Syntheses and Catalytic Hydrogenation Performance of Cationic Bis(phosphine) Cobalt(I) Diene and Arene Compounds

Revisiting the Preparation and Catalytic Performance of a Phosphine-Modified Co(II) Hydroformylation Precatalyst

In light of recent conflicting reports regarding the hydroformylation catalytic activity derived from cationic Co(II) precatalysts of the form [Co(acac)(bis(phosphine))]BF4, the synthetic procedures and characterization of [Co(acac)(dppBz)]BF4, 1, are evaluated. Leveraging calibrated ESI-TOF MS methodologies, substantial quantities of Co(acac)2(dppBz), 2, were observed within samples of 1. The source of the impurity, 2, is determined to derive from incomplete protonolysis of the Co(acac)2 precursor and ligand scrambling occurring during the synthesis of 1. Revised synthetic procedures using lower temperature conditions and longer reaction times afford analytically pure samples of 1 based on ESI-TOF MS and NMR spectroscopic analysis. Complex 1 is demonstrated to act as a hydroformylation precatalyst for the conversion of 1-hexene to 1-heptanal under relatively mild conditions at 51.7 bar and 140 °C. The presence of impurity 2 is shown to dramatically decrease the catalytic performance derived from 1.

Asymmetric Hydrogenation of Indazole-Containing Enamides Relevant to the Synthesis of Zavegepant Using Neutral and Cationic Cobalt Precatalysts

The cobalt-catalyzed asymmetric hydrogenation of indazole-containing enamides relevant to the synthesis of the calcitonin gene-related peptide (CGRP) receptor antagonist, zavegepant (1), approved for the treatment of migraines, is described. Both neutral bis(phosphine)cobalt(II) and cationic bis(phosphine)cobalt(I) complexes served as efficient precatalysts for the enamide hydrogenation reactions, providing excellent yield and enantioselectivities (up to >99.9%) for a range of related substrates, though key reactivity differences were observed. Hydrogenation of indazole-containing enamide, methyl (Z)-2-acetamido-3-(7-methyl-1H-indazol-5-yl)acrylate, was performed on a 20 g scale.

Cobalt-Catalyzed Asymmetric Hydrogenation: Substrate Specificity and Mechanistic Variability

Asymmetric hydrogenation finds widespread application in academia and industry. And indeed, a number of processes have been implemented for the production of pharma and agro intermediates as well as flavors & fragrances. Although these processes are all based on the use of late transition metals as catalysts, there is an increasing interest in the use of base metal catalysis in view of their lower cost and the expected different substrate scope. Catalysts based on cobalt have already shown their potential in enantioselective hydrogenation chemistry. This review outlines the impressive progress made in recent years on cobalt-catalyzed asymmetric hydrogenation of different unsaturated substrates. We also illustrate the ligand dependent substrate specificity as well as the mechanistic variability in detail. This may well guide further catalyst development in this research area.

Expedient and divergent synthesis of unnatural peptides through cobalt-catalyzed diastereoselective umpolung hydrogenation

The development of a reliable method for asymmetric synthesis of unnatural peptides is highly desirable and particularly challenging. In this study, we present a versatile and efficient approach that uses cobalt-catalyzed diastereoselective umpolung hydrogenation to access noncanonical aryl alanine peptides. This protocol demonstrates good tolerance toward various functional groups, amino acid sequences, and peptide lengths. Moreover, the versatility of this reaction is illustrated by its successful application in the late-stage functionalization and formal synthesis of various representative chiral natural products and pharmaceutical scaffolds. This strategy eliminates the need for synthesizing chiral noncanonical aryl alanines before peptide formation, and the hydrogenation reaction does not result in racemization or epimerization. The underlying mechanism was extensively explored through deuterium labeling, control experiments, HRMS identification, and UV-Vis spectroscopy, which supported a reasonable CoI/CoIII catalytic cycle. Notably, acetic acid and methanol serve as safe and cost-effective hydrogen sources, while indium powder acts as the terminal electron source.

Experimental and Computational Studies on Cobalt(I)-Catalyzed Regioselective Allylic Alkylation Reactions

Here, we report the development of cobalt(I)-catalyzed regioselective allylic alkylation reactions of tertiary allyl carbonates with 1,3-dicarbonyl compounds. A family of well-defined tetrahedral cobalt(I) complexes bearing commercially available bidentate bis(phosphine) ligands [(P,P)Co(PPh3 )Cl] are synthesized and explored as catalysts in allylic alkylation reactions. The catalyst [(dppp)Co(PPh3 )Cl] (dppp=1,3-Bis(diphenylphosphino)propane) enables the alkylation of a large variety of tertiary allyl carbonates with high yields and excellent regioselectivity for the branched product. Remarkably, this methodology is selective for the activation of tertiary allyl carbonates even in the presence of secondary allyl carbonates. This contrasts with the selectivity observed in cobalt-catalyzed allylic alkylations enabled by visible light photocatalysis. Mechanistic insights by means of experimental and computational investigations support a Co(I)/Co(III) catalytic cycle.

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.

Co-Catalyzed Asymmetric Hydrogenation. The Same Enantioselection Pattern for Different Mechanisms

The mechanism of the recently reported catalyzed asymmetric hydrogenation of enyne 1 catalyzed by the Co-(R,R)-QuinoxP* complex was studied by DFT. Conceivable pathways for the Co(I)-Co(III) mechanism were computed together with a Co(0)-Co(II) catalytic cycle. It is commonly assumed that the exact nature of the chemical transformations taking place along the actually operating catalytic pathway determine the sense and level of enantioselection of the catalytic reaction. In this work, two chemically different mechanisms reproduced the experimentally observed perfect stereoselection of the same handedness. Moreover, the relative stabilities of the transition states of the stereo induction stages were controlled via exactly the same weak disperse interactions between the catalyst and the substrate.

Precise Synthesis of Chiral Z-Allylamides by Cobalt-Catalyzed Asymmetric Sequential Hydrogenations

Asymmetric sequential hydrogenations of conjugated enynes have been developed using a Ph-BPE-Co^I catalyst for the precise synthesis of chiral Z-allylamides in high activity (up to 1000 substrate/catalyst (S/C)) and with excellent enantioselectivity (up to >99 % enantiomeric excess (ee)). Mechanism experiments and theoretical calculations support a cationic Co^I /Co^III redox catalytic cycle. The catalytic activity difference between cobalt complexes of Ph-BPE and QuinoxP* was explained by the process decomposition of rate-determining step in the second hydrogenation.

Mechanistic Investigations of the Asymmetric Hydrogenation of Enamides with Neutral Bis(phosphine) Cobalt Precatalysts

The mechanism of the asymmetric hydrogenation of prochiral enamides by well-defined, neutral bis(phosphine) cobalt(0) and cobalt(II) precatalysts has been explored using(R,R)-^iPrDuPhos ((R,R)-^iPrDuPhos = (+)-1,2-bis[(2R,5R)-2,5-diisopropylphospholano]benzene) as a representative chiral bis(phosphine) ligand. A series of (R,R)-(^iPrDuPhos)Co(enamide) (enamide = methyl-2-acetamidoacrylate (MAA), methyl(Z)-α-acetamidocinnamate (MAC), and methyl(Z)-acetamido(4-fluorophenyl)acrylate (^4FMAC)) complexes (1-MAA, 1-MAC, and 1-^4FMAC), as well as a dinuclear cobalt tetrahydride, [(R,R)-(^iPrDuPhos)Co]₂(μ₂-H)₃(H) (2), were independently synthesized, characterized, and evaluated in both stoichiometric and catalytic hydrogenation reactions. Characterization of (R,R)-(^iPrDuPhos)Co(enamide) complexes by X-ray diffraction established the formation of the pro-(R) diastereomers in contrast to the (S)-alkane products obtained from the catalytic reaction. In situ monitoring of the cobalt-catalyzed hydrogenation reactions by UV-visible and freeze-quench electron paramagnetic resonance spectroscopies revealed (R,R)-(^iPrDuPhos)Co(enamide) complexes as the catalyst resting state for all the three enamides studied. Variable time normalization analysis kinetic studies of the cobalt-catalyzed hydrogenation reactions in methanol established a rate law that is first order in (R,R)-(^iPrDuPhos)Co(enamide) and H₂ but independent of the enamide concentration. Deuterium-labeling studies, including measurement of an H₂/D₂ kinetic isotope effect and catalytic hydrogenations with HD, established an irreversible H₂ addition step to the bound enamide. Density functional theory calculations support that this step is both rate and selectivity determining. Calculations, as well as HD-labeling studies, provide evidence for two-electron redox cycling involving cobalt(0) and cobalt(II) intermediates during the catalytic cycle. Taken together, these experiments support an unsaturated pathway for the [(R,R)-(^iPrDuPhos)Co]-catalyzed hydrogenation of prochiral enamides.

Azole-Directed Cobalt-Catalyzed Asymmetric Hydrogenation of Alkenes

The azole-directed cobalt-catalyzed asymmetric hydrogenation of alkenes has been developed with high efficiency. With this approach, chiral pyrazole compounds were obtained in quantitative yields and excellent enantioselectivities (up to 99 % ee) under mild conditions, and the hydrogenation was conducted on a gram scale with up to 2000 TON. Several useful applications were demonstrated including the convenient introduction of β-chirality to a drug intermediate containing an azole ring.

Activator-free single-component Co(I)-catalysts for regio- and enantioselective heterodimerization and hydroacylation reactions of 1,3-dienes. New reduction procedures for synthesis of [L]Co(I)-complexes and comparison to in situ generated catalysts

Although cobalt(I) bis-phosphine complexes have been implicated in many selective C-C bond-forming reactions, until recently relatively few of these compounds have been fully characterized or have been shown to be intermediates in catalytic reactions. In this paper we present a new practical method for the synthesis and isolation of several cobalt(I)-bis-phosphine complexes and their use in Co(I)-catalyzed reactions. We find that easily prepared (in situ generated or isolated) bis-phosphine and (2,6-N-aryliminoethyl)pyridine (PDI) cobalt(II) halide complexes are readily reduced by 1,4-bis-trimethylsilyl-1,4-dihydropyrazine or commercially available lithium nitride (Li₃N), leaving behind only innocuous volatile byproducts. Depending on the structures of the bis-phosphines, the cobalt(I) complex crystallizes as a phosphine-bridged species [(P∼P)(X)Co^I[μ-(P∼P)]Co^I(X)(P∼P)] or a halide-bridged species [(P∼P)Co^I[μ-(X)]₂Co^I(P∼P)]. Because the side-products are innocuous, these methods can be used for the in situ generation of catalytically competent Co(I) complexes for a variety of low-valent cobalt-catalyzed reactions of even sensitive substrates. These complexes are also useful for the synthesis of rare cationic [(P∼P)Co^I-η⁴-diene]⁺ X^- or [(P∼P)Co^I-η⁶-arene]⁺ X^- complexes, which are shown to be excellent single-component catalysts for the following regioselective reactions of dienes: heterodimerizations with ethylene or methyl acrylate, hydroacylation and hydroboration. The reactivity of the single-component catalysts with the in situ generated species are also documented.

Mechanism and Origins of Enantioselectivity of Cobalt-Catalyzed Intermolecular Hydroarylation/Cyclization of 1,6-Enynes with N-Pyridylindoles

Density functional theory calculations were performed to investigate the cobalt-catalyzed intermolecular hydroarylation/cyclization of 1,6-enynes with N-pyridylindoles. The computations reveal that the reaction begins with the oxidative cyclization of 1,6-enyne to afford the five-membered cobaltacycle, from which the metal-assisted σ-bond metathesis/C-C reductive elimination led to the final hydroarylation/cyclization product. The initial oxidative cyclization constitutes the rate-determining step of the overall reaction. The steric repulsion and π···π interaction were found to play a crucial role in dictating the experimentally observed enantioselectivity.

Three-Component Coupling of Arenes, Ethylene, and Alkynes Catalyzed by a Cationic Bis(phosphine) Cobalt Complex: Intercepting Metallacyclopentenes for C-H Functionalization

A cobalt-catalyzed intermolecular three-component coupling of arenes, ethylene, and alkynes was developed using the well-defined air-stable cationic bis(phosphine) cobalt(I) complex, [(dcype)Co(η⁶-C₇H₈)][BAr^F₄] (dcype = 1,2-bis(dicyclohexylphosphino)ethane; BAr^F₄ = B[(3,5-(CF₃)₂)C₆H₃]₄), as the precatalyst. All three components were required for turnover and formation of ortho-homoallylated arene products. A range of directing groups including amide, ketone, and 2-pyridyl substituents on the arene promoted the reaction. The cobalt-catalyzed method exhibited broad functional group tolerance allowing for the late-stage functionalization of two drug molecules, fenofibrate and haloperidol. A series of control reactions, deuterium labeling studies, resting state analysis, as well as synthesis of substrate- and product-bound η⁶-arene complexes supported a pathway involving C(sp²)–H activation from a cobalt(III) metallacycle.

Recent Advances in the Enantioselective Synthesis of Chiral Amines via Transition Metal-Catalyzed Asymmetric Hydrogenation

Chiral amines are key structural motifs present in a wide variety of natural products, drugs, and other biologically active compounds. During the past decade, significant advances have been made with respect to the enantioselective synthesis of chiral amines, many of them based on catalytic asymmetric hydrogenation (AH). The present review covers the use of AH in the synthesis of chiral amines bearing a stereogenic center either in the α, β, or γ position with respect to the nitrogen atom, reported from 2010 to 2020. Therefore, we provide an overview of the recent advances in the AH of imines, enamides, enamines, allyl amines, and N-heteroaromatic compounds.

Nickel-Catalyzed Asymmetric Hydrogenation for the Synthesis of a Key Intermediate of Sitagliptin

Nickel-catalyzed enantioselective hydrogenation of enamines leading to the efficient synthesis of 3-R-Boc-amino-4-(2,4,5-trifluorophenyl)butyric esters, the key intermediate of the blockbuster antidiabetic drug (R)-SITAGLIPTIN, is described. The sitagliptin motifs were isolated in more than 99% yield and with 75-92% ee using the earth-abundant nickel catalyst. Upon chiral resolution with (R)- and (S)-1-phenylethylamines, the partially enantioenriched (R)- and (S)-Boc-3-amino-4-(2,4,5-trifluorophenyl)butanoic acids provided >99.5% ee of the crucial sitagliptin intermediate. The asymmetric hydrogenation protocol was scaled up to 10 g with consistency in yield and ee, and has been reproduced in multiple batches.

A New Paradigm in Enantioselective Cobalt Catalysis: Cationic Cobalt(I) Catalysts for Heterodimerization, Cycloaddition, and Hydrofunctionalization Reactions of Olefins

One of the major challenges facing organic synthesis in the 21st century is the utilization of abundantly available feedstock chemicals for fine chemical synthesis. Regio- and enantioselective union of easily accessible 1,3-dienes and other feedstocks like ethylene, alkyl acrylates, and aldehydes can provide valuable building blocks adorned with latent functionalities for further synthetic elaboration. Through an approach that relies on mechanistic insights and systematic examination of ligand and counterion effects, we developed an efficient cobalt-based catalytic system [(P∼P)CoX2/Me3Al] (P∼P = bisphosphine) to effect the first enantioselective heterodimerization of several types of 1,3-dienes with ethylene. In addition to simple cyclic and acyclic dienes, siloxy-1,3-dienes participate in this reaction, giving highly functionalized, nearly enantiopure silyl enolates, which can be used for subsequent C-C and C-X bond-forming reactions. As our understanding of the mechanism of this reaction improved, our attention was drawn to more challenging partners like alkyl acrylates (one of the largest volume feedstocks) as the olefin partners instead of ethylene. Prompted by the intrinsic limitations of using aluminum alkyls as the activators for this reaction, we explored the fundamental chemistry of the lesser known (P∼P)Co(I)X species and discovered that in the presence of halide sequestering agents, such as sodium tetrakis[3,5-bis(trifluoromethyl)phenyl]borate (NaBARF) or (C6F5)3B, certain chiral bisphosphine complexes are superb catalysts for regio- and enantioselective heterodimerization of 1,3-dienes and alkyl acrylates. We have since found that these cationic Co(I) catalysts, most conveniently prepared in situ by reduction of the corresponding cobalt(II) halide complexes by zinc in the presence of NaBARF, promote enantioselective [2 + 2]-cycloaddition between alkynes and an astonishing variety of alkenyl derivatives to give highly functionalized cyclobutenes. In reactions between 1,3-enynes and ethylene, the [2 + 2]-cycloaddition between the alkyne and ethylene is followed by a 1,4-addition of ethylene in a tandem fashion to give nearly enantiopure cyclobutanes with an all-carbon quaternary center, giving a set of molecules that maps well into many medicinally relevant compounds. In another application, we find that the cationic Co(I)-catalysts promote highly selective hydroacylation and 1,2-hydroboration of prochiral 1,3-dienes. Further, we find that a cationic Co(I)-catalyst promotes cycloisomerization followed by hydroalkenylation of 1,6-enynes to produce highly functionalized carbo- and heterocyclic compounds. Surprisingly the regioselectivity of the alkene addition depends on whether it is a simple alkene or an acrylate, and the acrylate addition produces an uncommon Z-adduct. This Account will provide a summary of the enabling basic discoveries and the attendant developments that led to the unique cationic Co(I)-complexes as catalysts for disparate C-C and C-B bond-forming reactions. It is our hope that this Account will stimulate further work with these highly versatile catalysts which are derived from an earth-abundant metal.

Asymmetric hydrogenation for the synthesis of 2-substituted chiral morpholines

Asymmetric hydrogenation of unsaturated morpholines has been developed by using a bisphosphine-rhodium catalyst bearing a large bite angle. With this approach, a variety of 2-substituted chiral morpholines could be obtained in quantitative yields and with excellent enantioselectivities (up to 99% ee). The hydrogenated products could be transformed into key intermediates for bioactive compounds.

2-Substituted chiral morpholines were synthesized via a newly developed asymmetric hydrogenation of dehydromorpholines catalyzed by a bisphosphine–rhodium complex bearing a large bite angle.

Nickel-catalyzed enantioselective umpolung hydrogenation for stereoselective synthesis of β-amido esters

Nickel complexes ligated by strongly donating diphosphines catalyze enantioselective hydrogenation for the preparation of acyclic and cyclic β-amido esters. A combination of acetic acid and indium powder provides protons and electrons to form nickel hydrido complexes under umpolung hydrogenation conditions.

Recent Advances in Asymmetric Hydrogenation Catalysis Utilizing Spiro and Other Rigid C-Stereogenic Phosphine Ligands

Transition-metal-catalyzed asymmetric reactions have been a powerful tool in organic synthesis for many years. The design of chiral ligands with the right configuration is fundamental to induce high regio- and stereoselectivity to catalytic reactions and to achieve high turnover numbers and high yields. A challenge is the control of prochiral centers with similar electronic properties in a similar steric environment within the same molecule. Over the last 10 years, a range of novel rigid C-stereogenic chiral phosphine ligands has been developed and successfully applied in various types of asymmetric transformations. Many of these ligands are of a di-, tri-, or multidentate nature. The purpose of this Perspective is to highlight recent synthetic achievements (since 2010) with spiro-phosphines and other rigid phosphines and discuss some mechanistic aspects of the catalytic reactions.

Enantioselective Synthesis of Pyridines with All-Carbon Quaternary Carbon Centers via Cobalt-Catalyzed Desymmetric [2+2+2] Cycloaddition

A Co-catalyzed enantioselective desymmetric [2+2+2] cycloaddition for synthesis of pyridines with all-carbon quaternary carbon centers has been developed. The regio- and enantioselectivities are controlled by the inherent nature of terminal alkynes and the substituents on the bisoxazolinephosphine ligands. Pyridines with 5-substitutents could be obtained with >20:1 regioselectivity and up to 94 % ee when terminal alkyl, alkenyl or silyl alkynes and DTBM/Ph-based NPN* ligand L6 were used. Terminal aryl alkynes and Ph/Bn-based ligand L4 leads to formation of pyridines with 6-substitutents in up to 99 % ee.

α- and β-Functionalized Ketones from 1,3-Dienes and Aldehydes: Control of Regio- and Enantioselectivity in Hydroacylation of 1,3-Dienes

Ketones are among the most widely used intermediates in organic synthesis, and their synthesis from inexpensive feedstocks could be quite impactful. Regio- and enantioselective hydroacylation reactions of dienes provide facile entry into useful ketone-bearing chiral motifs with an additional latent functionality (alkene) suitable for further elaboration. Three classes of dienes, 2- or 4-monosubstituted and 2,4-disubstituted 1,3-dienes, undergo cobalt(I)-catalyzed regio- and enantioselective hydroacylation, giving products with high enantiomeric ratios (er). These reactions are highly dependent on the ligands, and we have identified the most useful ligands and reaction conditions for each class of dienes. 2-Substituted and 2,4-disubstituted dienes predominantly undergo 1,2-addition, whereas 4-substituted terminal dienes give highly enantioselective 4,1- or 4,3-hydroacylation depending on the aldehyde, aliphatic aldehydes giving 4,1-addition and aromatic aldehydes giving 4,3-addition. Included among the substrates are feedstock dienes, isoprene (US$1.4/kg) and myrcene (US$129/kg), and several common aldehydes. We propose an oxidative dimerization mechanism that involves a Co(I)/Co(III) redox cycle that appears to be initiated by a cationic Co(I) intermediate. Studies of reactions using isolated neutral and cationic Co(I) complexes confirm the critical role of the cationic intermediates in these reactions. Enantioselective 1,2-hydroacylation of 2-trimethylsiloxy-1,3-diene reveals a hitherto undisclosed route to chiral siloxy-protected aldols. Finally, facile syntheses of the anti-inflammatory drug (S)-Flobufen (2 steps, 92% yield, >99:1 er) and the food additive (S)-Dihydrotagetone (1 step, 83% yield; 96:4 er) from isoprene illustrate the power of this method for the preparation of commercially relevant compounds.

Ligand Substitution and Electronic Structure Studies of Bis(phosphine)Cobalt Cyclooctadiene Precatalysts for Alkene Hydrogenation

Diene self-exchange reactions of the 17-electron, formally cobalt(0) cyclooctadienyl precatalyst, (R,R)-(^iPrDuPhos)Co(COD) (P ₂ CoCOD, (R,R)-^iPrDuPhos = 1,2-bis((2R,5R)-2,5-diisopropylphospholano)benzene, COD = 1,5-cyclooctadiene) were studied using natural abundance and deuterated 1,5-cyclooctadiene. Exchange of free and coordinated diene was observed at ambient temperature in benzene-d ₆ solution and kinetic studies support a dissociative process. Both neutral P ₂ CoCOD and the 16-electron, cationic cobalt(I) complex, [(R,R)-(^iPrDuPhos)Co(COD)][BAr^F ₄] (BAr^F ₄ = B[(3,5-(CF₃)₂)C₆H₃]₄) underwent instantaneous displacement of the 1,5-cyclooctadiene ligand by carbon monoxide and generated the corresponding carbonyl derivatives. The solid-state parameters, DFT-computed Mulliken spin density and analysis of molecular orbitals suggest an alternative description of P ₂ CoCOD as low-spin cobalt(II) with the 1,5-cyclooctadiene acting as a LX₂-type ligand. This view of the electronic structure provides insight into the nature of the ligand substitution processes and the remarkable stability of the neutral cobalt complexes toward protic solvents observed during catalytic alkene hydrogenation.

Cobalt-Catalyzed Chemo- and Enantioselective Hydrogenation of Conjugated Enynes

Asymmetric hydrogenation is one of the most powerful methods for the preparation of single enantiomer compounds. However, the chemo- and enantioselective hydrogenation of the relatively inert unsaturated group in substrates possessing multiple unsaturated bonds remains a challenge. We herein report a protocol for the highly chemo- and enantioselective hydrogenation of conjugated enynes while keeping the alkynyl bond intact. Mechanism studies indicate that the accompanying Zn²⁺ generated from zinc reduction of the Co^II complex plays a critical role to initiate a plausible Co^I /Co^III catalytic cycle. This approach allows for the highly efficient generation of chiral propargylamines (up to 99.9 % ee and 2000 S/C) and further useful chemical transformations.

Highly Enantioselective Cobalt-Catalyzed (3+2) Cycloadditions of Alkynylidenecyclopropanes

Low-valent cobalt complexes equipped with chiral ligands can efficiently promote highly enantioselective (3+2) cycloadditions of alkyne-tethered alkylidenecyclopropanes. The annulation allows to assemble bicyclic systems containing five-membered rings in good yields and with excellent enantiomeric ratios. We also present a mechanistic discussion based on experimental and computational data, which support the involvement of Co^I /Co^III catalytic cycles.

Fluoroarene Separations in Metal-Organic Frameworks with Two Proximal Mg2+ Coordination Sites

Fluoroarenes are widely used in medicinal, agricultural, and materials chemistry, and yet their production remains a critical challenge in organic synthesis. Indeed, the nearly identical physical properties of these vital building blocks hinders their purification by traditional methods, such as flash chromatography or distillation. As a result, the Balz-Schiemann reaction is currently employed to prepare fluoroarenes instead of more atom-economical C-H fluorination reactions, which produce inseparable mixtures of regioisomers. Herein, we propose an alternative solution to this problem: the purification of mixtures of fluoroarenes using metal-organic frameworks (MOFs). Specifically, we demonstrate that controlling the interaction of fluoroarenes with adjacent coordinatively unsaturated Mg²⁺ centers within a MOF enables the separation of fluoroarene mixtures with unparalleled selectivities. Liquid-phase multicomponent equilibrium adsorption data and breakthrough measurements coupled with van der Waals-corrected density functional theory calculations reveal that the materials Mg₂(dobdc) (dobdc^4- = 2,5-dioxidobenzene-1,4-dicarboxylate) and Mg₂(m-dobdc) (m-dobdc^4- = 2,4-dioxidobenzene-1,5-dicarboxylate) are capable of separating the difluorobenzene isomers from one another. Additionally, these frameworks facilitate the separations of fluoroanisoles, fluorotoluenes, and fluorochlorobenzenes. In addition to enabling currently unfeasible separations for the production of fluoroarenes, our results suggest that carefully controlling the interaction of isomers with not one but two strong binding sites within a MOF provides a general strategy for achieving challenging liquid-phase separations.

Asymmetric hydrogenation catalyzed by first-row transition metal complexes

This review focuses on asymmetric direct and transfer hydrogenation with first-row transition metal complexes. The reaction mechanisms and the models of enantiomeric induction were summarized and emphasized.

Recent Developments in Transition-Metal-Catalyzed Asymmetric Hydrogenation of Enamides

The catalytic asymmetric hydrogenation of prochiral olefins is one of the most widely studied and utilized transformations in asymmetric synthesis. This straightforward, atom economical, inherently direct and sustainable strategy induces chirality in a broad range of substrates and is widely relevant for both industrial applications and academic research. In addition, the asymmetric hydrogenation of enamides has been widely used for the synthesis of chiral amines and their derivatives. In this review, we summarize the recent work in this field, focusing on the development of new catalytic systems and on the extension of these asymmetric reductions to new classes of enamides.

1 Introduction

2 Asymmetric Hydrogenation of Trisubstituted Enamides

2.1 Ruthenium Catalysts

2.2 Rhodium Catalysts

2.3 Iridium Catalysts

2.4 Nickel Catalysts

2.5 Cobalt Catalysts

3 Asymmetric Hydrogenation of Tetrasubstituted Enamides

3.1 Ruthenium Catalysts

3.2 Rhodium Catalysts

3.3 Nickel Catalysts

4 Asymmetric Hydrogenation of Terminal Enamides

4.1 Rhodium Catalysts

4.2 Cobalt Catalysts

5 Rhodium-Catalyzed Asymmetric Hydrogenation of Miscellaneous Enamides

6 Conclusions

Enantioselective Cobalt-Catalyzed Intermolecular Hydroacylation of 1,6-Enynes

We report a cobalt-catalyzed hydroacylation of 1,6-enynes with exogenous aldehydes in a domino sequence to construct enantioenriched ketones. The products were obtained in good yields with excellent regio-, diastereo-, and enantioselectivity. Furthermore, the chiral products served as valuable precursors to access complex spirocyclic scaffolds with three contiguous stereocenters. The asymmetric hydroacylation process exhibited no C-H crossover and no KIE, thus indicating that the C-H bond cleavage was not involved in the turnover-limiting step.

Cobalt/Lewis Acid Catalysis for Hydrocarbofunctionalization of Alkynes via Cooperative C-H Activation

A catalytic system comprising a cobalt-diphosphine complex and a Lewis acid (LA) such as AlMe₃ has been found to promote hydrocarbofunctionalization reactions of alkynes with Lewis basic and electron-deficient substrates such as formamides, pyridones, pyridines and related azines, imidazo[1,2-a]pyridines, and azole derivatives through site-selective C-H activation. Compared with known Ni/LA catalytic systems for analogous transformations, the present catalytic systems not only feature convenient setup using inexpensive and bench-stable precatalyst and ligand such as Co(acac)₃ and 1,3-bis(diphenylphosphino)propane (dppp) but also display distinct site-selectivity toward C-H activation of pyridone and pyridine derivatives. In particular, a completely C4-selective alkenylation of pyridine has been achieved for the first time. Meanwhile, the present catalytic system proved to promote exclusively C5-selective alkenylation of imidazo[1,2-a]pyridine derivatives. Mechanistic studies including DFT calculations on the Co/Al-catalyzed addition of formamide to alkyne have suggested that the reaction involves cleavage of the carbamoyl C-H bond as the rate-limiting step, which proceeds through a ligand-to-ligand hydrogen transfer (LLHT) mechanism leading to an alkenyl(carbamoyl)cobalt intermediate.

Cobalt-catalyzed highly enantioselective hydrogenation of α,β-unsaturated carboxylic acids

Asymmetric hydrogenation of α,β-unsaturated acids catalyzed by noble metals has been well established, whereas, the asymmetric hydrogenation with earth-abundant-metal was rarely reported. Here, we describe a cobalt-catalyzed asymmetric hydrogenation of α,β-unsaturated carboxylic acids. By using chiral cobalt catalyst bearing electron-donating diphosphine ligand, high activity (up to 1860 TON) and excellent enantioselectivity (up to >99% ee) are observed. Furthermore, the cobalt-catalyzed asymmetric hydrogenation is successfully applied to a broad spectrum of α,β-unsaturated carboxylic acids, such as various α-aryl and α-alkyl cinnamic acid derivatives, α-oxy-functionalized α,β-unsaturated acids, α-substituted acrylic acids and heterocyclic α,β-unsaturated acids (30 examples). The synthetic utility of the protocol is highlighted by the synthesis of key intermediates for chiral drugs (6 cases). Preliminary mechanistic studies reveal that the carboxy group may be involved in the control of the reactivity and enantioselectivity through an interaction with the metal centre.

A large number of marketed drugs contains a chiral carboxylic acid scaffold. Here, the authors report the asymmetric hydrogenation of α,β-unsaturated carboxylic acids to α-chiral carboxylic acids using a cobalt catalyst bearing an electron-donating chiral diphosphine ligand.

Synthesis and Reactivity of Organometallic Intermediates Relevant to Cobalt-Catalyzed Hydroformylation

Intermediates relevant to cobalt-catalyzed alkene hydroformylation have been isolated and evaluated in fundamental organometallic transformations relevant to aldehyde formation. The 18-electron (R,R)-(^iPr DuPhos)Co(CO)₂ H has been structurally characterized, and it promotes exclusive hydrogenation of styrene in the presence of 50 bar of H₂ /CO gas (1:1) at 100 °C. Deuterium-labeling studies established reversible 2,1-insertion of styrene into the Co-D bond of (R,R)-(^iPr DuPhos)Co(CO)₂ D. Whereas rapid β-hydrogen elimination from cobalt alkyls occurred under an N₂ atmosphere, alkylation of (R,R)-(^iPr DuPhos)Co(CO)₂ Cl in the presence of CO enabled the interception of (R,R)-(^iPr DuPhos)Co(CO)₂ C(O)CH₂ CH₂ Ph, which upon hydrogenolysis under 4 atm H₂ produced the corresponding aldehyde and cobalt hydride, demonstrating the feasibility of elementary steps in hydroformylation. Both the hydride and chloride derivatives, (X=H^- , Cl^- ), underwent exchange with free ¹³ CO. Under reduced pressure, (R,R)-(^iPr DuPhos)Co(CO)₂ Cl underwent CO dissociation to form (R,R)-(^iPr DuPhos)Co(CO)Cl.

Cobalt-Catalyzed Enantioselective Hydroarylation of 1,6-Enynes

An asymmetric hydroarylative cyclization of enynes involving a C-H bond cleavage is reported. The cobalt-catalyzed cascade generates three new bonds in an atom-economical fashion. The products were obtained in excellent yields and excellent enantioselectivities as single diastereo- and regioisomers. Preliminary mechanistic studies indicate that the reaction shows no intermolecular C-H crossover. This work highlights the potential of cobalt catalysis in C-H bond functionalization and enantioselective domino reactivity.

A Structurally Characterized Cobalt(I) σ-Alkane Complex

A cobalt σ-alkane complex, [Co(Cy2 P(CH2 )4 PCy2 )(norbornane)][BArF 4 ], was synthesized by a single-crystal to single-crystal solid/gas hydrogenation from a norbornadiene precursor, and its structure was determined by X-ray crystallography. Magnetic data show this complex to be a triplet. Periodic DFT and electronic structure analyses revealed weak C-H→Co σ-interactions, augmented by dispersive stabilization between the alkane ligand and the anion microenvironment. The calculations are most consistent with a η1 :η1 -alkane binding mode.