enantioselective Divergent Synthesis of (−)‐cis‐α‐ and (−)‐cis‐γ‐Irone by Using Wilkinson’s Catalyst

A simple, efficient synthesis is reported for (?)‐cis‐α‐ and (?)‐cis‐γ‐irone, two precious constituents of iris oils, in ≥99 % diastereomeric and enantioselective ratios. The two routes diverge from a common intermediate prepared from (?)‐epoxygeraniol. Of general interest in this approach is the installation of the enone moiety of irones through a NHC?Au^I‐catalyzed Meyer–Schuster‐like rearrangement of a propargylic benzoate and the use of Wilkinson’s catalyst for the stereoselective hydrogenation of a prostereogenic exocyclic double bond to secure the critical cis stereochemistry of the alkyl groups at C2 and C6 of the irones. The stereochemical aspects of this reaction are rationally supported by DFT calculation of the conformers of the substrates undergoing the hydrogenation and by a modeling study of the geometry of the rhodium η² complexes involved in the diastereodifferentiation of the double bond faces. Thus, computational investigation of the η² intermediates formed in the catalytic cycle of prostereogenic alkene hydrogenation by using Wilkinson’s catalyst could be highly predictive of the stereochemistry of the products.     

Highly Efficient Synthesis of Heterocyclic and Alicyclic β2‐Amino Acid Derivatives by Catalytic Asymmetric Hydrogenation

A valuable class of new heterocyclic and alicyclic prochiral α‐aminomethylacrylates has been conveniently synthesized through a three‐step transformation involving a Baylis–Hillman reaction, O‐acetylation, and a subsequent allylic amination. The corresponding novel β²‐amino acid derivatives were prepared with excellent enantioselectivities and high yields by catalytic asymmetric hydrogenation using the catalyst rhodium(Et‐Duphos) (Et‐Duphos=2′,5′,2′′,5′′‐tetraethyl‐1,2‐bis(phospholanyl)benzene)) under mild reaction conditions (up to 99 % ee and S/C=1000). The influence of the substrate on the enantioselectivity and reactivity is investigated, and the most suitable substrate configuration for the highly efficient enantioselective hydrogenation of β‐substituted α‐aminomethylacrylates under the Rh–Duphos system is reported. The current protocol provides a very practical, facile, and scalable method for the preparation of heterocyclic and alicyclic β²‐amino acids and their derivatives.     

Highly Productive CNN Pincer Ruthenium Catalysts for the Asymmetric Reduction of Alkyl Aryl Ketones

Chiral pincer ruthenium complexes of formula [RuCl(CNN)(Josiphos)] ( 2 – 7 ; Josiphos=1‐[1‐(dicyclohexylphosphano)ethyl]‐2‐(diarylphosphano)ferrocene) have been prepared by treating [RuCl₂(PPh₃)₃] with (S,R)‐Josiphos diphosphanes and 1‐substituted‐1‐(6‐arylpyridin‐2‐yl)methanamines (HCNN; substituent=H ( 1 a ), Me ( 1 b ), and tBu ( 1 c )) with NEt₃. By using 1 b and 1 c as a racemic mixture, complexes 4 – 7 were obtained through a diastereoselective synthesis promoted by acetic acid. These pincer complexes, which display correctly matched chiral PP and CNN ligands, are remarkably active catalysts for the asymmetric reduction of alkyl aryl ketones in basic alcohol media by both transfer hydrogenation (TH) and hydrogenation (HY), achieving enantioselectivities of up to 99 %. In 2‐propanol, the enantioselective TH of ketones was accomplished by using a catalyst loading as low as 0.002 mol % and afforded a turnover frequency (TOF) of 10⁵–10⁶ h^?1 (60 and 82 °C). In methanol/ethanol mixtures, the CNN pincer complexes catalyzed the asymmetric HY of ketones with H₂ (5 atm) at 0.01 mol % relative to the complex with a TOF of ≈10⁴ h^?1 at 40 °C.     

Furfural Hydrogenation on Nickel‐promoted Cu‐containing Catalysts Prepared from Hydrotalcite‐Like Precursors

The nickel‐promoted Cu‐containing catalysts (Cu_xNi_y‐MgAlO) for furfural (FFR) hydrogenation were prepared from the hydrotalcite‐like precursors, and characterized by X‐ray powder diffraction, inductively‐coupled plasma atomic emission spectroscopy, N₂ adsorption‐desorption, UV‐Vis diffuse reflectance spectra and temperature‐programmed reduction with H₂ in the present work. The obtained catalysts were observed to exhibit a better catalytic property than the corresponding Cu‐MgAlO or Ni‐MgAlO samples in FFR hydrogenation, and the CuNi‐MgAlO catalyst with the actual Cu and Ni loadings of 12.5 wt% and 4.5 wt%, respectively, could give the highest FFR conversion (93.2%) and furfuryl alcohol selectivity (89.2%). At the same time, Cu⁰ species from the reduction of Cu²⁺ ions in spinel phases were deduced to be more active for FFR hydrogenation.     

Mechanism of the Asymmetric Hydrogenation of Exocyclic α,β‐Unsaturated Carbonyl Compounds with an Iridium/BiphPhox Catalyst: NMR and DFT Studies

The mechanism of the asymmetric hydrogenation of exocyclic α,β‐unsaturated carbonyl compounds with the (aS)‐Ir/iPr‐BiphPhox catalyst was studied by NMR experiments and DFT computational analyses. Computed optical yields of the asymmetric hydrogenation proceeding by an iridium(I)/iridium(III) mechanism involving a transition state stabilized through two intramolecular hydrogen bonds are in good accordance with the experimental ee values.     

Facile Synthesis of Chiral Cyclic Ureas through Hydrogenation of 2‐Hydroxypyrimidine/Pyrimidin‐2(1H)‐one Tautomers

A facile access to optically active cyclic ureas was developed through palladium‐catalyzed asymmetric hydrogenation of pyrimidines containing tautomeric hydroxy group with up to 99 % ee. Mechanistic studies indicated that reaction pathway proceed through hydrogenation of C=N of the oxo tautomer pyrimidin‐2(1H)‐one, acid‐catalyzed isomerization of enamine–imine, and hydrogenation of imine pathway. In addition, the chiral cyclic ureas are readily converted into useful chiral 1,3‐diamine and thiourea derivatives without loss of optical purity.     

Synthesis and Structures of Two Dinuclear Transition Metal Complexes and Their Catalytic Applications in Hydrogenation of Ketones

The complexes [Ni₂(L)₂]₂ · H₂O ( 1 ) and [Cu₂(L)₂(H₂O)] · 2CH₃OH ( 2 ) were prepared by reaction of the chiral Schiff base ligand N‐[(1R,2S)‐2‐hydroxy‐1,2‐diphenyl]‐acetylacetonimine (H₂L) with Ni^II and Cu^II ions, respectively, aiming to develop economically and environmentally‐friendly catalysts for the hydrogenation of ketones. They have a dinuclear skeleton with axial vacant sites. The catalytic effects of the two complexes for hydrogenation of ketones were tested using dihydrogen gas as hydrogen source. They present some catalytic effects in hydrogenation of acetophenone, which has a dependence on the temperature and base used in these reactions. However, no apparent catalytic effects were found for the two complexes in hydrogenation of 4‐nitroacetophenone and 4‐methylacetophenone. Although the catalytic conversion in these hydrogenation reactions is low, they do represent a kind of cheap and environmentally‐friendly hydrogenation catalyst.     

The Role of Pd2+/Pd0 in Hydrogenation by [Pd(2‐pymo)2]n: An X‐ray Absorption and IR Spectroscopic Study

The metal–organic framework (MOF) [Pd(2‐pymo)₂]_n (2‐pymo=2‐pyrimidinolate) was used as catalyst in the hydrogenation of 1‐octene. During catalytic hydrogenation, the changes at the metal nodes and linkers of the MOF were investigated by in situ X‐ray absorption spectroscopy (XAS) and IR spectroscopy. With the help of extended X‐ray absorption fine structure and X‐ray absorption near edge structure data, Quick‐XAS, and IR spectroscopy, detailed insights into the catalytic relevance of Pd²⁺/Pd⁰ in the hydrogenation of 1‐octene could be achieved. Shortly after exposure of the catalyst to H₂ and simultaneously with the hydrogenation of 1‐octene, the aromatic rings of the linker molecules are hydrogenated rapidly. Up to this point, the MOF structure remained intact. After completion of linker hydrogenation, the linkers were also protonated. When half of the linker molecules were protonated, the onset of reduction of the Pd²⁺ centers to Pd⁰ was observed and the hydrogenation activity decreased, followed by fast reduction of the palladium centers and collapse of the MOF structure. Major fractions of Pd⁰ are only observed when the hydrogenation of 1‐octene is almost finished. Consequently, the Pd²⁺ nodes of the MOF [Pd(2‐pymo)₂]_n are identified as active centers in the hydrogenation of 1‐octene.     

Cooperative Effects in Catalytic Hydrogenation Regulated by both the Cation and Anion of an Ionic Liquid

The use of transition‐metal nanoparticles/ionic liquid (IL) as a thermoregulated and recyclable catalytic system for hydrogenation has been investigated under mild conditions. The functionalized ionic liquid was composed of poly(ethylene glycol)‐functionalized alkylimidazolium as the cation and tris(meta‐sulfonatophenyl)phosphine ([P(C₆H₄‐m‐SO₃)₃]^3?) as the anion. Ethyl acetate was chosen as the thermomorphic solvent to avoid the use of toxic organic solvents. Due to a cooperative effect regulated by both the cation and anion of the ionic liquid, the nanocatalysts displayed distinguished temperature‐dependent phase behavior and excellent catalytic activity and selectivity, coupled with high stability. In the hydrogenation of α,β‐unsaturated aldehydes, the ionic‐liquid‐stabilized palladium and rhodium nanoparticles exhibited higher selectivity for the hydrogenation of the C?C bonds than commercially available catalysts (Pd/C and Rh/C). We believe that the anion of the ionic liquid, [P(C₆H₄‐m‐SO₃)₃]^3?, plays a role in changing the surrounding electronic characteristics of the nanoparticles through its coordination capacity, whereas the poly(ethylene glycol)‐functionalized alkylimidazolium cation is responsible for the thermomorphic properties of the nanocatalyst in ethyl acetate. The present catalytic systems can be employed for the hydrogenation of a wide range of substrates bearing different functional groups. The catalysts could be easily separated from the products by thermoregulated phase separation and efficiently recycled ten times without significant changes in their catalytic activity.     

Chemoselective Hydrogenation of Nitroaromatics at the Nanoscale Iron(III)–OH–Platinum Interface

Catalytic hydrogenation of nitroaromatics is an environment‐benign strategy to produce industrially important aniline intermediates. Herein, we report that Fe(OH)_x deposition on Pt nanocrystals to give Fe(OH)_x/Pt, enables the selective hydrogenation of nitro groups into amino groups without hydrogenating other functional groups on the aromatic ring. The unique catalytic behavior is identified to be associated with the Fe^III‐OH‐Pt interfaces. While H₂ activation occurs on exposed Pt atoms to ensure the high activity, the high selectivity towards the production of substituted aniline originates from the Fe^III‐OH‐Pt interfaces. In situ IR, X‐ray photoelectron spectroscopy (XPS), and isotope effect studies reveal that the Fe³⁺/Fe²⁺ redox couple facilitates the hydrodeoxygenation of the ‐NO₂ group during hydrogenation catalysis. Benefitting from Fe^III‐OH‐Pt interfaces, the Fe(OH)_x/Pt catalysts exhibit high catalytic performance towards a broad range of substituted nitroarenes.     

Mechanism of asymmetric hydrogenation of enamides with [Rh(BisP*)]+ catalyst: Model DFT study

The potential energy profile for the [Rh(R,R)‐Et‐BisP*]⁺‐catalyzed asymmetric hydrogenation of prochiral enamides, α‐acetamidoacrylonitrile, was studied by the nonlocal density functional method (B3LYP). As illustrated, this hydrogenation is exothermic and goes mainly through association of [Rh(R,R)‐Et‐BisP*]⁺ with α‐acetamidoacrylonitrile, oxidative addition of hydrogen, migratory insertion of an olefin carbon into a Rh? H bond, reductive elimination of the C? H bond from the alkyl hydride, and dissociation of product–catalyst adducts with regeneration of the catalyst. The turnover‐limiting step for this reaction is the oxidative addition of hydrogen. The main product predicted theoretically is the R‐conformer. Our results are consistent with available empirical data and experiments for rhodium‐catalyzed asymmetric hydrogenation. © 2004 Wiley Periodicals, Inc. Int J Quantum Chem, 2005     

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

The 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 NiB_x as a catalyst and used water as the oxygen source for the oxygenation of various organic compounds. NiB_x 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.     

Solution Structure and Behavior of Benzophenone-based Achiral Bisphosphine Ligands in Noyori-Type Ru（Ⅱ）-Catalysts

We herein report on solution structural studies of Ru^Ⅱ catalysts （3a, 9） composed of achiral bisphosphine ligands （4, 8） and the enantiopure 1,2-diphenylethylenediamine （DPEN）. Complete chiral induction from enantiopure （R,R）-DPEN to achiral bisphosphine ligand 3a was observed in solution, with the complex adopting a single, stable and non-fluxional （even at 70 ℃） configuration. The coordination of the C=O moiety in 4 to the cationic Run center is considered to be of key importance in providing the higher thermodynamic and kinetic rotation barrier for the flexible bisphosphine ligand in the complex. The obtained enantioselectivity （91% enantiomeric excess） and sense of chiral induction in the hydrogenation of acetophenone were found to be solely dependent on the chirality of the 1,2-diamine. Consistent with the hydrogenation product, the （R,R）-DPEN induces a M-conformation （fight-handed） chirality for flexible phosphine ligand 4 in the complex, resulting in a 2,2-configuration about the Ru^Ⅱ center.     

Inverting External Asymmetric Induction via Selective Energy Transfer Catalysis: A Strategy to β‐Chiral Phosphonate Antipodes

Enantiodivergent, catalytic reduction of activated alkenes relays stereochemical information encoded in the antipodal chiral catalysts to the pro‐chiral substrate. Although powerful, the strategy remains vulnerable to costs and availability of sourcing both catalyst enantiomers. Herein, a stereodivergent hydrogenation of α,β‐unsaturated phosphonates is disclosed using a single enantiomer of the catalyst. This enables generation of the R‐ or S‐configured β‐chiral phosphonate with equal and opposite selectivity. Enantiodivergence is regulated at the substrate level through the development of a facile E → Z isomerisation. This has been enabled for the first time by selective energy transfer catalysis using anthracene as an inexpensive organic photosensitiser. Synthetically valuable in its own right, this process enables subsequent Rh^I‐mediated stereospecific hydrogenation to generate both enantiomers of the product using only the S‐catalyst (up to 99:1 and 3:97 e.r.). This strategy out‐competes the selectivities observed with the E‐substrate and the R‐catalyst.     

Chiral Iridium Spiro Aminophosphine Complexes: Asymmetric Hydrogenation of Simple Ketones,Structure, and Plausible Mechanism

The iridium complexes of chiral spiro aminophophine ligands, especially the ligand with 3,5‐di‐tert‐butylphenyl groups on the P atom ( 1c ) were demonstrated to be highly efficient catalysts for the asymmetric hydrogenation of alkyl aryl ketones. In the presence of KOtBu as a base and under mild reaction conditions, a series of chiral alcohols were synthesized in up to 97 % ee with high turnover number (TON up to 10 000) and high turnover frequency (TOF up to 3.7×10⁴ h⁻¹). Investigation on the structures of the iridium complexes of ligands (R)‐ 1a and 1c by X‐ray analyses disclosed that the 3,5‐di‐tert‐butyl groups on the P‐phenyl rings of the ligand are the key factor for achieving high activity and enantioselectivity of the catalyst. Study of the catalysts generated from the Ir‐(R)‐ 1c complex and H₂ by means of ESI‐MS and NMR spectroscopy indicated that the early formed iridium dihydride complex with one (R)‐ 1c ligand was the active species, which was slowly transformed into an inactive iridium dihydride complex with two (R)‐ 1c ligands. A plausible mechanism for the reaction was also suggested to explain the observations of the hydrogenation reactions.     

Transfer hydrogenation of ketones catalysed by half‐sandwich (η6‐p‐cymene) ruthenium(II) complexes incorporating benzoylhydrazone ligands

Neutral half‐sandwich η⁶‐p ‐cymene ruthenium(II) complexes of general formula [Ru(η⁶‐p ‐cymene)Cl(L)] (HL = monobasic O, N bidendate benzoylhydrazone ligand) have been synthesized from the reaction of [Ru(η⁶‐p ‐cymene)(μ‐Cl)Cl]₂ with acetophenone benzoylhydrazone ligands. All the complexes have been characterized using analytical and spectroscopic (Fourier transform infrared, UV–visible, ¹H NMR, ¹³C NMR) techniques. The molecular structures of three of the complexes have been determined using single‐crystal X‐ray diffraction, indicating a pseudo‐octahedral geometry around the ruthenium(II) ion. All the ruthenium(II) arene complexes were explored as catalysts for transfer hydrogenation of a wide range of aromatic, cyclic and aliphatic ketones with 2‐propanol using 0.1 mol% catalyst loading, and conversions of up to 100% were obtained. Further, the influence of other variables on the transfer hydrogenation reaction, such as base, temperature, catalyst loading and substrate scope, was also investigated.     

Noncovalent anchoring of asymmetric hydrogenation catalysts on a new mesoporous aluminosilicate: application and solvent effects

A new Brønsted acidic aluminosilicate, AlTUD‐1, with ideal characteristics for catalyst immobilisation (mesoporous structure, high surface area, and high Al_tetrahedral/Si ratio), was used successfully for the noncovalent anchoring of two well‐established asymmetric hydrogenation catalysts: [Rh^I(cod){(R,R)‐MeDuPHOS}]BF₄ ( 1 ) and [Rh^I(cod){(S,S)‐DiPAMP}]BF₄ ( 2 ). The new heterogeneous catalysts, 1 ‐AlTUD‐1 and 2 ‐AlTUD‐1, prepared by a straightforward ion‐exchange procedure, were highly active and selective in the asymmetric reduction of dimethyl itaconate ( 3 ) and methyl 2‐acetamidoacrylate ( 4 ), giving enantiomeric excesses of up to >98 %. The catalysts showed similar behaviour to their homogeneous counterparts. Catalyst 2 ‐AlTUD‐1 could be re‐used multiple times without loss of enantioselectivity or activity. Leaching of Rh showed a significant dependence on the polarity of the solvent in which the catalysis was performed. By applying tert‐butylmethyl ether (MTBE) as solvent, the loss of Rh could be reduced to <0.1 %. The solvent also had a noteworthy effect on the enantioselectivity in the hydrogenation of 4 (an effect not seen with 3 as substrate), that is, in MeOH the ee was 92 %, in MTBE it dropped to 26 % when using 2 ‐AlTUD‐1 as catalyst.     

Catalytic Hydrogenation of Acetic Acid to Acetaldehyde: Synergistic Effect of Bifunctional Co/Ce‐Fe Oxide Solid Solution Catalysts

The large‐scale industrial production of acetic acid (HAc) from carbonylation of methanol has enabled intense research interest from direct hydrogenation of HAc to acetaldehyde (AA). Herein, a series of cerium‐iron oxide solid solution supported metallic cobalt catalysts were prepared by modified sol‐gel method and were applied in gas‐phase hydrogenation of HAc to AA. A synergistic effect between the hydrogenation metal cobalt and Ce‐Fe oxide solid solution is revealed. Specifically, oxygen vacancies provide the active sites for adsorption of HAc, while highly uniformly dispersed metallic Co adsorbs H₂ and activates the reduction of HAc into AA. Moreover, the metallic Co can also assist the cyclical conversion between Fe³⁺/Fe²⁺ and Ce³⁺/Ce⁴⁺ on the surface of Ce_1‐xFe_xO_2‐δ supports. The unique effect substantially enhances the ability of the support material to rapidly capture oxygen atoms from HAc. It is found that the catalyst of 5% Co/Ce_0.8Fe_0.2O_2‐δ with the highest concentration of oxygen vacancy presents the best catalytic performance (i.e. acetaldehyde yield reaches 49.9%) under the optimal reaction conditions (i.e. 623 K and H₂ flow rate = 10 mL/min). This work indicates that the Co/Ce‐Fe oxide solid solution catalyst can be potentially used for the selective hydrogenation from HAc to AA. The synergy between the metallic Co and Ce_1‐xFe_xO_2‐δ revealed can be extended to the design of other composite catalysts.     

Divergent Coupling of Anilines and Enones by Integration of C−H Activation and Transfer Hydrogenation

Cp*Rh^III/Ir^III complexes are known to play important roles in both C?H activation and transfer hydrogenation (TH). However, these two areas evolved separately. They have been integrated in redox‐ and chemodivergent coupling reactions of N‐pyridylanilines with enones. The iridium‐catalyzed coupling with enones leads to the efficient synthesis of tetrahydroquinolines through TH from ⁱPrOH. Counterintuitively, ⁱPrOH does not serve as the sole hydride source, and the major reaction pathway involves disproportionation of a dihydroquinoline intermediate, followed by the convergent and iterative reduction of quinolinium species.