ortho ‐Allylation of 1‐Arylpyrazoles with Allyl Phenyl Ether via Iron‐Catalyzed CH Bond Activation under Mild Conditions

Rh(III)-Catalyzed C-H Allylation of Aromatic Ketoximes with Vinylaziridines

The Rh(III)-catalyzed reaction of aromatic ketoximes with 2-vinylaziridines affords ortho-allylation products of the phenyl rings of aromatic ketoximes in moderate to excellent yields. The reaction requires 0.5 equiv of NaOAc as a base and occurs under mild conditions. The protocol exhibits ortho-monoallylation selectivity, wide scope of substrates, and good compatibility of functional groups.

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.

Experimental and Computational Studies on RuII -Catalyzed C7-Allylation of Indolines with Allyl Bromide

The selective C7-allylation of indolines with allyl bromide under ruthenium catalysis has been revealed here. Under established reaction conditions, C7-allylation of various indolines, including drug compounds, was accomplished with good selectivity and yields. Based on combined experimental and density functional theory (DFT) studies, the olefin insertion route was energetically favorable among four possible pathways. Experimental and DFT studies further revealed that the C-H activation is a reversible rate-limiting step.

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.

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.

Rh(III)-Catalyzed C-H Cyanation of 2H-Indazole with N-Cyano-N-phenyl-p-toluenesulfonamide

A Rh(III)-catalyzed direct cyanation of 2H-indazoles with N-cyano-N-phenyl-p-toluenesulfonamide has been realized via a chelation-assisted strategy. The methodology enables regioselective access to various ortho-cyanated phenylindazoles in good yields with a broad substrate scope and good functional group compatibility. The obtained cyanated indazoles could further be converted into other value-added chemicals. Importantly, the current protocol is featured with several characteristics, including a novel cyanating agent, good regioselectivity, and operational convenience.

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.

Chromium(III)-Catalyzed C(sp2)-H Alkynylation, Allylation, and Naphthalenation of Secondary Amides with Trimethylaluminum as Base

Among base metals used for C-H activation reactions, chromium(III) is rather unexplored despite its natural abundance and low toxicity. We report herein chromium(III)-catalyzed C(sp²)-H functionalization of an ortho-position of aromatic and α,β-unsaturated secondary amides using readily available AlMe₃ as a base and using bromoalkynes, allyl bromide, and 1,4-dihydro-1,4-epoxynaphthalene as electrophiles. This redox-neutral reaction taking place at 70-90 °C, requires as low as 1-2 mol % of CrCl₃ or Cr(acac)₃ as a catalyst without any added ligand, and tolerates functional groups such as aryl iodide, boronate, and thiophene groups. Stoichiometric and kinetics studies as well as kinetic isotope effects suggest that the catalytic cycle consists of a series of thermally stable but reactive intermediates bearing two molecules of the amide substrate on one chromium atom and also that one of these chromate(III) complexes takes part in the alkynylation, allylation, and naphthalenation reactions. The proposed mechanism accounts for the effective suppression of methyl group delivery from AlMe₃ for ortho-C-H methylation.

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.

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.

Direct Functionalization of C-H Bonds by Iron, Nickel, and Cobalt Catalysis

Non-precious-metal-catalyzed reactions are of increasing importance in chemistry due to the outstanding ecological and economic properties of these metals. In the subfield of metal-catalyzed direct C-H functionalization reactions, recent years have shown an increasing number of publications dedicated to this topic. Nickel, cobalt, and last but not least iron, have started to enter a field which was long dominated by precious metals such as palladium, rhodium, ruthenium, and iridium. The present review article summarizes the development of iron-, nickel-, and cobalt-catalyzed C-H functionalization reactions until the end of 2016, and discusses the scope and limitations of these transformations.

Iron-Catalyzed C-H Bond Activation

Catalytic C-H bond activation, which was an elusive subject of chemical research until the 1990s, has now become a standard synthetic method for the formation of new C-C and C-heteroatom bonds. The synthetic potential of C-H activation was first described for ruthenium catalysis and is now widely exploited by the use of various precious metals. Driven by the increasing interest in chemical utilization of ubiquitous metals that are abundant and nontoxic, iron catalysis has become a rapidly growing area of research, and iron-catalyzed C-H activation has been most actively explored in recent years. In this review, we summarize the development of stoichiometric C-H activation, which has a long history, and catalytic C-H functionalization, which emerged about 10 years ago. We focus in this review on reactions that take place via reactive organoiron intermediates, and we excluded those that use iron as a Lewis acid or radical initiator. The contents of this review are categorized by the type of C-H bond cleaved and the type of bond formed thereafter, and it covers the reactions of simple substrates and substrates possessing a directing group that anchors the catalyst to the substrate, providing an overview of iron-mediated and iron-catalyzed C-H activation reported in the literature by October 2016.

Synthesis of Functionalized Pyrazoles via 1,3-Dipolar Cycloaddition of α-Diazo-β-ketophosphonates, Sufones and Esters with Electron-Deficient Alkenes

1,3-Dipolar cycloaddition of diazo compounds with olefinic substrates is a promising atom-economic strategy for the construction of functionalized pyrazoles. Over the last few years, our group has been engaged in the synthesis of phosphonyl/sulfonylpyrazoles and pyrazole esters by employing Bestmann-Ohira Reagent (BOR) and its sulfur and ester analogs as 1,3-dipole precursors with various dipolarophiles. This account describes the novel synthetic methods developed in our laboratory, in the perspective of closely related work by others, for the synthesis of phosphonyl/sulfonylpyrazoles, pyrazole esters and the total synthesis of Withasomnine, a natural product, by using 1,3-dipolar cycloaddition as the key step.

Mild metal-catalyzed C–H activation: examples and concepts

C–H Activation reactions that proceed under mild conditions are more attractive for applications in complex molecule synthesis. Mild C–H transformations reported since 2011 are reviewed and the different concepts and strategies that have enabled their mildness are discussed.

Temperature-controlled redox-neutral ruthenium(ii)-catalyzed regioselective allylation of benzamides with allylic acetates

Temperature controlled regioselective synthesis of ortho allyl and vinyl benzamides under redox-free conditions was reported. Detailed mechanistic investigation of the C–H bond activation and isomerization reactions was carried out.

Pincer-Nickel-Catalyzed Allyl-Aryl Coupling between Allyl Methyl Ethers and Arylzinc Chlorides

The P,N,N-pincer nickel complex [Ni(Cl){N(2-Ph2PC6H4)(2'-Me2NC6H4)}]-catalyzed allyl-aryl coupling was studied. The reaction of allyl methyl ethers, including (1-methoxyallyl)arenes and (3-methoxyprop-1-en-1-yl)arenes, with arylzinc chlorides afforded linear (E)-alkenes in high yields, whereas the reaction of (E)-1-methoxytridec-2-ene with p-Me2NC6H4ZnCl generated a mixture of linear and branched alkenes.

Ruthenium-Catalyzed Oxidant-Free Allylation of Aromatic Ketoximes with Allylic Acetates at Room Temperature

Substituted aromatic ketoximes reacted efficiently with allylic acetates in the presence of {[RuCl2(p-cymene)]2} and AgSbF6 in 1,2-dichloroethane at ambient temperature, providing ortho-allyl aromatic ketoximes in a highly regioselective manner without an oxidant. In the reaction, the acetate group of allyl acetate acts as a base to activate the C-H bond of aromatics. Later, ortho-allyl aromatic ketoximes were converted into ortho-allyl aromatic ketones in the presence of HCl.

Co(III)-catalyzed C-H activation/formal SN-type reactions: selective and efficient cyanation, halogenation, and allylation

The first cobalt-catalyzed cyanation, halogenation, and allylation via C-H activation have been realized. These formal SN-type reactions generate valuable (hetero)aryl/alkenyl nitriles, iodides, and bromides as well as allylated indoles using a bench-stable Co(III) catalyst. High regio- and mono-selectivity were achieved for these reactions. Additionally, allylation proceeded efficiently with a turnover number of 2200 at room temperature, which is unprecedented for this Co(III) catalyst. Alkenyl substrates and amides have been successfully utilized in Cp*Co(III)-catalyzed C-H activation for the first time.