Cobalt-catalyzed hydrogenation of esters to alcohols: unexpected reactivity trend indicates ester enolate intermediacy

Ammonia-Borane Dependent Transfer Hydrogenation of Carboxylic Esters to Primary Alcohols

Here, we present a user-friendly protocol that uses bench-stable NH3·BH3 (AB) as the hydrogen transfer agent for the reduction of both aromatic and aliphatic esters without an external catalyst and base, delivering a structurally diverse array of primary alcohols (80-98% yields). The broad functional-group tolerance (halogen, boronic ester, -NO2, -OH, etc.) under environmentally acceptable conditions implies high practical utility. Further, a tandem catalytic conversion of a plastic waste polyethylene terephthalate (PET) bottle to 1,4-benzenedimethanol including fatty esters into respective fatty alcohols was shown.

Biomimetic Frustrated Lewis Pair Catalysts for Hydrogenation of CO to Methanol at Low Temperatures

The industrial production of methanol through CO hydrogenation using the Cu/ZnO/Al2O3 catalyst requires harsh conditions, and the development of new catalysts with low operating temperatures is highly desirable. In this study, organic biomimetic FLP catalysts with good tolerance to CO poison are theoretically designed. The base-free catalytic reaction contains the 1,1-addition of CO into a formic acid intermediate and the hydrogenation of the formic acid intermediate into methanol. Low-energy spans (25.6, 22.1, and 20.6 kcal/mol) are achieved, indicating that CO can be hydrogenated into methanol at low temperatures. The new extended aromatization-dearomatization effect involving multiple rings is proposed to effectively facilitate the rate-determining CO 1,1-addition step, and a new CO activation model is proposed for organic catalysts.

Cobalt catalyzed practical hydroboration of terminal alkynes with time-dependent stereoselectivity

Stereodefined vinylboron compounds are important organic synthons. The synthesis of E-1-vinylboron compounds typically involves the addition of a B-H bond to terminal alkynes. The selective generation of the thermodynamically unfavorable Z-isomers remains challenging, necessitating improved methods. Here, such a proficient and cost-effective catalytic system is introduced, comprising a cobalt salt and a readily accessible air-stable CNC pincer ligand. This system enables the transformation of terminal alkynes, even in the presence of bulky substituents, with excellent Z-selectivity. High turnover numbers (>1,600) and turnover frequencies (>132,000 h-1) are achieved at room temperature, and the reaction can be scaled up to 30 mmol smoothly. Kinetic studies reveal a formal second-order dependence on cobalt concentration. Mechanistic investigations indicate that the alkynes exhibit a higher affinity for the catalyst than the alkene products, resulting in exceptional Z-selective performance. Furthermore, a rare time-dependent stereoselectivity is observed, allowing for quantitative conversion of Z-vinylboronate esters to the E-isomers.

Efficient and chemoselective imine synthesis catalyzed by a well-defined PN3-manganese(II) pincer system

The highly efficient reductive amination of aldehydes with ammonia (NH3) and hydrogen (H2) to form secondary imines is described, as well as the dehydrogenative homocoupling of benzyl amines. Using an air-stable, well-defined PN3-manganese(II) pincer complex as a catalyst precursor, various aldehydes are easily converted directly into secondary imines using NH3 as a nitrogen source under H2 in a one-pot reaction. Importantly, the same catalyst facilitates the dehydrogenative homocoupling of various benzylamines, exclusively forming imine products. These reactions are conducted under very mild conditions, without the addition of any additives, yielding excellent selectivities and high yields of secondary imines in a green manner by minimizing wastes.

Advances in Group VI Metal-Catalyzed Homogeneous Hydrogenation and Dehydrogenation Reactions

Transition metal-catalyzed homogeneous hydrogenation and dehydrogenation reactions for attaining plethora of organic scaffolds have evolved as a key domain of research in academia and industry. These protocols are atom-economic, greener, in line with the goal of sustainability, eventually pave the way for numerous novel environmentally benign methodologies. Appealing progress has been achieved in the realm of homogeneous catalysis utilizing noble metals. Owing to their high cost, less abundance along with toxicity issues led the scientific community to search for sustainable alternatives. In this context, earth- abundant base metals have gained substantial attention culminating enormous progress in recent years, predominantly with pincer-type complexes of nickel, cobalt, iron, and manganese. In this regard, group VI chromium, molybdenum and tungsten complexes have been overlooked and remain underdeveloped despite their earth-abundance and bio-compatibility. This review delineates a comprehensive overview in the arena of homogeneously catalysed (de)hydrogenation reactions using group VI base metals chromium, molybdenum, and tungsten till date. Various reactions have been described; hydrogenation, transfer hydrogenation, dehydrogenation, acceptorless dehydrogenative coupling, hydrogen auto transfer, along with their scope and brief mechanistic insights.

Bis(N-Heterocyclic Carbene) Manganese(I) Complexes: Efficient and Simple Hydrogenation Catalysts

The use of bis(NHC) manganese(I) complexes 3 as catalysts for the hydrogenation of esters was investigated. For that purpose, a series of complexes has been synthesized via an improved two step procedure utilizing bis(NHC)-BEt3 adducts. By applying complexes 3 with KHBEt3 as additive, various aromatic and aliphatic esters were hydrogenated successfully at mild temperatures and low catalyst loadings, highlighting the efficiency of the novel catalytic system. The versatility of the developed catalytic system was further demonstrated by the hydrogenation of other substrate classes like ketones, nitriles, N-heteroarenes and alkenes. Mechanistic experiments and DFT calculations indicate an inner sphere mechanism with the loss of one CO ligand and reveal the role of BEt3 as cocatalyst.

Cobalt-Catalyzed Enantioselective Hydrogenation of Diaryl Ketones with Ferrocene-Based Secondary Phosphine Oxide Ligands

A new class of cobalt catalytic system for asymmetric hydrogenation of ketones was herein reported, involving the development of novel ferrocene-based secondary phosphine oxide ligands. An unusual P-O bidentate coordination pattern with cobalt was confirmed by an X-ray diffraction study. The bichelating tetrahedral cobalt(II) complexes afforded high reactivities (up to 99% yield) and good to excellent enantioselectivities (up to 92% ee) in the AH of various ortho-substituted diaryl ketones. In addition, the diferrocenyl cobalt complex was characterized with intriguing UV-vis absorption and electrochemical properties.

Designing New Magnesium Pincer Complexes for Catalytic Hydrogenation of Imines and N-Heteroarenes: H2 and N-H Activation by Metal-Ligand Cooperation as Key Steps

Utilization of main-group metals as alternatives to transition metals in homogeneous catalysis has become a hot research area in recent years. However, their application in catalytic hydrogenation is less common due to the difficulty in heterolytic cleavage of the H-H bond. Employing aromatization/de-aromatization metal-ligand cooperation (MLC) highly enhances the H₂ activation process, offering an efficient approach for the hydrogenation of unsaturated molecules catalyzed by main-group metals. Herein, we report a series of new magnesium pincer complexes prepared using PNNH-type pincer ligands. The complexes were characterized by NMR and X-ray single-crystal diffraction. Reversible activation of H₂ and N-H bonds by MLC employing these pincer complexes was developed. Using the new magnesium complexes, homogeneously catalyzed hydrogenation of aldimines and ketimines was achieved, affording secondary amines in excellent yields. Control experiments and DFT studies reveal that a pathway involving MLC is favorable for the hydrogenation reactions. Moreover, the efficient catalysis was extended to the selective hydrogenation of quinolines and other N-heteroarenes, presenting the first example of hydrogenation of N-heteroarenes homogeneously catalyzed by early main-group metal complexes. This study provides a new strategy for hydrogenation of C═N bonds catalyzed by magnesium compounds and enriches the research of main-group metal catalysis.

Aluminium-Catalyzed Selective Hydroboration of Esters and Epoxides to Alcohols: C-O Bond Activation

In this work, the molecular aluminium dihydride complex bearing an N, N'-chelated conjugated bis-guanidinate (CBG) ligand is used as a catalyst for reducing a wide range of aryl and alkyl esters with good tolerance of alkene (C=C), alkyne (C≡C), halides (Cl, Br, I and F), nitrile (C≡N), and nitro (NO₂ ) functionalities. Further, we investigated the catalytic application of aluminium dihydride in the C-O bond cleavage of alkyl and aryl epoxides into corresponding branched Markovnikov ring-opening products. In addition, the chemoselective intermolecular reduction of esters over other reducible functional groups, such as amides and alkenes, has been established. Intermediates are isolated and characterized by NMR and HRMS studies, which confirm the probable catalytic cycles for the hydroboration of esters and epoxides.

Vapour-Phase Selective Hydrogenation of γ-Valerolactone to 2-Methyltetrahydrofuran Biofuel over Silica-Supported Copper Catalysts

2-Methyltetrahydrofuran (MTHF) is a desirable biomass-based platform chemical with excellent potential as an ideal biofuel, green solvent, and raw material for synthesizing downstream chemicals. In this work, a series of copper nanoparticles encapsulated on SiO₂ were prepared by the wet impregnation method and evaluated as efficient non-noble metal catalysts for the vapour-phase hydrogenation of γ-valerolactone (GVL) to MTHF in a fixed-bed reactor under mild reaction conditions. The obtained catalyst properties were determined by XRD, FE-SEM, TEM, UV-DRS, TPR, NH₃-TPD, N₂O decomposition and pore size distribution measurements. Meanwhile, the parameters/variables tuning their catalytic performance (activity, conversion, selectivity and stability) were examined. Various Cu loadings featured on the SiO₂ support are essential for tuning the catalytic activity. Among the catalysts tested, a 5 wt% Cu/SiO₂ catalyst showed a 97.2% MTHF selectivity with 71.9% GVL conversion, and showed a stability for 33 h time-on-stream, achieved at 260 °C and atmospheric pressure conditions. It was found that a huge dispersion of Cu metal in support, hydrogen activation ability, abundant acidic sites and surface area are all beneficial for improved MTHF selectivity.

Electronic insights into aminoquinoline-based PNHN ligands: protonation state dictates geometry while coordination environment dictates N-H acidity and bond strength

A variety of transition metal complexes bearing aminoquinoline PN^HH'-R ligands R = Ph (L1H), Cy (L2H) and their amido analogues are reported for rhodium(I) ([Rh(L1H)(PPh₃)]⁺1 and Rh(L1)(PPh₃) 2), cobalt(II) (Co(L2)(Cl) 3), and iron(II) ([Fe(L1H)₂]²⁺5, Fe(L1)₂6, and [Fe(C₅Me₅)(L1H)]PF₆7). The acid-base and redox properties of the amido complexes 2, 6, and their protio parent complexes 1, and 5 permit the determination of the pK_a and bond dissociation free energy (BDFE) of their N-H bonds while the ligand scaffold is coordinated to metal centres of square planar and octahedral geometry, respectively. From relative concentrations obtained by the use of ³¹P{¹H} NMR spectroscopy, a pK_a^THF value of 14 is calculated for rhodium complex 1, 6.4 for iron complex 5, and 24 for iron complex 7. These data, when combined with elecrochemical potentials obtained via cyclic voltammetry, allow the calculations of BDFE values for the N-H bond of 69 kcal mol^-1 for 1, and of 55 kcal mol^-1 for 5.

Catalytic and mechanistic studies of a highly active and E-selective Co(II) PNNH pincer catalyst system for transfer-semihydrogenation of internal alkynes

Herein we report the application of a Co(II) PNNH pincer catalyst system (PNNH = 2-(5-(t-butyl)-1H-pyrazol-3-yl)-6-(dialkylphosphinomethyl)pyridine) for the highly E-selective transfer semihydrogenation of internal diaryl alkynes using methanol and ammonia borane...

Room temperature Z-selective hydrogenation of alkynes by hemilabile and non-innocent (NNN)Co(ii) catalysts

Room temperature chemo- and stereoselective hydrogenation of alkynes is described using a well-defined and phosphine-free hemilabile cobalt catalyst.

Ammonia-Borane Dehydrogenation Catalyzed by Dual-Mode Proton-Responsive Ir-CNNH Complexes

Metal complexes incorporating proton-responsive ligands have been proved to be superior catalysts in reactions involving the H₂ molecule. In this contribution, a series of Ir^III complexes based on lutidine-derived CNN^H pincers containing N-heterocyclic carbene and secondary amino NHR [R = Ph (4a), tBu (4b), benzyl (4c)] donors as flanking groups have been synthesized and tested in the dehydrogenation of ammonia–borane (NH₃BH₃, AB) in the presence of substoichiometric amounts (2.5 equiv) of tBuOK. These preactivated derivatives are efficient catalysts in AB dehydrogenation in THF at room temperature, albeit significantly different reaction rates were observed. Thus, by using 0.4 mol % of 4a, 1.0 equiv of H₂ per mole of AB was released in 8.5 min (turnover frequency (TOF_50%) = 1875 h^–1), while complexes 4b and 4c (0.8 mol %) exhibited lower catalytic activities (TOF_50% = 55–60 h^–1). 4a is currently the best performing Ir^III homogeneous catalyst for AB dehydrogenation. Kinetic rate measurements show a zero-order dependence with respect to AB, and first order with the catalyst in the dehydrogenation with 4a (−d[AB]/dt = k[4a]). Conversely, the reaction with 4b is second order in AB and first order in the catalyst (−d[AB]/dt = k[4b][AB]²). Moreover, the reactions of the derivatives 4a and 4b with an excess of tBuOK (2.5 equiv) have been analyzed through NMR spectroscopy. For the former precursor, formation of the iridate 5 was observed as a result of a double deprotonation at the amine and the NHC pincer arm. In marked contrast, in the case of 4b, a monodeprotonated (at the pincer NHC-arm) species 6 is observed upon reaction with tBuOK. Complex 6 is capable of activating H₂ reversibly to yield the trihydride derivative 7. Finally, DFT calculations of the first AB dehydrogenation step catalyzed by 5 has been performed at the DFT//MN15 level of theory in order to get information on the predominant metal–ligand cooperation mode.

Iridium complexes based on CNN^H ligands containing two potential proton-responsive sites—a lutidine scaffold and a secondary amino group—have been tested in the dehydrogenation of ammonia-borane. Upon reaction with base, depending on the amino group acidity, mono- or doubly deprotonated species exhibiting significantly different catalytic activities were observed.

Recent Advances in Chemistry of Unsymmetrical Phosphorus-Based Pincer Nickel Complexes: From Design to Catalytic Applications

Pincer complexes play an important role in organometallic chemistry; in particular, their use as homogeneous catalysts for organic transformations has increased dramatically in recent years. The high catalytic activity of such bis-cyclometallic complexes is associated with the easy tunability of their properties. Moreover, the phosphorus-based unsymmetrical pincers showed higher catalytic activity than the corresponding symmetrical analogues in several catalytic reactions. However, in modern literature, an increasing interest in the development of catalysts based on non-precious metals is observed. For example, nickel, which is an affordable and sustainable analogue of platinum and palladium, known for its low toxicity, has attracted increasing attention in the catalytic chemistry of transition metals in recent years. Thus, this mini-review is devoted to the recent advances in the chemistry of unsymmetrical phosphorus-based pincer nickel complexes, including the ligand design, the synthesis of nickel complexes and their catalytic applications.

Amine-Boranes as Transfer Hydrogenation and Hydrogenation Reagents: A Mechanistic Perspective

Transfer hydrogenation (TH) has historically been dominated by Meerwein-Ponndorf-Verley (MPV) reactions. However, with growing interest in amine-boranes, not least ammonia-borane (H3 N⋅BH3 ), as potential hydrogen storage materials, these compounds have also started to emerge as an alternative reagent in TH reactions. In this Review we discuss TH chemistry using H3 N⋅BH3 and their analogues (amine-boranes and metal amidoboranes) as sacrificial hydrogen donors. Three distinct pathways were considered: 1) classical TH, 2) nonclassical TH, and 3) hydrogenation. Simple experimental mechanistic probes can be employed to distinguish which pathway is operating and computational analysis can corroborate or discount mechanisms. We find that the pathway in operation can be perturbed by changing the temperature, solvent, amine-borane, or even the substrate used in the system, and subsequently assignment of the mechanism can become nontrivial.

Selective and Additive-Free Hydrogenation of Nitroarenes Mediated by a DMSO-Tagged Molecular Cobalt Corrole Catalyst

We report on the first cobalt corrole that effectively mediates the homogeneous hydrogenation of structurally diverse nitroarenes to afford the corresponding amines. The given catalyst is easily assembled prior to use from 4-tert-butylbenzaldehyde and pyrrole followed by metalation of the resulting corrole macrocycle with cobalt(II) acetate. The thus-prepared complex is self-contained in that the hydrogenation protocol is free from the requirement for adding any auxiliary reagent to elicit the catalytic activity of the applied metal complex. Moreover, a containment system is not required for the assembly of the hydrogenation reaction set-up as both the autoclave and the reaction vessels are readily charged under a regular laboratory atmosphere.

Ligand-enabled and magnesium-activated hydrogenation with earth-abundant cobalt catalysts

Site-selective hydrogenation of PAHs and olefins through a Mg preactivated diketimine/CoBr2 or diketimine–Co complex.

Ester Hydrogenation with Bifunctional Metal-NHC Catalysts: Recent Advances

Hydrogenation of ester to alcohol is an essential reaction in organic chemistry due to its importance in the production of a wide range of bulk and fine chemicals. There are a number of homogeneous and heterogeneous catalyst systems reported in the literature for this useful reaction. Mostly, phosphine-based bifunctional catalysts, owing to their ability to show metal-ligand cooperation during catalytic reactions, are extensively used in these reactions. However, phosphine-based catalysts are difficult to synthesize and are also highly air- and moisture-sensitive, restricting broad applications. In contrast, N-heterocyclic carbenes (NHCs) can be easily synthesized, and their steric and electronic attributes can be fine-tuned easily. In recent times, many phosphine ligands have been replaced by potent σ-donor NHCs, and the resulting bifunctional metal-ligand systems are proven to be very efficient in several important catalytic reactions. This mini-review focuses the recent advances mainly on bifunctional metal-NHC complexes utilized as (pre)catalysts in ester hydrogenation reactions.

Heterogeneously Catalyzed γ-Valerolactone Hydrogenation into 1,4-Pentanediol in Milder Reaction Conditions

Hydrogenation of γ-valerolactone (GVL) in polar solvents (n-butanol, 1,4-dioxane) to 1,4-pentanediol (PDO) and 2-methyltetrahydrofuran (MTHF) was performed at 363–443 K in a fixed bed reactor under overall H2 pressure of 0.7–1.3 MPa. Preliminary screening in a batch reactior was performed with a series of Ru, Ir, Pt, Co, and Cu catalysts, earlier efficiently applied for levulinic acid hydrogenation to GVL. The fresh catalysts were analyzed by transmission electron microscopy (TEM), X-ray fluorescent analysis (XRF), temperature programmed reduction by H2 (H2-TPR), and N2 physisorption. Cu/SiO2 prepared by reduction of copper hydroxosilicate with chrysocolla mineral structure provided better selectivity of 67% towards PDO at 32% GVL conversion in a continuous flow reactor. This catalyst was applied to study the effect of temperature, hydrogen pressure, and contact time. The main reaction products were PDO, MTHF, and traces of pentanol, while no valeric acid was observed. Activity and selectivity to PDO over Cu/SiO2 did not change over 9 h, indicating a fair resistance of copper to leaching.

Reaction of H2 with mitochondria-relevant metabolites using a multifunctional molecular catalyst

The Krebs cycle is the fuel/energy source for cellular activity and therefore of paramount importance for oxygen-based life. The cycle occurs in the mitochondrial matrix, where it produces and transfers electrons to generate energy-rich NADH and FADH₂, as well as C₄-, C₅-, and C₆-polycarboxylic acids as energy-poor metabolites. These metabolites are biorenewable resources that represent potential sustainable carbon feedstocks, provided that carbon-hydrogen bonds are restored to these molecules. In the present study, these polycarboxylic acids and other mitochondria-relevant metabolites underwent dehydration (alcohol-to-olefin and/or dehydrative cyclization) and reduction (hydrogenation and hydrogenolysis) to diols or triols upon reaction with H₂, catalyzed by sterically confined iridium-bipyridyl complexes. The investigation of these single-metal site catalysts provides valuable molecular insights into the development of molecular technologies for the reduction and dehydration of highly functionalized carbon resources.

Formylation of Amines by CO2 Hydrogenation Using Preformed Co(II)/Nickel(II) Complexes

Preparation of formamides by CO₂ hydrogenation requires an efficient catalyst and temperatures around 100 °C or higher, but most catalysts reported so far incorporate rare and toxic precious metals. Five cobalt(II) or nickel(II) complexes with dmpe or PNN (dmpe = 1,2-bis(dimethylphosphino)ethane; PNN = [(2-(di-tert-butylphosphinomethyl)-6-diethylaminomethyl)pyridine) have been evaluated as precatalysts for the hydrogenation of CO₂ to prepare formamides from the corresponding secondary amines. The most active catalyst for these reactions was found to be [NiCl₂(dmpe)] in DMSO, producing dimethylformamide (DMF) from CO₂, H₂, and dimethylamine in up to 6300 TON, the highest activity reported for this reaction with an abundant metal-phosphine complex.

Cobalt-catalysed selective synthesis of aldehydes and alcohols from esters

Efficient and selective reduction of esters to aldehydes and alcohols is reported in which a simple cobalt pincer catalyst catalyses both transformations using diethylsilane as a reductant. Remarkably, the reaction selectivity is controlled by the stoichiometry of diethylsilane.

Hydrosilylation of Esters Catalyzed by Bisphosphine Manganese(I) Complex: Selective Transformation of Esters to Alcohols

Selective and efficient hydrosilylations of esters to alcohols by a well-defined manganese(I) complex with a commercially available bisphosphine ligand are described. These reactions are easy alternatives for stoichiometric hydride reduction or hydrogenation, and employing cheap, abundant, and nonprecious metal is attractive. The hydrosilylations were performed at 100 °C under solvent-free conditions with low catalyst loading. A large variety of aromatic, aliphatic, and cyclic esters bearing different functional groups were selectively converted into the corresponding alcohols in good yields.

Migratory Hydrogenation of Terminal Alkynes by Base/Cobalt Relay Catalysis

Migratory functionalization of alkenes has emerged as a powerful strategy to achieve functionalization at a distal position to the original reactive site on a hydrocarbon chain. However, an analogous protocol for alkyne substrates is yet to be developed. Herein, a base and cobalt relay catalytic process for the selective synthesis of (Z)-2-alkenes and conjugated E alkenes by migratory hydrogenation of terminal alkynes is disclosed. Mechanistic studies support a relay catalytic process involving a sequential base-catalyzed isomerization of terminal alkynes and cobalt-catalyzed hydrogenation of either 2-alkynes or conjugated diene intermediates. Notably, this practical non-noble metal catalytic system enables efficient control of the chemo-, regio-, and stereoselectivity of this transformation.

Transfer Hydrogenation of Ketones Catalyzed by Symmetric Imino-N-heterocyclic Carbene Co(III) Complexes

The synthesis of new moisture-sensitive imine-functionalized N-heterocyclic carbene (NHC) precursor salts [1-(2-[(hydroxyl-benzylidene)-amino]-ethyl)-3-R-3H-imidazole-1-ium bromide; R = methyl (1a), ethyl (1b), and benzyl (1c)] is reported. Subsequent deprotonation of 1a-c and coordination of the in situ generated NHC ligands to CoBr₂ led to the isolation of air-stable six-coordinate Co(III) complexes 2a-c, respectively. All the salts and complexes were fully characterized. Single-crystal X-ray analysis of 2a and 2c showed octahedral Co centers hexacoordinated to two NHC carbons, two imine nitrogen atoms, and two phenolate oxygens in the form [C^N^O(Co³⁺)C^N^O]. The complexes were used in the catalytic transfer hydrogenation (CTH) of a range of ketones in 2-propanol as the solvent and hydrogen donor. Based on a low catalyst concentration of 0.4 mol %, significant conversions in the range of 70-99% were recorded at high turnover frequencies up to 1635 h^-1. A mechanism to account for the steps involved in the CTH of cyclohexanone by complex 2a is proposed and supported by data from cyclic voltammetry, low-resolution mass spectrometry, UV, and IR spectroscopic techniques.

Cobalt-catalyzed alkylation of methyl-substituted N-heteroarenes with primary alcohols: direct access to functionalized N-heteroaromatics

Phosphine free, air and moisture stable Co(NNN) complex catalyzed alkylation of various methyl-substituted N-heteroarenes with alcohols is reported. Following the borrowing hydrogen methodology, a variety of methyl-substituted N-heteroarenes can be functionalized efficiently. To understand the mechanism of this reaction various kinetic and control experiments were carried out.

Highly selective hydrogenation of amides catalysed by a molybdenum pincer complex: scope and mechanism

A series of molybdenum pincer complexes has been shown for the first time to be active in the catalytic hydrogenation of amides.

A series of molybdenum pincer complexes has been shown for the first time to be active in the catalytic hydrogenation of amides. Among the tested catalysts, Mo-1a proved to be particularly well suited for the selective C–N hydrogenolysis of N-methylated formanilides. Notably, high chemoselectivity was observed in the presence of certain reducible groups including even other amides. The general catalytic performance as well as selectivity issues could be rationalized taking an anionic Mo(0) as the active species. The interplay between the amide C Created by potrace 1.16, written by Peter Selinger 2001-2019 ]]> O reduction and the catalyst poisoning by primary amides accounts for the selective hydrogenation of N-methylated formanilides. The catalyst resting state was found to be a Mo–alkoxo complex formed by reaction with the alcohol product. This species plays two opposed roles – it facilitates the protolytic cleavage of the C–N bond but it encumbers the activation of hydrogen.

Mechanistic insights into the non-bifunctional hydrogenation of esters by Co(ii) pincer complexes: a DFT study

A recent experiment has revealed that additive free ester hydrogenation by Co-pincer complexes might follow an unusual non-bifunctional mechanism, however, the detailed mechanistic pathway is missing. It has been predicted that several intermediates and transition states are involved, having their essential role in the catalytic performances. Detailed theoretical studies are therefore essential in this regard for achieving more efficient ester hydrogenation catalysts. On the basis of first principles calculations, performed over Co(PNP)/(PNN) complexes, we present here the energetics and mechanistic details, showing the distinct orientations of different possible intermediates and transition states, and find the minimum energy pathway for the conversion of esters to alcohols. In the way, we find that some intermediates must undergo structural distortion for achieving the lowest potential energy barrier which must have a severe impact on the catalytic turnover frequency.

Unmasking the Ligand Effect in Manganese-Catalyzed Hydrogenation: Mechanistic Insight and Catalytic Application

Manganese-catalyzed hydrogenation reactions have attracted broad interest since the first report in 2016. Among the reported catalytic systems, Mn catalysts supported by tridentate PNP- and NNP-pincer ligands have most commonly been used. For example, a number of PNP-Mn pincer catalysts have been reported for the hydrogenation of aldehydes, aldimines, ketones, nitriles, and esters. Furthermore, various NNP-Mn pincer catalysts have been shown to be active in the hydrogenation of less-reactive substrates such as amides, carbonates, carbamates, and urea derivations. These observations indicated that Mn catalysts supported by NNP-pincer ligands exhibit higher reactivity in hydrogenation reactions than their PNP counterparts. Such a ligand effect in Mn-catalyzed hydrogenation reactions has yet to be confirmed. Herein, we investigated the origin and applicability of this ligand effect. A combination of experimental and theoretical investigations showed that NNP-pincer ligands on the Mn complexes were more electron-rich and less sterically hindered than their PNP counterparts, leading to higher reactivity in a series of Mn-catalyzed hydrogenation reactions. Inspired by the ligand effect on Mn-catalyzed hydrogenations, we developed the first Mn-catalyzed hydrogenation of N-heterocycles. Specifically, NNP-Mn pincer catalysts hydrogenated a series of N-heterocycles (32 examples) with up to 99% yields, and the corresponding PNP-Mn pincer catalysts afforded low reactivity under the same conditions. This verified that such a ligand effect is generally applicable in hydrogenation reactions of both carbonyl and noncarbonyl substrates based on Mn catalysis.

Dehydroalkylative Activation of CNN- and PNN-Pincer Ruthenium Catalysts for Ester Hydrogenation

Ruthenium-pincer complexes bearing CNN- and PNN-pincer ligands with diethyl- or diisopropylamino side groups, which have previously been reported to be active precatalysts for ester hydrogenation, undergo dehydroalkylation on heating in the presence of tricyclohexylphosphine to release ethane or propane, giving five-coordinate ruthenium(0) complexes containing a nascent imine functional group. Ethane or propane is also released under the conditions of catalytic ester hydrogenation, and time-course studies show that this release is concomitant with the onset of catalysis. A new PNN-pincer ruthenium(0)-imine complex is a highly active catalyst for ester hydrogenation at room temperature, giving up to 15 500 turnovers with no added base. This complex was shown to react reversibly at room temperature with two equivalents of hydrogen to give a ruthenium(II)-dihydride complex, where the imine functionality has been hydrogenated to give a protic amine side group. These observations have potentially broad implications for the identities of catalytic intermediates in ester hydrogenation and related transformations.

Liquid-Phase Hydrogenation of Nitriles to Amines Facilitated by a Co(II)/Zn(0) Pair: A Ligand-Free Catalytic Protocol

The given report introduces a simple and user-friendly in situ method for the production of catalytically active cobalt particles. The approach circumvents the use of air- and moisture-sensitive reductants as well as the application of anhydrous Co-precursor salts. Accordingly, the described catalytic system is readily assembled under open-flask conditions by simply combining the components in the reaction vessel. Therefore, the arduous charging procedure of the reaction autoclave in a glovebox under an inert gas atmosphere is no longer necessary. In fact, the catalytically active material is obtained upon treatment of readily available Co(OAc)₂·4 H₂O with benign commercial Zn powder. The catalytic performance of the resultant material was tested in the heterogeneous hydrogenation of nitriles to the corresponding primary amines. Both activity and selectivity of the cobalt catalyst are significantly enhanced if a triflate-based Lewis acid and ammonia is added to the reaction mixture.

Hydrogenation of Polyesters to Polyether Polyols

The amount of plastic waste is continuously increasing. Besides conventional recycling, one solution to deal with this problem could be to use this waste as a resource for novel materials. In this study, polyesters are hydrogenated to give polyether polyols by using in situ-generated Ru-Triphos catalysts in combination with Lewis acids. The choice of Lewis acid and its concentration relative to the ruthenium catalyst are found to determine the selectivity of the reaction. Monitoring of the molecular weight during the reaction confirms a sequential mechanism in which the diols that are formed by hydrogenation are etherified to the polyethers. To probe the applicability of this tandem hydrogenation etherification approach, a range of polyester substrates is investigated. The oligoether products that form in these reactions have the chain lengths that are appropriate for application in the adhesives and coatings industries. This strategy makes polyether polyols accessible that are otherwise difficult to obtain from conventional fossil-based feedstocks.