Chiral Pincer Ruthenium and Osmium Complexes for the Fast and Efficient Hydrogen Transfer Reduction of Ketones

Robust and efficient transfer hydrogenation of carbonyl compounds catalyzed by NN-Mn(I) complexes

A series of manganese(I) carbonyl complexes bearing structurally related NN- and NNN-chelating ligands have been synthesized and assessed as catalysts for transfer hydrogenation (TH). Notably, the NN-systems based on N-R functionalized 5,6,7,8-tetrahydroquinoline-8-amines, proved the most effective in the manganese-promoted conversion of acetophenone to 1-phenylethanol. In particular, the N-isopropyl derivative, Mn1, when conducted in combination with t-BuONa, was the standout performer mediating not only the reduction of acetophenone but also a range of carbonyl substrates including (hetero)aromatic-, aliphatic- and cycloalkyl-containing ketones and aldehydes with especially high values of TON (up to 17 200; TOF of 3550 h-1). These findings, obtained through a systematic variation of the N-R group of the NN ligand, are consistent with an outer-sphere mechanism for the hydrogen transfer. As a more general point, this Mn-based catalytic TH protocol offers an attractive and sustainable alternative for producing alcoholic products from carbonyl substrates.

Enantioseparation of amino acid and mandelic acid enantiomers using Josiphos-metal complexes as chiral extractants

The development of new and efficient chiral extractants has always been the research hotspot and difficulty in the field of chiral extraction. Josiphos, a famous ferrocene derivative catalyst, is employed as a chiral extractant in enantioseparation of amino acid and mandelic acid enantiomers. The influences of metal ions, organic solvents, pH of the aqueous solution, extractant concentrations, and extraction temperature on enantioselectivities are systematically studied. The result reveals that Josiphos-Pd has good capabilities to enantioseparate 4-nitro-phenylalanine (Nphe), 3-chloro-phenylglycine (Cpheg), and mandelic acid (MA) with separation factors (α) of 3.30, 2.65, and 2.18, respectively. The pH of the aqueous phase and Josiphos-Pd concentration affect the extraction significantly, whereas extraction temperature shows little influence. After optimizing by response surface method, the mathematical models for extractions are established. And the highest experimental performance factors (pf) for Nphe, Cpheg, and MA are 0.1843, 0.1335, and 0.08884, respectively.

Catalysis with cycloruthenated complexes

Cycloruthenated complexes have been studied extensively over the last few decades. Many accounts of their synthesis, characterisation, and catalytic activity in a wide variety of transformations have been reported to date. Compared with their non-cyclometallated analogues, cycloruthenated complexes may display enhanced catalytic activities in known transformations or possess entirely new reactivity. In other instances, these complexes can be chiral, and capable of catalysing stereoselective reactions. In this review, we aim to highlight the catalytic applications of cycloruthenated complexes in organic synthesis, emphasising the recent advancements in this field.

We discuss recent advances in the applications of cycloruthenated complexes in organic synthesis, comprising C–H activation, chiral-at-metal catalysis, Z-selective olefin metathesis, transfer hydrogenation, enantioselective cyclopropanations and cycloadditions.

Efficient transfer hydrogenation of ketones by molybdenum complexes through comprehensively verifying auxiliary ligands

Molybdenum complexes, ligated with N1,N1-dialkyl-N2-(5,6,7,8-tetrahydroquinolin-8-yl)ethane-1,2-diamines along with auxiliary ligands, provide various structural features as [NNH/NNHN]Mo(CO)4/3 (Mo1 – Mo3), [NNHN]Mo(CO)2Br] (Mo4 – Mo5), [NNH]Mo(CO)(η3-C3H5)Br](Mo6) and [NNHN/S] Mo(CO)(PPh3)2] (Mo7 – Mo8). All...

Cyclometalated (NNC)Ru(II) complex catalyzed β-methylation of alcohols using methanol

Indolyl fragment containing phenanthroline based new ligands and their corresponding Ru(II) complexes were synthesized and fully characterized by various spectroscopic techniques. Catalytic activity of these newly synthesized cyclometalated (NNC)Ru(II) complexes...

Base-free transfer hydrogenation of aldehydes and ketones catalyzed by imidazolin-2-iminato actinide complexes

A series of thorium and uranium complexes decorated with unsymmetrical imidazolin-2-iminato ligands were found to be effective as catalysts in the transfer hydrogenation of aldehydes and ketones with 2-propanol to form the corresponding alcohols.

Chiral Tridentate Ligands in Transition Metal-Catalyzed Asymmetric Hydrogenation

Asymmetric hydrogenation (AH) of double bonds has been one of the most effective methods for the preparation of chiral molecules and for the synthesis of important chiral building blocks. In the past 60 years, noble metals with bidentate ligands have shown marvelous reactivity and enantioselectivity in asymmetric hydrogenation of a series of prochiral substrates. In recent years, developing chiral tridentate ligands has played an increasingly important role in AH. With modular frameworks and a variety of functionalities on the side arms, chiral tridentate ligand complexes enable both reactivities and stereoselectivities. Although great achievements have been made for noble metal catalysts with chiral tridentate ligands since the 1990s, the design of chiral tridentate ligands for earth abundant metal catalysts has still been in high demand. This review summarizes the development of chiral tridentate ligands for homogeneous asymmetric hydrogenation. The philosophy of ligand design and the reaction mechanisms are highlighted and discussed as well.

Preparation of Neutral trans - cis [Ru(O2CR)2P2(NN)], Cationic [Ru(O2CR)P2(NN)](O2CR) and Pincer [Ru(O2CR)(CNN)P2] (P = PPh3, P2 = diphosphine) Carboxylate Complexes and their Application in the Catalytic Carbonyl Compounds Reduction

The diacetate complexes trans-[Ru(κ¹-OAc)₂(PPh₃)₂(NN)] (NN = ethylenediamine (en) (1), 2-(aminomethyl)pyridine (ampy) (2), 2-(aminomethyl)pyrimidine (ampyrim) (3)) have been isolated in 76-88% yield by reaction of [Ru(κ²-OAc)₂(PPh₃)₂] with the corresponding nitrogen ligands. The ampy-type derivatives 2 and 3 undergo isomerization to the thermodynamically most stable cationic complexes [Ru(κ¹-OAc)(PPh₃)₂(NN)]OAc (2a and 3a) and cis-[Ru(κ¹-OAc)₂(PPh₃)₂(NN)] (2b and 3b) in methanol at RT. The trans-[Ru(κ¹-OAc)₂(P₂)₂] (P₂ = dppm (4), dppe (5)) compounds have been synthesized from [Ru(κ²-OAc)₂(PPh₃)₂] by reaction with the suitable diphosphine in toluene at 95 °C. The complex cis-[Ru(κ¹-OAc)₂(dppm)(ampy)](6) has been obtained from [Ru(κ²-OAc)₂(PPh₃)₂] and dppm in toluene at reflux and reaction with ampy. The derivatives trans-[Ru(κ¹-OAc)₂P₂(NN)] (7-16; NN = en, ampy, ampyrim, 8-aminoquinoline; P₂ = dppp, dppb, dppf, (R)-BINAP) can be easily synthesized from [Ru(κ²-OAc)₂(PPh₃)₂] with a diphosphine and treatment with the NN ligands at RT. Alternatively these compounds have been prepared from trans-[Ru(OAc)₂(PPh₃)₂(NN)] by reaction with the diphosphine in MEK at 50 °C. The use of (R)-BINAP affords trans-[Ru(κ¹-OAc)₂((R)-BINAP)(NN)] (NN = ampy (11), ampyrim (15)) isolated as single stereoisomers. Treatment of the ampy-type complexes 8-15 with methanol at RT leads to isomerization to the cationic derivatives [Ru(κ²-OAc)P₂(NN)]OAc (8a-15a; NN = ampy, ampyrim; P₂ = dppp, dppb, dppf, (R)-BINAP). Similarly to 2, the dipivalate trans-[Ru(κ¹-OPiv)₂(PPh₃)₂(ampy)] (18) is prepared from [Ru(κ²-OPiv)₂(PPh₃)₂] (17) and ampy in CHCl₃. The pincer acetate [Ru(κ¹-OAc)(CNN^OMe)(PPh₃)₂] (19) has been synthesized from [Ru(κ²-OAc)₂(PPh₃)₂] and HCNN^OMe ligand in 2-propanol with NEt₃ at reflux. In addition, the dppb pincer complexes [Ru(κ¹-OAc)(CNN)(dppb)] (CNN = AMTP (20), AMBQ^Ph (21)) have been obtained from [Ru(κ²-OAc)₂(PPh₃)₂], dppb, and HAMTP or HAMBQ^Ph with NEt₃, respectively. The acetate NN and pincer complexes are active in transfer hydrogenation with 2-propanol and hydrogenation with H₂ of carbonyl compounds at S/C values of up to 10000 and with TOF values of up to 160000 h^-1.

Cyclometalated Osmium Compounds and beyond: Synthesis, Properties, Applications

The synthesis of cyclometalated osmium complexes is usually more complicated than of other transition metals such as Ni, Pd, Pt, Rh, where cyclometalation reactions readily occur via direct activation of C–H bonds. It differs also from their ruthenium analogs. Cyclometalation for osmium usually occurs under more severe conditions, in polar solvents, using specific precursors, stronger acids, or bases. Such requirements expand reaction mechanisms to electrophilic activation, transmetalation, and oxidative addition, often involving C–H bond activations. Osmacycles exhibit specific applications in homogeneous catalysis, photophysics, bioelectrocatalysis and are studied as anticancer agents. This review describes major synthetic pathways to osmacycles and related compounds and discusses their practical applications.

Efficient methylation of anilines with methanol catalysed by cyclometalated ruthenium complexes

Cyclometalated ruthenium allow the effective and selective N-methylation of anilines under mild conditions.

Diastereodivergent chiral aldehyde catalysis for asymmetric 1,6-conjugated addition and Mannich reactions

Chiral aldehyde catalysis is a burgeoning strategy for the catalytic asymmetric α-functionalization of aminomethyl compounds. However, the reaction types are limited and to date include no examples of stereodivergent catalysis. In this work, we disclose two chiral aldehyde-catalysed diastereodivergent reactions: a 1,6-conjugate addition of amino acids to para-quinone methides and a bio-inspired Mannich reaction of pyridinylmethanamines and imines. Both the syn- and anti-products of these two reactions can be obtained in moderate to high yields, diastereo- and enantioselectivities. Four potential reaction models produced by DFT calculations are proposed to explain the observed stereoselective control. Our work shows that chiral aldehyde catalysis based on a reversible imine formation principle is applicable for the α-functionalization of both amino acids and aryl methylamines, and holds potential to promote a range of asymmetric transformations diastereoselectively.

Zinc Hydride-Catalyzed Hydrofuntionalization of Ketones

Three new dimeric bis-guanidinate zinc(II) alkyl, halide, and hydride complexes [LZnEt]₂ (1), [LZnI]₂ (2) and [LZnH]₂ (3) were prepared. Compound 3 was successfully employed for the hydrosilylation and hydroboration of a vast number of ketones. The catalytic performance of 3 in the hydroboration of acetophenone exhibits a turnover frequency, reaching up to 5800 h^-1, outperforming that of reported zinc hydride catalysts. Notably, both intra- and intermolecular chemoselective hydrosilylation and hydroboration reactions have been investigated.

Cyclometalated Ruthenium Pincer Complexes as Catalysts for the α-Alkylation of Ketones with Alcohols

Ruthenium PNP pincer complexes bearing supplementary cyclometalated C,N‐bound ligands have been prepared and fully characterized for the first time. By replacing CO and H⁻ as ancillary ligands in such complexes, additional electronic and steric modifications of this topical class of catalysts are possible. The advantages of the new catalysts are demonstrated in the general α‐alkylation of ketones with alcohols following a hydrogen autotransfer protocol. Herein, various aliphatic and benzylic alcohols were applied as green alkylating agents for ketones bearing aromatic, heteroaromatic or aliphatic substituents as well as cyclic ones. Mechanistic investigations revealed that during catalysis, Ru carboxylate complexes are predominantly formed whereas neither the PNP nor the CN ligand are released from the catalyst in significant amounts.

CNN pincer ruthenium complexes for efficient transfer hydrogenation of biomass-derived carbonyl compounds

The ligand HCNNOMe (6-(4-methoxyphenyl)-2-aminomethylpyridine) is easily prepared from the commercially available 6-(4-methoxyphenyl)pyridine-2-carbaldehyde by the reaction of hydroxylamine and hydrogenation (H2, 1 atm) with Pd/C. The pincer complexes cis-[RuCl(CNNOMe)(PPh3)2] (1) and [RuCl(CNNOMe)(PP)] (PP = dppb, 2; and dppf, 3) are synthesized from [RuCl2(PPh3)3], HCNNOMe and PP (for 2 and 3) in 2-propanol with NEt3 at reflux and are isolated in 85-93% yield. Carbonylation of 1 (CO, 1 atm) gives [RuCl(CNNOMe)(CO)(PPh3)] (4) (79% yield) which cleanly reacts with Na[BArf4] and PCy3, affording the cationic trans-[Ru(CNNOMe)(CO)(PCy3)(PPh3)][BArf4] (5) (92% yield). These robust pincer complexes display remarkably high catalytic activity in the transfer hydrogenation (TH) of lignocellulosic biomass carbonyl compounds, using 2-propanol at reflux in a basic medium (NaOiPr or K2CO3). Thus, furfural, 5-(hydroxymethyl)furfural and Cyrene are reduced to the corresponding alcohols with 2 and 3, at S/C in the range of 10 000-100 000, within minutes or hours (TOF up to 1 500 000 h-1). The monocarbonyl complex 5 was found to be extremely active in the TH of cinnamaldehyde, vanillin derivatives and ethyl levulinate at S/C in the range of 10 000-50 000. Vanillyl alcohol is also obtained by the TH of vanillin with 5 (S/C = 500) in 2-propanol in the presence of K2CO3.

Catalytic hydrogenation of carbonyl and nitro compounds using an [N,O]-chelate half-sandwich ruthenium catalyst

A series of N,O-chelate half-sandwich ruthenium complexes have been synthesized, which exhibited high activity for the catalytic hydrogenation of carbonyl and nitro compounds in aqueous solution.

An air and moisture tolerant iminotrihydroquinoline-ruthenium(ii) catalyst for the transfer hydrogenation of ketones

Both amine- and imine-containing E and F are highly effective in the transfer hydrogenation of a wide range of ketones with F significantly operating in bench quality 2-propanol and in vessels open to the air.

RuII, IrIII and OsII mesoionic carbene complexes: efficient catalysts for transfer hydrogenation of selected functionalities

Pyridine- and pyrimidine-appended triazolylidene donors are used as ligands for the transfer hydrogenation of a series of functionalities.

Non-symmetric pincer ligands: complexes and applications in catalysis

Pincer ligands have become ubiquitous in organometallic chemistry and homogeneous catalysis. Recently, new varieties of pincer ligands with non-symmetrical backbones and/or ligating groups have been reported and their application in transition metal complexes has been exploited in a variety of catalytic transformations. This non-symmetric approach vastly increases the structural and electronic diversity of this class of ligand. This approach has proven beneficial in a variety of ways, such as the use of a single weakly coordinating moiety, which can dissociate and thereby create a vacant coordination site to increase the catalyst activity. Additionally, this provides further access to chiral ligands and complexes for asymmetric induction. This perspective highlights recent, important examples of non-symmetric pincer ligands, which feature aryl or pyridine backbones, and the synthesis and use of subsequent complexes in catalytic transformations, and discusses the future potential of this type of ligand system.

Easy To Synthesize, Robust Organo-osmium Asymmetric Transfer Hydrogenation Catalysts

Asymmetric transfer hydrogenation (ATH) is an important process in organic synthesis for which the Noyori-type Ru(II) catalysts [(arene)Ru(Tsdiamine)] are now well established and widely used. We now demonstrate for the first time the catalytic activity of the osmium analogues. X-ray crystal structures of the 16-electron Os(II) catalysts are almost identical to those of Ru(II). Intriguingly the precursor complex was isolated as a dichlorido complex with a monodentate amine ligand. The Os(II) catalysts are readily synthesised (within 1 h) and exhibit excellent enantioselectivity in ATH reactions of ketones.

Ru(II), Os(II), and Ir(III) complexes with chelating pyridyl-mesoionic carbene ligands: structural characterization and applications in transfer hydrogenation catalysis

Chelating ligands with one pyridine donor and one mesoionic carbene donor are fast establishing themselves as privileged ligands in homogeneous catalysis. The synthesis of several new Ir(III)-Cp*- and Os(II)-Cym complexes (Cp* = pentamethylcyclopentadienyl, Cym = p-cymene=4-isopropyl-toluene) derived from chelating pyridyltriazolylidenes where the additional pyridine donor was incorporated via the azide part of the triazole is presented. Furthermore, different 4-substituted phenylacetylene building blocks have been used to introduce electronic fine-tuning in the ligands. The ligands thus can be generally described as 4-(4-R-phenyl)-3-methyl-1-(pyridin-2-yl)-1H-1,2,3-triazol-5-ylidene (with R being H (L(1)), Me (L(2)), OMe (L(3)), CN (L(4)), CF3 (L(5)), Br (L(6)) or NO2 (L(7))). The corresponding complexes (Ir-1 to Ir-7 and Os-1 to Os-7) were characterized by standard spectroscopic methods, and the expected three-legged, piano-stool type coordination was unambiguously confirmed by X-ray diffraction analysis of selected compounds. Together with Ru(II) analogues previously reported by us, a total of 21 complexes were tested as (pre)catalysts for the transfer hydrogenation of carbonyl groups, showing a remarkable reactivity even at very low catalyst loadings. The electronic effects of the ligands as well as different substrates were investigated. Some mechanistic elucidations are also presented.

Highly regioselective osmium-catalyzed hydroformylation

The first highly regioselective and general osmium-catalyzed hydroformylation of olefins to aldehydes is reported. The combination of Os3(CO)12 and imidazoyl-substituted phosphine ligands allows n-selective (up to 99%) hydroformylation of bulk aliphatic as well as functional alkenes in good yields (64-87%).

Recent advances in osmium-catalyzed hydrogenation and dehydrogenation reactions

CONSPECTUS: A current issue in metal-catalyzed reactions is the search for highly efficient transition-metal complexes affording high productivity and selectivity in a variety of processes. Moreover, there is also a great interest in multitasking catalysts that are able to efficiently promote different organic transformations by careful switching of the reaction parameters, such as temperature, solvent, and cocatalyst. In this context, osmium complexes have shown the ability to catalyze efficiently different types of reactions involving hydrogen, proving at the same time high thermal stability and simple synthesis. In the catalytic reduction of C═X (X = O, N) bonds by both hydrogenation (HY) and transfer hydrogenation (TH) reactions, the most interest has been focused on homogeneous systems based on rhodium, iridium, and in particular ruthenium catalysts, which have proved to catalyze chemo- and stereoselective hydrogenations with remarkable efficiency. By contrast, osmium catalysts have received much less attention because they are considered less active on account of their slower ligand exchange kinetics. Thus, this area remained almost neglected until recent studies refuted these prejudices. The aim of this Account is to highlight the impressive developments achieved over the past few years by our and other groups on the design of new classes of osmium complexes and their applications in homogeneous catalytic reactions involving the hydrogenation of carbon-oxygen and carbon-nitrogen bonds by both HY and TH reactions as well as in alcohol deydrogenation (DHY) reactions. The work described in this Account demonstrates that osmium complexes are emerging as powerful catalysts for asymmetric and non-asymmetric syntheses, showing a remarkably high catalytic activity in HY and TH reactions of ketones, aldehydes, imines, and esters as well in DHY reactions of alcohols. Thus, for instance, the introduction of ligands with an NH function, possibly in combination with a pyridine ring, led to a new family of [OsCl2(PP)(NN)] (NN = diamine, 2-aminomethylpyridine; PP = diphosphine) and pincer [OsCl(CNN)(PP)] (HCNN = 6-aryl-2-aminomethylpyridine, 2-aminomethylbenzo[h]quinoline) complexes, which are outstanding catalysts for (asymmetric) HY and TH of carbonyl compounds and DHY of alcohols with turnover numbers and turnover frequencies up to 10(5) and 10(6) h(-1), respectively. In addition, PNN osmium complexes containing the 2-aminomethylpyridine motif have been found to be among the most active catalysts for HY of esters. These complexes have shown catalytic activities that are comparable and in some cases superior to those reported for analogous ruthenium systems. These results give an idea of the potential of Os complexes for the design of new highly productive and robust catalysts for the synthesis of chiral and nonchiral alcohols and amines as well as ketones from alcohols. Thus, we hope that this report will promote increased interest in the chemistry of these metal complexes, opening novel opportunities for new catalytic processes as well as the improvement of existing ones.