Chelation‐Assisted Nickel‐Catalyzed Oxidative Annulation via Double C−H Activation/Alkyne Insertion Reaction

Transition Metal-Catalyzed Dual C-H Activation/Annulation Reactions Involving Internal Alkynes

Recently, transition metal-catalyzed ortho-C-H bond activation/annulations involving two internal alkyne molecules have been extensively used to synthesize highly substituted polycyclic aromatic scaffolds. Such reactions have emerged as a powerful atom and step-economical strategy for the assembly of multifunctional bioactive molecules. In this context, we focused on the recent achievements of dual C-H bond activation/annulations, as well as functionalization reactions involving diaryl/alkyl alkynes.

Regioselective Twofold Annulation of Propargyl Acetates

The regioselective annulation of unsymmetrical alkynes has always been a focused research topic. A Pd-catalyzed double annulation of unsymmetrical alkynes (i.e., yne-acetates) with aryl diazonium salts for the synthesis of substituted naphthalene derivatives is developed. The process addresses intrinsic regioselectivity challenges in the annulations of unsymmetrical alkynes. Mechanistic investigations shed light on the crucial role of the acetate-directing groups in determining the regiochemical reaction outcome.

Nickel-Catalyzed Tandem Cyclization of 1,6-Diynes with Indolines/Indoles through Dual C-H Bond Activation

A nickel-catalyzed site-selective tandem cyclization of 1,6-diynes with substituted indolines or indoles through consecutive dual C-H bond activation is described. In the reaction, substituted fused indole and carbazole derivatives were observed in good to excellent yields, in which three consecutive C-C bonds formed in one pot. Later, in the presence of DDQ, the aromatization of the indoline derivative was converted to the indole derivative. A possible reaction mechanism involving dual C-H bond activation as a key step was proposed to account for the present reaction.

Carbonylative Formal Cycloaddition between Alkylarenes and Aldimines Enabled by Palladium-Catalyzed Double C-H Bond Activation

Double C-H bond activation can enable an expeditious reaction pathway to cyclic compounds, offering an efficient tool to synthesize valuable molecules. However, cyclization reaction enabled by double C-H bond activation at one carbon atom is nearly unknown. Herein, we report a carbonylative formal cycloaddition of alkylarenes with imines via double benzylic C-H bond activation at one carbon atom, allowing a straightforward synthesis of β-lactams from readily accessible alkylarenes and imines, which paves the way for developing an annulation reaction through double C-H bond activation at one carbon atom.

Switching the Reactivity of the Nickel-Catalyzed Reaction of 2-Pyridones with Alkynes: Easy Access to Polyaryl/Polyalkyl Quinolinones

A Ni-catalyzed C6 followed by C5 cascade C-H activation/[2 + 2 + 2] annulation of 2-pyridone with alkynes has been achieved. A change in the reaction pathway was achieved by tuning the reaction conditions and incorporating a directing group. A wide variety of substrates and alkynes are amenable to this transformation. The key to success for this transformation is the use of sodium iodide as an additive. More importantly, we detected the five-membered metallacycle intermediate through HRMS wherein iodide is ligated to the metal.

Nickel-catalyzed [2 + 2 + 2] benzannulation of alkynes: a new route to the synthesis of highly substituted naphthalenes

The Ni(II)-catalyzed aromatic homologation of various directing groups with alkynes to afford highly substituted naphthalene products via C-X bond cleavage followed by alkyne insertion and C-H activation is described. This reaction proceeded with a wide range of directing groups such as pyrazoles, imidazopyridines, benzoimidazothiazoles, thiazoles and triazoles with alkynes to afford highly substituted naphthalenes in moderate to good yields. This transformation is promoted by a simple, straightforward combination of a Ni(II)-complex, Zn dust and a base.

Multiple annulations of inert C(sp2)-H bonds with alkynes

Transition-metal catalyzed directing group (DG) assisted annulation of inert C-H bonds leads to the formation of complex molecular frameworks from readily accessible substrates. Thus, multiple annulation of less functionalized substrates with unsaturated species leads to the construction of structurally diverse fused poly(hetero)cycles. The directed inert C(arene)-H bond activation and the mode of TM-migration in this process could enabled obatining L-type [involves DG heteroatom, o-C(arene)-H bond, and C(arene)-H bond of aryl-motif in alkyne], Y-type [involves two heteroatoms of the DG and o-,o'-C(arene)-H bonds], and B-type [involves o-C(arene)-H bond and m-C(arene)-H bond] π-extended annulation products. The coordination preference of the DG heteroatom makes the transformation chemo- and regio-selective. This article underlines the conceptual development of unsymmetrical multiple annulation of arene C(sp²)-H bonds with alkynes, which is exceedingly appealing and highly important.

RhodiumIII-Catalyzed Remote Difunctionalization of Arenes Assisted by a Relay Directing Group

Rhodium-catalyzed diverse tandem twofold C-H bond activation reactions of para olefin-tethered arenes have been realized, with unsaturated reagents such as internal alkynes, dioxazolones, and isocyanates being the coupling partner as...

Synthesis and Photophysical Study of Heteropolycyclic and Carbazole Motif: Nickel-Catalyzed Chelate-Assisted Cascade C-H Activations/Annulations

Herein, nickel-catalyzed synthesis of polyarylcarbazole through sequential C-H bond activations has been described. Regioselective indole C2/C3 functionalization has been achieved in the presence of indole C7-H, which is quite challenging. In addition, this approach also gives easy access to building a heteropolycyclic motif through C6/C7 C-H functionalization of indoline. This methodology is not limited to aromatic internal alkynes as coupling partners; aliphatic alkynes have also shown good tolerance. Notably, during the optimization the catalytic enhancement with sodium iodide as an additive has been observed. We have also studied the photophysical properties of these highly conjugated molecules.

Synthesis of π-Extended Heterocycles via Rh(III)-Catalyzed Oxidative Annulation of 5-Aryl Pyrazinones with Alkynes

The Rh(III)-catalyzed C-H functionalization and subsequent oxidative annulation between 5-aryl pyrazinones and internal alkynes are reported. This protocol provides facile access to a wide range of pyrazinone-linked naphthalenes via the C(sp²)-H alkenylation and subsequent annulation. This transformation is characterized by mild conditions, simplicity, and excellent functional group compatibility. Notably, it is a first report of the utilization of pyrazinones as directing groups in C-H functionalization.

Cobalt-catalyzed coupling reactions of 2-halobenzamides with alkynes: investigation of ligand-controlled dual pathways

The Co-catalyzed reactions of 2-halobenzamides and alkynes to form quinolinones or 2-vinyl benzamides are described here. These two reactions can be controlled merely by ligands.

Iridium(III) Catalysts with an Amide-Pendant Cyclopentadienyl Ligand: Double Aromatic Homologation Reactions of Benzamides by Fourfold C-H Activation

The synthesis, characterization, and catalytic performance of iridium(III) catalysts that bear an amide-pendant cyclopentadienyl ligand ([Cp^A IrI₂ ]₂ ) is reported. These [Cp^A IrI₂ ]₂ catalysts were obtained from the complexation of a Cp^A ligand precursor with [Ir(cod)OAc]₂ followed by oxidation. Double aromatic homologation reactions of benzamides with alkynes by fourfold C-H activation proceeded in good to high yield with these [Cp^A IrI₂ ]₂ catalysts, demonstrating their superior catalytic performance in this challenging transformation.

N-Heterocyclic Carbene Complexes in C-H Activation Reactions

In this contribution, we provide a comprehensive overview of C-H activation methods promoted by NHC-transition metal complexes, covering the literature since 2002 (the year of the first report on metal-NHC-catalyzed C-H activation) through June 2019, focusing on both NHC ligands and C-H activation methods. This review covers C-H activation reactions catalyzed by group 8 to 11 NHC-metal complexes. Through discussing the role of NHC ligands in promoting challenging C-H activation methods, the reader is provided with an overview of this important area and its crucial role in forging carbon-carbon and carbon-heteroatom bonds by directly engaging ubiquitous C-H bonds.

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.

N-Iminopyridinium ylide-directed, cobalt-catalysed coupling of sp2 C-H bonds with alkynes

N-Iminopyridinium ylides are competent monodentate directing groups for cobalt-catalysed annulation of sp2 C-H bonds with internal alkynes. The pyridine moiety in the ylide serves as an internal oxidant and is cleaved during the reaction. The annulation reactions possess excellent compatibility with heterocyclic substrates, tolerating furan, thiophene, pyridine, pyrrole, pyrazole, and indole functionalities.

Rhodium-Catalyzed [2+1+2+1] Cycloaddition of Benzoic Acids with Diynes through Decarboxylation and C≡C Triple Bond Cleavage

It has been established that an electron-deficient cyclopentadienyl rhodium(III) (Cp^E Rh^III ) complex catalyzes the oxidative and decarboxylative [2+1+2+1] cycloaddition of benzoic acids with diynes through C≡C triple bond cleavage, leading to fused naphthalenes. This cyclotrimerization is initiated by directed ortho C-H bond cleavage of a benzoic acid, and the subsequent regioselective alkyne insertion and decarboxylation produce a five-membered rhodacycle. The electron-deficient nature of the Cp^E Rh^III complex promotes reductive elimination giving a cyclobutadiene-rhodium(I) complex rather than the second intermolecular alkyne insertion. The oxidative addition of the thus generated cyclobutadiene to rhodium(I) (formal C≡C triple bond cleavage) followed by the second intramolecular alkyne insertion and reductive elimination give the corresponding [2+1+2+1] cycloaddition product. The synthetic utility of the present [2+1+2+1] cycloaddition was demonstrated in the facile synthesis of a donor-acceptor [5]helicene and a hemi-hexabenzocoronene by a combination with the chemoselective Scholl reaction.

Enantioselective Synthesis of C-N Axially Chiral N-Aryloxindoles by Asymmetric Rhodium-Catalyzed Dual C-H Activation

The first enantioselective Satoh-Miura-type reaction is reported. A variety of C-N axially chiral N-aryloxindoles have been enantioselectively synthesized by an asymmetric rhodium-catalyzed dual C-H activation reaction of N-aryloxindoles and alkynes. High yields and enantioselectivities were obtained (up to 99 % yield and up to 99 % ee). To date, it is also the first example of the asymmetric synthesis of C-N axially chiral compounds by such a C-H activation strategy.

Computational Study of the Ni-Catalyzed C-H Oxidative Cycloaddition of Aromatic Amides with Alkynes

The mechanism of Ni-catalyzed ortho C(sp²)-H oxidative cycloaddition of aromatic amides with internal alkynes containing 2-pyridinylmethylamine directing group was investigated using density functional theory (DFT) calculations. The C-H cleavage step proceeds via σ-complex-assisted metathesis (σ-CAM) with an alkenyl-Ni(II) complex. This is in contrast to the more common carboxylate/carbonate-assisted concerted metalation-deprotonation mechanism in related Ni-catalyzed C-H bond functionalization reactions with N,N-bidentate directing groups. In this reaction, the alkyne not only serves as the coupling partner, but also facilitates the σ-CAM C-H metalation both kinetically and thermodynamically. The subsequent functionalization of the five-membered nickelacycle proceeds via alkyne insertion into the Ni-C bond to form a seven-membered nickelacycle. This process proceeds with high levels of regioselectivity to form a C-C bond with sterically more encumbered alkyne terminus. This unusual regioselectivity is due to steric repulsions with the directing group that is coplanar with the alkyne in the migratory insertion transition state. The C-N bond reductive elimination to form the isoquinolone cycloadduct is promoted by PPh₃ complexation to the Ni center and the use of flexible 2-pyridinylmethylamine directing group. The origin of the cis-trans isomerism of alkene byproduct was also explained by computations.

Nickel-catalyzed oxidative C-H/N-H annulation of N-heteroaromatic compounds with alkynes

The reaction of N-heteroaromatic compounds, such as 2-aryl-pyrrole, benzimidazole, imidazole, indole, and pyrazole derivatives, with alkynes in the presence of a catalytic amount of a nickel complex results in C-H/N-H oxidative annulation. The reaction shows a high functional group compatibility. While both Ni(0) and Ni(ii) complexes show a high catalytic activity, Ni(0) is proposed as a key catalytic species in the main catalytic cycle. In the case of the Ni(ii) system, the presence of a catalytic amount of a strong base, such as KOBu t , is required for the reaction to proceed. In sharp contrast, a base is not required in the case of the Ni(0) system. The proposed mechanism is supported by DFT studies.

Recent advances and prospects in nickel-catalyzed C–H activation

Nickel-catalyzed C–H activation has become a predominant and ubiquitous research area in organic chemistry.

A comprehensive overview of directing groups applied in metal-catalysed C-H functionalisation chemistry

The present review is devoted to summarizing the recent advances (2015-2017) in the field of metal-catalysed group-directed C-H functionalisation. In order to clearly showcase the molecular diversity that can now be accessed by means of directed C-H functionalisation, the whole is organized following the directing groups installed on a substrate. Its aim is to be a comprehensive reference work, where a specific directing group can be easily found, together with the transformations which have been carried out with it. Hence, the primary format of this review is schemes accompanied with a concise explanatory text, in which the directing groups are ordered in sections according to their chemical structure. The schemes feature typical substrates used, the products obtained as well as the required reaction conditions. Importantly, each example is commented on with respect to the most important positive features and drawbacks, on aspects such as selectivity, substrate scope, reaction conditions, directing group removal, and greenness. The targeted readership are both experts in the field of C-H functionalisation chemistry (to provide a comprehensive overview of the progress made in the last years) and, even more so, all organic chemists who want to introduce the C-H functionalisation way of thinking for a design of straightforward, efficient and step-economic synthetic routes towards molecules of interest to them. Accordingly, this review should be of particular interest also for scientists from industrial R&D sector. Hence, the overall goal of this review is to promote the application of C-H functionalisation reactions outside the research groups dedicated to method development and establishing it as a valuable reaction archetype in contemporary R&D, comparable to the role cross-coupling reactions play to date.

Nickel-catalyzed direct C-H trifluoroethylation of heteroarenes with trifluoroethyl iodide

A highly selective nickel-catalyzed C-H trifluoroethylation of heteroarenes was developed with the assistance of a monodentate directing group. This protocol provides efficient access to various trifluoroethyl-substituted heteroarenes, including indoles, pyrroles, furans, and thiophenes, with commercially available CF₃CH₂I as an alkylation reagent. This robust catalytic procedure is scalable and tolerates a broad range of functional groups. Moreover, multifluoroalkylation of indoles is also viable.

Nickel catalyzed site selective C–H functionalization of α-aryl-thioamides

A nickel catalyzed C–H bond functionalization reaction has been used for the first time to study an intramolecular site-selective C–S bond formation of arenes.

Ni(i)–Ni(iii) vs. Ni(ii)–Ni(iv): mechanistic study of Ni-catalyzed alkylation of benzamides with alkyl halides

DFT investigations into the mechanism of Ni-catalyzed alkylation of benzamides with alkyl halides are reported. Computational results show that the Ni(ii)–Ni(iv) catalytic cycle is favorable; meanwhile, the oxidative addition of alkylbromide forms a Ni(iv) intermediate and is the rate-determining step of the whole catalytic cycle.

The mechanism of directed Ni(ii)-catalyzed C-H iodination with molecular iodine

This computational study reveals electrophilic cleavage pathways for substrates with N,N-bidentate directing centers in Ni(ii)-catalyzed C–H iodination with molecular iodine.

The density functional theory method is used to elucidate the elementary steps of Ni(ii)-catalyzed C(sp²)–H iodination with I₂ and substrates bearing N,N′-bidentate directing centers, amide-oxazoline (AO) and 8-aminoquinoline (AQ). The relative stability of the lowest energy high- and low-spin electronic states of the catalyst and intermediates is found to be an important factor for all of the steps in the reaction. As a result, two-state reactivity for these systems is reported, where the reaction is initiated on the triplet surface and generates a high energy singlet nickelacycle. It is shown that the addition of Na₂CO₃ base to the reaction mixture facilitates C–H activation. The presence of I₂ in the reaction provides the much needed driving force for the C–H activation and nickelacycle formation and ultimately reacts to form a new C–I bond through either a redox neutral electrophilic cleavage (EC) pathway or a one-electron reductive cleavage (REC) pathway. The previously proposed Ni(ii)/Ni(iv) and homolytic cleavage pathways are found to be higher in energy. The nature of the substrate is found to have a large impact on the relative stability of the lowest electronic states and on the stability of the nickelacycle resulting from C–H activation.

Nickel-catalyzed C-H/N-H annulation of aromatic amides with alkynes in the absence of a specific chelation system

The Ni-catalyzed reaction of aromatic amides with alkynes in the presence of KOBu t involves C-H/N-H oxidative annulation to give 1(2H)-isoquinolinones. A key to the success of the reaction is the use of a catalytic amount of strong base, such as KOBu t . The reaction shows a high functional group compatibility. The reaction with unsymmetrical alkynes, such as 1-arylalkynes, gives the corresponding 1(2H)-isoquinolinones with a high level of regioselectivity. This discovery would lead to the development of Ni-catalyzed chelation-assisted C-H functionalization reactions without the need for a specific chelation system.

Computational Study of Ni-Catalyzed C-H Functionalization: Factors That Control the Competition of Oxidative Addition and Radical Pathways

The mechanisms of Ni-catalyzed C-H arylation, alkylation, and sulfenylation with N,N-bidentate directing groups are investigated using density functional theory (DFT) calculations. While the C-H cleavage occurs via the concerted metalation-deprotonation (CMD) mechanism in all types of reactions, the subsequent C-C and C-X bond formation steps may occur via either oxidative addition to form a Ni(IV) intermediate or radical pathways involving Ni(III) complexes generated from homolytic dissociation of disulfides/peroxides or halide-atom transfer from alkyl halides. DFT calculations revealed that radical mechanisms are preferred in reactions with sterically hindered coupling partners with relatively low bond dissociation energies (BDE) such as dicumyl peroxide, heptafluoroisopropyl iodide and diphenyl disulfide. In contrast, these radical processes are highly disfavored when generating unstable phenyl and primary alkyl radicals. In such cases, the reaction proceeds via an oxidative addition/reductive elimination mechanism involving a Ni(IV) intermediate. These theoretical insights into the substrate-controlled mechanisms in the C-H functionalizations were employed to investigate a number of experimental phenomena including substituent effects on reactivity, chemo- and regioselectivity and the effects of oxidant in the intermolecular oxidative C-H/C-H coupling reactions.