Employing Arynes in Diels-Alder Reactions and Transition-Metal-Free Multicomponent Coupling and Arylation Reactions

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 Novel Hexahydroimidazo[1,2-a]pyridine Derivatives by One-Pot, Multicomponent Reaction under Ambient Conditions

An efficient one-pot multicomponent reaction for the synthesis of novel tetrasubstituted hexahydroimidazo[1,2-a]pyridines starting from readily available cinnamaldehydes, ethylenediamines, and 1,3-dicarbonyl compounds catalyzed by AcOH is described. Two new cycles and four new bonds are constructed with all reactants being efficiently utilized in this transformation. The products could be obtained in 1-3 h under ambient conditions exclusively as a single isomer (trans). Single-crystal X-ray analysis confirmed the trans derivative as the only isomer.

Copper(I)/DDQ-Mediated Double-Dehydrogenative Diels-Alder Reaction of Aryl Butenes with 1,4-Diketones and Indolones

A copper(I)/DDQ-mediated double-dehydrogenative Diels-Alder (DDDA) reaction of simple butenes with 1,4-diketones and indolones has been established for the first time. This strategy is based on a tandem double-dehydrogenation/Diels-Alder reaction from nonprefunctionalized starting materials, in which both a diene and dienophile were in situ generated via activation of fourfold inert C(sp³)-H bonds in one catalytic system.

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.

Macrocyclization of 3-triflyloxybenzynes with tetrahydrofuran via an anionic thia-Fries rearrangement

A novel and uncommon macrocyclization reaction between 3-triflyloxybenzynes and tetrahydrofuran has been developed for the first time, providing a direct method for the synthesis of a range of functionalized 19-membered polyether macrocycles in moderate to good yields. The process was proposed to proceed through an anionic thia-Fris rearrangement under transition metal-free conditions, leading to the formation of four new C-O bonds and one new C-S bond in a single step.

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.

Synthesis of Sulfamate-Fused 2-Aminopyrroles via an Isocyanide-Based Three Component [1+2+2] Annulation

An efficient preparation of sulfamate-fused 2-aminopyrroles was achieved through an isocyanide-based three-component [1+2+2] annulation of isocyanides, dialkyl acetylenedicarboxylates, and sulfamate-derived cyclic imines in good to excellent yields (up to 99 %). This reaction proceeds smoothly without any activation or modification of substances under neutral and metal-free conditions. The reaction could also be conveniently performed on a gram scale.

Uncovering the Neglected Similarities of Arynes and Donor-Acceptor Cyclopropanes

Arynes and donor-acceptor (D-A) cyclopropanes are two classes of strained systems having the potential for numerous applications in organic synthesis. The last two decades have witnessed a renaissance of interest in the chemistry of these species primarily because of the mild and robust methods for their generation or activation. Commonly, arynes as easily polarizable systems result in 1,2-disubstitution, whereas D-A cyclopropanes as polarized systems lead to 1,3-bisfunctionalization thereby showing striking similarities. Transformations with 1,2- and 1,3-dipoles afford cyclic structures. With arynes, emerging four-membered rings as intermediates might react further, whereas the analogous five-membered rings obtained from D-A cyclopropanes are most often the final products. However, there are a few cases where these intermediates behave surprisingly differently. This Minireview highlights the parallels in reactivity between arynes and D-A cyclopropanes thereby shedding light on the neglected similarities of these two reactive species.

Strategies toward Aryne Multifunctionalization via 1,2-Benzdiyne and Benzyne

Polysubstituted arenes are prevalent in numerous natural products, medicines, agrochemicals, and organic functional materials. Among methods to prepare polysubstituted arenes, pathways involving benzyne intermediates are particularly attractive given they can readily assemble highly diverse vicinal difunctionalized benzenes in a step-economical manner under transition-metal-free conditions. In order to incorporate more than two substituents on a benzene ring via a benzyne intermediate, methodologies involving benzdiyne and benztriyne have been developed, which significantly expand the current difunctionalization strategies in benzyne chemistry. In the past years, our group has been focusing on pushing the frontier of traditional benzyne chemistry and exploring new applications. In an aim to efficiently and conveniently construct polysubstituted arenes, we developed several aryne multifunctionalization strategies. The first strategy is through the 1,2-benzdiyne processes, which can be divided into a domino aryne approach and stepwise 1,2-benzdiyne approach. In our domino aryne study, we developed three domino aryne reagents as "sesquibenzyne" synthons, which are complementary in terms of reactivity and could adapt different modes of cascade transformations. By employing these domino aryne precursors, we were able to accomplish several cascade transformations, including double nucleophilic reactions, i.e., the reaction with carbonyl protected benzothioamides, 1,2-diamination, and 1,3-diamination. In addition, two cascade processes involving nucleophilic and pericyclic reactions, namely, domino aryne annulation via nucleophilic-ene cascade and domino aryne nucleophilic, Diels-Alder process, were successfully achieved as well. Meanwhile, with our desire to expand the scope of 1,2-benzdiyne transformations, we employed stepwise 1,2-benzdiyne tactics to access polysubstituted arenes. Depending on the property of the substituent on the C3-position of a benzyne intermediate, either an electron-withdrawing or electron-donating group, the incoming ortho groups were chosen accordingly. Consequently, we realized a [2 + 2] cycloaddition-Grob fragmentation process using 3-triflyloxybenzyne and a 1,2-benzdiyne process using 3-(trimethylsilyl)benzyne. Our second research strategy is to use single benzyne to access trisubstituted arenes, which represents a more atom- and step-economical protocol. With our deliberate design, we discovered a tandem benzyne S═O bond insertion/C-H functionalization process using single benzyne and aryl allyl sulfoxides, furnishing three chemical bonds of different types in a single process. This transformation is the first 1,2,3-trisubstitution example using single benzyne intermediate. At last, we developed an oxidative dearomatization strategy on Kobayashi benzyne precursors, which led to the preparation of various cyclohexenynone precursors with diverse substituents in highly efficient manner. In this study, we also demonstrated a new reaction mode of these cyclohexenynones with allyl sulfoxides, which involves a deeper utilization of the cyclohexyne triple bond. This work is the first example of successful connection of precursors of benzyne with cyclohexyne together, revealing a new research direction in the field of cyclohexyne.

Diverse reactivity of isatin-based N,N′-cyclic azomethine imine dipoles with arynes: synthesis of 1′-methyl-2′-oxospiro [indene-1,3′-indolines] and 3-aryl-3-pyrazol-2-oxindoles

Reaction of aryne with isatin based N,N′-cyclic AMI 1,3-dipole afforded 3,3-disubstituted oxindole while methyl substitution on the pyrrolidine ring of 1,3-dipole directed [3+2] cycloaddition products.

Aerobic Cu-catalyzed oxidative 1 : 2 coupling of benzynes with terminal alkynes

Aryl-Cu(iii) may serve as an intermediate in Cu-catalyzed aerobic 1 : 2 couplings of arynes with terminal alkynes, allowing for one-step assemblies of arenediynes.

Recent advances in the synthesis of fluorinated compounds via an aryne intermediate

In this review, the usefulness of aryne intermediates for the synthesis of various fluorinated organic compounds during the last decade has been demonstrated.

Revisiting Kekulene: Synthesis and Single-Molecule Imaging

Four decades after the first (and only) reported synthesis of kekulene, this emblematic cycloarene has been obtained again through an improved route involving the construction of a key synthetic intermediate, 5,6,8,9-tetrahydrobenzo[m]tetraphene, by means of a double Diels-Alder reaction between styrene and a versatile benzodiyne synthon. Ultra-high-resolution AFM imaging of single molecules of kekulene and computational calculations provide additional support for a molecular structure with a significant degree of bond localization in accordance with the resonance structure predicted by the Clar model.

1,2-Aminohalogenation of arynes with amines and organohalides

An unprecedented use of inexpensive organohalides as halogen electrophiles to trap the zwitterion intermediates generated from amines and arynes has been developed to access structurally diverse tertiary 2-haloanilines. Effective organohalides include carbon tetrachloride, hexachloroethane, N-chlorosuccinimide, carbon tetrabromide, fluorotribromomethane, N-bromosuccinimide, carbon tetraiodide, and N-iodosuccinimide.

Cyclic Alkyne Approach to Heteroatom-Containing Polycyclic Aromatic Hydrocarbon Scaffolds

We report a modular synthetic strategy for accessing heteroatom-containing polycyclic aromatic hydrocarbons (PAHs). Our approach relies on the controlled generation of transient heterocyclic alkynes and arynes. The strained intermediates undergo in situ trapping with readily accessible oxadiazinones. Four sequential pericyclic reactions occur, namely two Diels-Alder/retro-Diels-Alder sequences, which can be performed in a stepwise or one-pot fashion to assemble four new carbon-carbon (C-C) bonds. These studies underscore how the use of heterocyclic strained intermediates can be harnessed for the preparation of new organic materials.

Cycloadditions of Oxacyclic Allenes and a Catalytic Asymmetric Entryway to Enantioenriched Cyclic Allenes

The chemistry of strained cyclic alkynes has undergone a renaissance over the past two decades. However, a related species, strained cyclic allenes, especially heterocyclic derivatives, have only recently resurfaced and represent another class of valuable intermediates. We report a mild and facile means to generate the parent 3,4-oxacyclic allene from a readily accessible silyl triflate precursor, and then trap it in (4+2), (3+2), and (2+2) reactions to provide a variety of cycloadducts. In addition, we describe a catalytic, decarboxylative asymmetric allylic alkylation performed on an α-silylated substrate, to ultimately permit access to an enantioenriched allene. Generation and trapping of the enantioenriched cyclic allene occurs with complete transfer of stereochemical information in a Diels-Alder cycloaddition through a point-chirality, axial-chirality, point-chirality transfer process.