Radical Cascades Initiated by Intermolecular Radical Addition to Alkynes and Related Triple Bond Systems

Cu(I)-catalysed chemo-, regio-, and stereoselective radical 1,2-carboalkynylation with two different terminal alkynes

Transition-metal-catalysed asymmetric multicomponent reactions with two similar substrates often suffer from the lack of strategies to control the chemo-, regio-, and stereoselectivity of these substrates due to the close similarity in the chemical structures and properties of each reagent. Here, we describe a Cu(I)-catalysed asymmetric radical 1,2-carboalkynylation of two different terminal alkynes and alkyl halides with high chemo-, regio-, and stereoselectivity by using sterically bulky chiral tridentate anionic N,N,P-ligands and modulating alkynes with different electronic properties to circumvent above-mentioned challenges. This method features good substrate scope, high functional group tolerance of two different terminal alkynes, and diverse alkyl halides, providing universal access to a series of useful axially chiral 1,3-enyne building blocks.

Efficient synthesis of pyrrolo[1,2-a]indol-3-ones through a radical-initiated cascade cyclization reaction

A radical cascade cyanoisopropylation/cyclization reaction of 2,2'-azobis(2-methylpropionitrile) (AIBN) with 1-methacryloyl-3-phenyl-1H-indole-2-carbonitrile has been realized, providing an efficient strategy to access various pyrrolo[1,2-a]indol-3-ones in good to excellent yields with good functional group compatibility. The notable features of this protocol include avoiding the use of a photocatalyst and a transition metal, scalability, and ethanol as the green solvent. Moreover, mechanistic studies have been conducted and a plausible mechanism has been proposed.

Metal-free iodosulfonylation of alkynes to access (E)-β-iodovinyl sulfones in water

With the escalating concerns regarding environmental pollution caused by organic solvents, chemists are increasingly focusing on conducting organic reactions in water, nature's chosen solvent for chemical synthesis. Herein, the development of metal-free iodosulfonylation of alkynes with p-toluenesulfonyl cyanide and NIS for the synthesis of (E)-β-iodovinyl sulfones in water is reported. This reaction gives the desired products in good to excellent yields with high regio- and stereoselectivity by using water as a green solvent at room temperature. This reaction could be readily scaled up, and synthetic application was also studied.

Recent progress in the catalytic transformation of acetylene

Acetylene, a traditional industrial raw material, has garnered increasing attention in modern organic synthesis over the past two decades. Its catalytic transformation has emerged as an atom-economical and efficient strategy for producing a variety of high value-added compounds. This review comprehensively summarizes recent advancements and breakthroughs in the catalytic conversion of acetylene, focusing on two main categories: transition-metal-catalyzed transformations and photo-catalyzed/promoted transformations. The discussions center on various reaction intermediates, including alkenylmetals, acetylides, metallacyclopentadienes or heterometallacycles, gold carbenes, alkenyl-Ni complexes, and vinyl radicals. Furthermore, this review delves into the detailed mechanisms and diverse derivatizations of these reactions, highlighting their significance in the development of versatile acetylene catalytic transformations.

Synthesis through C(sp3)-C(sp2) Bond Scission in Alkenes and Ketones

ConspectusThe homolytic cleavage of C-C bonds adjacent to functional groups has recently become a popular strategy for restructuring the skeletons of complex organic molecules. In contrast to the traditional reactivity profiles of polar bond disconnections, homolytic scission furnishes carbon-centered free radicals primed for controlled termination with a diverse range of radicophiles. Beyond standard radical capture, transition-metal catalysis facilitates sophisticated C-C and C-heteroatom bond-forming reactions. Intensive efforts have been focused over many years into the cleavage of the neighboring C-C bonds of carboxylic acids and alcohols. Despite the ubiquity of alkenes and ketones in natural products, feedstock chemicals, and common synthetic intermediates, much less attention has been paid to exploiting their potential in diversifying chiral pool materials, such as terpenes and terpenoids. Defunctionalization in this manner is a powerful approach for synthesizing high-value chemicals and advanced synthetic intermediates because of the possibility to reconstruct and further decorate chirality-bearing carbon skeletons. Motivated by synthetic necessity, since 2018 our group has focused on developing ozonolysis-based dealkenylative molecular diversification, and we expanded into deacylation in 2025. In this Account, we chronicle our initial motivation, describe the historical background, and summarize our research into dealkenylative and deacylative synthesis. Our dealkenylative approach capitalizes on the ozonolysis of alkenes in MeOH to generate α-methoxyhydroperoxides primed for a reaction with reducing agents. Their reduction through single electron transfer, mediated by a transition metal, leads to the formation of an alkoxyl radical that undergoes rapid β-scission, furnishing both a carbon-centered free radical and an ester group derived from the acetal carbon atom. The produced free radical can be strategically terminated by radicophiles, thereby delivering remodeled chiral molecules. Using this concept, we have developed hydrodealkenylation (through hydrogen atom transfer from benzenethiol), dealkenylative thiylation (through thiyl group transfer from diaryl disulfides), alkenylation (through addition/elimination with nitrostyrenes), and oxodealkenylation (through treatment with TEMPO followed by oxidation). Furthermore, kinetic analysis has enabled the development of a catalytic FeII/vitamin C system for dealkenylative alkynylation and halodealkenylation. Synergizing ozonolysis and copper catalysis has recently enabled aminodealkenylation through net-redox-neutral C-C cleavage followed by C-N bond formation. Although the high oxidation potential of ozone relative to organic compounds makes alkene-to-peroxide conversion possible, it also limits the applicability of dealkenylative techniques for substrates featuring ozone-sensitive functional groups. We recently overcame this constraint by first applying Isayama-Mukayiama peroxidation to olefins and then using a novel catalytic system─catalytic FeIII and PhSH with stoichiometric γ-terpinene─for ozone-free hydrodealkenylation. Beyond alkenes, we have developed a straightforward methodology for the homolytic deacylative cleavage of ketones as well, including cycloalkanones. This process is applicable in total syntheses and in the late-stage modifications of complex ketone-containing natural products.

A Consecutive Two-Step Radical-Mediated Cyclization of gem-Difluorinated Diynes to Access gem-Difluorinated Cedrenes

A consecutive two-step radical-mediated cyclization of gem-difluoromethylenated bis(3-arylpropagyl)-indane-1,3-diones to access structurally unique gem-difluoromethylenated cedrenes is described. Substituents located on the aryl rings of the two propagyl units play an important role in governing the consecutive cyclization pattern. Upon treatment of gem-difluoromethylenated 1,3-diane with Bu3SnH/AIBN, a tributylstannyl radical-mediated radical cyclization between the two diyne units chemoselectively took place, leading to the corresponding gem-difluoromethylenated acoradienes in good yields, after removal of tributylstannyl group by TFA. Subsequently, Bu3SnH/AIBN promoted reductive cleavage of the phenylsulfanyl group leading to difluoroalkyl radicals which spontaneously underwent radical cyclization to give a series of gem-difluoromethylenated cedrenes.

Electrochemically promoted tandem cyclization of functionalized methylenecyclopropanes: synthesis of tetracyclic benzazepine derivatives

In this study, an electrocatalytic tandem cyclization reaction of amide-tethered methylenecyclopropanes has been developed, which can realize the rapid construction of tetracyclic benzazepine derivatives in moderate yields with good functional group compatibility under relatively mild conditions. In this transformation, the catalytic amount of ferrocene serves as the electrocatalytic medium, and electron transfer on electrodes can replace oxidants or reducing agents, which is more environmentally friendly than and economically comparable to traditional photocatalysis or metal catalysis. Moreover, the origin of the regiochemistry is well elucidated through density functional theory (DFT) calculations.

Copper-Catalyzed Asymmetric Cyanation of Propargylic Radicals Derived From Silyl-Substituted Allenes and Alkynes

Here, we report an efficient method to synthesize enantiomerically enriched propargyl nitriles via copper-catalyzed asymmetric cyanation of propargylic radicals, which are generated from silyl-substituted allenes or alkynes. These reactions proceeded through a highly site-selective hydrogen atom abstraction (HAA) with Cu(II)-bound nitrogen-centered radicals (NCRs). Notably, silyl-substituted allenes demonstrate exceptional allenic sp2 C─H bond activation selectivity, outcompeting alternative reactive sp3 C─H bonds (benzylic, allylic, and heteroatom-adjacent) in HAA processes. This chemo-selectivity profile enables precise enantiocontrol and site-specific functionalization of complex molecular architectures.

Radical-Induced Selective C─C Bond Activation at the Air-Solid Interface

Radicals are of great interest to trigger reactions in synthetic chemistry due to their high efficiency and unique reactivity, but their uncontrollable nature poses challenges in achieving selectivity. This study explores the influence of a surface/interface on radical reactions, leveraging a low-temperature plasma ionization source for radical generation. Combining insights from tip-enhanced Raman spectroscopy, mass spectrometry, and X-ray photoelectron spectroscopy, the selectivity of radical reactions of a model organic compound, biphenylthiol is investigated, under both homogeneous and heterogeneous conditions. The metal surface, acting as a template with interfacial water, is found to significantly modify the radical reaction pathway. The surface-immobilized BPT exhibited selective radical reaction products, forming 4-mercaptophenol molecules on Au(111) via the cleavage of C─C bonds. Such a high selectivity of radical reactions is unique and only achieved at the air/solid interface as compared to reactions in the gas, liquid, and solid phases. The present work highlights the potential of surfaces and interfaces in tailoring radical reaction pathways with high selectivity.

Photoinduced Carbamoylarylation of Alkynes with N-Aryl Oxamic Acids

1,2-Difunctionalization of alkynes is an attractive synthetic protocol, because it can achieve a high step economy and provide various complex organic molecules. This study demonstrates the visible-light-induced carbamoylarylation of terminal alkynes using N-aryl oxamic acids as bifunctional reagents. The transformation involves the addition of carbamoyl radicals to alkenes, resulting in 1,4-aryl migration via C(aryl)-N bond cleavage to afford the corresponding arylacrylamides in moderate to good yields.

Three-component Minisci reaction involving trifluoromethyl radicals promoted by TBHP

A three-component Minisci reaction based on radical polarity inversion and mediated by a trifluoromethyl radical is reported in this work. Under transition metal-free conditions, the electrophilic trifluoromethyl radical was added to an electron-rich alkene, generating a nucleophilic alkyl radical, which underwent a Minisci reaction with various electron-deficient N-heteroarenes. This protocol is easily executed using CF3SO2Na as the trifluoromethyl radical source under air- and moisture-free conditions, providing a versatile way to obtain trifluoromethyl-containing N-heteroarenes. Pyridines, quinolines, isoquinoline, phenanthridine and acridine were tolerated under the reaction conditions and reacted with both acyclic and cyclic alkenes. The potential of this reaction was demonstrated through a gram-scale preparation of trifluoromethyl-containing N-heteroarene and their various synthetic applications in synthesizing divergent and structurally complex molecules.

Exploring the Adsorption and Reactions of Methyl Radicals on M(111) Surfaces (M=Cu, Ag, Au): A DFT Study

It was reported that adsorbed methyl radicals produce ethane with Ag0- and Au0-nanoparticles in aqueous media, whereas on Cu0-powders, the product is methanol. The source of these differences was explored computationally, using the DFT method. The results indicate that up to six radicals can be adsorbed on Ag(111) and Au(111), (top site), while only four can be adsorbed on Cu(111) (fcc site), each surface containing eight atoms. The diffusion of the radicals on the surface is very easy on silver and copper, as this is achieved with a very low barrier (0.06 eV and 0.15 eV for Ag(111) and Cu(111), respectively), while on Au(111), the barrier is higher, 0.51 eV. The formation of ethane via a reaction of two adsorbed radicals is thermodynamically plausible for all studied coverage ratios on the three surfaces, but kinetically, it is plausible at room temperature only on Au(111) and Ag(111) at full coverage. Ethane can also be produced on Au(111) and Ag(111) by a collision of a solvated radical and an adsorbed radical. This is a barrierless process. On Cu(111), the yield of such a process is CH4(aq), and an adsorbed CH2 which reacts further with a non-adsorbed water molecule to produce adsorbed CH3OH.

Application of Lead-Free Metal Halide Perovskite Heterojunctions for the Carbohalogenation of C-C Multiple Bonds

A graphitic carbon nitride/lead-free double perovskite heterojunction (g-C3N4/Cs2AgBiCl6) has been adopted as a heterogeneous photocatalyst under visible light irradiation. The employed material enabled the atom transfer radical addition-type carbohalogenation of multiple C-C bonds, including (internal) alkenes and alkynes, with alkyl halides. The protocol showed a remarkable functional group tolerance, compatible with the late-stage functionalization of natural and pharmaceutical derivatives, and could be easily scaled up, delivering >1 g of the desired products.

Photocatalytic Amine-Promoted Selective Hydrochlorination and sp3 C-O Acylation of Alkyne-Tethered Methyl Ethers with Aldehydes

We present a photoredox and alkylamine-assisted approach for the selective hydrochlorination and acylation of sp3 C-O bonds in alkynyl methyl ethers using aldehydes. This method leverages a cascade of radical processes─including chlorine radical addition, hydrogen atom transfer, in situ imine radical addition, and spin-center shift─to enable selective hydrochlorination of alkynes and the spontaneous cleavage of sp3 C-O bonds. The transformation accommodates a broad range of internal alkyne-tethered ethers and aldehydes, providing an efficient and streamlined pathway to chloro-alkenyl ketones. Utilizing only a photocatalyst, chloride, and propylamine under light irradiation, this strategy offers a practical and complementary alternative to previous sp3 C-O cleavage protocols.

Catalytic stereoselective synthesis of all-carbon tetra-substituted alkenes via Z-selective alkyne difunctionalization

We report a Ni-catalyzed cascade reaction leading to the arylation of an alkyne-induced acyl migration and the formation of all-carbon tetra-substituted alkenes in good yields with exclusive Z-selectivity. This transformation involves the generation of a nucleophilic vinyl-Ni species through regioselective syn-aryl nickelation of the alkynes, followed by an intramolecular acyl migration. The steric and electronic properties of the phosphine ligands are crucial for achieving high regio- and stereocontrol in this migratory carbo-acylation process. The synthetic utility of the resulting Z-tetra-substituted alkenes is also demonstrated.

Enantioselective Multicomponent Electrochemical Difunctionalization of Terminal Alkynes

The direct functionalization of alkyne triple bonds using a radical strategy provides an efficient platform for creating a wide range of substituted alkenes. However, developing a multicomponent enantioselective radical reaction using feedstock alkynes to forge all-carbon quaternary stereocenters─while addressing challenges related to compatibility, selectivity, and efficiency─remains relatively rare. Here we report an enantioselective electrochemical nickel-catalyzed three-component cross-coupling of readily available terminal alkynes, diverse racemic alkyl radical precursors, and group transfer reagents (such as (TMS)3Si-H, RSe-SeR, RTe-TeR, and CHI3), achieving excellent regio-, stereo-, and enantioselectivities (more than 70 examples, up to 95% ee). Electricity-mediated difunctionalizations significantly expand the scope of both aliphatic and aromatic alkynes, demonstrating excellent functional group compatibility. The key to success lies in the rational design of anodically generated nickel-bound tertiary radical intermediates, which stereoselectively capture alkynes to form vinyl radicals and participate in subsequently diverse group transfer processes to enable the intermolecular and anti-stereoselective difunctionalization of alkynes. This approach allows the transformation of terminal alkynes into diverse structural entities with α-quaternary stereogenic centers.

Photoinduced stereoselective reactions using pyridinium salts as radical precursors

Pyridinium salts are amine surrogates that are abundant in nature and the redox active nature of the pyridinium salts allows them to serve as precursors for generating radical species under mild conditions that can be initiated by light, heat or metal catalysis. The stereoselective formation of products has always been a topic of interest for synthetic chemists worldwide. In this context, pyridinium salts can readily undergo single electron reduction to form a neutral radical, and the N-X bond's subsequent fragmentation furnishes the X radical without any harsh reaction conditions. As a consequence, the past decade has witnessed an increased effort in utilizing pyridinium salts to photocatalytically generate radicals for the regioselective, diastereoselective as well as enantioselective formation of products that have been summarised in this review.

Radical-Triggered Bicyclization and Aryl Migration of 1,7-Diynes with Diphenyl Diselenide for the Synthesis of Selenopheno[3,4-c]quinolines

The translocation of an aryl group from selenium into carbon enabled by the cleavage of the C-Se bond is reported by using nitrogen atom-linked 1,7-diynes and diaryl diselenides as starting materials, leading to various selenophene derivatives in a regioselective manner. This method enables the construction of two C-Se bonds and two C-C bonds through sequential radical bicyclization and 1,2-aryl migration under metal-free conditions. Control experiments and mechanistic studies suggest that this reaction proceeds through the cleavage of the inert C(Ph)-Se bond, facilitating the aryl translocation process. This transformation enables the one-step conversion of simple diselenides into diverse selenopheno[3,4-c]quinolines via a radical-promoted process, holding significant potential for new seleniferous heterocycles.

Radical Addition Reactions: Hierarchical Ab Initio Benchmark and DFT Performance Study

We performed a hierarchical ab initio benchmark study of the gas-phase radical addition reactions of X⋅+C2H2 and X⋅+C2H4 (X⋅ = CH3⋅, NH2⋅, OH⋅, SH⋅). The hierarchical series of ab initio methods (HF, MP2, CCSD, CCSD(T)) were paired with a hierarchal series of Dunning basis sets with and without diffuse functions ((aug)-cc-pVDZ, (aug)-cc-pVTZ, (aug)-cc-pVQZ). The HF ground-state wavefunctions were transformed into quasi-restricted orbital (QRO) reference wavefunctions to address spin contamination. Following extrapolation to the CBS limit, the energies from our highest- QRO-CCSD(T)/CBS+ level converged within 0.0-3.4 kcal mol-1 and 0.0-1.0 kcal mol-1 concerning the ab initio method and basis set, respectively. Our QRO-CCSD(T)/CBS+ reference data was used to evaluate the performance of 98 density functional theory (DFT) approximations. The MAE of the best functionals for reaction barriers and energies were: OLYP (1.9 kcal mol-1), BMK (1.0 kcal mol-1), M06-2X (0.9 kcal mol-1), MN12-SX (0.8 kcal mol-1) and CAM-B3LYP (0.8 kcal mol-1). These functionals also accurately reproduce key geometrical parameters of the stationary points within an average 2 % deviation from the reference QRO-CCSD(T)/cc-pVTZ level.

Stereoretentive Decarboxylative Amidation of α,β-Unsaturated Carboxylic Acids to Access Enamides

Enamides have emerged as robust alternatives for enamines, exhibiting versatile reactivity for further synthetic modifications, including nucleophilic addition, cycloaddition, and asymmetric hydrogenation. While transition-metal-catalyzed cross-coupling of alkenyl (pseudo)halides with amides has been widely employed to construct this valuable scaffold, it suffers from some limitations, such as the need for transition-metal catalysts and the preparative synthesis of alkenyl (pseudo)halides. In this study, we report a mild and convenient stereoretentive decarboxylative amidation of α,β-unsaturated carboxylic acids with easily procurable 1,4,2-dioxazol-5-ones, providing a practical synthetic route to enamides. Density functional theory (DFT) calculations revealed a plausible reaction mechanism, which involves the nucleophilic addition of a carboxylate onto dioxazolone, followed by sequential concerted rearrangements.

Radical Alkylcyanation of 1,6-Enynes with Isonitriles as Bifunctional Reagents

We report a radical cyano-cyclization of 1,6-enynes with isonitriles via photochemically driven nickel catalysis, forging alkenyl nitrile-tethered γ-lactams under mild conditions. This reaction leverages the photoexcitation of in situ generated nickel (isonitrile) species to facilitate isonitriles serving as alkyl radical precursors and cyanide sources. The reaction accommodates a wide range of substrates, exhibiting excellent regioselectivity and Z/E stereoselectivity.

Regio- and Stereoselective β-Sulfonylamination of Alkynes via Photosensitized Bifunctional N-S Bond Homolysis

Nitrogen central radicals (NCRs) are versatile synthetic intermediates for creating functional nitrogen-containing molecules. Herein, a photosensitized β-sulfonylamination of terminal alkynes as well as acetylene has been established by employing N-sulfonyl heteroaromatics as bifunctional reagents (BFRs) to efficiently deliver versatile (E)-β-sulfonylvinylamines with excellent regio- and stereoselectivities. Mechanistic studies suggest a base-accelerated energy transfer (EnT) photocatalysis involving aromatic NCR formation, radical addition to alkynes, and sulfonylation processes.

BF3·OEt2-catalyzed/mediated alkyne cyclization: a comprehensive review of heterocycle synthesis with mechanistic insights

The quest for efficient and versatile methods for heterocycle synthesis continues to drive innovation in organic chemistry. In this context, the cyclization of alkynes catalyzed or mediated by boron trifluoride diethyl etherate (BF3·OEt2) has emerged as a powerful and widely applicable strategy. This review provides a comprehensive and authoritative overview of BF3·OEt2-catalyzed/mediated alkyne cyclization reactions, covering the scope, mechanisms, and applications of these processes. We discuss the synthesis of a diverse range of heterocyclic compounds, including dihydropyrans, quinolines, dehydropiperidines, oxindoles and others, and highlight the unique advantages of BF3·OEt2 as a catalyst/mediator. Recent advances, challenges, and future directions in this rapidly evolving field are also addressed. This review aims to serve as a valuable resource for synthetic chemists, inspiring further research and applications in this exciting area.

Alkenylation of unactivated alkanes: synthesis of Z-alkenes via dual Co-TBADT catalysis

Hydroalkylation of terminal alkynes via C-H activation is the most atom-economical and straightforward method for synthesizing alkenes. They remain confined to using C(sp2)-H or activated C(sp3)-H bonds. A chelating group enabled the alkenylation of C(sp3)-H bonds, resulting in E alkenes. Protocols by which alkenylation of unactivated C(sp3)-H bonds occurs without a chelating group via metal-hydride or radical pathways remain unknown. Our cobalt-HAT catalysis achieves the desired Z alkene with excellent regio- and diastereoselectivity via C-H activation.

Silver-Mediated Acetoxyselenylation of Alkynes: Mild Stereoselective Access to Bifunctional Alkenes

Herein, we report a AgF-mediated regio- and stereoselective acetoxyselenylation of terminal/internal alkynes from iodobenzene dicarboxylate [PhI(OCOR)2] and diorganyl diselenides via multiple-site functionalization to afford β-selenyl enol esters in good yields. Alkynes derived from bioactive molecules, such as l(-)-borneol, l-menthol, and acyne oxalate, are also suitable for this transformation and afford the expected compounds.

Photoredox-Catalyzed Multicomponent α-Sulfonylation of Terminal Alkynes

A generality-oriented and adaptive α-sulfonylation of alkynes via photoinduced multicomponent radical cross-coupling of terminal alkynes with sulfinates and a variety of alcohols, thiophenols, or selenophenols has been explored. This protocol features mild conditions, good functional group tolerability, broad substrate scope, excellent chemo-, site-, and stereoselectivity, and applicability to late-stage functionalization. It provides a modular platform for the synthesis of value-added structurally diverse α-sulfonyl-containing multisubstituted alkenes from simple precursors.

Combining Trifunctionalization of Alkynoic Acids, Arene ortho C-H Functionalization and Amination: An Approach to Unsymmetrical 2,3-Diaryl Substituted Indoles

Here we report a simultaneous construction of two C-C and two C-N bonds in a unified procedure that incorporates alkynoic acid trifunctionalization, ortho C-H functionalization, and amination cascade. In an ordered process, the regioselective alkyne insertion reaction is favored over the decarboxylation process. The presence of the carboxyl group in alkynoic acid ensures the high regioselectivity in the carbopalladation process, paving the way for a novel method to synthesize unsymmetrically 2,3-diaryl substituted indole scaffolds with excellent regioselectivity. The protocol is demonstrated to be suitable for gram-scale synthesis.

Three-Component Photochemical Cyclization/Dithiocarbamate Formation of gem-Difluoro Quinolin-2(1H)-ones

Herein, a novel visible-light-induced 6-exo-trig difluoromethylation cyclization and subsequent carbo-thioesterification reaction is described. This protocol allows efficient access to valuable gem-difluoro quinolin-2(1H)-ones in moderate to excellent yields under mild conditions. Broad amino sources compatibility, including cyclic morpholine, thiazolidine, thiomorpholine, pyrrolidine, 1,4-oxazepane, 2,6-dimethylmorpholine, tert-butyl piperazine-1-carboxylate and noncyclic diethylamine, N-ethylpropan-1-amine, N-benzylethanamine, N-benzyl-trimethylsilanamine, dibenzylamine, and N-(4-methoxybenzyl)ethanamine, demonstrated the practicability of this strategy. A radical-radical crossover route was proposed on the basis of radical inhibition experiments, visible light irradiation on-off test, apparent quantum efficiency (AQE) calculation, and fluorescence quenching studies.

The Progress of Reductive Coupling Reaction by Iron Catalysis

The transition metal catalyzed coupling reaction has revolutionized the strategies for forging the carbon-carbon bonds. In contrast to traditional cross-coupling methods using pre-prepared nucleophilic organometallic reagents, reductive coupling reactions for the C-C bonds formation provide some advantages. Because both coupling partners are reduced in the final products using a stoichiometric amount of a reductant, this approach not only avoids the need to use sensitive organometallic species, but also provides an orthogonal and complementary access to classical coupling reaction. Notably, the reductive coupling reactions feature readily available fragments, promote good step economy, exhibit high functional group tolerance and unique chemoselectivity, which have propelled their increasingly popular in the organic synthesis. In recent years, due to the low price, minimal toxicity, and environmentally benign character, iron-catalyzed carbon-carbon coupling reactions have garnered significant attention from the organic synthetic chemists and pharmacologists, especially the iron-catalyzed reductive coupling. This review aims to provide an insightful overview of recent advances in iron-catalyzed reductive coupling reactions, and to illustrate their possible reaction mechanisms.

Boryl radical-mediated halogen-atom transfer (XAT) enables the Sonogashira-like alkynylation of alkyl halides

Alkynes are a crucial class of materials with application across the wide range of chemical disciplines. The alkynylation of alkyl halides presents an ideal strategy for assembling these materials. Current methods rely on the intrinsic electrophilic nature of alkyl halides to couple with nucleophilic acetylenic systems, but these methods faces limitations in terms of applicability and generality. Herein, we introduce a different approach to alkynylation of alkyl halides that proceeds via radical intermediates and uses alkynyl sulfones as coupling partners. This strategy exploits the ability of amine-ligated boryl radicals to activate alkyl iodides and bromides through halogen-atom transfer (XAT). The resulting radicals then undergo a cascade of α-addition and β-fragmentation with the sulfone reagent, leading to the construction of C(sp3)-C(sp) bonds. The generality of the methodology has been demonstrated by its successful application in the alkynylation of complex and high-value molecules.

Electron-donor-acceptor (EDA) complex-driven regioselective vicinal and oxidative geminal functionalization of alkynes

A visible-light-initiated electron-donor-acceptor (EDA) complex-driven regioselective vicinal and oxidative geminal thiosulfonylation of alkynes is presented. Organic thiosulfonates act as an acceptor, producing either sulfonyl (RSO2˙) or thiyl (RS˙) radicals under base and solvent switchable conditions. Simultaneous installation of three different functionalities, viz carbonyl, sulfonyl, and thiyl, takes place under one condition, while another condition leads to vicinal thiolation and sulfonylation.

Radical 6-Endo Addition Enables Pyridine Synthesis under Metal-Free Conditions

Metal-free synthesis of heterocycles is highly sought after in the pharmaceutical industry and has garnered widespread attention due to eliminating the need to remove trace metal catalysts from the reaction. We report a radical 6-endo addition method for pyridine synthesis from cyclopropylamides and alkynes under metal-free conditions. Various terminal and substituted alkynes are inserted as C2 units into cyclopropylamides to synthesize versatile pyridines with 57 examples. Mechanistic investigations and computational studies indicate the unprecedented 6-endo-trig addition of vinyl radicals to the imine nitrogen atom rather than the conventional 5-exo-trig addition to the imine carbon atom, in which the hypervalent iodine(III) plays a critical role. This reaction easily scales up with excellent functional group compatibility and suits the late-stage pyridine installation on complex molecules.

Copper-Catalyzed Regiodivergent Asymmetric Difunctionalization of Terminal Alkynes

We herein describe the first example of ligand-controlled, copper-catalyzed regiodivergent asymmetric difunctionalization of terminal alkynes through a cascade hydroboration and hydroallylation process. The catalytic system, consisting of (R)-DTBM-Segphos and CuBr, could efficiently achieve asymmetric 1,1-difunctionalization of aryl terminal alkynes, while ligand switching to (S,S)-Ph-BPE could result in asymmetric 1,2-difunctionalization exclusively. In addition, alkyl substituted terminal alkynes, especially industrially relevant acetylene and propyne, were also valid feedstocks for asymmetric 1,1-difunctionalization. This protocol is characterized by good functional group tolerance, a broad scope of substrates (>150 examples), and mild reaction conditions. We also showcase the value of this method in the late-stage functionalization of complicated bioactive molecules and simplifying the synthetic routes toward the key intermediacy of natural product (bruguierol A). Mechanistic studies combined with DFT calculations provide insight into the mechanism and origins of this ligand-controlled regio- and stereoselectivity.

Oxy-difluoroallylation of Ynamides by Nickel-Catalyzed Tandem Alkoxylation/Claisen Rearrangement

A nickel-catalyzed tandem alkoxylation/claisen rearrangement strategy for the oxy-difluoroallylation of ynamides has been developed. In this reaction, 3,3-difluoroallyl alcohol was used as a fluorine-containing building block to construct the C-CF2 bond for the first time. This approach is recognized for its robust tolerance of functional groups, impressive yields, and excellent atomic efficiency, all achieved under mild reaction conditions. A series of β,β-difluoromethyleneamide derivatives were efficiently obtained through simple operations, and their practicality was confirmed through gram-scale synthesis and product derivatization.

Photoinduced Nickel-Catalyzed Homolytic C(sp3)-N Bond Activation of Isonitriles for Selective Carbo- and Hydro-Cyanation of Alkynes

The exploration of strong chemical bonds as synthetic handles offers new disconnection strategies for the synthesis of functionalized molecules via transition metal catalysis. However, the slow oxidative addition rate of these covalent bonds to a transition metal center hampers their synthetic utility. Here, we report a C(sp3)-N bond activation strategy that bypasses thermodynamically challenging 2e- or 1e- oxidative addition via a distinct pathway in nickel catalysis. This strategy leverages a previously unknown activation pathway of photoinduced inner-sphere charge transfer of low-valent nickel(isonitriles), triggering a C(sp3)-N bond cleavage distal to the metal-ligand interaction to deliver nickel(cyanide) and versatile alkyl radicals. Utilizing this catalytic strategy, the selective intermolecular 1,2-carbocyanation reaction of alkynes with alkyl isonitriles as both alkylating and cyanating agents can be achieved, delivering a wide array of trisubstituted alkenyl nitriles with excellent atom-economy, regio-, and stereoselectivity under mild conditions. Furthermore, Markovnikov-selective hydrocyanation of aliphatic alkynes can be accomplished through the synergistic action of a photocatalyst utilizing isonitriles as the cyanation agents. Mechanistic investigations support the photogeneration of low-valent Ni(isonitrile) complexes that undergo photochemical homolysis of the C(sp3)-N bond to engage catalytic cyanation with alkynes.

Bifunctional iron-catalyzed alkyne Z-selective hydroalkylation and tandem Z-E inversion via radical molding and flipping

The challenging synthesis of thermodynamic-unfavored cis-olefins through catalytic cross-coupling reactions requires the synergistic interaction of substrate-activating units and configuration-regulating catalysts. Successfully hitting these two birds with one stone, we herein develop a convenient photoredox access to Z-alkenes from alkynes and light alkanes with a bifunctional iron-catalyzed system possessing both C(sp3)-H activation and configuration-controlling abilities. The protocol exhibits 100% atom utilization, mild conditions, a broad substrate scope, and compatibility with multitudinous functional groups. The detailed reaction mechanism and the origin of geometry regulation are well investigated by experimental and computational studies. Progressively, a catalytic amount of diaryl disulfides is introduced for consecutive photoinduced Z-E isomerization via reversible radical addition and flipping. Big steric hindrance substituents assembled on the disulfide emerge necessity for suppressing double-bond migration. This tandem strategy paves a promising way for stereoselective alkene construction and will bring significant inspiration for the development of transition metal photocatalysis.

Photo-Induced Difluoromethylation-Cyclization and Domino Amination-Defluorination to 4-(Aminomethyl)-3-fluoro-quinolinones

Herein, we report a visible light-induced difluoromethylation cyclization and subsequent amination-defluorination reaction. This protocol allows efficient to valuable 3-fluoro-quinolinones in moderate to excellent yields. A sequential difluoromethylation-cyclization-amination-defluorination mechanism was proposed based on a mechanism study. Further density functional theory (DFT) calculations revealed that the base K2HPO4 could lower the energy due to the C═O···K+ electrostatic interaction to assist the elimination process, while the six-membered transition state located in situ was essential for the cleavage of N-H and C-F bonds during this SN2'-type process.

Palladium-Catalyzed Cross-Coupling of Aryl Bromides and Chlorides with Trimethylsilylalkynes under Mild Conditions

Herein, we disclose a palladium-catalyzed cross-coupling of aryl bromides and chlorides with trimethylsilylalkynes under mild reaction conditions. This method utilizes commercially available and air stable palladium precatalysts and avoids the use of copper cocatalysts. Moreover, it allows for the synthesis of a wide range of disubstituted alkynes in high yields with excellent functional group tolerance. The utility of the developed method was further demonstrated via the late-stage alkynylation of pharmaceuticals and natural bioactive compounds.

Carbosulfonylation of Alkynes: A Direct Conversion of sp-C to sp3-C through Visible Light-Mediated 3-Component Reaction

A 3-component metal-free carbosulfonylation of alkynes is reported using readily available alkyl carboxylic acids and arylsulfinates under visible light irradiation. This photochemical approach gives direct conversion of sp-C to sp3-C yielding highly functionalized alkyl sulfones. It employs feedstock chemicals as starting materials and shows a broad substrate scope and moderate diastereoselectivity. The method's utility is highlighted in the synthesis of sedum alkaloids. A single photocatalyst is proposed to be active in two distinct photocatalytic cycles operating in tandem.

Recent Advances in Synthetic Methods by Photocatalytic Single-Electron Transfer Chemistry of Pyridine N-Oxides

By adoption of the enabling technology of modern photoredox catalysis and photochemistry, the generation of reactive and versatile pyridine N-oxy radicals can be facilely achieved from single-electron oxidation of pyridine N-oxides. This Synopsis highlights recent methodologies mediated by pyridine N-oxy radicals in developing (1) pyridine N-oxide-based hydrogen atom transfer catalysts for C(sp3)-H functionalizations and (2) β-oxyvinyl radical-mediated cascade reactions. In addition, recent research revealed that direct photoexcitation of pyridine N-oxides allowed for the generation of alkyl carbon radicals from alkylboronic acids.

Copper-Catalyzed Vicinal Thiocyanosulfonylation of Alkenes and Alkynes

Efficient copper-catalyzed radical thiocyanosulfonylation of alkenes and alkynes with potassium thiocyanate and sodium phenylsulfinate is described. The reactions provide general and convenient methods toward the synthesis of β-thiocyanoalkyl sulfones and β-thiocyanoalkenyl sulfones, respectively, in satisfactory yields. Based on conducted mechanistic experiments, a mechanism involving oxidative generation of sulfonyl radicals and subsequent addition to alkenes followed by Cu-assisted thiocyanation is proposed. Moreover, the practicability of the reaction is successfully demonstrated by its successful application on a gram scale.

Mild ketyl radical generation and coupling with alkynes enabled by Cr catalysis: stereoselective access to E-exocyclic allyl alcohols

The mild catalytic generation of ketyl radicals for organic transformations remains an unsolved issue, although it facilitates the discovery of metal-catalyzed reactions with the features of high functional group tolerance. Here, we report the generation of the ketyl radicals and coupling with alkynes that was enabled by cost-effective chromium catalysis, allowing for the formation of valuable E-exocyclic allyl alcohols with high stereo- and chemoselectivity. A broad range of synthetically useful functional groups that are sensitive to strong reductants are compatible with the catalytic system, providing access to diverse substituted E-exocyclic allyl alcohols under mild conditions. Appended hydroxyl groups in products are facilely late-stage functionalized in accessing numerous derivatives, as well as the enantio-enrichment of exocyclic allyl alcohol using chiral ligands. Mechanistic studies suggest that bipyridine-ligated Cr(ii) complex serves as a reactive catalyst enabling the generation of the ketyl radical for coupling, giving vinyl radical, followed by the combination of Cr and transmetalation with Cp2ZrCl moiety in affording oxazirconiumacycle. This reaction provides a new opportunity for the mild formation of transient ketyl radicals from widely accessible aliphatic aldehydes for coupling with Earth-abundant metal catalysis.

Metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes

Metal-catalyzed highly Markovnikov-type selective hydrofunctionalization of terminal alkynes provides a straightforward and atom-economical route to access 1,1-disubstituted alkenes, which have a wide range of applications in organic synthesis. However, the highly Markovnikov-type selective transformations are challenging due to the electronic and steric effects during the addition process. With the development of metal-catalyzed organic synthesis, different metal catalysts have been developed to solve this challenge, especially for platinum group metal catalysts. In this perspective, we review homogeneous metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes according to the classified element types as well as reaction mechanisms. Future avenues for investigation are also presented to help expand this exciting field.

An Expedient Radical Approach for the Decarboxylative Synthesis of Stereodefined All-Carbon Tetrasubstituted Olefins

We report a user-friendly approach for the decarboxylative formation of stereodefined and complex tri- and tetra-substituted olefins from vinyl cyclic carbonates and amines as radical precursors. The protocol relies on easy photo-initiated α-amino-radical formation followed by addition onto the double bond of the substrate resulting in a sequence involving carbonate ring-opening, double bond relay, CO2 extrusion and finally O-protonation. The developed protocol is efficient for both mismatched and matched polarity substrate combinations, and the scope of elaborate stereodefined olefins that can be forged including drug-functionalized derivatives is wide, diverse and further extendable to other types of heterocyclic and radical precursors. Mechanistic control reactions show that the decarboxylation step is a key driving force towards product formation, with the initial radical addition under steric control.

Carbonylative cyclization of biaryl enones with aldehydes and oxamic acids

An oxidative radical-promoted carbonylative cyclization strategy for the synthesis of phenanthren-9-(10H)-one frameworks from biaryl enones using aldehydes as the carbonyl radical sources is disclosed. The reaction proceeds through a sequential addition of a carbonyl radical to the olefin followed by cyclization with an aryl ring. The method is further extended to carbamoyl radicals generated from oxamic acids to access the corresponding phenanthrenones with amide functionalities.

Fluorosulfonyl Arylation of Alkynes via Electron Donor-Acceptor Photoactivation

A radical fluorosulfonyl arylation of alkynes with sulfuryl chlorofluoride as the FSO2 radical precursor via electron donor-acceptor photoactivation driven by daylight or a blue light-emitting diode is disclosed. A series of valuable benzo-fused carbocycles and heterocycles have been produced with simple operation under mild conditions in the absence of any external catalysts or additives. The synthetic potential of this protocol has further demonstrated excellent scalability, as well as diverse postderivatizations, including SuFEx reactions and other useful cascade reactions.

Sulfur in Waste-Free Sustainable Synthesis: Advancing Carbon-Carbon Coupling Techniques

This review explores the pivotal role of sulfur in advancing sustainable carbon-carbon (C-C) coupling reactions. The unique electronic properties of sulfur, as a soft Lewis base with significant mesomeric effect make it an excellent candidate for initiating radical transformations, directing C-H-activation, and facilitating cycloaddition and C-S bond dissociation reactions. These attributes are crucial for developing waste-free methodologies in green chemistry. Our mini-review is focused on existing sulfur-directed C-C coupling techniques, emphasizing their sustainability and comparing state-of-the-art methods with traditional approaches. The review highlights the importance of this research in addressing current challenges in organic synthesis and catalysis. The innovative use of sulfur in photocatalytic, electrochemical and metal-catalyzed processes not only exemplifies significant advancements in the field but also opens new avenues for environmentally friendly chemical processes. By focusing on atom economy and waste minimization, the analysis provides broad appeal and potential for future developments in sustainable organic chemistry.

E-Selective Radical Difunctionalization of Unactivated Alkynes: Preparation of Functionalized Allyl Alcohols from Aliphatic Alkynes

Radical difunctionalization of aliphatic alkynes provides direct access to valuable multi-substituted alkenes, but achieving a high level of chemo- and stereo-control remains a formidable challenge. Herein a novel photoredox neutral alkyne di-functionalization is reported through functional group migration followed by a radical-polar crossover and energy transfer-enabled stereoconvergent isomerization of alkenes. In this sequence, a hydroxyalkyl and an aryl group are incorporated concomitantly into an alkyne, leading to diversely functionalized E-allyl alcohols. The scope of alkynes is noteworthy, and the reaction tolerates aliphatic alkynes containing hydrogen donating C─H bonds that are prone to intramolecular hydrogen atom transfer. The protocol features broad functional group compatibility, high product diversity, and exclusive chemo- and stereoselectivity, thus providing a practical strategy for the elusive radical di-functionalization of unactivated alkynes.

Reversible Radical Addition Guides Selective Photocatalytic Intermolecular Thiol-Yne-Ene Molecular Assembly

In modern organic chemistry, harnessing the power of multicomponent radical reactions presents both significant challenges and extraordinary potential. This article delves into this scientific frontier by addressing the critical issue of controlling selectivity in such complex processes. We introduce a novel approach that revolves around the reversible addition of thiyl radicals to multiple bonds, reshaping the landscape of multicomponent radical reactions. The key to selectivity lies in the intricate interplay between reversibility and the energy landscapes governing C-C bond formation in thiol-yne-ene reactions. The developed approach not only allows to prioritize the thiol-yne-ene cascade, dominating over alternative reactions, but also extends the scope of coupling products obtained from alkenes and alkynes of various structures and electron density distributions, regardless of their relative polarity difference, opening doors to more versatile synthetic possibilities. In the present study, we provide a powerful tool for atom-economical C-S and C-C bond formation, paving the way for the efficient synthesis of complex molecules. Carrying out our experimental and computational studies, we elucidated the fundamental mechanisms underlying radical cascades, a knowledge that can be broadly applied in the field of organic chemistry.

Radical revelations: the pnictogen effect in linear acetylenes

Acetylenes are essential building blocks in modern chemistry due to their remarkable modularity. The introduction of heteroatoms, such as pnictogens (X), is one of the simplest approaches to altering the C≡C bond. However, the chemistry of the resultant dipnictogenoacetylenes (DXAs) is strongly dependent on the nature of X. In this work, rigorous theoretical chemistry tools are employed to shed light on the origin of these differences, providing a detailed evaluation of the impact of X on the geometrical and electronic features of DXAs. Special emphasis is made on the study of the carbene character of the systems through the analysis of the interconversion mechanism between the linear and zigzag isomers. Our results show that second-period atoms behave drastically differently to the remaining X: down the group, a zwitterionic resonance form emerges at the expense of decreasing the carbenoid role, eventually resulting in an electrostatically driven ring closure. Furthermore, our findings pave the way to potentially unveiling novel routes for the promotion of free-radical chemistry.