Identification and Reactivity of Cyclometalated Iron(II) Intermediates in Triazole-Directed Iron-Catalyzed C–H Activation

Iron-Catalyzed C-H Alkylation/Ring Opening with Vinylbenzofurans Enabled by Triazoles

We report an unprecedented iron-catalyzed C-H annulation using readily available 2-vinylbenzofurans as the reaction pattern. The redox-neutral strategy, based on cheap, non-toxic, and earth-abundant iron catalysts, exploits triazole assistance to promote a cascade C-H alkylation, benzofuran ring-opening and insertion into a Fe-N bond, to form highly functionalized isoquinolones. Detailed mechanistic studies supported by DFT calculations fully disclosed the manifold of the iron catalysis.

Divergent Fe-Mediated C-H Activation Paths Driven by Alkali Cations

The association of the ferrous complex FeIICl2(dmpe)2 (1) with alkali bases M(hmds) (M = Li, Na, K) proves to be an efficient platform for the activation of Ar-H bonds. Two mechanisms can be observed, leading to either Ar-FeII species by deprotonative ferration or hydrido species Ar-FeII-H by oxidative addition of transient Fe0(dmpe)2 generated by reduction of 1. Importantly, the nature of the alkali cation in M(hmds) has a strong influence on the preferred path. Starting from the same iron precursor, diverse catalytic applications can be explored by a simple modulation of the MI cation. Possible strategies enabling cross-coupling using arenes as pro-nucleophiles, reductive dehydrocoupling, or deuteration of B-H bonds are discussed.

Coordination chemistry effects of the space-demanding solvent molecule N,N'-dimethylpropyleneurea

Crystallographic investigations of eight homoleptic N,N'-dimethylpropyleneurea (dmpu) coordinated metal ions in the solid state, [Mg(dmpu)5]I2 (1), [Ca(dmpu)6]I2 (2), [Ca(dmpu)6](ClO4)2 (3), [Ca(dmpu)6](CF3SO3)2 (4), [Sr(dmpu)6](CF3SO3)2 (5), [Ba(dmpu)6](CF3SO3)2 (6), [Sc(dmpu)6]I3 (7), and [Pr(dmpu)6]I(I3)2 (8), and the complex [CoBr2(dmpu)2] (9) as well as the structures of the dmpu coordinated calcium, strontium, barium, scandium(III) and cobalt(II) ions and the cobalt(II) bromide complex in dmpu solution as determined by EXAFS are reported. The methyl groups in the dmpu molecule are close to the oxygen donor atom, causing steric restrictions, and making dmpu space-demanding at coordination to metal ions. The large volume required by the dmpu ligand at coordination contributes to crowdedness around the metal ion with often lower coordination numbers than for oxygen donor ligands without such steric restrictions. The crowdedness is seen in M⋯H distances equal to or close to the sum of the van der Waals radii. To counteract the space-demand at coordination, the dmpu molecule has an unusual ability to increase the M-O-C bond angle to facilitate as large coordination numbers as possible. M-O-C bond angles in the range of 125-170° are reported depending on the crowdedness caused by the coordination figure and the M-O bond distance. All reported structures of dmpu coordinated metal ions in both the solid state and dmpu solution are summarized to study the relationship between the M-O-C bond angle and the crowdedness around the metal ion. However, highly symmetric complexes seem to be favoured in the solid state due to favourable lattice energies. As a result, the dmpu coordinated lanthanoid(III) ions are octahedral in the solid state, while they, except lutetium, are seven-coordinate in the dmpu solution.

Chelation-Assisted Iron-Catalyzed C-H Activations: Scope and Mechanism

ConspectusTo improve the resource economy of molecular syntheses, researchers have developed strategies to directly activate otherwise inert C-H bonds, thus avoiding cumbersome and costly substrate prefunctionalizations. During the past two decades, remarkable progress in coordination chemistry has set the stage for developing increasingly viable metal catalysts for C-H activations. Despite remarkable advances, C-H activations are largely dominated by precious 4d and 5d transition metal catalysts based primarily on palladium, ruthenium, iridium, and rhodium, thus decreasing the inherent sustainable nature of the C-H activation approach. Therefore, advancing catalytic reactions based on Earth-abundant and less toxic 3d transition metals, especially nontoxic and inexpensive iron, represents a desirable and attractive alternative. While research had previously focused on 8-aminoquinoline directing groups in C-H activations, we have devised easily accessible, tunable, and clickable triazoles, which feature widespread applications in bioactive compounds and drugs, among others, as peptide isosteres. Thus, in contrast to other directing groups, the triazole group is a highly desirable structural motif and functions as a bioisostere in medicine and biology, where it is exploited to mimic amide bonds.This Account summarizes the evolution of chelation-assisted iron-catalyzed C-H activations via C-H, C-H/N-H, and C-H/N-H/C-C bond cleavages, with a topical focus on the most recent contributions of our team. Thus, the triazole-enabled iron catalysis has surfaced as a transformative platform for a large variety of C-H transformations, including arylations, alkylations, alkenylations, allylations, annulations, and alkynylations, achieved through C-H activations with organometallic reagents, organohalides, alkynes, alkenes, allenes, and bicyclopropylidenes among others. Consequently, we developed widely applicable methods for the versatile preparation of decorated arenes and heteroarenes, providing access to benzamides, isoquinolones, pyrrolones, pyridones, phenones, indoles, and isoindolinones, among others. Most of these reactions employed 1,2-dichloroisobutane (DCIB) as an oxidant. Notably, chemical-oxidant-free strategies were also developed, with the major breakthroughs being the use of internal oxidants in oxidative annulations, the use of resource-economic electrocatalysis, and the development of well-defined iron(0)-mediated catalysis. In addition, a highly enantioselective inner-sphere C-H alkylation of (aza)indoles was developed by designing novel remotely decorated N-heterocyclic carbene ligands with dispersion energy donors. In addition, detailed mechanistic experiments including kinetic analyses, intermediate isolation, Mößbauer spectroscopy, and computation provided strong support for the mode of catalysis operation, enabling unprecedented C-H activations. Thereby, low-valent iron catalysts paved the way toward weakly coordinating ketones and enantioselective iron-catalyzed C-H activations through organometallic intermediates.

Modern Organometallic C-H Functionalizations with Earth-Abundant Iron Catalysts: An Update

Iron-catalyzed C-H activation has recently emerged as an increasingly powerful synthetic method for the step- and atom- economical direct C-H functionalizations of otherwise inert C-H bonds. Iron's low-cost and toxicity along with its catalytic versatility have encouraged the scientific community to elect this metal for the development of new C-H activation methodologies. Within this review, we aim to present a collection of the most recent examples of iron-catalyzed C-H functionalizations with a particular emphasis on modern synthetic strategies and mechanistic aspects.

Iron-Catalyzed para-Selective C-H Allylation of Aniline Derivatives

Transition-metal-catalyzed directed C-H allylation of arenes offers an efficient and straightforward approach to construct value-added allylic arenes. However, these reactions are often performed with precious transition-metal catalysts and mainly limited to ortho-C-H allylation of arenes. Herein, we disclose a novel iron-catalyzed para-C-H allylation of aniline derivatives with allyl alcohols via a chelation-induced strategy, providing various allylic arenes in good yields with excellent regio- and chemoselectivity. A simple FeCl3·6H2O is employed as a catalyst, serving a dual role in the reaction: (1) coordination with N-arylpicolinamide to alter the electronic property of the aromatic ring and (2) reaction with allyl alcohol to form allyl-Fe species.

Fe-Catalyzed Difunctionalization of Aryl Titanates Enabled by Fe/Ti Synergism

Fe-catalyzed difunctionalization of aryl titanates via double C-H activation has been developed, where aryl titanates were arylated via ortho C-H activation, followed by ipso electrophilic trapping of the C-Ti bond. The ortho C-H arylation should be promoted by a 1,2-Fe/Ti synergistic heterobimetallic arylene intermediate and represents an ortho C-H ferration directed by a readily transformable C-Ti group. Common benzamides, esters, and nitriles function as arylating reagents, which involves another ortho C-H activation directed by these functionalities.

Triazole-enabled, iron-catalysed linear/branched selective C-H alkylations with alkenes

Iron-catalysed C-H alkylations with alkenes were achieved on benzamides by N-triazole assistance. A notable switch of the regioselectivity from linear to branched was observed depending on the nature of the olefin employed. The approach allowed for the synthesis of a family of decorated benzamides with ample scope and high levels of chemo-, regio- and site-selectivity.

Synergism of Fe/Ti Enabled Regioselective Arene Difunctionalization

Regioselective difunctionalization of arenes remains a long-standing challenge in organic chemistry. We report a novel and general Fe/Ti synergistic methodology for regioselective synthesis of various polysubstituted arenes through either E/E' or Nu/E ortho difunctionalizations of arenes. Preliminary results showed that an unprecedented 1,2-Fe/Ti heterobimetallic arylene intermediate bearing two distinct C-M bonds is essential to the regioselective difunctionalization.

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.

Iron-catalysed alkene and heteroarene H/D exchange by reversible protonation of iron-hydride intermediates

The iron-catalysed C(sp2)–H bond H/D exchange reaction using CD3OD is reported for both heterocycles and alkenes. Characterisation of the key C–H metallation intermediates provided evidence for reversible protonation of the iron hydride catalyst.

Intermediates and mechanism in iron-catalyzed C-H methylation with trimethylaluminum

A mechanistic study is performed on the reaction method for iron-catalyzed C-H methylation with AlMe₃ reagent, previously proposed to involve cyclometalated iron(III) intermediates and an iron(III)/(I) reaction cycle. Detailed spectroscopic studies (⁵⁷Fe Mössbauer, EPR) during catalysis and in stoichiometric reactions identify iron(II) complexes, including cyclometalated iron(II) intermediates, as the major iron species formed in situ under catalytic reaction conditions. Reaction studies identify a cyclometalated iron(II)-methyl species as the key intermediate leading to C-H methylated product upon reaction with oxidant, consistent with a previously proposed iron(II)/iron(III)/iron(I) reaction manifold for C-H arylation.

Additive and Counterion Effects in Iron-Catalyzed Reactions Relevant to C-C Bond Formation

The use of iron catalysts in carbon-carbon bond forming reactions is of interest as an alternative to precious metal catalysts, offering reduced cost, lower toxicity, and different reactivity. While well-defined ligands such as N-heterocyclic carbenes (NHCs) and phosphines can be highly effective in these reactions, additional additives such as N-methylpyrrolidone (NMP), N,N,N',N'-tetramethylethylenediamine (TMEDA), and iron salts that alter speciation can also be employed to achieve high product yields. However, in contrast to well-defined iron ligands, the roles of these additives are often ambiguous, and molecular-level insights into how they achieve effective catalysis are not well-defined. Using a unique physical-inorganic in situ spectroscopic approach, detailed insights into the effect of additives on iron speciation, mechanism, and catalysis can inform further reaction development. In this Perspective, recent advances will be discussed as well as ongoing challenges and potential opportunities in iron-catalyzed reactions.

Experimental and computational studies of the mechanism of iron-catalysed C-H activation/functionalisation with allyl electrophiles

Synthetic methods that utilise iron to facilitate C–H bond activation to yield new C–C and C–heteroatom bonds continue to attract significant interest. However, the development of these systems is still hampered by a limited molecular-level understanding of the key iron intermediates and reaction pathways that enable selective product formation. While recent studies have established the mechanism for iron-catalysed C–H arylation from aryl-nucleophiles, the underlying mechanistic pathway of iron-catalysed C–H activation/functionalisation systems which utilise electrophiles to establish C–C and C–heteroatom bonds has not been determined. The present study focuses on an iron-catalysed C–H allylation system, which utilises allyl chlorides as electrophiles to establish a C–allyl bond. Freeze-trapped inorganic spectroscopic methods (⁵⁷Fe Mössbauer, EPR, and MCD) are combined with correlated reaction studies and kinetic analyses to reveal a unique and rapid reaction pathway by which the allyl electrophile reacts with a C–H activated iron intermediate. Supporting computational analysis defines this novel reaction coordinate as an inner-sphere radical process which features a partial iron–bisphosphine dissociation. Highlighting the role of the bisphosphine in this reaction pathway, a complementary study performed on the reaction of allyl electrophile with an analogous C–H activated intermediate bearing a more rigid bisphosphine ligand exhibits stifled yield and selectivity towards allylated product. An additional spectroscopic analysis of an iron-catalysed C–H amination system, which incorporates N-chloromorpholine as the C–N bond-forming electrophile, reveals a rapid reaction of electrophile with an analogous C–H activated iron intermediate consistent with the inner-sphere radical process defined for the C–H allylation system, demonstrating the prevalence of this novel reaction coordinate in this sub-class of iron-catalysed C–H functionalisation systems. Overall, these results provide a critical mechanistic foundation for the rational design and development of improved systems that are efficient, selective, and useful across a broad range of C–H functionalisations.

Experimental and computational studies support an inner-sphere radical pathway for iron-catalysed C–H activation/functionalisation with allyl electrophiles.

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.

Well-Defined Aryl-FeII Complexes in Cross-Coupling and C–H Activation Processes

Herein we explore the intrinsic organometallic reactivity of iron embedded in a tetradentate N₃C macrocyclic ligand scaffold that allows the stabilization of aryl-Fe species, which are key intermediates in Fe-catalyzed cross-coupling and C–H functionalization processes. This study covers C–H activation reactions using ^MeL_H and FeCl₂, biaryl C–C coupling product formation through reaction with Grignard reagents, and cross-coupling reactions using ^MeL_Br or ^HL_Br in combination with Fe⁰(CO)₅. Synthesis under light irradiation and moderate heating (50 °C) affords the aryl-Fe^II complexes [Fe^II(Br)(^MeL)(CO)] (1^Me) and [Fe^II(^HL)(CO)₂]Br (1^H). Exhaustive spectroscopic characterization of these rare low-spin diamagnetic species, including their crystal structures, allowed the investigation of their intrinsic reactivity.

Organic Electrochemistry: Molecular Syntheses with Potential

Efficient and selective molecular syntheses are paramount to inter alia biomolecular chemistry and material sciences as well as for practitioners in chemical, agrochemical, and pharmaceutical industries. Organic electrosynthesis has undergone a considerable renaissance and has thus in recent years emerged as an increasingly viable platform for the sustainable molecular assembly. In stark contrast to early strategies by innate reactivity, electrochemistry was recently merged with modern concepts of organic synthesis, such as transition-metal-catalyzed transformations for inter alia C-H functionalization and asymmetric catalysis. Herein, we highlight the unique potential of organic electrosynthesis for sustainable synthesis and catalysis, showcasing key aspects of exceptional selectivities, the synergism with photocatalysis, or dual electrocatalysis, and novel mechanisms in metallaelectrocatalysis until February of 2021.

Organic synthesis with the most abundant transition metal–iron: from rust to multitasking catalysts

The promising aspects of iron in synthetic chemistry are being explored for three-four decades as a green and eco-friendly alternative to late transition metals. This present review unveils these rich iron-chemistry towards different transformations.

Allenes for Versatile Iron-Catalyzed C-H Activation by Weak O-Coordination: Mechanistic Insights by Kinetics, Intermediate Isolation, and Computation

The iron-catalyzed hydroarylation of allenes was accomplished by weak phenone assistance. The C-H activation proceeded with excellent efficacy and high ortho-regioselectivity in proximity to the weakly coordinating carbonyl group for a range of substituted phenones and allenes. Detailed mechanistic studies, including the isolation of key intermediates, the structural characterization of an iron-metallacycle, and kinetic analysis, allowed the sound elucidation of a plausible catalytic working mode. This mechanistic rationale is supported by detailed computational density functional theory studies, which fully address multi-spin-state reactivity. Furthermore, in operando nuclear magnetic resonance monitoring of the catalytic reaction provided detailed insights into the mode of action of the iron-catalyzed C-H alkylation with allenes.

Iron-Catalyzed C-H Functionalizations under Triazole-Assistance

3d transition metals-catalyzed C-H bond functionalizations represent nowadays an important tool in organic synthesis, appearing as the most promising alternative to cross-coupling reactions. Among 3d transition metals, iron found widespread application due to its availability and benign nature, and it was established as an efficient catalyst in organic synthesis. In this context, the use of ortho-orientating directing groups (DGs) turned out to be necessary for promoting selective iron-catalyzed C-H functionalization reactions. Very recently, triazoles DGs were demonstrated to be more than an excellent alternative to the commonly employed 8-aminoquinoline (AQ) DG, as a result of their modular synthesis as well as the mild reaction conditions applied for their removal. In addition, their tunable geometry and electronics allowed for new unprecedented reactivities in iron-catalyzed C-H activation methodologies that will be summarized within this review.

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.

Iron-Electrocatalyzed C-H Arylations: Mechanistic Insights into Oxidation-Induced Reductive Elimination for Ferraelectrocatalysis

Despite major advances, organometallic C-H transformations are dominated by precious 5d and 4d transition metals, such as iridium, palladium and rhodium. In contrast, the unique potential of less toxic Earth-abundant 3d metals has been underexplored. While iron is the most naturally abundant transition metal, its use in oxidative, organometallic C-H activation has faced major limitations due to the need for superstoichiometric amounts of corrosive, cost-intensive DCIB as the sacrificial oxidant. To fully address these restrictions, we describe herein the unprecedented merger of electrosynthesis with iron-catalyzed C-H activation through oxidation-induced reductive elimination. Thus, ferra- and manganaelectro-catalyzed C-H arylations were accomplished at mild reaction temperatures with ample scope by the action of sustainable iron catalysts, employing electricity as a benign oxidant.