Active Species and Mechanistic Pathways in Iron-Catalyzed C–C Bond-Forming Cross-Coupling Reactions

Heterogeneous Iron-Based Catalysts for Organic Transformation Reactions: A Brief Overview

Iron (Fe) is considered to be one of the most significant elements due to its wide applications. Recent years have witnessed a burgeoning interest in Fe catalysis as a sustainable and cost-effective alternative to noble metal catalysis in organic synthesis. The abundance and low toxicity of Fe, coupled with its competitive reactivity and selectivity, underscore its appeal for sustainable synthesis. A lot of catalytic reactions have been performed using heterogeneous catalysts of Fe oxide hybridized with support systems like aluminosilicates, clays, carbonized materials, metal oxides or polymeric matrices. This review provides a comprehensive overview of the latest advancements in Fe-catalyzed organic transformation reactions. Highlighted areas include cross-coupling reactions, C-H activation, asymmetric catalysis, and cascade processes, showcasing the versatility of Fe across a spectrum of synthetic methodologies. Emphasis is placed on mechanistic insights, elucidating the underlying principles governing iron-catalyzed reactions. Challenges and opportunities in the field are discussed, providing a roadmap for future research endeavors. Overall, this review illuminates the transformative potential of Fe catalysis in driving innovation and sustainability in organic chemistry, with implications for drug discovery, materials science, and beyond.

Ruthenium-Catalyzed Chemo-Selective Carbene Insertion into C-H Bond of Styrene over Cyclopropanation: C-C Bond Formation

The C-C bond formation reactions are important in organic synthesis. Heck reaction is known to arylate the terminal carbon of olefins; however, direct alkylation of the terminal carbon of olefin is limited. Herein, we report a novel ruthenium-catalyzed selective cross-coupling reaction of styrene and α-diazoesters to form a new C-C bond over cyclopropanation via the C-H insertion process for the first time. Using this novel methodology, a wide variety of substrates have been utilized and a variety of α-vinylated benzylic esters and densely functionalized olefins have been synthesized with good stereoselectivity under mild reaction conditions. The overall reaction process proceeds through the carbene insertion into styrene to form the desired products in good to excellent yields with proper stereoselectivity. The selective C-H inserted product, wide substrate scope, and excellent functional group tolerance are the best features of this work.

Sequential Iron-Catalyzed C(sp2)-C(sp3) Cross-Coupling of Chlorobenzamides/Chemoselective Amide Reduction and Reductive Deuteration to Benzylic Alcohols

Benzylic alcohols are among the most important intermediates in organic synthesis. Recently, the use of abundant metals has attracted significant attention due to the issues with the scarcity of platinum group metals. Herein, we report a sequential method for the synthesis of benzylic alcohols by a merger of iron catalyzed cross-coupling and highly chemoselective reduction of benzamides promoted by sodium dispersion in the presence of alcoholic donors. The method has been further extended to the synthesis of deuterated benzylic alcohols. The iron-catalyzed Kumada cross-coupling exploits the high stability of benzamide bonds, enabling challenging C(sp²)-C(sp³) cross-coupling with alkyl Grignard reagents that are prone to dimerization and β-hydride elimination. The subsequent sodium dispersion promoted reduction of carboxamides proceeds with full chemoselectivity for the C-N bond cleavage of the carbinolamine intermediate. The method provides access to valuable benzylic alcohols, including deuterium-labelled benzylic alcohols, which are widely used as synthetic intermediates and pharmacokinetic probes in organic synthesis and medicinal chemistry. The combination of two benign metals by complementary reaction mechanisms enables to exploit underexplored avenues for organic synthesis.

Recent Advances in Carbon-Based Iron Catalysts for Organic Synthesis

Carbon-based iron catalysts combining the advantages of iron and carbon material are efficient and sustainable catalysts for green organic synthesis. The present review summarizes the recent examples of carbon-based iron catalysts for organic reactions, including reduction, oxidation, tandem and other reactions. In addition, the introduction strategies of iron into carbon materials and the structure and activity relationship (SAR) between these catalysts and organic reactions are also highlighted. Moreover, the challenges and opportunities of organic synthesis over carbon-based iron catalysts have also been addressed. This review will stimulate more systematic and in-depth investigations on carbon-based iron catalysts for exploring sustainable organic chemistry.

Cobalt-Catalyzed C(sp2)-C(sp3) Suzuki-Miyaura Cross-Coupling Enabled by Well-Defined Precatalysts with L,X-Type Ligands

Cobalt(II) halides in combination with phenoxy-imine (FI) ligands generated efficient precatalysts in situ for the C(sp²)-C(sp³) Suzuki-Miyaura cross coupling between alkyl bromides and neopentylglycol (hetero)arylboronic esters. The protocol enabled efficient C-C bond formation with a host of nucleophiles and electrophiles (36 examples, 34-95%) with precatalyst loadings of 5 mol%. Studies with alkyl halide electrophiles that function as radical clocks support the intermediacy of alkyl radicals during the course of the catalytic reaction. The improved performance of the FI-cobalt catalyst was correlated with decreased lifetimes of cage-escaped radicals as compared to diamine-type ligands. Studies of the phenoxy(imine)-cobalt coordination chemistry validate the L,X interaction leading to the discovery of an optimal, well defined, air-stable mono-FI cobalt(II) precatalyst structure.

Iron-catalyzed domino coupling reactions of π-systems

The development of environmentally benign, inexpensive, and earth-abundant metal catalysts is desirable from both an ecological and economic standpoint. Certainly, in the past couple decades, iron has become a key player in the development of sustainable coupling chemistry and has become an indispensable tool in organic synthesis. Over the last ten years, organic chemistry has witnessed substantial improvements in efficient synthesis because of domino reactions. These protocols are more atom-economic, produce less waste, and demand less time compared to a classical stepwise reaction. Although iron-catalyzed domino reactions require a mindset that differs from the more routine noble-metal, homogenous iron catalysis they bear the chance to enable coupling reactions that rival that of noble-metal-catalysis. This review provides an overview of iron-catalyzed domino coupling reactions of π-systems. The classifications and reactivity paradigms examined should assist readers and provide guidance for the design of novel domino reactions.

Iron-Catalyzed Cross-Coupling Reactions of Alkyl Grignards with Aryl Chlorobenzenesulfonates

Aryl sulfonate esters are versatile synthetic intermediates in organic chemistry as well as attractive architectures due to their bioactive properties. Herein, we report the synthesis of alkyl-substituted benzenesulfonate esters by iron-catalyzed C(sp2)-C(sp3) cross-coupling of Grignard reagents with aryl chlorides. The method operates using an environmentally benign and sustainable iron catalytic system, employing benign urea ligands. A broad range of chlorobenzenesulfonates as well as challenging alkyl organometallics containing β-hydrogens are compatible with these conditions, affording alkylated products in high to excellent yields. The study reveals that aryl sulfonate esters are the most reactive activating groups for iron-catalyzed alkylative C(sp2)-C(sp3) cross-coupling of aryl chlorides with Grignard reagents.

Fe-Catalyzed Intramolecular B-H/C-H Dehydrogenative Coupling: Synthesis of Carborane-Fused Nitrogen Heterocycles

We disclose herein the first example of iron-catalyzed regioselective intramolecular C-H/B-H dehydrogenative coupling, affording unprecedented C,B-substituted carborane-fused phenanthroline derivatives. The 8-aminoquinoline type auxiliaries not only serve as the bidentate directing groups but also ingeniously become the core part of the final products. The mechanistic hypothesis includes B-H activation, directing group rotation promoted by trans effect, C-H activation, and reductive elimination.

Nickel-catalyzed formation of quaternary carbon centers using tertiary alkyl electrophiles

This review provides a comprehensive summary of recent advances in nickel-catalyzed reactions employing tertiary alkyl electrophiles for the construction of quaternary carbon centers.

Efficient Pd-Catalyzed Direct Coupling of Aryl Chlorides with Alkyllithium Reagents

Organolithium compounds are amongst the most important organometallic reagents and frequently used in difficult metallation reactions. However, their direct use in the formation of C-C bonds is less established. Although remarkable advances in the coupling of aryllithium compounds have been achieved, Csp2 -Csp3 coupling reactions are very limited. Herein, we report the first general protocol for the coupling or aryl chlorides with alkyllithium reagents. Palladium catalysts based on ylide-substituted phosphines (YPhos) were found to be excellently suited for this transformation giving high selectivities at room temperature with a variety of aryl chlorides without the need for an additional transmetallation reagent. This is demonstrated in gram-scale synthesis including building blocks for materials chemistry and pharmaceutical industry. Furthermore, the direct coupling of aryllithiums as well as Grignard reagents with aryl chlorides was also easily accomplished at room temperature.

The Role of Organoferrates in Iron-Catalyzed Cross-Couplings

Recent groundbreaking studies on organoferrates have demonstrated that coordinatively unsaturated three-coordinate-σ-alkylferrates are active catalysts in Fe-catalyzed cross-couplings with Grignard reagents and that pronounced solvent and counterion effects dictate metalate speciation and catalyst activity. Thanks to modern spectroscopic methods, sensitive catalyst intermediates could be analyzed.

Intra- and intermolecular Fe-catalyzed dicarbofunctionalization of vinyl cyclopropanes

Design and implementation of the first (asymmetric) Fe-catalyzed intra- and intermolecular difunctionalization of vinyl cyclopropanes (VCPs) with alkyl halides and aryl Grignard reagents has been realized via a mechanistically driven approach. Mechanistic studies support the diffusion of alkyl radical intermediates out of the solvent cage to participate in an intra- or intermolecular radical cascade with a range of VCPs followed by re-entering the Fe radical cross-coupling cycle to undergo (stereo)selective C(sp2)-C(sp3) bond formation. This work provides a proof-of-concept of the use of vinyl cyclopropanes as synthetically useful 1,5-synthons in Fe-catalyzed conjunctive cross-couplings with alkyl halides and aryl/vinyl Grignard reagents. Overall, we provide new design principles for Fe-mediated radical processes and underscore the potential of using combined computations and experiments to accelerate the development of challenging transformations.

Iron-Catalyzed Reductive Coupling of Alkyl Iodides with Alkynes To Yield cis-Olefins: Mechanistic Insights from Computation

In a recent study, a new procedure for Z-selective olefin synthesis by reductive coupling of alkyl iodides with terminal alkynes in the presence of iron salts is described. This transformation is representative of many newly developed synthetic routes through the involvement of multiple species and phases, which makes mechanistic insight hard to obtain. Here, we report computational work aimed at exploring the possible reaction pathways. DFT calculations lead to two suggested routes, one involving C-I reduction by metallic zinc and radical addition to the alkyne and the other involving addition of two reduced iron species to the alkyne bond followed by reductive elimination. Comparison to experimental results as well as kinetic modeling is used to discuss the likelihood of these and related mechanisms.

Iron-Catalyzed C(sp2)-C(sp3) Cross-Coupling of Aryl Chlorobenzoates with Alkyl Grignard Reagents

Aryl benzoates are compounds of high importance in organic synthesis. Herein, we report the iron-catalyzed C(sp²)-C(sp³) Kumada cross-coupling of aryl chlorobenzoates with alkyl Grignard reagents. The method is characterized by the use of environmentally benign and sustainable iron salts for cross-coupling in the catalytic system, employing benign urea ligands in the place of reprotoxic NMP (NMP = N-methyl-2-pyrrolidone). It is notable that high selectivity for the cross-coupling is achieved in the presence of hydrolytically-labile and prone to nucleophilic addition phenolic ester C(acyl)-O bonds. The reaction provides access to alkyl-functionalized aryl benzoates. The examination of various O-coordinating ligands demonstrates the high activity of urea ligands in promoting the cross-coupling versus nucleophilic addition to the ester C(acyl)-O bond. The method showcases the functional group tolerance of iron-catalyzed Kumada cross-couplings.

Iron and cobalt catalysis: new perspectives in synthetic radical chemistry

Iron and cobalt complexes are at the origin of high valuable synthetic pathways involving radical intemediates.

Anticancer Molecule Discovery via C2-Substituent Promoted Oxidative Coupling of Indole and Enolate

C2, C3-disubstituted indole is one of the most frequently encountered motifs in bioactive alkaloids and medicinal chemistry. Thus, developing novel, concise, and efficient access to it is highly desired in drug discovery. Herein, we present such an approach to this scaffold by direct oxidative coupling of C2-substituted indoles and enolates. Compared with indole bearing no C2-substituent, higher yields (up to 96%) were obtained for C2-substituted indoles in most cases. Mechanistic studies showed the reaction went through a Fe-chelated radical-anion oxidative coupling procedure promoted by C2-substituent on indole by two means: (1) stabilizing C2-radical intermediate during the reaction; (2) reducing indole homocoupling. This approach serves as a synthetic useful tool to quickly build up bioactive small molecule library of C2, C3-disubstituted indoles, and several products showed promising anticancer activities. Besides, indomethacin and its analogs were conveniently prepared in three-step sequence efficiently, indicating the potential application of our approach in medicinal chemistry.

Stereoselective Alkyne Hydrogenation by using a Simple Iron Catalyst

The stereoselective hydrogenation of alkynes constitutes one of the key approaches for the construction of stereodefined alkenes. The majority of conventional methods utilize noble and toxic metal catalysts. This study concerns a simple catalyst comprised of the commercial chemicals iron(II) acetylacetonate and diisobutylaluminum hydride, which enables the Z-selective semihydrogenation of alkynes under near ambient conditions (1-3 bar H₂ , 30 °C, 5 mol % [Fe]). Neither an elaborate catalyst preparation nor addition of ligands is required. Mechanistic studies (kinetic poisoning, X-ray absorption spectroscopy, TEM) strongly indicate the operation of small iron clusters and particle catalysts.

Visible-Light-Promoted Iron-Catalyzed C(sp2 )-C(sp3 ) Kumada Cross-Coupling in Flow

A continuous‐flow, visible‐light‐promoted method has been developed to overcome the limitations of iron‐catalyzed Kumada–Corriu cross‐coupling reactions. A variety of strongly electron rich aryl chlorides, previously hardly reactive, could be efficiently coupled with aliphatic Grignard reagents at room temperature in high yields and within a few minutes’ residence time, considerably enhancing the applicability of this iron‐catalyzed reaction. The robustness of this protocol was demonstrated on a multigram scale, thus providing the potential for future pharmaceutical application.

Enhanced Fe-Centered Redox Flexibility in Fe-Ti Heterobimetallic Complexes

Previously, we reported the synthesis of Ti[N(o-(NCH₂P(ⁱPr)₂)C₆H₄)₃] and the Fe–Ti complex, FeTi[N(o-(NCH₂P(ⁱPr)₂)C₆H₄)₃], abbreviated as TiL (1), and FeTiL (2), respectively. Herein, we describe the synthesis and characterization of the complete redox families of the monometallic Ti and Fe–Ti compounds. Cyclic voltammetry studies on FeTiL reveal both reduction and oxidation processes at −2.16 and −1.36 V (versus Fc/Fc⁺), respectively. Two isostructural redox members, [FeTiL]⁺ and [FeTiL]⁻ (2^ox and 2^red, respectively) were synthesized and characterized, along with BrFeTiL (2-Br) and the monometallic [TiL]⁺ complex (1^ox). The solid-state structures of the [FeTiL]^+/0/– series feature short metal–metal bonds, ranging from 1.94–2.38 Å, which are all shorter than the sum of the Ti and Fe single-bond metallic radii (cf. 2.49 Å). To elucidate the bonding and electronic structures, the complexes were characterized with a host of spectroscopic methods, including NMR, EPR, and ⁵⁷Fe Mössbauer, as well as Ti and Fe K-edge X-ray absorption spectroscopy (XAS). These studies, along with hybrid density functional theory (DFT) and time-dependent DFT calculations, suggest that the redox processes in the isostructural [FeTiL]^+,0,– series are primarily Fe-based and that the polarized Fe–Ti π-bonds play a role in delocalizing some of the additional electron density from Fe to Ti (net 13%).

An isostructural redox series of Fe≡Ti complexes was investigated using a combination of spectroscopic methods and density functional theory to elucidate their electronic structures and to understand their polarized metal−metal bonding. Overall, the results support that the redox changes occur primarily at the Fe site though some electron density is delocalized to Ti. Hence, the Ti plays an important role in enhancing the redox flexibility of the single Fe site.

Computational Approach to Molecular Catalysis by 3d Transition Metals: Challenges and Opportunities

Computational chemistry provides a versatile toolbox for studying mechanistic details of catalytic reactions and holds promise to deliver practical strategies to enable the rational in silico catalyst design. The versatile reactivity and nontrivial electronic structure effects, common for systems based on 3d transition metals, introduce additional complexity that may represent a particular challenge to the standard computational strategies. In this review, we discuss the challenges and capabilities of modern electronic structure methods for studying the reaction mechanisms promoted by 3d transition metal molecular catalysts. Particular focus will be placed on the ways of addressing the multiconfigurational problem in electronic structure calculations and the role of expert bias in the practical utilization of the available methods. The development of density functionals designed to address transition metals is also discussed. Special emphasis is placed on the methods that account for solvation effects and the multicomponent nature of practical catalytic systems. This is followed by an overview of recent computational studies addressing the mechanistic complexity of catalytic processes by molecular catalysts based on 3d metals. Cases that involve noninnocent ligands, multicomponent reaction systems, metal-ligand and metal-metal cooperativity, as well as modeling complex catalytic systems such as metal-organic frameworks are presented. Conventionally, computational studies on catalytic mechanisms are heavily dependent on the chemical intuition and expert input of the researcher. Recent developments in advanced automated methods for reaction path analysis hold promise for eliminating such human-bias from computational catalysis studies. A brief overview of these approaches is presented in the final section of the review. The paper is closed with general concluding remarks.

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.

Cross-Coupling and Related Reactions: Connecting Past Success to the Development of New Reactions for the Future

Cross-coupling reactions, which were discovered almost 50 years ago, are widely used in both industry and academia. Even though cross-coupling reactions now represent mature technology, there is still a significant amount of research in this area that aims to improve the scope of these reactions, develop more efficient catalysts, and make reactions more practical. In this tutorial, a brief background to cross-coupling reactions is provided, and then the major advances in cross-coupling research over the last 20 years are described. These include the development of improved ligands and precatalysts for cross-coupling and the extension of cross-coupling reactions to a much wider range of electrophiles. For example, cross-coupling reactions are now common with sp3-hybridized electrophiles as well as ester, amide, ether, and aziridine substrates. For many of these more modern substrates, traditional palladium-based catalysts are less efficient than systems based on first-row transition metals such as nickel. Conventional cross-coupling reactions have also inspired the development of a range of related reactions, such as cross-electrophile and decarboxylative couplings as well as couplings based on metallaphotoredox chemistry. The development of these new reactions is probably at the same stage as traditional cross-coupling reactions 30 years ago, and this tutorial highlights how many of the same strategies used to improve cross-coupling reactions may also be applicable to making the new reactions more practical.

Mössbauer and mass spectrometry support for iron(ii) catalysts in enantioselective C–H activation

A combination of electrospray-ionization mass spectrometry and Mössbauer spectroscopy was used to investigate the species generated in situ in highly enantioselective Fe/NHC-catalyzed C–H alkylations.

Iron-catalyzed C(sp2)–C(sp3) cross-coupling at low catalyst loading

An operationally-simple protocol for the selective C(sp²)–C(sp³) iron-catalyzed cross-coupling of aryl chlorides with Grignard reagents at low catalyst loading is reported.