Further ‘tethered’ Ru(II) catalysts for asymmetric transfer hydrogenation (ATH) of ketones; the use of a benzylic linker and a cyclohexyldiamine ligand

The synthesis and characterisation of two new Ru(II) catalysts for the asymmetric transfer hydrogenation (ATH) of ketones is described. In the case of 4, the novelty lies in the use of a benzyl tethering group between the asymmetric ligand part (TsDPEN) and the η⁶-arene ring, which increases the complex rigidity. For 5, the use of a cyclohexyldiamine as a chiral ligand is described for the first time. In the ATH of ketones in formic acid/triethylamine, alcohols with ees of up to 97% were formed.     

Asymmetric transfer hydrogenation of quinolines using tethered Ru(II) catalysts

The first report of an asymmetric transfer hydrogenation, in formic acid/triethylamine, of quinolines is described. Using a Ru(II) catalyst containing a 4-carbon tether, products of up to 73% ee were formed, whilst a Rh(III)-tethered catalyst gave products of up to 94% ee.     

Ru/ZSM-5催化葡萄糖选择性加氢制备山梨醇(英)

采用无有机模板剂一步法制备了Ru/ZSM-5催化剂,利用X射线衍射、N₂吸附-脱附、NH₃-程序升温脱附和CO₂-程序升温脱附、扫描电镜和透射电镜等方法对催化剂进行了表征.考察了反应温度、钌负载量和催化剂重复利用等因素对Ru/ZSM-5上葡萄糖加氢反应性能的影响,并与浸渍法制备的Ru/ZSM-5催化剂进行了对比.结果表明,与传统浸渍法相比,一步法制备的Ru/ZSM-5催化剂钌粒子具有更高的分散性和稳定性.在120℃和4 MPa的温和反应条件下,葡萄糖接近完全转化,山梨醇选择性高达99.2%,催化剂可重复利用5次,仍保持较高活性.     

Cationic palladium(II) complexes of ferrocenylphosphines as homogeneous hydrogenation catalysts for olefins

Cationic palladium(II) complexes of ferrocenylphosphines [(L-L′)Pd(S)₂][ClO₄]₂ ((L-L′) = Fe(η⁵-C₅H₄P (C₆H₅)₂)₂ 1, or Fe(η⁵-C₅H₅)(η⁵C₅H₃(CHMeNMe₂)P(C₆H₅)₂-1,2) 2a: S=pyridine or dimethylformamide) were prepared and characterized. The derivatives of 2a are effective catalysts for the hydrogenation of simple olefins at 30°C (1 atm H₂). The rate of reduction of styrene depends on the substrate concentration, catalyst concentration and the solvent, and is only slightly inhibited (16%) by the addition of mercury. These observations are conistent with a homogeneous catalytic system.     

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

The ruthenium(II) complexes RuH₂(CO)₂(PⁿBu₃)₂, RuH₂(CO)₂(PPh₃)₂, and RuH₂(PPh₃)₄ are catalytically active in the hydrogenation of organic substrates containing a NN, N(O)N or NO₂ group. The reduction of the first two groups leads to hydrazine as intermediate and amine as the final product, while reducing a NO₂ 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 RuH₂(PPh₃)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.     

Asymmetric transfer hydrogenation of α,β-unsaturated, α-tosyloxy and α-substituted ketones

Asymmetric transfer hydrogenation of cyclic and acyclic α,β-unsaturated ketones catalysed by η⁶-p-cymene/ruthenium(II) and η⁵-pentamethylcyclopentadienyl/rhodium(III) complexes have been investigated. Cyclic α,β-unsaturated ketones appeared to be more suitable substrates for the synthesis of enantiomerically pure allylic alcohols than do acyclic α,β-unsaturated ketones. A proposed mechanism for the formation of 4-phenyl-[1,3]-dioxolan-2-one from α-tosyloxy- and halo-substituted acetophenones is discussed. The results of further investigations into the reduction of a range of α-tosyloxyacetophenones and the dynamic kinetic resolution of α-substituted ketones is presented.     

Synthesis,characterization, and transfer hydrogenation of Ru(II)-N-heterocyclic carbene complexes

A new series of ruthenium(II) N-heterocyclic carbene complexes [RuL^1,2,3(p-cymene)Cl₂] (3a–c) (where L is a N-heterocyclic carbene), have been synthesized via transmetalation. The new ruthenium(II)-NHC complexes were applied to transfer hydrogenation of acetophenone derivatives and aldehydes using 2-propanol as a hydrogen source and KOH as a co-catalyst. The results show that the corresponding alcohols could be obtained in good yield with high catalyst activity (up to 100%) under mild conditions. [RuL¹(p-cymene)Cl₂] (3a) is much more active than the other complexes in transfer hydrogenation. Reactions, catalyzed by 3a–c, showed the highest reaction rates and yields of alcohol when the substrates bear more electron-withdrawing substituents. All new compounds were characterized by IR, elemental analysis, LC–MS (ESI), and NMR spectroscopy.     

On the behaviour of Ru(I) and Ru(II) carbonyl acetates in the presence of H2 and/or acetic acid and their role in the catalytic hydrogenation of acetic acid

The reactivity of phosphine substituted ruthenium carbonyl carboxylates Ru(CO)₂(MeCOO)₂(PBu₃)₂, Ru₂(CO)₄(μ-MeCOO)₂(PBu₃)₂, Ru₄(CO)₈(μ-MeCOO)₄(PBu₃)₂ with H₂ and/or acetic acid was investigated by IR and NMR spectroscopy to clarify their role in the catalytic hydrogenation of acetic acid. Evidences were collected to suggest hydride ruthenium complexes as the catalytically active species. Equilibria among ruthenium hydrides and carboxylato complexes take place in the presence of hydrogen and acetic acid, that is in the conditions of the catalytic reaction. Nevertheless the presence of acetic acid reduces the rate of the formation of hydrides. Working at a very high temperature (180°C) polynuclear phosphido hydrides such as [Ru₆(μ-H)₆(CO)₁₀(μ-PHBu)(μ-PBu₂)₂(PBu₃)₂(μ₆-P)] were formed. These phosphido clusters are suggested as the resting state of the catalytic system.Furthermore the bi- or tetranuclear Ru(I) carboxylato complexes react with acetic acid giving a mononuclear ruthenium complex Ru(CO)₂(MeCOO)(μ-MeCOO)(PBu₃), containing a monodentate and a chelato acetato ligands. This complex was spectroscopically characterised. Its identity and structure were confirmed by its reactivity with stoichiometric amount of PPh₃ to give Ru(CO)₂(MeCOO)₂(PBu₃)(PPh₃), a new mononuclear ruthenium carbonyl carboxylate containing two different phosphines, that was fully characterised.     

"Tethered" Ru(II) catalysts for asymmetric transfer hydrogenation of ketones

Stereochemically well-defined ruthenium(II) catalysts have been applied to the asymmetric transfer hydrogenation of a series of ketones. In one case, statistical experimental design was employed to optimize the enantiomeric excess of the product. In the case of the TsDPEN-based systems, the replacement of trans-1,2-diphenyl substitution with cis-, or deletion of one of the phenyl groups, results in significant deterioration of the enantiomeric excess. A new method is described for the synthesis of tethered amino alcohol-containing catalysts.     

Ruthenium(II) complex catalysts bearing a pyridyl-supported pyrazolyl-imine ligand for transfer hydrogenation of ketones

A new class of hemilabile unsymmetrical 2-(1-arylimino)-6-(pyzazol-1-yl)pyridine ligands and their ruthenium(II) and nickel(II) NNN complexes were synthesized. The Ru(II) complex catalysts have been fully characterized and exhibited good to excellent catalytic activity in the transfer hydrogenation (TH) of ketones in refluxing 2-propanol. These results have demonstrated rare examples of active ruthenium(II) NNN complex catalysts that do not feature a N-H functionality for TH of ketones.     

Chiral bisphospholane ligands (Me-ketalphos): synthesis of their Rh(I) complexes and applications in asymmetric hydrogenation

Rhodium complexes of functionalized bisphospholane ligands (S,S,S,S-Me-ketalphos) 1 and (R,S,S,R-Me-ketalphos) 2 have been used as catalyst precursors for the asymmetric hydrogenation of several different types of functionalized olefins and have achieved high enantioselectivities.     

钌(II)-吡啶基NNN配合物催化酮的室温(不对称)氢转移反应

合成了一种基于吡啶骨架含有苯并咪唑和手性咪唑啉基团的三齿NNN配体及其二价钌(II)配合物,通过核磁共振波谱学和X射线单晶晶体结构测定确认了钌(II)配合物的分子结构.这些配合物在室温下催化酮的氢转移反应,表现出了优异的催化活性,收率和ee值最高分别可达99%和97%.     

Ruthenium-catalysed transfer hydrogenation reactions with dimethylamine borane

Dimethylamine-borane adduct has been used as the hydrogen source for the reduction of carbonyl compounds, imines, oximes, nitriles, nitroarenes and alkenes using [Ru(p-cymene)Cl₂]₂ as the catalyst.     

Ruthenium-catalyzed asymmetric transfer hydrogenation of ketones in ethanol

A ruthenium catalyst formed in situ by combining [Ru(p-cymene)Cl₂]₂ and an amino acid hydroxy-amide was found to catalyze efficiently the asymmetric reduction of aryl alkyl ketones under transfer hydrogenation conditions using ethanol as the hydrogen donor. The secondary alcohol products were obtained in moderate to good yields and with good to excellent enantioselectivity (up to 97% ee).     

Efficient and selective hydration of nitriles to amides in aqueous systems with Ru(II)-phosphaurotropine catalysts

A simple and efficient synthesis of amides by selective hydration of aromatic and aliphatic nitriles is described. The catalysts are prepared in situ from easily available Ru-precursors and ligands using water as the solvent. The most active catalyst, is obtained from [RuCl₂(dmso)₄] and benzylated 1,3,5-triaza-7-phosphaadamantane. Of the 16 substrates examined, 92–99% conversions of 14 nitriles were achieved in one hour at reflux temperature.     

Two pseudo-N3 ligands and the catalytic activity of their ruthenium(II) complexes in transfer hydrogenation and hydrogenation of ketones

Complex RuCl₂(PPh₃)(iBu-BTP) (5) was synthesized by the reaction of 2,6-bis(5,6-bis(iso-butyl)-1,2,4-triazin-3-yl)pyridine (iBu-BTP) and RuCl₂(PPh₃)₃ in refluxing toluene, and its molecular structure was confirmed by X-ray crystallographic determination. Complex 5 was applied as a catalyst for transfer hydrogenation of ketones and exhibited catalytic activity comparable to RuCl₂(PPh₃)(Me₄BPPy) (1) (Me₄BPPy = bis(3,5-dimethylpyrazol-1-yl)pyridine) in some cases. The difference between the catalytic activity of 5 and 1 is attributed to the significantly different arrangement and positions of the PPh₃ and chlorides and also to the different electron density on the N-heterocycles. Complex 1 exhibited good to excellent catalytic activity in hydrogenation of ketones under mild conditions. These results have suggested new applications of iBu-BTP and Me₄BPPy as promising planar tridentate pseudo-N₃ ligands to construct highly active transition-metal catalysts.     

Ru3(CO)12-catalyzed intramolecular allylic transfer reaction of tethered carbonyl group and allylic acetate moiety

A ruthenium catalyzed intramolecular allylic transfer reaction of allylic acetates containing aldehydes or ketones to form cis-homoallylic cyclopentanols and cyclohexanols as a major component is described. The use of Ru₃(CO)₁₂ (1 mol %) to promote reaction results in a convenient procedure for intramolecular allylation of carbonyl functionalities.     

Magnetically recoverable supported ruthenium catalyst for hydrogenation of alkynes and transfer hydrogenation of carbonyl compounds

A ruthenium (Ru) catalyst supported on magnetic nanoparticles (NiFe₂O₄) has been successfully synthesized and used for hydrogenation of alkynes at room temperature as well as transfer hydrogenation of a number of carbonyl compounds under microwave irradiation conditions. The catalyst shows excellent selectivity toward the desired products with very high yield even after five repeated uses.     

Rhodium(I) and platinum(II) complexes of aminomethylphosphines as hydrogenation and hydroformylation catalysts

Hydrogenation of α-acetamidocinnamic acid with chiral aminomethylphosphine complexes of rhodium(I), [Rh(cyclo-octa-1, 5-diene) {(R₂PCH₂)₂NR¹}]-PF₆ (R = Ph or Cy, R¹ = D(+)-CHMePh, L-CHMeCO₂Et, (R)(+)-bornyl) shows no asymmetric induction. The hydroformylation of styrene using the catalyst mixture [PtCl₂(P–P)]/SnCl₂ shows asymmetric induction with up to 31% enantiomeric excess of 2-phenyl-propanol being observed.     

Ruthenium(II) pincer complexes with oxazoline arms for efficient transfer hydrogenation reactions

Well-defined P^NN^CN pincer ruthenium complexes bearing both strong phosphine and weak oxazoline donors were developed. These easily accessible complexes exhibit significantly better catalytic activity in transfer hydrogenation of ketones compared to their PN³P analogs. These reactions proceed under mild and base-free conditions via protonation-deprotonation of the ‘NH’ group in the aromatization-dearomatization process.