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

Preparation and Study of Reusable Polymerized Catalysts for Ester Hydrogenation

A cross-linked catalyst organic framework was prepared by an alternating ring-opening olefin metathesis polymerization between dichloro{N,N'-bis({(2-diphenylphosphino)phenyl}methylidene)bicyclo[2.2.1]-hept-5-ene-2,3-diamine}ruthenium, 1,2-N-di(cis-5-norbornene-2,3-endo-dicarboximido)-ethane, and cis-cyclooctene catalyzed by RuCl2(=CHPh)(PCy3)2 in the presence of a BaSO4 support. The heterogenized catalyst hydrogenated methyl benzoate at a similar rate to the homogeneous catalyst (0.0025 mol % catalyst, 10 mol % KO t Bu, 80 °C, 50 atm, tetrahydrofuran, 21 h, ∼15 000 turnovers during the first 1 h). The catalyst was used five times for a total of 121 680 turnovers. A study on the reusability of this catalyst showed that ester hydrogenations with bifunctional catalysts slow as the reaction proceeds. This inhibition is removed by isolating and reusing the catalyst, suggesting that future catalyst design should emphasize avoiding product inhibition.

Cobalt-Catalyzed Reductive Alkylation of Amines with Carboxylic Acids

Direct reductive alkylation of amines with carboxylic acid is carried out by using an inexpensive, air-stable cobalt/triphos catalytic system with molecular hydrogen as the reductant. This efficient synthetic method proceeds through reduction and condensation, followed by reduction of the in situ-generated imine into the amine in a green catalytic process.

Cobalt-Catalyzed Aqueous Dehydrogenation of Formic Acid

Among the known liquid organic hydrogen carriers, formic acid attracts increasing interest in the context of safe and reversible storage of hydrogen. Here, the first molecularly defined cobalt pincer complex is disclosed for the dehydrogenation of formic acid in aqueous medium under mild conditions. Crucial for catalytic activity is the use of the specific complex 3. Compared to related ruthenium and manganese complexes 7 and 8, this optimal cobalt complex showed improved performance. DFT computations support an innocent non-classical bifunctional outer-sphere mechanism on the triplet state potential energy surface.

Electro- and Solar-Driven Fuel Synthesis with First Row Transition Metal Complexes

The synthesis of renewable fuels from abundant water or the greenhouse gas CO2 is a major step toward creating sustainable and scalable energy storage technologies. In the last few decades, much attention has focused on the development of nonprecious metal-based catalysts and, in more recent years, their integration in solid-state support materials and devices that operate in water. This review surveys the literature on 3d metal-based molecular catalysts and focuses on their immobilization on heterogeneous solid-state supports for electro-, photo-, and photoelectrocatalytic synthesis of fuels in aqueous media. The first sections highlight benchmark homogeneous systems using proton and CO2 reducing 3d transition metal catalysts as well as commonly employed methods for catalyst immobilization, including a discussion of supporting materials and anchoring groups. The subsequent sections elaborate on productive associations between molecular catalysts and a wide range of substrates based on carbon, quantum dots, metal oxide surfaces, and semiconductors. The molecule-material hybrid systems are organized as "dark" cathodes, colloidal photocatalysts, and photocathodes, and their figures of merit are discussed alongside system stability and catalyst integrity. The final section extends the scope of this review to prospects and challenges in targeting catalysis beyond "classical" H2 evolution and CO2 reduction to C1 products, by summarizing cases for higher-value products from N2 reduction, C x>1 products from CO2 utilization, and other reductive organic transformations.

A DFT study of Co(i) and Ni(ii) pincer complex-catalyzed hydrogenation of ketones: intriguing mechanism dichotomy by ligand field variation

Ligand field variation governs the mechanism dichotomy for isoelectronic catalysts.

Cobalt pincer complexes for catalytic reduction of nitriles to primary amines

An efficient cobalt pincer type complex catalyzes the hydrogenation of nitriles to amines under mild conditions with high yields.

Phosphine-free cobalt catalyst precursors for the selective hydrogenation of olefins

A phosphine-free Co(NNS)Cl₂ complex was used as a catalyst precursor for the highly selective hydrogenation of alkenes in the presence of other functional groups such as ketones.

PNN′ & P2NN′ ligands via reductive amination with phosphine aldehydes: synthesis and base-metal coordination chemistry

Phosphorus-donor “arms” are readily added to amines in order to enable sturdy base metal coordination.

First-Row Transition Metal (De)Hydrogenation Catalysis Based On Functional Pincer Ligands

The use of 3d metals in de/hydrogenation catalysis has emerged as a competitive field with respect to "traditional" precious metal catalyzed transformations. The introduction of functional pincer ligands that can store protons and/or electrons as expressed by metal-ligand cooperativity and ligand redox-activity strongly stimulated this development as a conceptual starting point for rational catalyst design. This review aims at providing a comprehensive picture of the utilization of functional pincer ligands in first-row transition metal hydrogenation and dehydrogenation catalysis and related synthetic concepts relying on these such as the hydrogen borrowing methodology. Particular emphasis is put on the implementation and relevance of cooperating and redox-active pincer ligands within the mechanistic scenarios.

Base-Free Iron Catalyzed Transfer Hydrogenation of Esters Using EtOH as Hydrogen Source

Herein, we report on the use of the iron pincer complex Iron-MACHO-BH, in the base-free transfer hydrogenation of esters with EtOH as a hydrogen source. More than 20 substrates including aromatic and aliphatic esters and lactones were reduced affording the desired primary alcohols and diols with moderate to excellent isolated yields. It is also possible to reduce polyesters to the diols with this method, enabling a novel way of plastic recycling. Reduction of the renewable substrate methyl levulinate proceeds to form 1,4-pentanediol directly. The yields are largely governed by the equilibrium between the alcohol and the ethyl ester.

Cobalt-Pincer Complexes in Catalysis

Non-noble metal catalysts based on pincer type compounds are of special interest for organometallic chemistry and organic synthesis. Next to iron and manganese, currently cobalt-pincer type complexes are successfully applied in various catalytic reactions. In this review the recent progress in (de)hydrogenation, transfer hydrogenation, hydroboration and hydrosilylation as well as dehydrogenative coupling reactions using cobalt-pincer complexes is summarised.

Hydrogenation of CO2 -Derived Carbonates and Polycarbonates to Methanol and Diols by Metal-Ligand Cooperative Manganese Catalysis

The first base-metal-catalysed hydrogenation of CO₂ -derived carbonates to alcohols is presented. The reaction proceeds under mild conditions in the presence of a well-defined manganese complex with a loading as low as 0.25 mol %. The non-precious-metal homogenous catalytic system provides an indirect route for the conversion of CO₂ into methanol with the co-production of value-added (vicinal) diols in yields of up to 99 %. Experimental and computational studies indicate a metal-ligand cooperative catalysis mechanism.

Cobalt Complexes as an Emerging Class of Catalysts for Homogeneous Hydrogenations

Catalytic hydrogenation using molecular hydrogen represents a green and practical approach for reductions of all kinds of organic chemicals. Traditionally, in the majority of these processes the presence of transition metal catalysts is required. In this regard, noble-metal-based catalysts have largely been implemented, such as the application of iridium, palladium, rhodium, ruthenium, and others. Recently, the employment of earth-abundant 3d metals has emerged to replace the utilization of scarce noble metals because of their availability, lower cost, and often reduced toxicity. In this respect, several cobalt complexes, in the form of either molecularly well-defined or in situ-formed complexes, are receiving increasing attention from the scientific community. Importantly, the stability and reactivity of the complexes have greatly been supported by multidentate ligands under steric and/or electronic influences. For instance, tridentate or tetradentate phosphine ligands indirectly tune the reactivity of the metal center to accelerate the overall process, whereas direct participation of the ligand in pincer-type complexes through ligand-metal cooperation regulates the elementary steps in the catalytic cycle. In this Account, we emphasize specifically the advancements in cobalt-catalyzed hydrogenations using molecular hydrogen accomplished in our group. A variety of substrate classes ranging from simple molecules (e.g., carbon dioxide) to complex compounds were explored under the mild and efficient catalytic conditions. Notable examples include the reduction of carbon dioxide to afford either formates using a Co(BF₄)₂·6H₂O/Tetraphos catalyst system or methanol employing a Co(acac)₃/Triphos complex in the presence of HNTf₂. As interesting examples of the synthesis of fine chemicals, cobalt-promoted hydrogenations of nitriles to primary amines and reductive alkylations of indoles using carboxylic acids as alkylating agents are highlighted. Moreover, highly selective hydrogenations of N-heteroarenes under additive-free conditions were possible by the application of specific cobalt complexes. More recently, a set of carboxylic esters could be hydrogenated to the corresponding alcohols with high efficiency by the use of a well-defined cobalt-PNP pincer catalyst. In particular, the decent reactivity of cobalt catalysts enabled high selectivity and functional group tolerance to be achieved. Throughout our studies, it was found that the pairing of a suitable cobalt precursor and an appropriate tridentate or tetradentate phosphine ligand plays a crucial role harnessing the desired reactivity, while other monodentate and bidentate phosphine ligands showed no reactivity in these investigations. Our developments could provide supervisory information for the future exploration of cobalt-catalyzed hydrogenation reactions and other types of reactions involving cobalt catalysis. Furthermore, relevant contributions from other groups, remaining challenges, and future perspectives in this research area are also presented.

Tailored Cobalt-Catalysts for Reductive Alkylation of Anilines with Carboxylic Acids under Mild Conditions

The first cobalt-catalyzed hydrogenative N-methylation and alkylation of amines with readily available carboxylic acid feedstocks as alkylating agents and H₂ as ideal reductant is described. Combination of tailor-made triphos ligands with cobalt(II) tetrafluoroborate significantly improved the efficiency, thus promoting the reaction under milder conditions. This novel protocol allows for a broad substrate scope with good functional group tolerance, even in the presence of reducible alkenes, esters, and amides.

Mechanistic insights into the iridium catalysed hydrogenation of ethyl acetate to ethanol: a DFT study

Density functional theory study of the hydrogenation of ethyl acetate catalysed by iridium complexes [Cp*Ir(bpy)OH2]2+ reveals a direct C-O bond cleavage mechanism with two cascade catalytic cycles for the hydrogenation of ethyl acetate to aldehyde and the hydrogenation of aldehyde to ethanol. Calculation results indicate that the rate-determining state in the whole catalytic reaction is the direct C-O bond cleavage for the formation of aldehyde and ethanol with a total free energy barrier of 25.5 kcal mol-1, which is 0.6 kcal mol-1 more favorable than the mechanism proposed by Goldberg and co-workers in their experimental study.

Hydrogenation of carbonyl compounds of relevance to hydrogen storage in alcohols

Alcohols are a promising source for the sustainable production of hydrogen that may also serve as rechargeable liquid organic hydrogen carriers (LOHCs). Metal-catalyzed acceptorless dehydrogenation of alcohols produces carbonyl derivatives as H2-depleted by-products, which by means of a hydrogenation reaction can be reconverted to the initial alcohols. Hence, reversible H2-storage systems based on pairs of secondary alcohols/ketones and primary alcohols/carboxylic acid derivatives may be envisaged. In this contribution, the hydrogenation of carbonyl derivatives, including ketones, esters, amides and carboxylic acids, is reviewed from the perspective of the hydrogen storage in alcohols.

A theoretical study on the mechanism of hydrogenation of carboxylic acids catalyzed by the Saito catalyst

The mechanism of the ruthenium carboxylate-catalyzed hydrogenation of carboxylic acids was investigated by using density functional theory (DFT) calculations. The novel mechanism including two hydrogenation cycles was proposed for this reaction. The first cycle is the hydrogenation of the carboxylic acid to an aldehyde, while the second cycle is the hydrogenation of the aldehyde to an alcohol. These two catalytic cycles share similar elementary steps, including H₂ heterolysis, hydride migration of the carboxylic acid or aldehyde, and catalyst regeneration. In this hydrogenation mechanism, the carboxylic acid is not only a reactant, but also an important proton source. Furthermore, the noncovalent interaction (e.g. hydrogen bonding interaction) between the ligand and carboxylic acid substrate could promote the hydrogenation of the carboxylic acid through stabilizing the transition state of the most energy-demanding step (i.e., hydride migration in the first catalytic cycle). Besides, the strong electron-donating ability of the dppb ligand could also facilitate the hydride migration.

Selective ligand-free cobalt-catalysed reduction of esters to aldehydes or alcohols

Activated cobalt metal salts are effective and versatile catalysts for the chemoselective reduction of esters to aldehydes or alcohols through hydrosilylation reaction.

Catalytic (de)hydrogenation promoted by non-precious metals – Co, Fe and Mn: recent advances in an emerging field

This review is aimed at introducing the remarkable progress made in the last three years in the development of base metal catalysts for hydrogenations and dehydrogenative transformations.

Phosphine-free cobalt pincer complex catalyzed Z-selective semi-hydrogenation of unbiased alkynes

A molecularly defined NNN-type cobalt pincer complex catalyzed semi-hydrogenation of unbiased alkynes to Z-selective alkenes is reported. The reaction operates at a low temperature (50–80 °C), phosphine ligand-free, and base-free conditions with no additive required.

Cobalt-catalysed reductive C-H alkylation of indoles using carboxylic acids and molecular hydrogen

The direct CH-alkylation of indoles using carboxylic acids is presented for the first time. The catalytic system based on the combination of Co(acac)3 and 1,1,1-tris(diphenylphosphinomethyl)-ethane (Triphos, L1), in the presence of Al(OTf)3 as co-catalyst, is able to perform the reductive alkylation of 2-methyl-1H-indole with a wide range of carboxylic acids. The utility of the protocol was further demonstrated through the C3 alkylation of several substituted indole derivatives using acetic, phenylacetic or diphenylacetic acids. In addition, a careful selection of the reaction conditions allowed to perform the selective C3 alkenylation of some indole derivatives. Moreover, the alkenylation of C2 position of 3-methyl-1H-indole was also possible. Control experiments indicate that the aldehyde, in situ formed from the carboxylic acid hydrogenation, plays a central role in the overall process. This new protocol enables the direct functionalization of indoles with readily available and stable carboxylic acids using a non-precious metal based catalyst and hydrogen as reductant.

CH2 Linkage Effects on the Reactivity of Bis(aminophosphine)-Ruthenium Complexes for Selective Hydrogenation of Esters into Alcohols

A novel ruthenium complex binding to two subtly different aminophosphine ligands, (o-PPh2C6H4CH2NH2)(o-PPh2C6H4NH2)RuCl2, was successfully isolated. This bis(aminophosphine)-ruthenium complex shows efficient activity in both dimethyl oxalate (DMO) and methyl benzoate (MB) hydrogenation. On the contrast, similar complexes (o-PPh2C6H4NH2)2RuCl2 and (o-PPh2C6H4CH2NH2)2RuCl2, can only effectively catalyze the hydrogenation of DMO and MB, respectively. Our experimental studies in combination of theoretical calculations reveal that the remarkable substrate selectivity in the hydrogenation of esters arises from the nonbonding interactions operated by the CH2 linkage of the ligand.

Non-Pincer-Type Manganese Complexes as Efficient Catalysts for the Hydrogenation of Esters

Catalytic hydrogenation of carboxylic acid esters is essential for the green production of pharmaceuticals, fragrances, and fine chemicals. Herein, we report the efficient hydrogenation of esters with manganese catalysts based on simple bidentate aminophosphine ligands. Monoligated Mn PN complexes are particularly active for the conversion of esters into the corresponding alcohols at Mn concentrations as low as 0.2 mol % in the presence of sub-stoichiometric amounts of KOt Bu base.

Unprecedented selective homogeneous cobalt-catalysed reductive alkoxylation of cyclic imides under mild conditions

The first general and efficient non-noble metal-catalysed reductive C2-alkoxylation of cyclic imides (phthalimides and succinimides) is presented. Crucial for the success is the use of [Co(BF₄)₂·6H₂O/triphos (L1)] combination and no external additives are required.

The first general and efficient non-noble metal-catalysed reductive C2-alkoxylation of cyclic imides (phthalimides and succinimides) is presented. Crucial for the success is the use of [Co(BF₄)₂·6H₂O/triphos (L1)] combination and no external additives are required. Using the optimal cobalt-system, the hydrogenation of the aromatic ring of the parent phthalimide is avoided and only one of the carbonyl groups is selectively functionalized. The resulting products, N- and aryl-ring substituted 3-alkoxy-2,3-dihydro-1H-isoindolin-1-one and N-substituted 3-alkoxy-pyrrolidin-2-one derivatives, are prepared under mild conditions in good to excellent isolated yields. Intramolecular reductive couplings can also be performed affording tricyclic compounds in a one-step process. The present protocol opens the way to the development of new base-metal processes for the straightforward synthesis of functionalized N-heterocyclic compounds of pharmaceutical and biological interest.