Fe-Catalyzed Reductive Couplings of Terminal (Hetero)Aryl Alkenes and Alkyl Halides under Aqueous Micellar Conditions

Si-Cl σ-Bond Cleavage by an Fe(0) Complex with Two Steps of One-Electron Transfer toward Hydrosilane Formation from Tetrachlorosilane

Iron catalysts exhibit unique reactivity that has not been found in precious metal catalysts. Herein, a new strategy for strong Si-Cl σ-bond cleavage of tetrachlorosilane (SiCl4) was successful using [{Fe(PNNP)}2(μ-N2)] (PNNP = 2,9-bis((diphenylphosphino)methyl)-1,10-phenanthroline). The resulting oxidative addition product [Fe(SiCl3)(Cl)(PNNP)] was fully identified, establishing the first example of Si-Cl σ-bond cleavage using an air-sensitive iron(0) complex. Theoretical study revealed that Si-Cl σ-bond cleavage occurs with radical character through two steps of single-electron transfer from [Fe(PNNP)] to the Si-Cl σ* antibonding orbital, which differs from usual concerted oxidative addition. This σ-bond cleavage reaction was successfully applied to HSiCl3 formation from SiCl4 and 1,10-dihydroanthracene via hydrogen atom transfer (HAT). Thus, we succeeded in performing Si-Cl σ-bond cleavage using the Fe(0) complex and its application to hydrosilane synthesis from SiCl4 using a mild hydrogen source. This iron(0) complex is expected to be active as catalyst for other difficult reaction.

Fe/Mn-Synergistic Promoted C(sp3)-Bi Cross-Coupling of Alkyl Chlorides with Chlorobismuthanes to Access Air-Stable Alkylbismuthanes

Alkylating bismuthanes containing C(sp3)-Bi bonds were first disclosed as early as 1850, but their development remains severely limited, especially the synthetic method. Herein, we developed the first Fe/Mn-synergistic promoted C(sp3)-Bi cross-coupling of alkyl halides with chlorobismuthanes, which can access alkylbismuthanes in satisfactory yields with various functional group compatibility (cyano, ester, aldehyde, and amide). Inactivated alkyl chlorides/bromides, as well as cyclic and acyclic alkyl electrophiles, smoothly yielded the corresponding products. Gram-scale synthesis and late-stage modification of drug molecules were also conducted. The synthesized alkylbismuthanes also could be alkylation reagents toward C(sp3)-C(sp2), C(sp3)-Se, C(sp3)-S, and C(sp3)-N bonds.

The Progress of Reductive Coupling Reaction by Iron Catalysis

The transition metal catalyzed coupling reaction has revolutionized the strategies for forging the carbon-carbon bonds. In contrast to traditional cross-coupling methods using pre-prepared nucleophilic organometallic reagents, reductive coupling reactions for the C-C bonds formation provide some advantages. Because both coupling partners are reduced in the final products using a stoichiometric amount of a reductant, this approach not only avoids the need to use sensitive organometallic species, but also provides an orthogonal and complementary access to classical coupling reaction. Notably, the reductive coupling reactions feature readily available fragments, promote good step economy, exhibit high functional group tolerance and unique chemoselectivity, which have propelled their increasingly popular in the organic synthesis. In recent years, due to the low price, minimal toxicity, and environmentally benign character, iron-catalyzed carbon-carbon coupling reactions have garnered significant attention from the organic synthetic chemists and pharmacologists, especially the iron-catalyzed reductive coupling. This review aims to provide an insightful overview of recent advances in iron-catalyzed reductive coupling reactions, and to illustrate their possible reaction mechanisms.

Hydroxylamine-Mediated C(sp2)-H Trifluoromethylation of Terminal Alkenes

Introduction of the trifluoromethyl (CF3) group into organic compounds has garnered substantial interest because of its significant role in pharmaceuticals and agrochemicals. Here, we report a hydroxylamine-mediated radical process for C(sp2)-H trifluoromethylation of terminal alkenes. The reaction shows good reactivity, impressive E/Z selectivity (up to >20 : 1), and broad functional group compatibility. Expansion of this approach to perfluoroalkylation and late-stage trifluoromethylation of bioactive molecules demonstrates its promising application potential. Mechanistic studies suggest that the reaction follows a radical addition and subsequent elimination pathway.

Green imine synthesis from amines using transition metal and micellar catalysis

Imines are a versatile class of chemicals with applications in pharmaceuticals and as synthetic intermediates. While imines are conventionally synthesized via aldehyde-amine condensation, their direct preparation from amines can avoid the need for the independent preparation of the aldehyde coupling partner and associated constraints with regard to aldehyde storage and purification. The direct preparation of imines from amines typically utilizes transition metal catalysis and is often well-aligned with green chemistry principles. This review provides a comprehensive overview of transition metal catalysed imine synthesis, with a particular focus on the copper-catalyzed oxidative coupling of amines. The emerging application of micellar catalysis for imine synthesis is also surveyed due to its potential to avoid the use of hazardous solvents and intensify these reactions through reduced catalyst loadings and locally increased reactant concentrations. Future directions relating to the confluence of these two areas are proposed towards the more sustainable preparation of imines.

Iron-Catalyzed Reductive Cross-Coupling of Alkyl Electrophiles with Olefins

In terms of its abundance and its minimal toxicity, iron has advantages relative to other transition metals. Although alkyl-alkyl bond construction is central to organic synthesis, examples of iron-catalyzed alkyl-alkyl couplings of alkyl electrophiles are relatively sparse. Herein we report an iron catalyst that achieves cross-coupling reactions of alkyl electrophiles wherein olefins, in the presence of a hydrosilane, are used in place of alkylmetal reagents. Carbon-carbon bond formation proceeds at room temperature, and the method employs commercially available components (Fe(OAc)2 , Xantphos, and Mg(OEt)2 ); interestingly, this set of reagents can be applied directly to a distinct hydrofunctionalization of olefins, hydroboration. Mechanistic studies are consistent with the generation of an alkyl radical from the alkyl electrophile, as well as with reversibility for elementary steps that precede carbon-carbon bond formation (olefin binding to iron and β-migratory insertion).

Regioselective Hydroalkylation of Vinyl- and Allylsilanes as Well as Vinylgermanes under Ni-H Catalysis

A Ni-H-catalyzed hydroalkylation of vinylsilanes and -germanes as well as allylsilanes with unactivated alkyl iodides is reported. Unlike related reactions of styrene or vinyl boronate esters, the addition across the C-C double bond proceeds with anti-Markovnikov selectivity to deliver the linear regioisomer. Mechanistic control experiments support a radical mechanism, and a competition experiment reveals that the chemoselectivity is in favor of the vinyl over the allyl group.

Deep Electroreductive Chemistry: Harnessing Carbon- and Silicon-based Reactive Intermediates in Organic Synthesis

This Viewpoint outlines our recent contribution in electroreductive synthesis. Specifically, we leveraged deeply reducing potentials provided by electrochemistry to generate radical and anionic intermediates from readily available alkyl halides and chlorosilanes. Harnessing the distinct reactivities of radicals and anions, we have achieved several challenging transformations to construct C-C, C-Si, and Si-Si bonds. We highlight the mechanistic design principle that underpinned the development of each transformation and provide a view forward on future opportunities in growing area of reductive electrosynthesis.

Fully Aqueous and Air-Compatible Cross-Coupling of Primary Alkyl Halides with Aryl Boronic Species: A Possible and Facile Method

Aqueous transformations confer many advantages, including decreased environmental impact and increased opportunity for biomolecule modulation. Although several studies have been conducted to enable the cross-coupling of aryl halides in aqueous conditions, until now a process for the cross-coupling of primary alkyl halides in aqueous conditions was missing from the catalytic toolbox and considered impossible. Alkyl halide coupling in water suffers from severe problems. The reasons for this include the strong propensity for β-hydride elimination, the need for highly air- and water-sensitive catalysts and reagents, and the intolerance of many hydrophilic groups to cross-coupling conditions. Here, we report a broadly applicable and readily accessible process for the cross-coupling of water-soluble alkyl halides in water and air by using simple and commercially available bench-stable reagents. The trisulfonated aryl phosphine TXPTS in combination with a water-soluble palladium salt Na2PdCl4 allowed for the Suzuki-Miyaura coupling of water-soluble alkyl halides with aryl boronic acids, boronic esters, and borofluorate salts in mild, fully aqueous conditions. Multiple challenging functionalities, including unprotected amino acids, an unnatural halogenated amino acid within a peptide, and herbicides can be diversified in water. Structurally complex natural products were used as testbeds to showcase the late-stage tagging methodology of marine natural products to enable liquid chromatography-mass spectrometry (LC-MS) detection. This enabling methodology therefore provides a general method for the environmentally friendly and biocompatible derivatization of sp3 alkyl halide bonds.

Surfactant Micellar and Vesicle Microenvironments and Structures under Synthetic Organic Conditions

Fluorescence lifetime imaging microscopy (FLIM) reveals vesicle sizes, structures, microenvironments, reagent partitioning, and system evolution with two chemical reactions for widely used surfactant-water systems under conditions relevant to organic synthesis, including during steps of Negishi cross-coupling reactions. In contrast to previous investigations, the present experiments characterize surfactant systems with representative organohalide substrates at high concentrations (0.5 M) that are reflective of the preparative-scale organic reactions performed and reported in water. In the presence of representative organic substrates, 2-iodoethylbenzene and 2-bromo-6-methoxypyridine, micelles swell into emulsion droplets that are up to 20 μm in diameter, which is 3-4 orders of magnitude larger than previously measured in the absence of an organic substrate (5-200 nm). The partitioning of reagents in these systems is imaged through FLIM─demonstrated here with nonpolar, amphiphilic, organic, basic, and oxidative-addition reactive compounds, a reactive zinc metal powder, and a palladium catalyst. FLIM characterizes the chemical species and/or provides microenvironment information inside micelles and vesicles. These data show that surfactants cause surfactant-dictated microenvironments inside smaller micelles (<200 nm) but that addition of a representative organic substrate produces internal microenvironments dictated primarily by the substrate rather than by the surfactant, concurrent with swelling. Addition of a palladium catalyst causes the internal environments to differ between vesicles─information that is not available through nor predicted from prior analytical techniques. Together, these data provide immediately actionable information for revising reaction models of surfactant-water systems that underpin the development of sustainable organic chemistry in water.

Iron-catalysed reductive coupling for the synthesis of polyfluorinated compounds

Iron-catalysed reductive cross-coupling of difluorobromo acetic acid derivatives with trifluoromethyl olefins to afford polyfluorinated molecules, containing a difluorenyl and difluoroalkyl group, with a broad substrate scope.

Tunable and Practical Homogeneous Organic Reductants for Cross-Electrophile Coupling

The syntheses of four new tunable homogeneous organic reductants based on a tetraaminoethylene scaffold are reported. The new reductants have enhanced air stability compared to current homogeneous reductants for metal-mediated reductive transformations, such as cross-electrophile coupling (XEC), and are solids at room temperature. In particular, the weakest reductant is indefinitely stable in air and has a reduction potential of -0.85 V versus ferrocene, which is significantly milder than conventional reductants used in XEC. All of the new reductants can facilitate C(sp²)-C(sp³) Ni-catalyzed XEC reactions and are compatible with complex substrates that are relevant to medicinal chemistry. The reductants span a range of nearly 0.5 V in reduction potential, which allows for control over the rate of electron transfer events in XEC. Specifically, we report a new strategy for controlled alkyl radical generation in Ni-catalyzed C(sp²)-C(sp³) XEC. The key to our approach is to tune the rate of alkyl radical generation from Katritzky salts, which liberate alkyl radicals upon single electron reduction, by varying the redox potentials of the reductant and Katritzky salt utilized in catalysis. Using our method, we perform XEC reactions between benzylic Katritzky salts and aryl halides. The method tolerates a variety of functional groups, some of which are particularly challenging for most XEC transformations. Overall, we expect that our new reductants will both replace conventional homogeneous reductants in current reductive transformations due to their stability and relatively facile synthesis and lead to the development of novel synthetic methods due to their tunability.

Regioselective Hydroalkylation of Vinylarenes by Cooperative Cu and Ni Catalysis

Disclosed here is a dual copper and nickel catalytic system with a silyl hydride source for promoting the linear selective hydroalkylation of vinylarenes. This carbon-carbon bond-forming protocol is applied to couple a variety of functionalized vinylarenes with alkyl halides applying a nickel(II) NNN pincer complex in the presence of an NHC-ligated copper catalyst. This combination allows for a 1 mol % loading of the nickel catalyst leading to turnover numbers of up to 72. Over 40 examples are presented, including applications for pharmaceutical diversification. Labeling experiments demonstrated the regioselectivity of the reaction and revealed that the copper catalyst plays a crucial role in enhancing the rate for formation of the reactive linear alkyl nickel complex. Overall, the presented work provides a complimentary approach for hydroalkylation reactions, whilst providing a preliminary mechanistic understanding of the cooperativity between the copper and nickel complexes.

Recent Progress of Metal Nanoparticle Catalysts for C–C Bond Forming Reactions

Over the past few decades, the use of transition metal nanoparticles (NPs) in catalysis has attracted much attention and their use in C–C bond forming reactions constitutes one of their most important applications. A huge variety of metal NPs, which have showed high catalytic activity for C–C bond forming reactions, have been developed up to now. Many kinds of stabilizers, such as inorganic materials, magnetically recoverable materials, porous materials, organic–inorganic composites, carbon materials, polymers, and surfactants have been utilized to develop metal NPs catalysts. This review classified and outlined the categories of metal NPs by the type of support.

Micelle enabled C(sp2)-C(sp3) cross-electrophile coupling in water via synergistic nickel and copper catalysis

A robust and sustainable C(sp2)-C(sp3) cross-electrophile coupling was developed via nickel/copper synergistic catalysis under micellar conditions. This protocol provided a general method to access alkylated arenes with good to excellent yields on a very large scale.

Metal-Micelle Cooperativity: Phosphine Ligand-Free Ultrasmall Palladium(II) Nanoparticles for Oxidative Mizoroki-Heck-type Couplings in Water at Room Temperature

The amphiphile PS-750-M generates stable, phosphine ligand-free, and catalytically active ultrasmall Pd(II) nanoparticles (NPs) from Pd(OAc)₂, preventing their precipitation, polymerization, and oxidation state changes. PS-750-M directly interacts with Pd(II) NP surfaces, as confirmed by high-resolution mass spectrometry and IR spectroscopy, resulting in their high stability. The Pd cations in NPs are most likely held together by hydroxides and acetate ions. The NPs were characterized by HRTEM, revealing their morphology and particle size distribution, and by HRMS and IR, providing evidence for NP-amphiphile interaction. The NP catalytic activity was examined in the context of oxidative Mizoroki-Heck-type couplings in water at room temperature. Hot filtration, hot extraction, and three-phase tests indicate heterogeneous catalysis occurring at the micellar interface rather than homogeneous catalysis occurring in the solution. NMR studies indicate that the catalytic activity stems from metal cation-π interactions of the styrene along with transmetalation by the arylboronic acid, followed by insertion and β-H elimination to furnish the coupled product along with the reoxidation of Pd by benzoquinone to complete the catalytic cycle. This method is very mild and sustainable, both in terms of NP synthesis and subsequent catalysis, and shows broad substrate scope while circumventing the need for organic solvents for this important class of couplings.

Iron-Based Catalyst for Borylation of Unactivated Alkyl Halides without Using Highly Basic Organometallic Reagents

The mild borylation of alkyl bromides and chlorides with bis(neopentylglycolato)diborane (B₂neop₂) mediated by iron-bis amide is described. The reaction proceeds with a broad substrate scope and good functional group compatibility. Moreover, sufficient catalytic activity was obtained for primary and secondary alkyl halides. Mechanistic studies indicate that the reaction proceeds through a radical pathway.

Recent advances on micellar catalysis in water

Water is the universal solvent in nature to catalyze the biological transformation processes. However, owing to the immiscibility of many reagents in water, synthesis chemistry relies heavily on organic solvent. Micellar media is a green alternative to traditional petroleum feedstock derived solvents, which is recently attracting increasing research attention. The present review deals with the recent advances in micellar catalysis with an emphasis on the new "tailor-made" surfactants for various reactions. A brief overview of commercial surfactants, including anionic micelles, cationic micelles, and nonionic micelles is presented. More importantly, an attempt was made to discuss systematically the recent research progress on new surfactants by introducing structures, micellar effects and recycling process, aiming to serve as the basis for future development of surfactants.

Iron- and zinc-mediated reductive coupling of styrenes and alkyl bromides: mechanistic investigation using DFT calculations

DFT calculations were performed to explore the mechanism for iron- and zinc-mediated reductive coupling of styrenes and alkyl bromides.

Electroreductive Carbofunctionalization of Alkenes with Alkyl Bromides via a Radical-Polar Crossover Mechanism

Electrochemistry grants direct access to reactive intermediates (radicals and ions) in a controlled fashion toward selective organic transformations. This feature has been demonstrated in a variety of alkene functionalization reactions, most of which proceed via an anodic oxidation pathway. In this report, we further expand the scope of electrochemistry to the reductive functionalization of alkenes. In particular, the strategic choice of reagents and reaction conditions enabled a radical-polar crossover pathway wherein two distinct electrophiles can be added across an alkene in a highly chemo- and regioselective fashion. Specifically, we used this strategy in the intermolecular carboformylation, anti-Markovnikov hydroalkylation, and carbocarboxylation of alkenes-reactions with rare precedents in the literature-by means of the electroreductive generation of alkyl radical and carbanion intermediates. These reactions employ readily available starting materials (alkyl halides, alkenes, etc.) and simple, transition-metal-free conditions and display broad substrate scope and good tolerance of functional groups. A uniform protocol can be used to achieve all three transformations by simply altering the reaction medium. This development provides a new avenue for constructing Csp3-Csp3 bonds.

Iron-Catalyzed Conjugate Addition of Aryl Iodides onto Activated Alkenes under Air in Water

The combination of commercially available FeCl3·6H2O with a water-soluble cationic 2,2′-bipyridyl catalytic system was found to enable the direct conjugate addition of aryl iodides onto activated alkenes, such as an α,β-unsaturated ester and a ketone, in a weakly acidic aqueous solution. This operationally simple protocol was carried out at 80 °C under air atmosphere in a potassium acetate-buffered aqueous solution for 12 h in the presence of Zn dust as a reductant to provide the desired 1,4-adducts in good yields.

Recent Advances in Asymmetric Iron Catalysis

Asymmetric transition-metal catalysis represents a fascinating challenge in the field of organic chemistry research. Since seminal advances in the late 60s, which were finally recognized by the Nobel Prize to Noyori, Sharpless and Knowles in 2001, the scientific community explored several approaches to emulate nature in producing chiral organic molecules. In a scenario that has been for a long time dominated by the use of late-transition metals (TM) catalysts, the use of 3d-TMs and particularly iron has found, recently, a widespread application. Indeed, the low toxicity and the earth-abundancy of iron, along with its chemical versatility, allowed for the development of unprecedented and more sustainable catalytic transformations. While several competent reviews tried to provide a complete picture of the astounding advances achieved in this area, within this review we aimed to survey the latest achievements and new concepts brought in the field of enantioselective iron-catalyzed transformations.

Amphiphilic Polymeric Nanoparticles for Photoredox Catalysis in Water

Photoredox catalysis has recently emerged as a powerful synthesis tool in organic and polymer chemistry. In contrast to the great achievements realized in organic solvents, performing photocatalytic processes efficiently in aqueous media encounters several challenges. Here, it is presented how amphiphilic single-chain polymeric nanoparticles (SCPNs) can be utilized as small reactors to conduct light-driven chemical reactions in water. By incorporating a phenothiazine (PTH) catalyst into the polymeric scaffold, metal-free reduction and C-C cross-coupling reactions can be carried out upon exposure to UV light under ambient conditions. The versatility of this approach is underlined by a large substrate scope, tolerance towards oxygen, and excellent recyclability. This approach thereby contributes to a sustainable and green way of implementing photoredox catalysis.

Concentrated solar radiation as a renewable heat source for a preparative-scale and solvent-free Biginelli reaction

Highly sustainable, economical, and faster Biginelli reaction has been established using concentrated solar radiation.