Direct and Transfer Hydrogenation of Ketones and Imines with a Ruthenium N‐Heterocyclic Carbene Complex

The dihydride ruthenium N‐heterocyclic carbene complex Ru(IMes)(PPh₃)₂CO(H)₂ ( 1 ) (IMes=1,3‐dimesityl‐1,3‐dihydro‐2H‐imidazol‐2‐ylidene) is an efficient catalyst for both direct hydrogenation and transfer hydrogenation of ketones and imines, in the absence of base.     

Mechanistic Aspects of Formation of Chiral Ruthenium Hydride Complexes from 16‐Electron Ruthenium Amide Complexes and Formic Acid: Facile Reversible Decarboxylation and Carboxylation

The 16‐electron amide complex, Ru[(R,R)‐TsNCHPhCHPhNH](η⁶‐p‐cymene) (Ts=p‐toluenesulfonyl, Ph=C₆H₅) readily reacts with formic acid to give the corresponding formate complex, which subsequently undergoes decarboxylation leading to the hydride complex with release of CO₂. The reaction of this hydride complex with CO₂ under mild reaction conditions, a pressure of 10 atm and even at −78 °C, proceeds rapidly to give the corresponding formate complex almost quantitatively. Thus, the reversible decarboxylation and carboxylation takes place with or without the aid of a metal‐NH bifunctional effect of the Ru complexes.     

Selective Transfer Hydrogenation of Carbonyl Compounds by Ruthenium Nanoclusters Supported on Alkali‐Exchanged Zeolite Beta

Selective transfer hydrogenation of aromatic ketones and β‐keto esters to the corresponding alcohols was achieved by using ruthenium nanoclusters supported on alkali‐exchanged zeolite beta catalyst. The high activity and selectivity of the catalyst is due to the presence of highly dispersed ruthenium clusters in combination with the large number of Brønsted acidic sites of zeolite.     

Efficient and Chemoselective Reduction of Pyridines to Tetrahydropyridines and Piperidines via Rhodium‐Catalyzed Transfer Hydrogenation

Promoted by iodide anion the rhodium complex dimer, [Cp*RhCl₂]₂, catalyzes efficiently the transfer hydrogenation of various quaternary pyridinium salts under mild conditions, affording not only piperidines but also 1,2,3,6‐tetrahydropyridines in a highly chemoselective fashion, depending on the substitution pattern at the pyridinium ring. The reduction is conducted in azeotropic formic acid/triethylamine (HCOOH‐Et₃N) mixture at 40 °C, with catalyst loadings as low as 0.005 mol% being feasible.     

Enantioselective Synthesis of α‐Trifluoromethyl Arylmethylamines by Ruthenium‐Catalyzed Transfer Hydrogenation Reaction

A simple combination of dichloro(para‐cymene)ruthenium(II) dimer, a chiral amino alcohol and isopropyl alcohol allowed for in‐situ generation of the bifunctional catalyst responsible for the transfer hydrogenation reaction of trifluoromethyl ketimines in excellent yields with high enantioselectivities (up to 93% ee). Herein, we describe the optimization, scope, limitations, and applications of the method.

     

Ruthenium Nanoparticle‐Catalyzed,Controlled and Chemoselective Hydrogenation of Nitroarenes using Ethanol as a Hydrogen Source

This communication describes a ruthenium nanoparticle‐catalyzed reduction of nitroarenes giving azoxyarenes, azoarenes, or anilines in good to excellent yields using ethanol as a hydrogen source.     

Enantioselective Hydrogenation of the Tetrasubstituted C=C Bond of Enamides Catalyzed by a Ruthenium Catalyst Generated in situ

The enantioselective hydrogenation of enamides bearing an endocyclic tetrasubstituted carbon‐carbon double bond has been performed in the presence of ruthenium catalyst precursors prepared from Ru(cod)(methallyl)₂, Duphos, or BPE as optically active ligand and HBF₄. This promising catalytic system makes possible the selective cis‐hydrogenation with satisfactory enantioselectivities (up to 72% ee) for this type of tetrasubstituted double bonds.     

Enantioselective Synthesis of Chiral β‐Aryloxy Alcohols by Ruthenium‐Catalyzed Ketone Hydrogenation via Dynamic Kinetic Resolution (DKR)

A highly efficient enantioselective synthesis of chiral β‐aryloxy alcohols by the {RuCl₂[(S)‐SDP][(R,R)‐DPEN]} [(S_a,R,R)‐ 1a ; SDP=7,7′‐bis(diarylphosphino)‐1,1′‐spirobiindane; DPEN=trans‐1,2‐diphenylethylenediamine] complex‐catalyzed asymmetric hydrogenation of racemic α‐aryloxydialkyl ketones via dynamic kinetic resolution (DKR) has been developed. Enantioselectivities of up to 99% ee with good to high cis/anti‐selectivities (up to>99:1) were achieved.     

Efficient Access to Conjugated Dienones and Diene‐diones from Propargylic Alcohols and Enolizable Ketones: A Tandem Isomerization/Condensation Process Catalyzed by the Sixteen‐Electron Allyl‐Ruthenium(II) Complex [Ru(η3‐2‐C3H4Me)(CO)(dppf)] [SbF6]

A large variety of conjugated dienones R¹R²CCHCHC(R³)C(O)R⁴ and diene‐diones R¹R²CCHCHC{C(O)R³}C(O)R⁴ have been synthesized in high yields by reacting terminal propargylic alcohols HCCCR¹R²(OH) with enolizable ketones R³CH₂C(O)R⁴ and β‐dicarbonyl compounds R³C(O)CH₂C(O)R⁴, respectively. The process, which is catalyzed by the 16e^‐ (η³‐allyl)‐ruthenium(II ) complex [Ru(η³‐2‐C₃H₄Me)(CO)(dppf)] [SbF₆] associated with CF₃CO₂H, involves the initial isomerization of the propargylic alcohol into the corresponding α,β‐unsaturated aldehyde R¹R²CCHCHO (Meyer–Schuster rearrangement) and subsequent aldol‐type condensation.     

[CpRu(IV)(π‐C3H5)(2‐quinolinecarboxylato)]PF6 Complex: A Robust Catalyst for the Cleavage and Formation of Allyl Ethers

A facile and efficient method for the quantitative synthesis of [CpRu(IV)(π‐C₃H₅)(2‐quinolinecarboxylato)]PF₆ from [CpRu(CH₃CN)₃]PF₆, 2‐quinolinecarboxylic acid, and 2‐propen‐1‐ol has been established. The cationic Ru(IV) complex is air‐ and moisture‐stable, and can be stored in a vial for at least six months. This complex realizes a simple and easy operation for both the deallylation of allyl ethers in methanol and the dehydrative allylation of alcohols. Furthermore, with removal of the volatile allyl methyl ether co‐product from the reaction system, the robust catalyst can attain a turnover of 10000 cycles of allyl ether cleavage.     

Hydrogenation of Carbon Dioxide to Formate Catalyzed by a Copper/1,8‐Diazabicyclo[5.4.0]undec‐7‐ene System

Hydrogenation of carbon dioxide to formate was achieved using copper (Cu) catalysts in the presence of strong organic bases including amidines and guanidines. Specifically, 1,8‐diazabicyclo[5.4.0]undec‐7‐ene (DBU) proved to be effective for the transformation of a 1:1 mixture of hydrogen and carbon dioxide into its formate salt under increased pressure in the presence of various Cu(I) and Cu(II) salts at 100 °C. A novel complex derived from copper iodide and DBU equally promoted the same reaction, indicating that DBU–Cu species are involved as real catalysts in this hydrogenation.

     

Catalytic Enantioselective Hydrophosphonylation of Aldehydes Using the Iron Complex of a Camphor‐Based Tridentate Schiff Base [FeCl(SBAIB‐d)]2

An iron(III)‐Schiff base‐catalyzed, highly enantioselective hydrophosphonylation of various aldehydes is described. Under the optimized reaction conditions, 5 mol% of the iron/camphor‐based tridentate Schiff base complex [FeCl(SBAIB‐d)]₂ produces high yields (up to 99%) of α‐hydroxy phosphonates in excellent enantioselectivities (up to 99%). The merits of this catalytic system are an easily synthesizable catalyst, inexpensive starting materials, practically simple aerobic reaction conditions, and low catalyst loading (5 mol%).

     

Mild and Efficient Heterogeneous Hydrogenation of Nitroarenes Facilitated by a Pyrolytically Activated Dinuclear Ni(II)-Ce(III) Diimine Complex

We communicate the assembly of a solid, Ce-promoted Ni-based composite that was applied as catalyst for the hydrogenation of nitroarenes to afford the corresponding organic amines. The catalytically active material described herein was obtained through pyrolysis of a SiO₂-pellet-supported bimetallic Ni-Ce complex that was readily synthesized prior to use from a MeO-functionalized salen congener, Ni(OAc)₂·4 H₂O, and Ce(NO₃)₃·6 H₂O. Rewardingly, the requisite ligand for the pertinent solution phase precursor was accessible upon straightforward and time-saving imine condensation of ortho-vanillin with 1,3-diamino-2,2′-dimethylpropane. The introduced catalytic protocol is operationally simple in that the whole reaction set-up is quickly put together on the bench without the need of cumbersome handling in a glovebox or related containment systems. Moreover, the advantageous geometry and compact-sized nature of the used pellets renders the catalyst separation and recycling exceptionally easy.     

A Novel Propargylation/Cycloisomerization Tandem Process Catalyzed by a Ruthenium(II)/Trifluoroacetic Acid System: One‐Pot Entry to Fully Substituted Furans from Readily Available Secondary Propargylic Alcohols and 1,3‐Dicarbonyl Compounds

A simple and highly efficient method for the preparation of tetrasubstituted furans starting from readily accessible propargylic alcohols and commercially available 1,3‐dicarbonyl compounds has been developed. The process, which proceeds in a one‐pot manner, involves the initial propargylation of the 1,3‐dicarbonyl compound promoted by trifluoroacetic acid, and subsequent cycloisomerization of the resulting γ‐ketoalkyne catalyzed by the 16‐electron allyl‐ruthenium(II ) complex [Ru(η³‐2‐C₃H₄Me)(CO)(dppf)][SbF₆].     

Compensation Effect of Benzene Hydrogenation on Pt(111) and Pt(100) Analyzed by the Selective Energy Transfer Model

Kinetic measurements at low temperatures (310–360 K) using gas chromatography (GC) for benzene hydrogenation on Pt(100) and Pt(111) single crystal surfaces have been carried out at Torr pressures. These kinetic measurements demonstrated a linear compensation effect for the production of cyclohexane. A detailed application of the model of selective energy transfer to the experimentally obtained results yields the vibrational frequency of the adsorbate leading to reaction. This frequency is attributed to ring distortion modes. The vibrational frequency of the heat bath, or catalyst, is ascribed to a Pt-H mode. An approximate heat of adsorption of the reacting molecule is also calculated from the model.     

Chemoenzymatic synthesis of the novel amphiphilic diblock copolymer poly[caprolactone‐block‐(glycidyl methacrylate)] from a bifunctional initiator and its micellization behavior

The chemoenzymatic synthesis of a novel diblock copolymer consisting of a hydrocarbon block of polycaprolactone (PCL) and an epoxy‐based block of poly(glycidyl methacrylate) (PGMA) was achieved by the combination of enzymatic ring‐opening polymerization (eROP) and atom transfer radical polymerization (ATRP). A trichloromethyl‐terminated PCL macrointiator was obtained via Novozyme 435‐catalyzed eROP of ε‐caprolactone from a bifunctional initiator, 2,2,2‐trichloroethanol, under anhydrous conditions. PCL‐b‐PGMA diblock copolymers were synthesized in a subsequent ATRP of glycidyl methacrylate. The kinetics analysis of ATRP indicated a ‘living’/controlled radical polymerization. The macromolecular structure and thermal properties of the PCL macroinitiator and of the diblock copolymer were characterized using NMR spectroscopy, gel permeation chromatography and differential scanning calorimetry. The well‐defined PCL‐b‐PGMA amphiphilic diblock copolymer self‐assembled in aqueous solution into nanoscale micelles. The size and shape of the resulting micelles were investigated using dynamic light scattering, transmission electron microscopy and tapping‐mode atomic force microscopy. Copyright © 2007 Society of Chemical Industry     

Oxidation of catechol and l-ascorbic acid by [Ru(tpy)(pic)(OH)] (tpy = 2,2′6′,2″-terpyridine; pic = picolinate): Kinetic and mechanistic studies

Kinetics of oxidation of catechol (H₂cat) to benzoquinone (BQ) and, ascorbate (HA⁻) to dehydroascorbic acid (A) by [Ru^III(tpy)(pic)(OH)]⁺ (1) (tpy = 2,2’6’,2”-terpyridine; pic⁻ = picolinate) have been studied as function of [H₂cat] (or [HA⁻]), ionic strength (0.01 − 0.25 M), temperature (10–30 °C) at a constant pH = 3.2, using stopped-flow and rapid-scan diode array spetrophotometric techniques. The rate of reaction of 1 with HA⁻ was found to be very fast as compared to that of with H₂cat. The kinetic data and activation parameters are interpreted in terms of an associative interchange mechanism. Analysis of spectral and kinetic data revealed that the reaction of 1 with catechol proceeds by a straightforward outer-sphere electron transfer pathway, whereas, reduction of 1 with HA⁻ involves ion-pair formation.     

Synthesis of a chiral stationary phase with poly[styrene‐b‐cellulose 2,3‐bis(3,5‐dimethylphenylcarbamate)] by surface‐initiated atom transfer radical polymerization and its chiral resolution efficiency

A chiral stationary phase (CSP) with poly[styrene‐b‐cellulose 2,3‐bis(3,5‐dimethylphenylcarbamate)] was synthesized by the surface‐initiated atom transfer radical polymerization (SI‐ATRP) of cellulose 2,3‐bis(3,5‐dimethylphenylcarbamate)‐6‐acrylate after the SI‐ATRP of styrene on the surface of silicon dioxide supports in pyridine. The successful preparation of the CSP with poly[styrene‐b‐cellulose 2,3‐bis(3,5‐dimethylphenylcarbamate)] was confirmed via Fourier transform infrared spectroscopy, field emission scanning electron microscopy, X‐ray photoelectron spectroscopy, elemental analysis, and thermal analysis. The applicability for the chiral resolution of the CSP with poly[styrene‐b‐cellulose 2,3‐bis(3,5‐diphenylcarbamate)] was evaluated with high‐performance liquid chromatography with 10 racemates under various mobile phases of hexane/alcohol, hexane/tetrahydrofuran (THF), and hexane/chloroform. The results show that the CSP with poly[styrene‐b‐cellulose 2,3‐bis(3,5‐diphenylcarbamate)] could be used in THF and chloroform as eluents. The chiral resolutions of the commercial Chiracel OD, the CSP with cellulose 2,3‐bis(3,5‐dimethylphenylcarabmate), and the CSP with poly[styrene‐b‐cellulose 2,3‐bis(3,5‐dimethylphenylcarbamate)] prepared by SI‐ATRP were examined. © 2011 Wiley Periodicals, Inc. J Appl Polym Sci, 2011     

Efficient and Green Route to γ‐Lactams by Copper‐Catalysed Reversed Atom Transfer Radical Cyclisation of α‐Polychloro‐N‐allylamides,using a Low Load of Metal (0.5 mol%)

The cyclisation of N‐allyl‐N‐substituted‐α‐polychloroamides is efficiently obtained through a copper‐catalysed activators regenerated by electron transfer–atom transfer radical cyclisation process, with a metal load of only 0.5 mol%. The redox catalyst is introduced in its inactive form as copper(II) chloride/[nitrogen ligand] complex, and continuously regenerated to the active copper(I) chloride/[nitrogen ligand] species by ascorbic acid. To preserve the catalyst integrity, the hydrochloric acid, released after each regeneration cycle, has been quenched by carbonate. The choice of the solvent is critical, the best performance being observed in ethyl acetate‐ethanol (3:1).