Ruthenium-Catalyzed Cross-Dehydrogenativeortho-N-Carbazolation of Diarylamines: Versatile Access to Unsymmetrical Diamines

C–N coupling reactions with arenes through C–H activation: the state-of-the-art versus the principles of green chemistry

Herein, we discuss the state-of-the-art in arene C–N coupling through C–H activation and to what extent it complies with the principles of green chemistry, with a focus on heterogeneously catalysed systems.

Catalyst-controlled site-selective N-H and C3-arylation of carbazole via carbene transfer reactions

A site-selective direct arylation reaction of carbazole and other N-heterocycles with diazo-naphthalen-2(1H)-ones has been developed. While Au(i)-NHC catalysts lead to selective C3-arylation, palladium acetate allows for selective N-H arylation, displaying complete site-selectivity each. To show the applicability of these arylation reactions, one-pot, two-fold diarylation reactions of carbazole were demonstrated.

Direct Amination of Aromatic C-H Bonds with Free Amines

Aromatic amines belong to a highly important class of organic compounds which are found in various natural products, functional materials, and pharmaceutical agents. Their prevalence has sparked continuing interest in the development of highly efficient and environmentally benign synthetic strategies for the construction of these compounds. Cross-dehydrogenative coupling reactions between two unmodified C(X)-H bonds have recently emerged as a versatile and powerful strategy for the fabrication of new C(X)-C(X) bonds. In this context, several procedures have been reported for the synthesis of aromatic amines through the direct amination of aromatic C-H bonds with free amines. This review highlights recent advances and progress in this appealing research arena, with special emphasis on the mechanistic features of the reactions.

Cu(II)-Catalyzed Ortho C(sp2)-H Diarylamination of Arylamines To Synthesize Triarylamines

A copper-catalyzed, directed ortho C-H diarylamination of indoles, indolines, anilines, and N-aryl-7-azaindoles has been established. Only copper salt as the catalyst and oxygen as the terminal oxidant are used to synthesize triarylamines using various diarylamines including carbazole and phenothiazine. Mechanistic interrogation reveals that copper plays a dual role.

Emerging unconventional organic solvents for C-H bond and related functionalization reactions

Solvent engineering is an increasingly essential topic in the chemical sciences. In this context, some recently appeared unconventional solvents have shown their large potential in the field of C-H bond functionalization reactions. This review aims not only at recognizing and classifying a short selection of these emerging solvents, in particular halogenated ones, but also at providing a medium term perspective of the possibilities they will offer for synthetic method development.

C-H Oxygenation Reactions Enabled by Dual Catalysis with Electrogenerated Hypervalent Iodine Species and Ruthenium Complexes

The catalytic generation of hypervalent iodine(III) reagents by anodic electrooxidation was orchestrated towards an unprecedented electrocatalytic C-H oxygenation of weakly coordinating aromatic amides and ketones. Thus, catalytic quantities of iodoarenes in concert with catalytic amounts of ruthenium(II) complexes set the stage for versatile C-H activations with ample scope and high functional group tolerance. Detailed mechanistic studies by experiment and computation substantiate the role of the iodoarene as the electrochemically relevant species towards C-H oxygenations with electricity as a sustainable oxidant and molecular hydrogen as the sole by-product. para-Selective C-H oxygenations likewise proved viable in the absence of directing groups.

One-pot ortho-amination of aryl C-H bonds using consecutive iron and copper catalysis

A one-pot approach for ortho-coupling of arenes with non-actived N-nucleophiles has been developed using sequential iron and copper catalysis. Regioselective ortho-activation of anisoles, anilines and phenols was achieved through iron(iii) triflimide catalysed iodination, followed by a copper(i)-catalysed, ligand-assisted coupling reaction with N-heterocycle, amide and sulfonamide-based nucleophiles. The synthetic utility of this one-pot, two-step method for the direct amination of ortho-aryl C-H bonds was demonstrated with the late-stage functionalisation of 3,4-dihydroquinolin-2-ones. This allowed the preparation of a TRIM24 bromodomain inhibitor and a series of novel analogues.

Visible Light Induced Metal-Free Carbene N-Carbazolation

Metal-free N-H functionalization reactions represent an important strategy for sustainable C-N coupling reactions. In this report, we describe the visible light photolysis of aryl diazoacetates in the presence of some N-heterocycles that enables mild, metal-free N-H functionalization reactions of carbazole and azepine heterocycles (15 examples, up to 83% yield).

Silver(i)-mediated oxidative C(sp3)–H amination of ethers with azole derivatives under mild conditions

A silver(i)-mediated oxidative N–H/C(sp³)–H coupling of NH-azoles with ethers has been developed.

Accelerated Ru-Cu Trinuclear Cooperative C-H Bond Functionalization of Carbazoles: A Kinetic and Computational Investigation

The mechanism of a trinuclear cooperative dehydrogenative C-N bond-forming reaction is investigated in this work, which avoids the use of chelate-assisting directing groups. Two new highly efficient Ru/Cu co-catalyzed systems were identified, allowing orders of magnitude greater TOFs than the previous state of the art. In-depth kinetic studies were performed in combination with advanced DFT calculations, which reveal a decisive rate-determining trinuclear Ru-Cu cooperative reductive elimination step (CRE).

Catalyst- and Reagent-Free Electrochemical Azole C-H Amination

Catalyst- and chemical oxidant-free electrochemical azole C-H aminations were accomplished via cross-dehydrogenative C-H/N-H functionalization. The catalyst-free electrochemical C-H amination proved feasible on azoles with high levels of efficacy and selectivity, avoiding the use of stoichiometric oxidants under ambient conditions. Likewise, the C(sp³ )-H nitrogenation proved viable under otherwise identical conditions. The dehydrogenative C-H amination featured ample scope, including cyclic and acyclic aliphatic amines as well as anilines, and employed sustainable electricity as the sole oxidant.

Cu-Catalyzed Cross-Dehydrogenative ortho-Aminomethylation of Phenols

A highly selective Cu^II‐catalyzed cross‐dehydrogenative ortho‐aminomethylation of phenols with aniline derivatives is described. The corresponding C(sp²)−C(sp³) coupling products were obtained in moderate to excellent yields under mild reaction conditions and with a broad substrate scope. A radical mechanism is proposed.

Computational Characterization of the Mechanism for the Oxidative Coupling of Benzoic Acid and Alkynes by Rhodium/Copper and Rhodium/Silver Systems

DFT calculations were applied to study the oxidative coupling between benzoic acid and 1-phenyl-1-propyne catalyzed by [CpRhCl₂ ]₂ (Cp=cyclopentadienyl) by using either Cu(OAc)₂ (H₂ O) or Ag(OAc) as the terminal oxidant, a process that has been experimentally shown to have subtleties related to regioselectivity (placement of the phenyl substituent of the alkyne in the isocoumarin product) and chemoselectivity (isocoumarin or naphthalene derivatives). Calculations reproduced the experimental results and showed the involvement of the oxidant throughout the catalytic cycle. The regioselectivity was found to be decided in the alkyne insertion step, in particular by the relative arrangement of the two phenyl groups. The high chemoselectivity towards isocoumarin associated to Cu(OAc)₂ (H₂ O) could be explained by the fact that the copper moiety blocks the CO₂ extrusion pathway, which would lead to naphthalene derivatives, something that does not happen if Ag(OAc) is used.

Cu(i)-Catalyzed amidation/imidation of N-arylglycine ester derivatives via C–N coupling under mild conditions

A copper-catalyzed cross-dehydrogenative amidation/imidation of N-arylglycine ester derivatives with benzamides, aromatic heterocyclic amides (oxindole, isatins), acyclic and cyclic amides (lactams) and imides (phthalimide, succinimide) under aerobic conditions has been developed.

Copper-Catalyzed Oxidative Dehydrogenative C(sp3 )-H Bond Amination of (Cyclo)Alkanes using NH-Heterocycles as Amine Sources

A copper-catalyzed oxidative C(sp³ )-H/N-H coupling of NH-heterocycles with affordable (cyclo)alkanes has been developed. This procedure involves C(sp³ )-N bond formation through a radical pathway generated by homolytic cleavage of di-tert-butyl peroxide and trapping of the radical(s) by copper catalysts. The reaction tolerates a series of functional groups, such as bromo, fluoro, ester, ketone, nitrile, methyl, and methoxy. free-NH-containing indoles, pyrroles, pyrazoles, indazoles, and benzotriazoles are successfully N-alkylated.

The cumene/O2 system: a very simple tool for the radical chain oxidation of some functional groups

Due to the relative stability of the cumyl radical, cumenes and α-methyl-styrenes are ideally structured to directly harvest the oxidizing reactivity of O2 and initiate radical chain reactions in catalyst-free conditions. In the absence of additional substrates, these processes can lead to acetophenones. In the presence of substrates, the cumene oxidation process can be intercepted in various chain reactions, affording very simple protocols for functional group oxidation.

Intermolecular Aryl C-H Amination through Sequential Iron and Copper Catalysis

A mild, efficient and regioselective method for para-amination of activated arenes has been developed through a combination of iron and copper catalysis. A diverse range of products were obtained from an operationally simple one-pot, two-step procedure involving bromination of the aryl substrate with the powerful Lewis acid iron(III) triflimide, followed by a copper(I)-catalysed N-arylation reaction. This two-step dehydrogenative process for the regioselective coupling of aromatic C-H bonds with non-activated amines was applicable to anisole-, phenol-, aniline- and acetanilide-type aryl compounds. Importantly, the arene substrates were used as the limiting reagent and required no protecting-group manipulations during the transformation.

Mechanically Induced Fe(III) Catalysis at Room Temperature: Solvent-Free Cross-Dehydrogenative Coupling of 3-Benzylic Indoles with Methylenes/Indoles

An Fe(III)-catalyzed solvent-free cross-dehydrogenative coupling of 3-benzylic indoles and compounds with acidic methylene groups has been achieved under high-speed ball-milling (HSBM) conditions at room temperature. The reactions afford desired 3-arylmethylindole derivatives in moderate to high yields within 21 min of grinding. Besides, both N-substituted and N-free indoles can take part in this mechanochemical reaction as efficient nucleophiles to give bisindoles with satisfactory results. Remarkably, this protocol displays the possibility to induce high activity when using iron catalysts under HSBM conditions.

Visible-Light-Promoted Direct Amination of Phenols via Oxidative Cross-Dehydrogenative Coupling Reaction

A transition-metal-free approach was disclosed for intermolecular aryl C-N bonds formation between phenols and cyclic anilines via cross-dehydrogenative coupling (CDC) amination that was mediated by visible light, wherein K2S2O8 served as an external oxidant. The salient features of this protocol include circumventing the requirement for prefunctionalized starting materials and achieving single regioselectivity of amination adducts at room temperature.

Heterocyclic Nanographenes and Other Polycyclic Heteroaromatic Compounds: Synthetic Routes, Properties, and Applications

Two-dimensionally extended, polycyclic heteroaromatic molecules (heterocyclic nanographenes) are a highly versatile class of organic materials, applicable as functional chromophores and organic semiconductors. In this Review, we discuss the rich chemistry of large heteroaromatics, focusing on their synthesis, electronic properties, and applications in materials science. This Review summarizes the historical development and current state of the art in this rapidly expanding field of research, which has become one of the key exploration areas of modern heterocyclic chemistry.

A General Method for Aminoquinoline-Directed, Copper-Catalyzed sp(2) C-H Bond Amination

An operationally simple and general method for copper-catalyzed, aminoquinoline-assisted amination of β-C(sp(2))-H bonds of benzoic acid derivatives is reported. The reaction employs Cu(OAc)2 or (CuOH)2CO3 catalysts, an amine coupling partner, and oxygen from air as a terminal oxidant. Exceptionally high generality with respect to amine coupling partners is observed. Specifically, primary and secondary aliphatic and aromatic amines, heterocycles, such as indoles, pyrazole, and carbazole, sulfonamides, as well as electron-deficient aromatic and heteroaromatic amines are competent coupling components.

Ru-Catalyzed C-H Arylation of Fluoroarenes with Aryl Halides

Although the ruthenium-catalyzed C–H arylation of arenes bearing directing groups with haloarenes is well-known, this process has never been achieved in the absence of directing groups. We report the first example of such a process and show that unexpectedly the reaction only takes place in the presence of catalytic amounts of a benzoic acid. Furthermore, contrary to other transition metals, the arylation site selectivity is governed by both electronic and steric factors. Stoichiometric and NMR mechanistic studies support a catalytic cycle that involves a well-defined η⁶-arene-ligand-free Ru(II) catalyst. Indeed, upon initial pivalate-assisted C–H activation, the aryl-Ru(II) intermediate generated is able to react with an aryl bromide coupling partner only in the presence of a benzoate additive. In contrast, directing-group-containing substrates (such as 2-phenylpyridine) do not require a benzoate additive. Deuterium labeling and kinetic isotope effect experiments indicate that C–H activation is both reversible and kinetically significant. Computational studies support a concerted metalation–deprotonation (CMD)-type ruthenation mode and shed light on the unusual arylation regioselectivity.

Copper(ii)-mediated regioselective N-arylation of pyrroles, indoles, pyrazoles and carbazole via dehydrogenative coupling

Copper-mediated regioselective N-arylation of pyrroles, indoles, pyrazoles and carbazole is described using 8-aminoquinoline amide as a directing group via dehydrogenative coupling. The protocol has a broad substrate scope with good yields at moderate temperature.

Reversed reactivity of anilines with alkynes in the rhodium-catalysed C-H activation/carbonylation tandem

Development of multicatalytic approach consisting of two or more mechanistically distinct catalytic steps using a single-site catalyst for rapid and straightforward access of structurally complex molecules under eco-benign conditions has significance in contemporary science. We have developed herein a rhodium-catalysed C-H activation strategy which uses an unprotected anilines and an electron-deficient alkynes to C-C bonded products as a potential intermediate in contrast to the archetypical C-N bonded products with high levels of regioselectivity. This is followed by carbonylation of C-H bond activated intermediate and subsequent annulation into quinolines has been described. This rhodium-catalysed auto-tandem reaction operates under mild, environmentally benign conditions using water as the solvent and CO surrogates as the carbonyl source with the concomitant generation of hydrogen gas. The strategy may facilitate the development of new synthetic protocols for the efficient and sustainable production of chemicals in an atom-economic way from simple, abundant starting materials.

A bio-inspired synthesis of oxindoles by catalytic aerobic dual C-H functionalization of phenols

We report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of oxygen.

Nitrogen-containing heterocycles are fundamentally important to the function of pharmaceuticals, agrochemicals and materials. Herein, we report a bio-inspired approach to the synthesis of oxindoles, which couples the energetic requirements of dehydrogenative C–N bond formation to the reduction of molecular oxygen (O₂). Our method is inspired by the biosynthesis of melanin pigments (melanogenesis), but diverges from the biosynthetic polymerization. Mechanistic analysis reveals the involvement of Cu^II-semiquinone radical intermediates, which enable dehydrogenative carbon–heteroatom bond formation that avoids a catechol/quinone redox couple. This mitagates the deleterious polarity reversal that results from phenolic dearomatization, and enables a high-yielding phenolic C–H functionalization under catalytic aerobic conditions. Our work highlights the broad synthetic utility and efficiency of forming C–N bonds via a catalytic aerobic dearomatization of phenols, which is currently an underdeveloped transformation.

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.