Arynes in Transition-Metal-Free Multicomponent Coupling Reactions

Efficient synthesis of β-aminonitriles from arynes and imines in acetonitrile

Multicomponent reactions are powerful strategies for synthesizing complex molecules in an efficient manner. In this work, we investigate a novel multicomponent reaction involving arynes, imines, and nitriles, leading to chiral β-aminonitriles. Notably, two new bonds (C-C and C-N) are formed in one step without the use of metal catalysts, showing the great potential of this transformation. We demonstrate that this synthetic methodology is compatible with different arynes and imines, and propose a reasonable reaction mechanism initiated by the nucleophilic addition of the imine to the aryne.

Aryne Chemistry: Generation Methods and Reactions Incorporating Multiple Arynes

Arynes hold significance for the efficient fusion of (hetero) arenes with diverse substrates, advancing the construction of complex molecular frameworks. Employing multiple equivalents of arynes is particularly effective in the rapid formation of polycyclic cores found in optoelectronic materials and bioactive compounds. However, the inherent reactivity of arynes often leads to side reactions, yielding unanticipated products and underlining the importance of a detailed investigation into the use of multiple arynes to fine-tune their reactivity. This review centers on methodologies and syntheses in organic reactions involving multiple arynes, categorizing based on mechanisms like cycloadditions, σ-bond insertions, nucleophilic additions, and ene reactions, and discusses aryne polymerization. The categorization based on these mechanisms includes two primary approaches: the first entails multiple aryne engagement within a single step while the second approach involves using a single equivalent of aryne sequentially across multiple steps, with both requiring strict reactivity control to ensure precise aryne participation in each respective step. Additionally, the review provides an in-depth analysis of the selection of aryne precursors, organized chronologically and by activation strategy, offering a comprehensive background that supports the main theme of multiple aryne utilization. The expectation remains that this comprehensive review will be invaluable in designing advanced syntheses engaging multiple arynes.

Light-Induced Generation and Cycloaddition Reactions of Benzyne: Synthesis of Naphthoxindoles E and Annulated Indolizines

By virtue of their high electrophilic nature, benzynes serve as reactive dienophiles in numerous cycloaddition reactions. However, in situ generation of benzyne involves either base-mediated thermal reactions, low-temperature conditions, or metal-catalyzed reactions of substituted arenes. This limits the applicability of benzynes as suitable dipolarophiles in cycloaddition reactions. Herein, we have reported a UVA (365 nM)-induced in situ generation of benzynes (from triazenyl benzoic acid) and subsequently their [4 + 2] Diels-Alder and [3 + 2] cycloaddition reactions with appropriate reaction partners such as N-protected alkylidene oxindole carboxylates and pyridinium ylides to afford naphthoxindoles E and pyrido[2,1-a]isoindole, respectively, in moderate to excellent yield. The reactions occurred at room temperature and under reagent-free reaction conditions. Each of these building blocks is pharmaceutically relevant; hence, this highlights an interesting strategy to access these classes of compounds.

Aryne and CO2-based formal [2 + 2 + 2] annulation to access tetrahydroisoquinoline-fused benzoxazinones

Tetrahydroisoquinoline and its fused polyheterocycles are prevalent structural motifs found in numerous natural products. In this study, we report a highly efficient and convergent synthetic approach for the construction of tetrahydroisoquinoline-fused polyheterocycles through a three-component formal [2 + 2 + 2] annulation process by combining 3,4-dihydroisoquinolines, CO2, and benzynes. Notably, electron-rich 3,4-dihydroisoquinolines and electron-deficient benzynes exhibit greater reactivity in this annulation. Moreover, this method benefits from the convergent synthesis and the utilization of carbon dioxide, providing a valuable strategy for the facile synthesis of tetrahydroisoquinoline-fused polyheterocycles, with potential applications in the discovery and development of novel organic molecules.

Synthesis of Sulfilimines via Aryne and Cyclohexyne Intermediates

An efficient and metal-free approach for the synthesis of sulfilimines from sulfenamides with aryne and cyclohexyne precursors has been developed. The reaction proceeds through unusual S-C bond formation, which offers a novel and practical entry to access a wide range of sulfilimines in moderate to good yields with excellent chemoselectivity. Moreover, this protocol is amenable to gram-scale synthesis and is applicable to the transformation of the products into useful sulfoximines.

Two Cascade Processes Initiated by the Insertion of Benzyne into the Se═O Bond

Two sets of cascade processes have been realized, both of which were initiated with a benzyne insertion into the Se═O bond. The key factors to differentiate these processes are based on the structures of diaryl selenium oxides and reaction conditions. When diaryl selenium oxides containing an ortho weak σ-electron-withdrawing group were used, triarylselenonium salts were obtained at room temperature, while ortho-(aryloxy)phenyl phenyl selanes could be produced from diaryl selenium oxides at 100 °C.

Arylation and Aryne Insertion into C-Acylimines: A Simple, Flexible, and Divergent Synthesis of C2-Aryl Indoles

We reveal a direct strategy for the flexible synthesis of C2-aryl/heteroaryl indoles without transition metal catalysts. The synthesis involves a one-pot, four-component reaction of readily available starting materials to offer diversity around the indole moiety with a broad substrate scope and high yields. The reaction proceeds via the Friedel-Crafts C-arylation of C-acylimine formed in situ, followed by N-arylation with aryne, a formal [3+2] cycloaddition, and a subsequent aromatization cascade.

Diverse Synthesis of Triarylselenonium Salts and o-(Alkoxy)aryl Aryl Selanes via Insertion of Benzyne into the Se═O Bond

An insertion of benzyne into the Se═O bond has been realized. In this reaction, two types of compounds containing selenium could be synthesized from different substrates. When diaryl selenium oxides were used, triarylselenonium salts were furnished, while o-(alkoxy)phenyl phenyl selanes could be produced from aryl alkyl selenium oxides.

CF3SO2Na-Mediated Five-Component Carbonylation of Triarylboroxines with TMSCF3 and THF/LiOH/NaI to Give Aroyloxyalkyl Iodides

Herein, we developed an efficient and transition-metal-free multicomponent coupling reaction for the synthesis of aroyloxyl alkyl iodides. In the reaction among 2,4,6-triarylboroxines, THF, TMSCF₃, LiOH, and NaI, five-component reactions could be precisely controlled by modulating CF₃SO₂Na, supplying one type of aroyloxyl alkyl iodides in moderate to high yields. The reaction exhibits good functional group tolerance and a wide substrate scope and can be easily transformed into other useful compounds. The mechanism is proposed on the basis of the control experiments.

Application of DMSO as a methylthiolating reagent in organic synthesis

In the past decades, DMSO has been widely used not only as a common solvent but also as an environmentally benign oxidant in various organic transformations. Most strikingly, DMSO can be used as a sulfur source to construct methylthiolated building blocks of potential biologically active molecules, which is a remarkable achievement in the field of organic sulfur chemistry. The purpose of this review article is to summarize and discuss the main developments in the application of DMSO as a methylthiolating reagent to introduce the -SMe functionality in organic synthesis.

Reactivity of Vinyl Epoxides/Oxetanes/Cyclopropanes toward Arynes: Access to Functionalized Phenanthrenes

The reactivity of vinyl epoxides/oxetanes/cyclopropanes toward arynes has been demonstrated under mild conditions to give the corresponding phenanthrenes in moderate to good yields. This transition-metal-free cascade process involves a series of Diels-Alder reaction, ring-opening aromatization, and ene reaction. Various functionalized phenanthrenes could be synthesized utilizing the versatile hydroxy group. Interestingly, vinyl epoxides/oxiranes experience preferentially the Diels-Alder reaction toward arynes over nucleophilic attack of epoxides/oxiranes.

Access to diverse organosulfur compounds via arynes: a comprehensive review on Kobayashi's aryne precursor

Arynes are highly reactive transient intermediates having enormous applications in organic synthesis. In the last three decades aryne chemistry has shown incredible developments in carbon-carbon and carbon-heteroatom bond formation reactions. After the discovery of Kobayashi's protocol for the generation of aryne intermediates in a mild way, this field of chemistry witnessed rapid growth in synthetic organic chemistry. One aspect of development in this field involves C-S bond formation under mild conditions which has a tremendous scope for the synthesis of various important organosulfur building blocks.

Three-Component, Diastereoselective [6 + 3] Annulation of Tropone, Imino Esters, and Arynes

A transition-metal-free, three-component, and diastereoselective [6 + 3] annulation reaction employing tropone, imino esters, and arynes allowing the synthesis of bridged azabicyclo[4.3.1]decadienes is demonstrated. The key nitrogen ylides for the [6 + 3] annulation were generated by the addition of imino esters to the arynes followed by a proton transfer. The nitrogen ylides undergo a regioselective addition to tropone to furnish the desired products in moderate to good yields with good functional group tolerance under mild conditions.

Could London Dispersion Force Control Regioselective (2 + 2) Cyclodimerizations of Benzynes? YES: Application to the Synthesis of Helical Biphenylenes

In recent years, London dispersion interactions, which are the attractive component of the van der Waals potential, have been found to play an important role in controlling the regio- and/or stereoselectivity of various reactions. Particularly, the dispersion interactions between substrates and catalysts (or ligands) are dominant in various selective catalyzes. In contrast, repulsive steric interactions, rather than the attractive dispersion interactions, between bulky substituents are predominant in most of the noncatalytic reactions. Herein, we demonstrate the first example of London dispersion-controlled noncatalytic (2 + 2) cyclodimerization of substituted benzynes to selectively afford proximal biphenylenes in high yields and regioselectivities, depending on the extent of dispersion interactions in the substituents. This method can be applied for the synthesis of novel helical biphenylenes, which would be fascinating for chemists as these compounds are potential skeletons for ligands, catalysts, and medicines.

Transition-Metal-Free C2-Functionalization of Pyridines through Aryne Three-Component Coupling

The direct C2-functionalization of pyridines through a transition-metal-free protocol by using aryne multicomponent coupling is demonstrated. The reaction allowed a broad-scope synthesis of C2-substituted pyridine derivatives bearing the -CF₃ group in good yields with α,α,α-trifluoroacetophenones as the third component. Activated keto esters could also be employed as the third component in this formal 1,2-di(hetero)arylation of ketones. Performing the reaction under dilute conditions inhibited the competing pyridine-aryne polymerization pathway. Nucleophilic attack by the initially generated pyridylidene intermediate on the carbonyl followed by an S_N Ar process resembling the Smiles rearrangement affords the desired products.

An Umpolung Oxa-[2,3] Sigmatropic Rearrangement Employing Arynes for the Synthesis of Functionalized Enol Ethers

An oxa-[2,3] sigmatropic rearrangement involving arynes is reported featuring the umpolung of ketones, where the C═O bond polarity is reversed. The in situ-generated sulfur ylides from β-keto thioethers and arynes undergo efficient rearrangement allowing the facile and robust synthesis of functionalized enol ethers in high yields and excellent functional group compatibility. Preliminary mechanistic studies rule out the possibility of Pummerer-type rearrangement operating in this case.

An efficient synthesis of 4-phenoxy-quinazoline, 2-phenoxy-quinoxaline, and 2-phenoxy-pyridine derivatives using aryne chemistry

Herein we report the mild and efficient synthesis of 4-phenoxyquinazoline, 2-phenoxyquinoxaline, and 2-phenoxypyridine derivatives from the starting materials viz. quinazolin-4(3H)-one, quinoxalin-2(1H)-one, and pyridin-2(1H)-one and aryne generated in situ from 2-(trimethylsilyl)phenyl trifluoromethanesulfonate and cesium fluoride. This synthetic methodology gives a new environmentally benign way for the preparation of several unnatural series of 4-phenoxyquinazoline, 2-phenoxyquinoxaline and 2-phenoxypyridine compounds with high yields and broad substrate scope.

Efficient synthesis of 4-phenoxyquinazoline, 2-phenoxyquinoxaline, 2-phenoxypyridine derivatives were generated of aryne from 2-(trimethylsilyl)phenyltrifluoromethanesulfonate with cesium fluoride.

Arynes as Radical Acceptors: TEMPO-Mediated Cascades Comprising Addition, Cyclization, and Trapping

The application of arynes as radical acceptors is described. The stable radical TEMPO (2,2,6,6-tetramethyl piperidine 1-oxyl) is shown to add to various ortho-substituted benzynes generating the corresponding aryl radicals which engage in 5-exo or 6-endo cyclizations. The cyclized radicals are eventually trapped by TEMPO. The introduced method provides ready access to various dihydrobenzofurans, oxindoles, and sultones by a conceptually novel approach.

Synthesis of α-Aminophosphonates via Phosphonylation of an Aryne-Imine Adduct

Multicomponent phosphonylation is accomplished upon the reaction of an imine with an aryne generated in situ in the presence of a dialkyl phosphite. This transition-metal-free protocol shows a broad substrate scope, providing a variety of α-aminophosphonates in moderate to good yields. A plausible mechanism for the reaction is proposed based on a deuterium exchange experiment.

Facile Synthesis of a Stable Side-on Phosphinyne Complex by Redox Driven Intramolecular Cyclisation

Alkyne complexes with vicinal substitution by a Lewis acid and a Lewis base at the coordinated alkyne are prospective frustrated Lewis pairs exhibiting a particular mutual distance and, hence, a specific activation potential. In this contribution, investigations on the generation of a WII alkyne complex bearing a phosphine as Lewis base and a carbenium group as Lewis acid are presented. Independently on potential substrates added, an intramolecular cyclisation product was always isolated. A subsequent deprotonation step led to an unprecedented side-on λ5 -phosphinyne complex, which is interpreted as highly zwitterionic according to visible absorption spectroscopy supported by TD-DFT. Low-temperature 31 P NMR and EPR spectroscopic measurements combined with time-dependent IR-spectroscopic monitoring provided insights in the mechanism of the cyclisation reaction. Decomposition of the multicomponent IR spectra by multivariate curve resolution and a kinetic hard-modelling approach allowed the derivation of kinetic parameters. Assignment of the individual IR spectra to potential intermediates was provided by DFT calculations.

Rapid Synthesis of Zwitterionic Phosphonium Benzoates by a Three-Component Coupling Involving Phosphines, Arynes and CO2

A mild and easy to perform multicomponent coupling involving phosphines, arynes generated from 2-(trimethylsilyl)aryl triflates, and CO₂ allowing the transition-metal-free synthesis of zwitterionic phosphonium benzoates has been developed. The reaction proceeds via the generation of 1 : 1 zwitterionic intermediates from phosphines and arynes followed by the interception with CO₂ to deliver the carboxylates in moderate to good yields instead of the anticipated benzooxaphosphol-3(1H)-ones.

One-Pot Generation of Functionalized Benzynes from Readily Available 2-Hydroxyphenylboronic Acids

We developed a one-pot method for the generation of benzynes from a range of readily available 2-hydroxyphenylboronic acids. This method features the in situ activation of both boronic acid and hydroxyl groups of the substrate to enhance benzyne generation at 60 °C. Such mild conditions facilitate the generation of functionalized benzynes that immediately react with diverse arynophiles to produce multisubstituted fused benzenes.

Reaction of Vinyl Aziridines with Arynes: Synthesis of Benzazepines and Branched Allyl Fluorides

We report the cycloaddition between vinyl aziridines and arynes. Depending on the reaction conditions and the choice of the aryne precursor, the aziridinium intermediate can be trapped through two distinct mechanistic pathways. The first one proceeds through a formal [5+2] cycloaddition to furnish valuable multi-substituted benzazepines. In the second pathway, the aziridinium is intercepted by a fluoride ion to afford allylic fluorides in good yields. Both reactions proceed stereospecifically and furnish enantiopure benzazepines and allylic fluorides.

Direct synthesis of ortho-methylthio allyl and vinyl ethers via three component reaction of aryne, activated alkene and DMSO

A transition-metal free synthetic strategy for the direct synthesis of ortho-methylthio allyl and vinyl ethers via cascade three-component coupling of aryne, activated alkene and DMSO.

Recent advances in the synthesis of organophosphorus compounds via Kobayashi's aryne precursor: a review

This review systematically summarizes the progress in aryne chemistry for the synthesis of organophosphorus compounds via aryne insertion into the C–P, P–N, P–P, P–O, PP, PN and PS bonds.

The Aza-hexadehydro-Diels-Alder Reaction

The generation of pyridynes from diyne nitriles is reported. These cyano-containing precursors are analogues of the triyne substrates typically used for the hexadehydro-Diels-Alder (HDDA) cycloisomerization reactions that produce ring-fused benzynes. Hence, the new processes described represent aza-HDDA reactions. Depending on the location of the nitrile, either 3,4-pyridynes (from 1,3-diynes containing a tethered cyano group) or 2,3-pyridynes (from 1-cyanoethyne derivatives containing a tethered alkyne) are produced. In situ trapping of these reactive intermediates leads to highly substituted and functionalized pyridine derivatives. In several instances, unprecedented pyridyne trapping reactions are seen. Differences in reaction energetics between the aza-HDDA substrates and that of their analogous HDDA (triyne) substrates are discussed.

Arynes and Their Precursors from Arylboronic Acids via Catalytic C-H Silylation

A new, operationally simple approach is presented to access arynes and their fluoride-activated precursors based on Ru-catalyzed C-H silylation of arylboronates. Chromatographic purification may be deferred until after aryne capture, rendering the arylboronates de facto precursors. Access to various new arynes and their derivatives is demonstrated, including, for the first time, those based on a 2,3-carbazolyne and 2,3-fluorenyne core, which pave the way for novel derivatizations of motifs relevant to materials chemistry.

Computational and experimental studies on copper-mediated selective cascade C-H/N-H annulation of electron-deficient acrylamide with arynes

An efficient and convenient copper-mediated method has been developed to achieve direct cascade C-H/N-H annulation to synthesize 2-quinolinones from electron-deficient acrylamides and arynes. This method highlights an emerging but simple strategy to transform inert C-H bonds into versatile functional groups in organic synthesis to provide a new method of synthesizing 2-quinolinones efficiently. Mechanistic investigations by experimental and density functional theory (DFT) studies suggest that an organometallic C-H activation via a Cu(iii) intermediate is likely to be involved in the reaction.

The aryne Sommelet–Hauser rearrangement

An aryne induced transition-metal-free and mild Sommelet–Hauser rearrangement of tertiary benzylamines for the synthesis of α-aryl amino acid derivatives in moderate to good yields and with broad substrate scope is presented.