Manganese- or iron-catalyzed homocoupling of grignard reagents using atmospheric oxygen as an oxidant

Nickel-Electrocatalyzed Synthesis of Bifuran-Based Monomers

Bifuran motifs can be accessed with nickel-bipyridine electrocatalyzed homocouplings of bromine-substituted methyl furancarboxylates, which, in turn, can be prepared from hemicellulose-derived furfural. The described protocol uses sustainable carbon-based graphite electrodes in the simplest setup - an undivided cell with constant current electrolysis. The reported method avoids using a sacrificial anode by employing triethanolamine as an electron donor.

Electrochemical deoxygenative homo-couplings of aromatic aldehydes

An electrochemical deoxygenative homo-coupling of aromatic aldehydes is achieved to selectively access bibenzyl and stilbene derivatives. The protocol allows the homo-coupling of aldehydes to occur after single-electron-reduction at the cathode. Taking advantage of the oxophilicity of triphenylphosphine, the electrochemical deoxygenation proceeds smoothly to give reductive homo-coupling products.

Homocoupling of benzyl pyridyl ethers via visible light-mediated deoxygenation

The photocatalytic coupling of ethers is uncommon because of the challenges in breaking C-O bonds and low selectivity. Herein, we report a visible light-mediated deoxygenation homocoupling of benzyl pyridyl ethers via their pyridium salts. This approach enables C(sp3)-O bond homolysis under mild conditions. Mechanistic experiments support the radical nature of the reaction. This method is highly compatible with electron-withdrawing groups and has potential applications for drug precursor synthesis.

Organolithium aggregation as a blueprint to construct polynuclear lithium nickelate clusters

By exploiting the high aggregation of aliphatic lithium acetylides, here we report the synthesis and structural analysis of polynuclear lithium nickelate clusters in which up to 10 equivalents of organolithium can co-complex per Ni(0) centre. Exposure of the Ni(0)-ate clusters to dry air provides an alternative route to homoleptic Ni(II)-ates.

P(III)-Promoted Reductive Coupling of Aromatic and Aliphatic Nitro Compounds with Grignard Reagents

A phosphine-promoted reductive coupling of nitro compounds with Grignard reagents is described. Polyfunctional and pharmaceutically relevant diarylamines were generated by this reaction in moderate to high yields. Aliphatic nitro compounds that are highly challenging substrates undergo a combination of α-arylation and reductive coupling to afford the α-arylated arylamines efficiently. A series of valuable biaryl compounds with polyfluorinated and heteroaryl rings are co-generated in 56-94% yields.

Mechanistic Facets of the Competition between Cross-Coupling and Homocoupling in Supporting Ligand-Free Iron-Mediated Aryl–Aryl Bond Formations

In the context of cross-coupling chemistry, the competition between the cross-coupling path itself and the oxidative homocoupling of the nucleophile is a classic issue. In that case, the electrophilic partner acts as a sacrificial oxidant. We investigate in this report the factors governing the cross- versus homocoupling distribution using aryl nucleophiles ArMgBr and (hetero)aryl electrophiles Ar′Cl in the presence of an iron catalyst. When electron-deficient electrophiles are used, a key transient heteroleptic [Ar₂Ar′Fe^II]⁻ complex is formed. DFT calculations show that an asynchronous two-electron reductive elimination follows, which governs the selective evolution of the system toward either a cross- or homocoupling product. Proficiency of the cross-coupling reductive elimination strongly depends on both π-accepting and σ-donating effects of the Fe^II-ligated Ar′ ring. The reactivity trends discussed in this article rely on two-electron elementary steps, which are in contrast with the usually described tendencies in iron-mediated oxidative homocouplings which involve single-electron transfers. The results are probed by paramagnetic ¹H NMR spectroscopy, experimental kinetics data, and DFT calculations.

KOtBu-promoted C3-homocoupling of quinoxalinones through single electron transfer from an sp2 carbanion intermediate

The C3-selective homodimerization of quinoxalinones is described. A C3-sp² carbanion species generated through deprotonation of quinoxalinone using potassium tert-butoxide (KO^tBu) transfers an electron (single electron transfer mechanism) to a second quinoxalinone, affording a radical-anion intermediate. The radical scavenging and electron paramagnetic resonance (EPR) experiments support the plausible radical reaction pathway. A mild reaction temperature and a short reaction time were attained under cost-effective conditions, which reveal the amenability of this protocol to pharmaceutical and chemical industries.

Accessing Grignard Reluctant Aldehyde in 2-Oxoaldehyde by Organocuprates to Give [1,2] Addition and Oxidative Coupling Reactions

Novel finding of aldehyde in 2-oxoaldehyde (2OA) is presented as it unprecedentedly disinclines to react with Grignard reagents but reacts with moderate organocuprate reagents in anaerobic condition to give [1,2] addition (α-hydroxyketones) reaction. In the presence of air, the reaction produces an efficient protocol for the synthesis of 1,2-diones through a copper-catalyzed oxidative cross-coupling reaction at room temperature. Mechanistic studies indicate that α-hydroxy ketone perhaps is generated before the hydrolysis step/acid work-up process. The α-keto group of 2OA causes to exhibit this peculiar aldehyde behavior toward these organometallic reagents.

Synthesis of Conjugated Dienes in Natural Compounds

This review describes the various synthetic methods commonly used to obtain molecules possessing conjugated dienes. We focus on methods involving cross-coupling reactions using various metals such as nickel, palladium, ruthenium, cobalt, cobalt/zinc, manganese, zirconium, or iron, mainly through examples that aimed to access natural molecules or their analogues. Among the natural molecules covered in this review, we discuss the total synthesis of a phytohormone, Acid Abscisic (ABA), carried out by our team involving the development of a conjugated diene chain.

Porphyrin N-Pincer Pd(II)-Complexes in Water: A Base-Free and Nature-Inspired Protocol for the Oxidative Self-Coupling of Potassium Aryltrifluoroborates in Open-Air

Metalloporphyrins (and porphyrins) are well known as pigments of life in nature, since representatives of this group include chlorophylls (Mg-porphyrins) and heme (Fe-porphyrins). Hence, the construction of chemistry based on these substances can be based on the imitation of biological systems. Inspired by nature, in this article we present the preparation of five different porphyrin, meso-tetraphenylporphyrin (TPP), meso-tetra(p-anisyl)porphyrin (TpAP), tetrasodium meso-tetra(p-sulfonatophenyl)porphyrin (TSTpSPP), meso-tetra(m-hydroxyphenyl)porphyrin (TmHPP), and meso-tetra(m-carboxyphenyl)porphyrin (TmCPP) as well as their N-pincer Pd(II)-complexes such as Pd(II)-meso-tetraphenylporphyrin (PdTPP), Pd(II)-meso-tetra(p-anisyl)porphyrin (PdTpAP), Pd(II)-tetrasodium meso-tetra(p-sulfonatophenyl)porphyrin (PdTSTpSPP), Pd(II)-meso-tetra(m-hydroxyphenyl)porphyrin (PdTmHPP), and Pd(II)-meso-tetra(m-carboxyphenyl)porphyrin (PdTmCPP). These porphyrin N-pincer Pd(II)-complexes were studied and found to be effective in the base-free self-coupling reactions of potassium aryltrifluoroborates (PATFBs) in water at ambient conditions. The catalysts and the products (symmetrical biaryls) were characterized using their spectral data. The high yields of the biaryls, the bio-mimicking conditions, good substrate feasibility, evading the use of base, easy preparation and handling of catalysts, and the application of aqueous media, all make this protocol very attractive from a sustainability and cost-effective standpoint.

Eco-friendly methoximation of aromatic aldehydes and ketones using MnCl2.4H2O as an easily accessible and efficient catalyst

Methoximes are important as a class of intermediates and products, among fine chemicals and specialties. The development of a new, facile and efficient method for their synthesis is reported. The methoximes were properly accessed from the corresponding aromatic aldehydes and ketones in good to excellent yields, under mild conditions, employing the inexpensive and environmentally friendly MnCl2.4H2O as a catalyst (at low loading and without the addition of ligand), in EtOH at 50°C. The scope of the process was systematically assessed.

Iron(II)-Catalyzed Aerobic Biomimetic Oxidation of Amines using a Hybrid Hydroquinone/Cobalt Catalyst as Electron Transfer Mediator

Herein we report the first Fe^II‐catalyzed aerobic biomimetic oxidation of amines. This oxidation reaction involves several electron transfer steps and is inspired by biological oxidation in the respiratory chain. The electron transfer from the amine to molecular oxygen is aided by two coupled catalytic redox systems, which lower the energy barrier and improve the selectivity of the oxidation reaction. An iron hydrogen transfer complex was utilized as the substrate‐selective dehydrogenation catalyst along with a bifunctional hydroquinone/cobalt Schiff base complex as a hybrid electron transfer mediator. Various primary and secondary amines were oxidized in air to their corresponding aldimines or ketimines in good to excellent yield.

Cyrene™ as a Neoteric Bio-Based Solvent for Catalyst-Free Microwave-Assisted Construction of Diverse Bipyridine Analogues for Heavy-Metal Sensing

An environment-friendly synthetic protocol was developed to access polyfunctionalized bipyridines from readily available amines, chromone-3-carboxaldehydes, and pyridinylacetonitriles under catalyst- and additive-free conditions using the bio-renewable neoteric solvent dihydrolevoglucosenone (Cyrene™). In this strategy, amines served as both a mild-base promoter and a substrate. In addition, water was the only by-product of this reaction. This multi-component protocol provided highly diverse 2,3-, 3,3-, and 3,4-bipyridines in good-to-excellent yields. Operational simplicity, short reaction time, excellent atom economy, and easily obtainable substrates are among the features of this microwave-assisted synthesis. Additionally, the compounds synthesized via this method have demonstrated the ability to detect heavy metals, specifically mercury(II), copper(II), and iron (III) ions.

Tandem Mn-I Exchange and Homocoupling Processes Mediated by a Synergistically Operative Lithium Manganate

Pairing lithium and manganese(II) to form lithium manganate [Li₂ Mn(CH₂ SiMe₃ )₄ ] enables the efficient direct Mn-I exchange of aryliodides, affording transient (aryl)lithium manganate intermediates which in turn undergo spontaneous C-C homocoupling at room temperature to furnish symmetrical (bis)aryls in good yields under mild reaction conditions. The combination of EPR with X-ray crystallographic studies has revealed the mixed Li/Mn constitution of the organometallic intermediates involved in these reactions, including the homocoupling step which had previously been thought to occur via a single-metal Mn aryl species. These studies show Li and Mn working together in a synergistic manner to facilitate both the Mn-I exchange and the C-C bond-forming steps. Both steps are carefully synchronized, with the concomitant generation of the alkyliodide ICH₂ SiMe₃ during the Mn-I exchange being essential to the aryl homocoupling process, wherein it serves as an in situ generated oxidant.

Iron(II)-Catalyzed Biomimetic Aerobic Oxidation of Alcohols

We report the first FeII -catalyzed biomimetic aerobic oxidation of alcohols. The principle of this oxidation, which involves several electron-transfer steps, is reminiscent of biological oxidation in the respiratory chain. The electron transfer from the alcohol to molecular oxygen occurs with the aid of three coupled catalytic redox systems, leading to a low-energy pathway. An iron transfer-hydrogenation complex was utilized as a substrate-selective dehydrogenation catalyst, along with an electron-rich quinone and an oxygen-activating Co(salen)-type complex as electron-transfer mediators. Various primary and secondary alcohols were oxidized in air to the corresponding aldehydes or ketones with this method in good to excellent yields.

Generation and Reactivity of Amidyl Radicals: Manganese-Mediated Atom-Transfer Reaction

A simple and efficient protocol to generate amidyl radicals from amine functionalities through a manganese-mediated atom-transfer reaction has been developed. This approach employs an earth-abundant and inexpensive manganese complex, Mn₂ (CO)₁₀ , as the catalyst and visible light as the energy input. Using this strategy, site-selective chlorination of unactivated C(sp³ )-H bonds of aliphatic amines and intramolecular/intermolecular chloroaminations of unactivated alkenes were readily realized under mild reaction conditions, thus providing efficient access to a range of synthetically valuable alkyl chlorides, chlorinated pyrrolidines, and vicinal chloroamine derivatives. These practical reactions exhibit a broad substrate scope and tolerate a wide array of functional groups, and complex molecules including various marketed drug derivatives.

Iron catalyzed stereoselective alkene synthesis: a sustainable pathway

Replacing expensive or toxic transition metals with iron has become an important trend. This article summarises the recent progresses of a wide range of Fe-catalyzed reactions for accessing various stereodefined alkenes.

Cu(II)-Catalyzed Homocouplings of (Hetero)Arylboronic Acids with the Assistance of 2-O-Methyl-d-Glucopyranose

This is the first report of a natural ligand improving the copper-catalyzed homocouplings of (hetero)arylboronic acids. Various important synthetic biaryl intermediates in organic synthesis could be assembled via this method. To gain insight into this reaction, in situ React IR technology was used to confirm the effectivity of this catalyst system. This protocol could provide important biaryl compounds in high yields within a short time.

Aerobic and Ligand-Free Manganese-Catalyzed Homocoupling of Arenes or Aryl Halides via in Situ Formation of Aryllithiums

Ligand-free manganese-catalyzed homocoupling of arenes or aryl halides can be carried out under aerobic conditions via the in situ formation of the corresponding aryllithiums. A wide range of biaryls and derivatives has been obtained, and a mechanism involving monomeric manganese-oxo complexes has been proposed on the basis of DFT calculations.

Ligand-Free Iron-Catalyzed Carbon (sp2)-Carbon (sp2) Oxidative Homo-Coupling of Alkenyllithiums

A new strategy was developed for the efficient synthesis of di-, tetra-, and hexa-substituted 1,3-butadienes. This one-pot procedure involves lithium-iodine exchange to generate the corresponding vinyllithium intermediates. A subsequent iron-catalyzed ligand-free oxidative homo-coupling eventually led to the formation of 1,3-butadienes in acceptable to excellent isolated yields.

3d Transition Metals for C-H Activation

C-H activation has surfaced as an increasingly powerful tool for molecular sciences, with notable applications to material sciences, crop protection, drug discovery, and pharmaceutical industries, among others. Despite major advances, the vast majority of these C-H functionalizations required precious 4d or 5d transition metal catalysts. Given the cost-effective and sustainable nature of earth-abundant first row transition metals, the development of less toxic, inexpensive 3d metal catalysts for C-H activation has gained considerable recent momentum as a significantly more environmentally-benign and economically-attractive alternative. Herein, we provide a comprehensive overview on first row transition metal catalysts for C-H activation until summer 2018.

One-Pot Sequential Kinetic Profiling of a Highly Reactive Manganese Catalyst for Ketone Hydroboration: Leveraging σ-Bond Metathesis via Alkoxide Exchange Steps

A comprehensive experimental and computational mechanistic study of the highly enantioselective hydroboration of ketones catalyzed by a manganese(II) alkyl boxmi pincer complex is reported. The catalyst operates at low catalyst loadings (down to 0.01 mol %) under very mild conditions (typically -40 °C) and facilitates the reduction of both aryl alkyl and dialkyl ketones with excellent selectivity (up to >95%ee). Catalyst activation pathways were investigated, demonstrating that a manganese(II) hydride and a manganese(II) alkoxide species are part of the catalytic cycle and can be generated via σ-bond metathesis of the alkyl precursor with the borane or by alcoholysis. Extensive kinetic experiments based on a "one-pot sequential kinetic profiling" approach under various conditions in combination with kinetic simulations reveal that two catalytic cycles are effective with this earth-abundant base metal catalyst: (i) a minor MnH/borane-mediated insertion cycle, in which the subsequent, product-releasing metathesis step is rate determining ( k _m = 0.076 s^-1), giving a background reaction, which is zeroth order in substrate concentrations, and (ii) a major MnOR/borane-based alkoxide exchange process, leveraging the high-barrier metathesis via the affiliation to an insertion step. The latter features non-integer reaction orders in both reagents due to a combination of an adduct formation step ( k _a = 2.12 M^-1 s^-1, k _-a = 0.49 s^-1) and a substrate insertion step of comparable rates ( k _ai = 3.74 M^-1 s^-1). The kinetic findings are underpinned by high-level density functional theory calculations of the mechanism, control experiments, and kinetic isotope effect/Hammett/Eyring analysis in different concentration regimes. The study highlights the role of a rigorous mechanistic understanding of homogeneous catalytic processes in 3d metals for rational catalyst discovery and optimization.

Perfluorocyclopentadienyl Radical Derivative as an Organocatalyst for Oxidative Coupling of Aryl- and Thienylmagnesium Compounds under Atmospheric Oxygen

The oxidative homocoupling reaction of Grignard reagents in the presence of atmospheric oxygen molecules proceeded in the presence of a heptafluorotolyl-substituted perfluorocyclopentadienyl radical. The turnover number (TON) was over 30 for the coupling reactions of PhMgBr to give biphenyl. The organocatalyst could couple thienylmagnesium compounds to give bithiophene derivatives in up to 94% yield. Furthermore, a gram-scale synthesis of 6,6'-dimethoxybiphenyl-2,2'-diyl-bis(phosphonic acid diethyl ester) was demonstrated. Stabilization of the phenyl radical for the inhibition of the side reaction was also considered using DFT calculations.

A Copper-Catalyzed Reductive Defluorination of β-Trifluoromethylated Enones via Oxidative Homocoupling of Grignard Reagents

An efficient copper-catalyzed reductive defluorination of β-trifluoromethylated enones via an oxidative homocoupling of Grignard reagents is reported. The reaction proceeded smoothly with a wide range of substrates, including the ones bearing aromatic heterocycles, such as furyl and thienyl ring systems in high yields (80-92%, except one example) under mild conditions. This provides a practical method for synthesis of gem-difluoroolefin ketones. It is also worth noting that this homocoupling process of Grignard reagents using β-trifluoromethylated enones as "oxidizing reagents" is effective for both the Csp²-Csp² and Csp³-Csp³ bond formations, including for substrates whose β-hydride elimination of the corresponding transition metal alkyl complex is particularly facile, affording coupling products with high yields (83-95%), simultaneously.

Fe-polyaniline composite nanofiber catalyst for chemoselective hydrolysis of oxime

A facile chemoselective one-pot strategy for the deprotection of oxime has been developed using Fe⁰-polyaniline composite nanofiber (Fe⁰-PANI), as a catalyst. Nano material based Fe⁰-PANI catalyst has been synthesized via in-situ polymerization of ANI monomer and followed by reductive deposition of Fe⁰ onto PANI matrix. The catalyst was characterized by FE-SEM, HR-TEM, BET, XRD, ATR-FTIR, XPS and VSM techniques. The scope of the transformation was studied for aryl, alkyl and heteroarylketoxime with excellent chemoselectivity (>99%). Mechanistic investigations suggested the involvement of a cationic intermediate with Fe³⁺ active catalytic species. Substituent effect showed a linear free energy relationship. The activation energy (E_a) was calculated to be 17.46 kJ mol^-1 for acetophenone oxime to acetophenone conversion. The recyclability of the catalyst demonstrated up to 10 cycles without any significant loss of efficiency. Based on the preliminary experiments a plausible mechanism has been proposed involving a carbocationic intermediate.

Ruthenium-catalyzed decarboxylative C–S cross-coupling of carbonothioate: synthesis of allyl(aryl)sulfide

A novel ruthenium-catalyzed decarboxylative cross-coupling of carbonothioate is disclosed. This method provides straightforward access to the corresponding allyl(aryl)sulfide derivatives in generally good to excellent yields under mild conditions and features a broad substrate scope, wide group tolerance and in particular, no need to use halocarbon precursors.

A method for the construction of a C–S bond via the ruthenium-catalyzed decarboxylative cross-coupling of carbonothioate under mild conditions is described.

Syntheses, Properties, and Catalytic Activities of Metal(II) Complexes and Free Bases of Redox-Switchable 20π, 19π, and 18π 5,10,15,20-Tetraaryl-5,15-diazaporphyrinoids

In spite of significant advances in redox-active porphyrin-based materials and catalysts, little attention has been paid to 20π and 19π porphyrins because of their instability in air. Here we report the meso-modification of 5,10,15,20-tetraarylporphyrin with two nitrogen atoms, which led to redox-switchable 20π, 19π, and 18π 5,10,15,20-tetraaryl-5,15-diazaporphyrinoids (TADAPs). Three kinds of metal(II) complexes and free bases of TADAP were prepared by the metal-templated annulation of the corresponding metal-bis(dipyrrin) complexes. The inductive and resonance effects of the meso-nitrogen atoms on the aromatic, optical, electrochemical, and magnetic properties of the entire TADAP π-systems were assessed by using various spectroscopic measurements and density functional theory calculations. The aromaticity and π-π* electronic transition energies of the TADAPs varied considerably, and were shown to be dependent on the oxidation states of the π-systems. In contrast to the isoelectronic 5,10,15,20-tetraarylporphyrin derivatives, the 20π and 19π TADAPs were chemically stable under air. In particular, the 19π TADAP radical cations were extremely stable towards dioxygen, moisture, and silica gel. This reflected the low-lying singly occupied molecular orbitals of their π-systems and the efficient delocalization of their unshared electron spin. The capability of MgTADAP to catalyze aerobic biaryl formation from aryl Grignard reagents was demonstrated, which presumably involved a 19π/20π redox cycle.