The cross-dehydrogenative coupling of C(sp3)-H bonds: a versatile strategy for C-C bond formations

Transition-metal-catalyzed ortho C-H functionalization of 2-arylquinoxalines

Recently, direct C-H bond activation and functionalization has become a prodigious and hot topic among synthetic organic chemists due to its step-economic nature and substantial synthetic versatility. On the other hand, quinoxaline, a fused bicycle of benzene and pyrazine, has omnipresent applications in medicinal-, industrial- and materials chemistry. The presence of the N-1 atom in 2-arylquinoxaline enables chelation formation with a metal catalyst leading to the formation of ortho-substituted products. In this review, all articles related to the ortho C-H bond functionalization of 2-arylquinoxalines published up to May 2022 are highlighted.

Synthesis of nuevamine and a cyano-chilenine analog via divergent C(sp3)-H bond functionalization of isoindolinone derivatives

Divergent C(sp³)-H bond functionalizations of isoindolinone derivatives were developed to synthesize nuevamine, a cyano-chilenine derivative, and two related analogs. A copper-catalyzed C-H cross-dehydrogenative coupling (via cation formation) allowed the formation of a new C-C bond leading to the direct assembly of the isoindolo[1,2-a]isoquinolinone tetracyclic system of the nuevamine. The syntheses of the cyano-chilenine derivatives were carried out by installing two nitrile groups under basic conditions (via anion formation). Then, the isoindolobenzazepinic system of the chilenine skeleton was constructed by a Houben-Hoesch cyclization process. The present methodology has the advantage of not requiring the use of pre-functionalized substrates.

Regioselective α-Cyanation of Unprotected Alicyclic Amines

Secondary alicyclic amines are converted to α-aminonitriles via addition of TMSCN to their corresponding imines, intermediates that are produced in situ via the oxidation of amine-derived lithium amides with simple ketone oxidants. Amines with an existing α-substituent undergo regioselective α'-cyanation even if the C-H bonds at that site are less activated. Amine α-arylation can be combined with α'-cyanation to generate difunctionalized products in a single operation.

Formal β-C-H Arylation of Aldehydes and Ketones by Cooperative Nickel and Photoredox Catalysis

α-C-H-functionalization of ketones and aldehydes has been intensively explored in organic synthesis. The functionalization of unactivated β-C-H bonds in such carbonyl compounds is less well investigated and developing a general method for their β-C-H arylation remains challenging. Herein we report a method that uses cooperative nickel and photoredox catalysis for the formal β-C-H arylation of aldehydes and ketones with (hetero)aryl bromides. The method features mild conditions, remarkable scope and wide functional group tolerance. Importantly, the introduced synthetic strategy also allows the β-alkenylation, β-alkynylation and β-acylation of aldehydes under similar conditions. Mechanistic studies revealed that this transformation proceeds through a single electron oxidation/Ni-mediated coupling/reductive elimination cascade.

Pd(ii)-catalyzed meta-C-H bromination and chlorination of aniline and benzoic acid derivatives

The classic electrophilic bromination leads to ortho- and para-bromination of anilines due to their electron-rich properties. Herein we report the development of an unprecedented Pd-catalyzed meta-C-H bromination of aniline derivatives using commercially available N-bromophthalimide (NBP), which overcomes the competing ortho/para-selectivity of electrophilic bromination of anilines. The addition of acid additives is crucial for the success of this reaction. A broad range of substrates with various substitution patterns can be tolerated in this reaction. Moreover, benzoic acid derivatives bearing complex substitution patterns are also viable with this mild bromination reaction, and meta-C-H chlorination is also feasible under similar reaction conditions. The ease of the directing group removal and subsequent diverse transformations of the brominated products demonstrate the application potential of this method and promise new opportunities for drug discovery.

Transition metal catalyzed selective B(3)-H or B(4)-H amination of o-carboranes via dehydrogenative BH/NH cross-coupling

A unique approach to vertex-selective catalytic B-H amination at either the B(3)- or B(4)-position in o-carboranes has been developed. Using different transition metal catalysts, dehydrogenative BH/NH cross-coupling of o-carboranes and free amines has been achieved, leading to a wide variety of cage B(3)- or B(4)-aminated o-carboranes in moderate to high yields with excellent regioselectivity, where carboranyl carboxylic acids and amines can serve as competent coupling partners without any pre-functionalization. The isolation and structural identification of a key intermediate provide an insight into the reaction mechanism in the catalytic B(4)-H amination.

Electrochemical Migratory Cyclization of N-Acylsulfonamides

Benzoxathiazine dioxide, as a bioisostere of the clinically widely used diazoxide, exhibits interesting biological activity. However, limited success has been achieved in terms of its concise and direct synthesis. We report herein a facile electrochemical migratory cyclization of N-acylsulfonamides to access a diverse array of benzoxathiazine dioxides. The inclusion of electrochemistry is crucial for realizing such a novel transformation, which is substantiated both by the experiments and density-functional-theory calculations.

A Tandem Dehydrogenation-Driven Cross-Coupling between Cyclohexanones and Primary Amines for Construction of Benzoxazoles

Herein, we report a transition metal-free, operationally simple, general method for straightforward syntheses of 2-substituted benzoxazoles from readily available cyclohexanones and aliphatic primary amines by an imine α-oxygenation-initiated cascade reaction sequence. The key to achieving high selectivity and excellent functional-group tolerance is the use of TEMPO as a mild oxidant that selectively oxidizes the reaction intermediates through its multiple reactivity modes, thus facilitating the individual steps to proceed in succession. More than 70 substrate combinations are disclosed, demonstrating the reliability of this protocol to synthesize structurally diverse products, including marketed drugs, drug candidate, and natural products that are unattainable by the existing methods.

Ortho C-H Functionalizations of 2-Aryl-2H-Indazoles

C-H Functionalization is ubiquitously considered as a powerful, efficient and handy tool for installing various functional groups in complex organic heterocycles in an easier and step-economic way. Similarly, indazole is endowed as a potent heterocycle and is eminent for its profound impact in biological, medicinal and industrial chemistry. In this scenario, C-H functionalization at the selective ortho position of 2-arylindazole in assistance of a metal catalyst is also becoming an appealing approach in synthetic organic chemistry. This review addressed the recent findings and developments on ortho C-H functionalization of 2-aryl-2H-indazazoles with literature coverage extending from 2018 to May 2022.

Iridium-Catalyzed Borylation of 6-Fluoroquinolines: Access to 6-Fluoroquinolones

The Ir-catalyzed C–H borylation of fluoroquinolines has been realized. The quinoline boronic ester formed undergoes a range of important transformations of relevance to medicinal chemistry. Judicious choice of the substituent at C4 on the quinoline facilitated the unmasking of a fluoroquinolone—the core structure of many antibiotics.

(NH4)2S2O8-Mediated Metal-Free Decarboxylative Formylation/Acylation of α-Oxo/Ketoacids and Its Application to the Synthesis of Indole Alkaloids

A metal-free method for the formylation/acylation of indoles and β-carbolines with (NH₄)₂S₂O₈ via direct decarboxylative cross-coupling of α-oxo/ketoacids in moderate to good yields is described. The reaction occurs between ambient temperature and 40 °C under mild reaction conditions with commercially available starting materials. This methodology can be expanded to some biologically active indole alkaloids like pityriacitrins, eudistomins Y₁ and Y₃, and marinacarbolines A-D.

Visible-Light-Promoted Cross Dehydrogenative/Decarboxylative Coupling Cascades of Glycine Ester Derivatives and β-Keto Acids

A visible-light-induced dehydrogenative/decarboxylative coupling reaction of arylglycine derivatives and β-keto acids is described. This photocatalyst- and additive-free protocol can be applied in the efficient synthesis of γ-keto glycine derivatives under ambient conditions. Further uses of this methodology and a plausible mechanism are also demonstrated.

O2-Mediated Te(II)-Redox Catalysis for the Cross-Dehydrogenative Coupling of Indoles

Very few elements in the periodic system can catalytically activate O₂, such as in the context of cross-dehydrogenative couplings. The development of O₂-activating catalysts is essential to enable new and sustainable reactivity concepts to emerge, because these catalysts also often feature specific activating interactions with the target substrates. In this context, the unprecedented Te(II)/Te(III) catalyzed dehydrogenative C3-C2 dimerization of indoles is described herein. The fact that O₂ can be directly utilized as a terminal oxidant in this reaction, as well as the absence of any background reactivity without the redox-active Te catalyst, constitute very important milestones for the fields of cross-dehydrogenative couplings and tellurium catalysis.

TEMPO-Promoted Mono- and Bisimidation of Tertiary Anilines: Synthesis of Symmetric and Unsymmetric N-Mannich Bases

A TEMPO-promoted method was developed for the synthesis of symmetric bis-N-Mannich bases via sequential activation of two α,α'-amino C(sp³)-H bonds of N,N-dimethylanilines under mild conditions. This methodology was further extended for monoimidation of α-amino-functionalized methylanilines to give unsymmetric N-Mannich bases in good to high yields. Several control experiments were performed, and the coupling reaction outcomes indicated that the oxoammonium (TEMPO⁺) species is involved in the reaction.

Direct phosphorylation of benzylic C-H bonds under transition metal-free conditions forming sp3C-P bonds

Direct phosphorylation of benzylic C-H bonds was achieved in a biphasic system under transition metal-free conditions. A selective radical/radical sp3C-H/P(O)-H cross coupling was proposed, and various substituted toluenes were applicable. The transformation provided a promising method for constructing sp3C-P bonds.

Synthetic applications of hydride abstraction reactions by organic oxidants

Carbon-hydrogen bond functionalizations provide an attractive method for streamlining organic synthesis, and many strategies have been developed for conducting these transformations. Hydride-abstracting reactions have emerged as extremely effective methods for oxidative bond-forming processes due to their mild reaction conditions and high chemoselectivity. This review will predominantly focus on the mechanism, reaction development, natural product synthesis applications, approaches to catalysis, and use in enantioselective processes for hydride abstractions by quinone, oxoammonium ion, and carbocation oxidants. These are the most commonly employed hydride-abstracting agents, but recent efforts illustrate the potential for weaker ketone and triaryl borane oxidants, which will be covered at the end of the review.

Advances in free-radical alkylation and arylation with organoboronic acids

Organoboronic acids act as carbon-centered radical precursors that are widely utilized to construct diverse C-C bonds. This review summarizes the advances in this field. The content is divided into four parts according to the different categories of coupling partners with organoboronic acids. The reaction conditions as well as the mechanisms are demonstrated in each part.

Photocatalytic C(sp3) radical generation via C-H, C-C, and C-X bond cleavage

C(sp³) radicals (R˙) are of broad research interest and synthetic utility. This review collects some of the most recent advancements in photocatalytic R˙ generation and highlights representative examples in this field. Based on the key bond cleavages that generate R˙, these contributions are divided into C–H, C–C, and C–X bond cleavages. A general mechanistic scenario and key R˙-forming steps are presented and discussed in each section.

C(sp³) radicals (R˙) are of broad research interest and synthetic utility.

Formal C-H/C-I Metathesis: Site-Selective C-H Iodination of Anilines Using Aryl Iodides

Functional group metathesis has the potential to render mild reaction conditions for C-H functionalization. Protocols for the meta- and ortho-C-H iodination of aniline derivatives via formal C(sp²)-H/C(sp²)-I metathesis using 2-nitrophenyl iodides as mild iodinating reagents are reported herein. These protocols led to the production of a range of valuable iodinated aniline derivatives. These results demonstrate the potential of developing novel site-selective C-H activation reactions with electron-rich compounds, since mild reagents can often been utilized in functional group metathesis reactions.

Oxidase‐Type C−H/C−H Coupling Using an Isoquinoline‐Derived Organic Photocatalyst

Oxidase‐type oxidation is an attractive strategy in organic synthesis due to the use of O₂ as the terminal oxidant. Organic photocatalysis can effect metal‐free oxidase chemistry. Nevertheless, current methods are limited in reaction scope, possibly due to the lack of suitable photocatalysts. Here we report an isoquinoline‐derived diaryl ketone‐type photocatalyst, which has much enhanced absorption of blue and visible light compared to conventional diaryl ketones. This photocatalyst enables dehydrogenative cross‐coupling of heteroarenes with unactivated and activated alkanes as well as aldehydes using air as the oxidant. A wide range of heterocycles with various functional groups are suitable substrates. Transient absorption and excited‐state quenching experiments point to an unconventional mechanism that involves an excited state “self‐quenching” process to generate the N‐radical cation form of the sensitizer, which subsequently abstracts a hydrogen atom from the alkane substrate to yield a reactive alkyl radical.

Direct Dehydrogenative Access to Unsymmetrical Phenones

The first non-directed dehydrogenative phenone coupling method of methylarenes with aromatic C-H bonds, displaying a large substrate scope, is herein reported. This reaction represents a far more direct atom- and step-efficient alternative to the classical Friedel-Crafts or Suzuki-Miyaura derived acylation reactions. The method can be carried out on a gram scale and was successfully applied to the synthesis of several Ketoprofen drug analogues.

A novel quinoline-based NNN-pincer Cu(II) complex as a superior catalyst for oxidative esterification of allylic C(sp3)-H bonds

We report for the first time that the quinoline-based NNN-pincer Cu(II) complex acts as an air stable superior catalyst for the oxidative cross-coupling of the allyl sp³ C-H bond with an acid for the synthesis of allyl esters in a homogeneous system at ambient temperature. The synthesized catalyst, 1, has been well characterized by various analytical techniques (HRMS, single crystal X-ray diffraction, CV, EPR, UV-vis spectroscopy) and showed excellent catalytic activity for the oxidative esterification of allylic C(sp³)-H bonds at 40 °C within a very short period of time (1 h) using only 1 mol% of the catalyst. A wide variety of aromatic allylic esters were synthesized in moderate to good yields, which could be extended to aliphatic allyl esters as well.

CF3SO2Na-Mediated Visible-Light-Induced Cross-Dehydrogenative Coupling of Heteroarenes with Aliphatic C(sp3)-H Bonds

Minisci-type reaction is one of the important means to construct C(sp³)-H functionalization of heteroarenes. According to traditional methods, stoichiometric amounts of precious transition metal catalysts and chemical oxidants were required at high temperatures. Here, a green and gentle novel Minisci-type method was developed via visible-light-induced cross-dehydrogenative coupling of heteroarenes with aliphatic C(sp³)-H bonds under oxidant-free and transition-metal-catalyst-free conditions. Only the catalytic equivalent of CF₃SO₂Na and room temperature were required to maintain an efficient reaction.

Copper-Catalyzed Phosphorylation of N,N-Disubstituted Hydrazines: Synthesis of Multisubstituted Phosphorylhydrazides as Potential Anticancer Agents

An efficient copper-catalyzed aerobic oxidative cross-dehydrogenative coupling reaction for the synthesis of multisubstituted phosphorylhydrazides from N,N-disubstituted hydrazines and hydrogen phosphoryl compounds is accomplished. The reaction proceeds under mild conditions without the addition of any external oxidants and bases. This work reported here represents a direct P(═O)-N-N bond formation with the advantages of being operationally simple, good functional group tolerance, and high atom and step economy. Furthermore, the selected compounds exhibit potential inhibitory activity against tumor cells, which can be used in the field of screening of anticancer agents as new chemical entities.

Native carboxyl group-assisted C-H acetoxylation of hydrocinnamic and phenylacetic acids

The use of a native directing group to promote C-H activation is highly desirable. Herein, we report a method of native carboxyl-assisted, Pd(II)-catalyzed ortho-C-H acetoxylation of both hydrocinnamic and phenylacetic acids that can be found in many biologically active molecules as the key moieties. Based on the broad scope and the application potential showcased with drug molecules, it is anticipated that this C-H acetoxylation reaction will find attractive applicability in future synthetic endeavors.

Visible-Light-Induced α,γ-C(sp3)-H Difunctionalization of Piperidines

Herein, we describe a novel protocol for visible-light-induced α,γ-C(sp³)-H difunctionalization of piperidines. This redox-neutral, atom-economical protocol, which exhibits a broad substrate scope and good functional group compatibility, constitutes a concise, practical method for constructing piperidine-containing bridged-ring molecules. Preliminary mechanistic studies indicated that highly regioselective activation of the inert γ-C(sp³)-H bond of piperidines was achieved through a 1,5-hydrogen atom transfer reaction of a nitrogen radical generated in situ.