Iron-catalyzed C(sp(2))-H bond functionalization with organoboron compounds

Regioselective Propargylic Suzuki-Miyaura Coupling by SciPROP-Iron Catalyst

The iron-catalyzed Suzuki-Miyaura cross-coupling of secondary propargyl electrophiles with lithium organoborates has been established. A propyl-bridged bulky bisphosphine ligand, SciPROP-TB, cooperated with the bulky TIPS substituent at the alkyne terminal position to achieve the cross-coupling reaction with exclusive propargylic selectivity. The reaction features high functional group compatibility, regioselectivity, and yield with a broad substrate scope. The reaction of an optically active chiral propargyl bromide proceeds with complete racemization, supporting a mechanism involving propargyl radical formation.

Expedient access to polysubstituted acrylamides via strain-release-driven dual phosphine and palladium catalysis

Polysubstituted acrylamides are ubiquitous in bioactive molecules and natural products. However, synthetic methods for the assembly of these important motifs remain underdeveloped. Herein, we report the expedient synthesis of structurally diverse and synthetically challenging polysubstituted acrylamides from readily available aromatic amines, cyclopropenones (CpOs), and aryl halides via the synergistic merging of nucleophilic phosphine-mediated amidation and palladium-catalyzed C-H arylation. The reaction is scalable, and some obtained acrylamides proved to be solid state luminogens with obvious aggregation-induced emission (AIE) properties, demonstrating the synthetic potential in drug discovery and material development.

DMSO-promoted direct δ-selective arylation of p-quinone methenylpiperidine bearinides to generate fuchsones under metal-free conditions by employing p-QMs themselves or substituted phenols as aryl sources

Fuchsones have wide applications in modern society. Present methods for generating fuchsones have many disadvantages and there are significant limitations for further exploration of fuchsone applications. Herein, we describe a DMSO-promoted direct δ-selective arylation of p-QMs to synthesize symmetrical and unsymmetrical fuchsones under metal-free conditions by employing p-QMs themselves or substituted phenols as aryl sources. As unprecedented methods, these novel strategies present a great advantage and significance for further exploration of fuchsones and the development of new applications.

Recent Advances in Alkenyl sp2 C-H and C-F Bond Functionalizations: Scope, Mechanism, and Applications

Alkenes and their derivatives are featured widely in a variety of natural products, pharmaceuticals, and advanced materials. Significant efforts have been made toward the development of new and practical methods to access this important class of compounds by selectively activating the alkenyl C(sp²)-H bonds in recent years. In this comprehensive review, we describe the state-of-the-art strategies for the direct functionalization of alkenyl sp² C-H and C-F bonds until June 2022. Moreover, metal-free, photoredox, and electrochemical strategies are also covered. For clarity, this review has been divided into two parts; the first part focuses on currently available alkenyl sp² C-H functionalization methods using different alkene derivatives as the starting materials, and the second part describes the alkenyl sp² C-F bond functionalization using easily accessible gem-difluoroalkenes as the starting material. This review includes the scope, limitations, mechanistic studies, stereoselective control (using directing groups as well as metal-migration strategies), and their applications to complex molecule synthesis where appropriate. Overall, this comprehensive review aims to document the considerable advancements, current status, and emerging work by critically summarizing the contributions of researchers working in this fascinating area and is expected to stimulate novel, innovative, and broadly applicable strategies for alkenyl sp² C-H and C-F bond functionalizations in the coming years.

Divergent regioselective Heck-type reaction of unactivated alkenes and N-fluoro-sulfonamides

The control of regioselectivity in Heck-type reaction of unactivated alkenes represents a longstanding challenge due to several detachable hydrogens in β–H elimination step, which generally afford either one specific regioisomer or a mixture. Herein, a copper-catalyzed intermolecular Heck-type reaction of unactivated alkenes and N-fluoro-sulfonamides with divergent regioselectivities is reported. The complete switch of regioselectivity mainly depends on the choice of different additives. Employment of alcohol solvent gives access to vinyl products, while the addition of carboxylate leads to the formation of allylic products. In addition, exclusion of these two promoting factors results in β-lactams via a C–N reductive elimination. This protocol shows a broad substrate scope for both alkenes and structurally diverse N-fluoro-sulfonamides, producing the corresponding products with excellent regio- and stereoselectivities. Further control experiments and DFT calculations provide in-depth insights into the reaction mechanism, highlighting the distinct effect of the additives on a bidentate auxiliary-stabilized Cu(III) intermediate.

The control of regioselectivity in Heck-type reactions of unactivated alkenes is challenging. Here, the authors realize regiodivergent Heck-type reactions of unactivated alkenes and N-fluoro-sulfonamides.

Ruthenium Catalyzed Stereo- and Chemoselective Oxidative Coupling of Vinyl Ketones: An Efficient Access to (E,E)-1,6-dioxo-2,4-dienes

A Ru-catalyzed direct oxidative coupling reaction of vinyl ketones was developed. It offers a straightforward and atom-economical protocol for synthesis of functionalized (E,E)-1,6-dioxo-2,4-diene derivatives in moderate to good yields with...

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.

Palladium-Catalyzed Arylation of C(sp2)-H Bonds with 2-(1-Methylhydrazinyl)pyridine as the Bidentate Directing Group

Palladium-catalyzed C(sp²)-H arylation of ortho C-H bonds involving 2-(1-methylhydrazinyl)pyridine (MHP) as the directing group has been investigated. The reaction proceeds smoothly under an air atmosphere to generate biaryl derivatives in an environmentally friendly manner while tolerating a wide range of functional groups. Notably, the directing group present in the product could be easily removed under mild reductive conditions.

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.

A Study of an 8-Aminoquinoline-Directed C(sp2)-H Arylation Reaction on the Route to Chiral Cyclobutane Keto Acids from Myrtenal

This work outlines a synthetic route that can be used to access chiral cyclobutane keto acids with two stereocenters in five steps from the inexpensive terpene myrtenal. Furthermore, the developed route includes an 8-aminoquinoline-directed C(sp2)-H arylation as one of its key steps, which allows a wide range of aryl and heteroaryl groups to be incorporated into the bicyclic myrtenal scaffold prior to the ozonolysis-based ring-opening step that furnishes the target cyclobutane keto acids. This synthetic route is expected to find many applications connected to the synthesis of natural product-like compounds and small molecule libraries.

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.

RhIII-Catalyzed C-H (Het)arylation/Vinylation of N-2,6-Difluoroaryl Acrylamides

Rh^III-catalyzed sp² C-H cross-coupling of acrylamides with organoboron reactants has been accomplished using a commercially available N-2,6-difluoroaryl acrylamide auxiliary. A broad range of aryl and vinyl boronates as well as a variety of heterocyclic boronates with strong coordinating ability can serve as the coupling partners. This transformation proceeds under moderate reaction conditions with excellent functional group tolerance and high regioselectivity.

Ruthenium-catalyzed formal sp3 C-H activation of allylsilanes/esters with olefins: efficient access to functionalized 1,3-dienes

Ru-catalysed oxidative coupling of allylsilanes and allyl esters with activated olefins has been developed via isomerization followed by C(allyl)-H activation providing efficient access to stereodefined 1,3-dienes in excellent yields. Mild reaction conditions, less expensive catalysts, and excellent regio- and diastereoselectivity ensure universality of the reaction. In addition, the unique power of this reaction was illustrated by performing the Diels-Alder reaction, and enantioselective synthesis of highly functionalized cyclohexenone and piperidine and finally synthetic utility was further demonstrated by the efficient synthesis of norpyrenophorin, an antifungal agent.

Copper(II)-Catalyzed Direct C-H (Hetero)arylation at the C3 Position of Indoles Assisted by a Removable N,N-Bidentate Auxiliary Moiety

The regioselective arylation of inert C3-H bonds in indoles reacting with arylboronates via effective copper-mediated catalysis with the aid of a facile and removable 2-pyridinylisopropyl (PIP) group without ligand participation is reported. This newly established method features high compatibility with diverse functional groups between coupling partners, including both indole substrates and arylboron reagents, consequentially leading to operational simplicity and providing access to generate the desired arylated products in good to excellent yields of up to 97%. Synthetically, the PIP-derived amide moiety could subsequently be readily removed under mild reaction conditions to produce useful indole carboxylic acids for further transformation.

Iron-catalyzed para-selective C–H silylation of benzamide derivatives with chlorosilanes

This paper developed the para-selective silylation of benzamide derivatives with chlorosilanes using FeCl₂ catalysis.

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.

Copper-catalyzed cross-coupling of vinyliodonium salts and diboron reagents to generate alkenyl boronic esters

An efficient approach for the synthesis of alkenyl boronic esters through the copper-catalyzed cross-coupling of vinyliodonium salts and diboron reagents is reported. This method is distinguished by its mild conditions and short reaction time of less than 30 min, which should provide an additional way for the construction of alkenyl boronic esters.

Cu-Catalyzed C-H Alkenylation of Benzoic Acid and Acrylic Acid Derivatives with Vinyl Boronates

An efficient Cu-catalyzed C-H alkenylation with acyclic and cyclic vinyl boronates was realized for the first time under mild conditions. The scope of the vinyl borons and the compatibility with functional groups including heterocycles are superior than Pd-catalyzed C-H coupling with vinyl borons, providing a reliable access to multisubstituted alkenes and dienes. Subsequent hydrogenation of the product from the internal vinyl borons will lead to installation of secondary alkyls.

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.

Regioselective Arylation of Quinoline N-Oxides (C8), Indolines (C7) and N-tert-Butylbenzamide with Arylboronic Acids

Herein, we disclose Ru(II)-catalyzed regioselective distal C(sp2)-H arylation of quinoline N-oxide with arylboronic acids to 8-arylquinolines. In the developed method, the Ru(II)-catalyst shows dual activity, that is, distal C-H activation of quinoline N-oxides followed by in situ deoxygenation of arylated quinoline N-oxide in the same pot. The current catalytic method features use of Ru metal as the catalyst and arylboronic acids as the arylating source under mild reaction conditions. Use of the Rh(III)-catalyst in place of Ru(II) under the same conditions afforded 8-arylquinoline N-oxides with excellent regioselectivity. Furthermore, the developed Ru(II) catalytic system is also extended for the C(sp2)-H arylation of indolines, N-tert-butylbenzamide, and 6-(5H)-phenanthridinone. Formation of the quinoline N-oxide coordinated ruthenium adduct is found to be the key reaction intermediate, which has been characterized by single crystal X-ray diffraction and NMR spectroscopy.

Bidentate Directing Groups: An Efficient Tool in C-H Bond Functionalization Chemistry for the Expedient Construction of C-C Bonds

During the past decades, synthetic organic chemistry discovered that directing group assisted C-H activation is a key tool for the expedient and siteselective construction of C-C bonds. Among the various directing group strategies, bidentate directing groups are now recognized as one of the most efficient devices for the selective functionalization of certain positions due to fact that its metal center permits fine, tunable, and reversible coordination. The family of bidentate directing groups permit various types of assistance to be achieved, such as N,N-dentate, N,O-dentate, and N,S-dentate auxiliaries, which are categorized based on the coordination site. In this review, we broadly discuss various C-H bond functionalization reactions for the formation of C-C bonds with the aid of bidentate directing groups.

The ambiguous behaviour of diphosphines towards the quasilinear iron(i) complex [Fe(N(SiMe3)2)2]− – between inertness, P–C bond cleavage and C–C double bond isomerisation

A quasilinear iron(i) complex facilitates C–C bond isomerisation or P-aryl bond cleavage of diphosphines but is otherwise inert to simple phosphine coordination.

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-Catalyzed C-H Activation with Propargyl Acetates: Mechanistic Insights into Iron(II) by Experiment, Kinetics, Mössbauer Spectroscopy, and Computation

An iron‐catalyzed C−H/N−H alkyne annulation was realized by using a customizable clickable triazole amide under exceedingly mild reaction conditions. A unifying mechanistic approach combining experiment, spectroscopy, kinetics, and computation provided strong support for facile C−H activation by a ligand‐to‐ligand hydrogen transfer (LLHT) mechanism. Combined Mössbauer spectroscopic analysis and DFT calculations were indicative of high‐spin iron(II) species as the key intermediates in the C−H activation manifold.

Iron-Catalyzed ortho C-H Arylation and Methylation of Pivalophenone N-H Imines

Iron-catalyzed ortho C-H arylation and methylation reactions of pivalophenone N-H imines are reported. The pivaloyl N-H imine proved an excellent directing group for the arylation with diarylzinc reagents in the presence of an iron-diphosphine catalyst and 2,3-dichlorobutane at room temperature. A similar catalytic system also allowed methylation with Me₃ Al at 70 °C. The pivaloyl imine of the product could be readily converted to a cyano group, thus allowing convenient preparation of ortho-functionalized benzonitriles.

An Efficient, One-Pot Transamidation of 8-Aminoquinoline Amides Activated by Tertiary-Butyloxycarbonyl

The efficient, one-pot access to the transamidation of 8-aminoquinoline (8-AQ), notorious for its harsh removal conditions, has been widely employed as an auxiliary in C⁻H functionalization reactions due to its strong directing ability. In this study, the facile and mild Boc protection of the corresponding 8-AQ amide was critical to activate the amide C(acyl)⁻N bond by twisting its geometry to lower the amidic resonance energy. Both aryl and alkyl amines proceeded transamidation in one-pot, user-friendly conditions with excellent yields.

Design and Exploration of Catalytic Activity of Two-Dimensional Surface-Engineered Graphene Oxide Nanosheets in the Transannulation of N-Heterocyclic Aldehydes or Ketones with Alkylamines

In this work, pharmaceutically and biologically important compounds containing imidazo[1,5-a]pyridine nuclei have been synthesized via transannulation of N-heteroaryl aldehydes or ketones with alkylamines using a graphene oxide-supported copper catalyst. The nanocatalyst was fabricated by the covalent immobilization of 4-aminoantipyrine onto an amine-functionalized graphene oxide nanosupport followed by its metallation with copper acetate. Structural analysis by transmission electron microscopy, scanning electron microscopy, X-ray photoelectron spectroscopy (XPS), and X-ray diffraction demonstrates that the two-dimensional sheet-like structure of graphene oxide is maintained even after the chemical modifications, whereas XPS revealed crucial information related to elemental composition and surface electronic states of the metal present in the catalyst. Apart from this, Fourier transform infrared spectroscopy helped in identifying the degree of oxidation and the presence of oxygenated groups in graphene oxide nanocomposites. As a heterogeneous catalyst, this graphene oxide-supported copper complex showed moderate to good catalytic activity in the C(sp3)-H bond activation/amination of a variety of substrates. This superior catalytic performance originated from the unique 2-dimensional structure of graphene oxide-based material which provided space between graphitic overlayers due to appropriate positioning of metal on their basal planes, decreasing the diffusion resistances of reactant surfaces, thus making it function as a nanoreactor. More importantly, this nanomaterial could be recovered easily and reused repeatedly by simple washing without chemical treatment with no appreciable loss in its catalytic activity, showing good potential for increasing the overall turnover number of this synthetically useful catalyst.

Lithium diisobutyl-tert-butoxyaluminum hydride (LDBBA) catalyzed hydroboration of alkynes and imines with pinacolborane

Lithium diisobutyl-tert-butoxyaluminum hydride (LDBBA) catalyzed hydroboration of alkynes and imines with pinacolborane (HBpin) was demonstrated.

Unveiling the mechanism and regioselectivity of iron-dipyrrinato-catalyzed intramolecular C(sp3)–H amination of alkyl azides

The mechanism of iron-catalyzed C(sp³)–H amination was established, in which regioselectivity arose from both radical stability and ring strain.

Rhodium-catalyzed alkenyl C–H functionalization with amides

Rhodium-catalyzed pyridyl-directed alkenyl C–H bond functionalization with aroyl- or acryl-amides via C–H/C–N activation was carried out.

Iron-catalysed enantioselective Suzuki–Miyaura coupling of racemic alkyl bromides

The first iron-catalyzed enantioselective Suzuki–Miyaura coupling reaction has been established by using electron-deficient P-chiral bisphosphine ligand (R,R)-QuinoxP*.

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.