RuII-Catalyzed/NH2-Assisted Selective Alkenyl C–H [5 + 1] Annulation of Alkenylanilines with Sulfoxonium Ylides to Quinolines

Free Amine-Directed Redox Neutral Ruthenium(II) Catalysis toward Regioselective Synthesis of Heterobiaryls

A regioselective coupling of ortho-heteroaryl anilines and 7-oxabenzonorbornadienes has been developed by leveraging free amine-directed redox-neutral Ru(II) catalysis. This protocol facilitates formal C-2 arylation of the indole moiety under mild conditions to offer valuable heterobiaryls in high yields. The reaction displays a broad substrate generality and scalability and retains efficacy in the presence of diverse pharmacophore scaffolds. Moreover, products bearing a free amine group were successfully employed in Mg(NTf2)2-catalyzed double Michael addition cascade, which led to the synthesis of intricate indole- and pyrrole-fused azaheterocycles.

A visible-light-induced photosensitizer-free decarbonylative Minisci-type reaction

This study presents a green and practical visible-light-induced photosensitizer-free decarbonylative Minisci-type reaction using aldehydes as alkyl radical precursors. The photocatalytic system exhibits a broad substrate scope and synthetically useful yields. Mechanistic experiments revealed that alkyl radicals could be generated through auto-oxidation of aldehydes under irradiation, which is a mild and effective method for achieving late-stage functionalization of N-heteroarenes. Some biologically active N-heteroarenes could be alkylated using this photocatalytic system smoothly.

Stereoselective Synthesis of Complex Polyenes through Sequential α-/β-C-H Functionalization of trans-Styrenes

Sequential C-H functionalization of molecules containing multiple C-H bonds can efficiently lead to structural diversity. Herein we present the first chelation-assisted sequential α-/β-C-H functionalization of E-styrenes with simple alkenes and alkynes in excellent regio- and stereo-selectivity. The process involves α C-H functionalization by six-membered exo-cyclopalladation to result in tri- and tetrasubstituted 1,3-dienes and β C-H functionalization through seven-membered endo-cyclopalladation to produce tetra- and pentasubstituted 1,3,5-trienes in up to 97 % yield with up to >99/1 E/Z selectivity, both enabled by the chelation assistance of pyrazinamide. The protocol is demonstrated to be widely applicable, tolerant to a wide range of functional groups and bioactive fragments, and suitable for gram-scale synthesis as well as one-pot and two step preparation of trienes. Mechanistic experiments and density functional theory (DFT) calculations were performed to elucidate the selectivity and reactivity.

Copper(II)/DBU Relay Catalyzed Annulation of α-Carbonyl-γ-alkynyl Sulfoxonium Ylides for Accessing N-Sulfonamido 2H-Isoindoles

A copper(II)/DBU relay catalyzed annulation of α-carbonyl-γ-alkynyl sulfoxonium ylides as a new class of sulfoxonium ylide reagents with sulfonyl hydrazides is reported, enabling intramolecular oxygen migration to produce a series of N-sulfonamido 2H-isoindoles with good yields. The present annulation proceeded readily by combining the Cu(II)-catalyzed 6-endo-dig oxo-cyclization with the DBU-catalyzed isochromene skeletal rearrangement, resulting in the formation of multiple new chemical bonds.

Lewis Acid Catalyzed [4 + 2] Annulation of Propargylic Alcohols with 2-Vinylanilines

An elegant Lewis acid catalyzed, protection-free, and straightforward synthetic strategy for the assembly of a series of sophisticated polycyclic quinoline skeletons employing propargylic alcohols and 2-vinylanilines as the substrates in the presence of Yb(OTf)3 (10 mol %) and AgOTf (10 mol %) in tetrahydrofuran has been described. This annulation protocol, which proceeds through a sequential Meyer-Schuster rearrangement/nucleophilic substitution/deprotonation sequence, provides a versatile, practical, and atom-economical approach for accessing quinoline derivatives in moderate-to-good yields.

Asymmetric [5+1] Annulation via C-H Activation/1,4-Rh Migration/Double Bond Shift Using a Transformable Pyridazine Directing Group

N-Heterocycle-assisted C-H activation/annulation reactions have provided new concepts for the construction and transformation of azacycles. In this work, we disclose a [5+1] annulation reaction using a novel transformable pyridazine directing group (DG). The DG-transformable reaction mode led to the construction of a new heterocyclic ring accompanied by transformation of the original pyridazine directing group via a C-H activation/1,4-Rh migration/double bond shift pathway, affording the skeleton of pyridazino[6,1-b]quinazolines with a good substrate scope under mild conditions. Diverse fused cyclic compounds can be achieved by derivatization of the product. The asymmetric synthesis of the skeleton was also realized to afford the enantiomeric products with good stereoselectivity.

Functionality-Directed Regio- and Enantio-Selective Olefinic C-H Functionalization of Aryl Alkenes

Aryl alkenes represents one of the most widely occurring structural motif in countless drugs and natural products, and direct C-H functionalization of aryl alkenes provides atom- step efficient access toward valuable analogues. Among them, group-directed selective olefinic α- and β-C-H functionalization, bearing a directing group on the aromatic ring, has attracted remarkable attentions, including alkynylation, alkenylation, amino-carbonylation, cyanation, domino cyclization and so on. These transformations proceed by endo- and exo-C-H cyclometallation and provide aryl alkene derivatives in excellent site- stereo-selectivity. Enantio-selective α- and β- olefinic C-H functionalization were also covered to synthesis axially chiral styrenes.

Synthesis of α-Carbonyl-α'-amide Sulfoxonium Ylides from Isocyanates with Complete Atom Economy

An efficient catalyst- and additive-free facile synthesis of α-carbonyl-α'-amide sulfoxonium ylides from isocyanates and β-ketosulfoxonium ylides with complete atom economy has been described. The β-ketosulfoxonium ylides and isocyanates adorned with various functional groups were well-tolerated and afforded moderate to high yields of the α-carbonyl-α'-amide sulfoxonium ylide derivatives. Finally, using large-scale reactions and converting the synthesized ylides into other valuable compounds, we demonstrated the practicality of this synthetic method.

α-Carbonyl sulfoxonium ylides in transition metal-catalyzed C-H activation: a safe carbene precursor and a weak directing group

Transition metal-catalyzed cross-coupling of sp² C-H bonds with diazo compounds via carbene migratory insertion represents an efficient strategy for the construction of C-C and C-heteroatom bonds in organic synthesis. Despite the popularity of diazo compounds as coupling partners in C-H activation, they pose serious safety and stability issues due to potential exothermic reactions linked with the release of N₂ gas. However, compared with diazo compounds, sulfoxonium ylides are generally crystalline solids, more stable, widely used in industrial scales, and easier/safer to prepare. Therefore, recent years have witnessed an upsurge in employing α-carbonyl sulfoxonium ylides as an alternative carbene surrogate in transition metal-catalyzed C-H activation. Unlike diazo compounds, α-carbonyl sulfoxonium ylides contain inherent potential to serve as a coupling partner as well as a weak directing group. This review will summarize the progress made in both categories of reactions.

Hydroxyl-Directed Ru(II)-Catalyzed Synthesis of Fused Dihydrofurans Using 1,4-Dioxane and Sulfoxonium Ylides as Annulating Agents

An unprecedented annulation reaction is developed for the synthesis of dihydrofuran-fused compounds. In this Ru-catalyzed hydroxyl-group-directed reaction, easily affordable sulfoxonium ylides and 1,4-dioxane were used as the annulating partners. This is the first example of the use of 1,4-dioxane as a methylene source to construct a heterocyclic scaffold. A wide range of dihydrofuran0fused coumarins and naphthalenes were synthesized using this three-component reaction.

Selective Synthesis of Pyrazolonyl Spirodihydroquinolines or Pyrazolonyl Spiroindolines under Aerobic or Anaerobic Conditions

Presented herein is a condition-controlled selective synthesis of pyrazolonyl spirodihydroquinolines or pyrazolonyl spiroindolines through formal [5 + 1] or [4 + 1] spiroannulation of 2-alkenylanilines with diazopyrazolones. Mechanistically, the formation of the title products involves initial generation of a pyrazolonyl spiro-fused seven-membered ruthenacycle species serving as a key intermediate through Ru(II)-catalyzed C-H/N-H bonds metalation, carbene formation, and its migratory insertion. When the reaction is carried out under air, the key intermediate undergoes reductive elimination to afford spirodihydroquinoline. When the reaction is run under argon, the key intermediate undergoes protonation and intramolecular nucleophilic addition to furnish spiroindoline. This work provides an atom-economical protocol for the effective functionalization of alkenyl C(sp²)-H bond, allowing rapid and selective assembly of valuable spiroscaffolds with a broad range of substrates.

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.

Synthesis of I(III)/S(VI) reagents and their reactivity in photochemical cycloaddition reactions with unsaturated bonds

The development of novel methodologies for the introduction of the sulfoxonium group under mild conditions is appealing but remains underexplored. Herein we report the synthesis of a class of hypervalent iodine reagents with a transferrable sulfoxonium group. These compounds enable mixed iodonium-sulfoxonium ylide reactivity. These well-defined reagents are examined in visible-light-promoted cyclization reactions with a wide range of unsaturated bonds including alkenes, alkynes, nitriles, and allenes. Two distinct cyclization pathways are identified, which are controlled by the substituent of the unsaturated bond. The cycloaddition protocol features simple operation, mild reaction conditions, and excellent functional group tolerance, affording a broad range of sulfoxonium-containing cyclic structures in moderate to excellent yields. Furthermore, the sufoxonium group in the product can be transformed into diverse functional groups and structural motifs via single electron transfer and transition-metal catalysis.

Synthesis of Tetrahydro-2H-thiopyran 1,1-Dioxides via [1+1+1+1+1+1] Annulation: An Unconventional Usage of a Tethered C-S Synthon

An unprecedented [1+1+1+1+1+1] annulation process has been developed for the construction of tetrahydro-2H-thiopyran 1,1-dioxides. Notably, rongalite acted as a tethered C-S synthon in this reaction and can be chemoselectively used as triple C1 units and as a source of sulfone. Mechanistic investigation indicated that two different carbon-increasing models are involved in this reaction in which rongalite serves as C1 units.

Condition-Dependent Selective Synthesis of Indolo[1,2-c]quinazolines and Indolo[3,2-c]quinolines from 2-(1H-Indol-2-yl)anilines and Sulfoxonium Ylides

In this paper, a selective synthesis of indolo[1,2-c]quinazolines and indolo[3,2-c]quinolines through the cascade reactions of 2-(1H-indol-2-yl)anilines with sulfoxonium ylides is presented. The formation of products involves the generation of a carbene species from sulfoxonium ylide and its N-H bond insertion reaction with 2-(1H-indol-2-yl)aniline followed by deoxygenative imine formation, intramolecular N- or C- nucleophilic addition and deoxygenative aromatization. This switchable synthesis was condition-dependent. In the presence of K₂CO₃ in CH₃CN, the reaction mainly furnished indolo[1,2-c]quinazolines. In the presence of HOAc in dioxane, it selectively afforded indolo[3,2-c]quinolines. In addition, direct C-H/N-H functionalization of the products obtained provides a convenient and direct access to polycyclic heteroaromatic compounds. These novel protocols have advantages such as readily accessible substrates, easily tunable selectivity, good compatibility with diverse functional groups, and the use of air as a cost-free and sustainable oxidant.

Synthesis of N-Arylindoles from 2-Alkenylanilines and Diazonaphthalen-2(1H)-ones through Simultaneous Indole Construction and Aryl Introduction

In this paper, an efficient synthesis of N-arylindoles through the cascade reaction of 2-alkenylanilines with diazonaphthalen-2(1H)-ones is presented. Mechanistically, this reaction involves the generation of a Ru-carbene complex from diazonaphthalen-2(1H)-one, followed by carbene N-H bond insertion with 2-alkenylaniline, intramolecular cyclization, and oxidative aromatization. In this reaction, the Ru(II) complex acts as a multifunctional catalyst to promote not only the carbene formation but also the intramolecular cyclization and the dehydrogenative aromatization. Meanwhile, air acts as a green and cost-effective oxidant. To our knowledge, this is the first example in which N-arylindoles were synthesized through simultaneous introduction of the N-aryl unit and construction of the indole scaffold. Notable advantages of this method include readily accessible and halide-free substrates, additive-free reaction conditions, good efficiency, excellent atom economy, and compatibility with diverse functional groups. In addition, the utility of the product thus obtained was showcased by its diverse structural transformations.

Metal-free three-component assemblies of anilines, α-keto acids and alkyl lactates for quinoline synthesis and their anti-inflammatory activity

A new and metal-free three-component method for the synthesis of 2,4-disubstituted quinolines via the reactions of anilines, α-keto acids and alkyl lactates is reported. The reactions proceed in the presence of p-toluene sulfonic acid (p-TSA) and tert-butyl peroxybenzoate (TBPB) to provide diverse quinoline products via the construction of new CC double, C-C single and CN double bonds without producing any organic mass-based side product. Notably, the anti-inflammatory activity of the quinolines has been investigated by measuring their ability to inhibit NO release by lipopolysaccharide (LPS) induced RAW264.7 cells, leading to the identification of 4i, 4t and 4x as potent anti-inflammatory compounds in vitro.

Photo-Thermo-Mechanochemical Approach to Synthesize Quinolines via Addition/Cyclization of Sulfoxonium Ylides with 2-Vinylanilines Catalyzed by Iron(II) Phthalocyanine

A novel photo-thermo-mechanochemical approach to assembling quinolines catalyzed by iron(II) phthalocyanine has been realized for the first time. This transformation features a cost-efficient catalytic system and operational simplicity, is free of solvent, and shows good substrate tolerance, providing a green alternative to existing thermal approaches. Mechanistic experiments demonstrate that the in-situ-formed secondary amine may be the key intermediate for the further cyclization/aromatization process.

Construction of axial chirality by asymmetric alpha C–H alkenylation of aryl alkenes

An asymmetric α-C–H alkenylation of aryl alkenes has been disclosed to provide axially chiral aryl 1,3-dienes, proceeding through six-membered exo-cyclopalladation, assisted by an aldehyde/l-t-leucine derived transient chiral auxiliary.

Access to axially chiral aryl 1,3-dienes by transient group directed asymmetric C-H alkenylations

We present a Pd-catalyzed atroposelective preparation of aryl 1,3-dienes from readily available styrenes and olefins through aldehyde derived transient chiral auxiliary, proceeding by enantioselective olefinic C-H alkenylation of styrenes via...

Regio- and stereo-selective olefinic C–H functionalization of aryl alkenes in ethanol

We report on α- and β-olefinic C–H alkenylation of 2-alkenyl benzylamine/benzoic acid derivatives in ethanol to afford aryl dienes/trienes with excellent selectivities, proceeding through 6-/7-membered exo-/endo-cyclometallation.

Rhodium-Catalyzed C-H Annulation of Free Anilines with Vinylene Carbonate as a Bifunctional Synthon

Chemical transformation with vinylene carbonate as an emerging synthetic unit has recently attracted considerable attention. This report is a novel conversion pattern with vinylene carbonate, in which such a vibrant reagent unprecedentedly acts as a difunctional coupling partner to complete the C-H annulation of free anilines. From commercially available substrates, this protocol leads to the rapid construction of synthetically versatile 2-methylquinoline derivatives (43 examples) with excellent functionality tolerance.

Copper(I)-catalyzed tandem synthesis of 2-acylquinolines from 2-ethynylanilines and glyoxals

An efficient one-step synthesis of 2-acylquinolines using a copper-catalyzed tandem reaction of 2-ethynylanilines with glyoxals in the presence of piperidine has been developed. This new protocol successfully avoids multi-step operation and the use of highly toxic cyanides required in traditional methods, and provides a practical tool for synthetic and pharmaceutical chemists. Various 2-acylquinolines are obtained with perfect regioselectivity in moderate to good yields (up to 86%). The potential synthetic utility of this method is exemplified by a large-scale experiment and synthetic transformation of the products.

Pd-Catalyzed Sequential Formation of C-C Bonds: A New Strategy for the Synthesis of (E)-α,β-Unsaturated Carbonyl Compounds from Sulfoxonium Ylides and 1-Iodo-2-((2-methylallyl)oxy)benzene Compounds

α,β-Unsaturated carbonyl compounds are significant moieties in many biological molecules and have attracted considerable attention in organic synthetic chemistry. A Pd-catalyzed cascade cyclization for the synthesis of (E)-α,β-unsaturated carbonyl compounds with the sequential formation of C-C bonds was developed. This method offers high efficiency, good functional group tolerance, and moderate to excellent yields and generally displays high stereoselectivity.

Free Amine, Hydroxyl and Sulfhydryl Directed C-H Functionalization and Annulation: Application to Heterocycle Synthesis

Transition metal-catalyzed direct C-H bond functionalization is recognized as an efficient strategy to assemble heterocyclic frameworks. For this purpose, directing groups (DGs) installation on an organic molecule has remained a widely exploited strategy for the years. The installation of directing groups, especially for the amine, alcohol and thiol containing reactants and their removal after the reaction need additional steps. In this regard, the use of free amine, hydroxyl and sulfhydryl as directing groups in native form is advantageous and in recent times, these transformations have stirred undisputable advancements for applications to heterocycle synthesis. In this review, the aromatic sp² -C attached free amine, hydroxyl and sulfhydryl as native functionalities are shown to be useful for the construction of five to seven-membered N-, O- and S-heterocycles.

Zinc-catalyzed C-H alkenylation of quinoline N-oxides with ynones: a new strategy towards quinoline-enol scaffolds

A zinc-catalyzed C-H alkenylation of quinoline N-oxides with ynones has been developed to rapidly assemble a broad collection of valuable quinoline-enol organic architectures. Uncommonly, this novel reaction involves C-H functionalization, and N-O, C-C and C[triple bond, length as m-dash]C bond cleavage in one operation, and leads exclusively to the formation of an enol rather than a keto product. Application of the enols generated was highlighted by further derivative transformation and preparation of a series of "BODIPY" analogues with high quantum yields (up to 86%).

Metal-Free C-H [5 + 1] Carbonylation of 2-Alkenyl/Pyrrolylanilines Using Dioxazolones as Carbonylating Reagents

A novel metal-free C-H [5 + 1] carbonylative annulation of 2-alkenyl/pyrrolylanilines with dioxazolones has been established for the assembly of the privileged quinolinones and pyrrolyl-fused quinoxalinones. Entirely differing from the existing reports, the dioxazolones herein behave with an innovative chemistry and first emerge as carbonylating reagents to participate in annulation reactions. Moreover, this process features exceedingly simple operation (only solvent) and tolerates both vinyl and aryl substrates. Comprehensive mechanistic studies indicate that the formed isocyanate intermediate plays a crucial role in enabling the carbonylation annulation.

Annulation of Conjugated Azine-Imine with a Sulfoxonium Ylide in a Noncarbenoid Route to Synthesize Multisubstituted Imidazole-Fused Heterocycles

A new [4+1]-annulation of in situ generated heterocyclic azine-aldimines with β-keto sulfoxonium ylides has been developed. The reaction constructs N-fused imidazole rings. In the reaction, the ylides play a dual-functional role of a nucleophilic 1,1-dipolar one-carbon synthon and a source of an internal oxidant, dimethyl sulfoxide, that promotes in situ dehydrogenation to product scaffolds. The method enables access to imidazo-pyridine, pyrazine, and pyrimidine heteroaromatics.

CuII-Catalyzed Coupling with Two Ynone Units by Selective Triple and Sigma C-C and C-H Bond Cleavages

We report a new copper-catalyzed [2 + 2 + 1] annulation process through the selective cleavage of sigma and triple C-C and C-H bonds using two ynone units. This new methodology involves breaking multiple chemical bonds in a single operation, including C≡C, C-C, C-H, and N-O. These high-value adducts lead to a diverse collection of synthetically challenging trisubstituted indolizines by the simultaneous engagement of different bond-breaking events and show excellent fluorescence in green aqueous solutions.

RhIII-Catalyzed formal [5 + 1] cyclization of 2-pyrrolyl/indolylanilines using vinylene carbonate as a C1 synthon

A rhodium(iii)-catalyzed formal C–H [5 + 1] cyclization of 2-pyrrolyl/indolylanilines with vinylene carbonate has been explored towards the potent assembly of diverse 4-methylpyrrolo[1,2-a]quinoxalines.

Ruthenium-catalyzed coupling of α-carbonyl phosphoniums with sulfoxonium ylides via C–H activation/Wittig reaction sequences

A Ru(ii)-catalyzed coupling of various α-carbonyl phosphoniums with sulfoxonium ylides has been realized for the facile synthesis of 1-naphthols in good to excellent yields.

Ru(ii)-Catalyzed C–H activation/annulation reactions of N-aryl-pyrazolidinones with sulfoxonium ylides: synthesis of cinnoline-fused pyrazolidinones

The first Ru(ii)-catalyzed cascade C–H activation/annulation reactions of N-aryl-pyrazolidinones with sulfoxonium ylides are reported.

Merging CF3SO2Na photocatalysis with palladium catalysis to enable decarboxylative cross-coupling for the synthesis of aromatic ketones at room temperature

By merging CF₃SO₂Na-mediated photocatalysis with palladium catalysis, an efficient decarboxylative coupling strategy of α-keto acids and aryl boronic acids has been developed for the synthesis of aromatic ketones.

Recent advances in chelation-assisted site- and stereoselective alkenyl C–H functionalization

This review highlights recent advances in vicinal- and geminal-group-directed alkenyl C–H functionalizations which proceeded by endo- and exo-cyclometallation.

Metal-free tandem carbene N-H insertions and C-C bond cleavages

A metal-free C-H [5 + 1] annulation reaction of 2-arylanilines with diazo compounds has been achieved, giving rise to two types of prevalent phenanthridines via highly selective C-C cleavage. Compared to the simple N-H insertion manipulation of diazo, this method elegantly accomplishes a tandem N-H insertion/SEAr/C-C cleavage/aromatization reaction, and the synthetic utility of this new transformation is exemplified by the succinct syntheses of trisphaeridine and bicolorine alkaloids.

Ruthenium(II)-Catalyzed Homocoupling of α-Carbonyl Sulfoxonium Ylides Under Mild Conditions: Methodology Development and Mechanistic DFT Study

A mild ruthenium(II)-catalyzed homocoupling of α-carbonyl sulfoxonium ylides was developed and the detailed mechanism was understood based on DFT calculations in the current report. The catalytic system utilizes the α-carbonyl sulfoxonium ylide as both the directing group for ortho-sp2 C-H activation and the acylmethylating reagent for C-C coupling. Various substituents are compatible in the transformation and a variety of isocoumarin derivatives were synthesized at room temperature without any protection. The theoretical results disclosed that the full catalytic cycle contains eight elementary steps, and in all the cationic Ru(II) monomer is involved as the catalytic active species. The acid additive is responsible for protonation of the ylide carbon prior to the intramolecular nucleophilic addition and C-C bond cleavage. Interestingly, the intermediacy of free acylmethylation intermediate or its enol isomer is not necessary for the transformation.