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.

Silica gel-induced aryne generation from o-triazenylarylboronic acids as stable solid precursors

We developed o-triazenylarylboronic acids as stable solid aryne precursors, which generate arynes under mild conditions using silica gel as the sole reagent and undergo reactions with a range of arynophiles both in solution and in the solid-state.

Construction of 6H-benzo[c]thiochromenes via a tandem reaction of arynes with thionoesters

A mild and transition-metal free method has been proposed for the synthesis of 6H-benzo[c]thiochromenes via the tandem reactions of arynes with thionoesters.

The mechanism of conversion of substituted glycals to chiral acenes via Diels-Alder reaction: a computational study

Synthesis of linearly fused aromatic systems using a glycal-based diene with an aryne is a long-standing topic of interest in glycal chemistry. We have examined the mechanistic pathways for the transformation of substituted glycals to chiral fused aromatic cores via Diels-Alder (DA) reaction using the SMDACN-M06-2X/6-31G(d) level of theory. The DA reactions of E (1a) and Z (1a') forms of C-2 alkenyl glycal and an aryl glycal (1b) as a diene were examined with a benzyne intermediate generated as a dienophile. The computational results reveal that 1a and 1b can only be transformed into the fused aromatic cores by the base-catalyzed reaction because a [1,5] sigmatropic hydrogen shift is not feasible. The activation free energy barrier for the base-catalyzed proton abstraction process is 4.2 kcal mol-1 and there is almost no barrier for stereoisomeric 1a DA-complexes. The activation free energy barrier values for stereoisomeric 1b DA-complexes for the base-catalyzed proton abstraction process are 10.8 and 12.4 kcal mol-1. The appropriate orientation of glycal-ring-oxygen and hydrogen at the 5th position of Z (1a') forms of C-2 alkenyl glycal facilitates the [1,5] sigmatropic hydrogen shift; however, the base-catalyzed reaction is energetically more favored than the former case. The rate-determining step for 1a and 1a' is the ring-opening step (18.2 and 19.5 kcal mol-1 for the S-stereoisomer), whereas the DA adduct formation step is the rate-determining step for 1b (16.1 kcal mol-1 for the S-stereoisomer). The structural analysis reveals the formation of the preferred S-stereoisomer over the R-stereoisomer with the respective dienes.

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.

One-step synthesis of benzo[b]thiophenes by aryne reaction with alkynyl sulfides

An aryne reaction with alkynyl sulfides affording benzo[b]thiophenes is disclosed. A wide range of 3-substituted benzothiophenes were synthesized from easily available o-silylaryl triflates and alkynyl sulfides in a one-step intermolecular manner. The synthesis of diverse multisubstituted benzothiophene derivatives involving a pentacyclic compound was achieved by virtue of the good functional group tolerance and versatile C2 functionalizations.

Transition-Metal-Free Tandem Cyclization/N-Arylation Reaction: A Method To Access Biaryl Sultam Derivatives via a Diradical Pathway

A novel procedure for the transition-metal-free tandem cyclization/N-arylation reaction sequence of an aryne with a 1,2,3,4-benzothiatriazine-1,1-dioxide is reported. This reaction goes through the intramolecular homolytic cyclization to generate an N-H biaryl sultam intermediate, which enables aryne insertion to access diversely functionalized biaryl sultam derivatives with high yields. The mechanism study indicates that homolytic cyclization is executed by a diradical species, initiated from the thermal decomposition of 1,2,3,4-benzothiatriazine-1,1-dioxide to release a nitrogen molecule.

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.

Sulfoxide synthesis from sulfinate esters under Pummerer-like conditions

A facile synthetic method for the preparation of allyl sulfoxides by S-allylation of sulfinate esters proceeds through sulfonium intermediates without [3,3]-sigmatropic rearrangement and further Pummerer-type reactions of the resulting allyl sulfoxides. On the basis of the plausible reaction mechanism involving sulfonium salt intermediates, S-alkynylation and S-arylation were also accomplished.

Functionalization of a Single C-F Bond of Trifluoromethylarenes Assisted by an ortho-Silyl Group Using a Trityl-Based All-in-One Reagent with Ytterbium Triflate Catalyst

Catalytic thiolation and azidation of a single C-F bond of trifluoromethylarenes were achieved assisted by an ortho-silyl group with all-in-one reagents to generate a trityl cation and nucleophiles. The reactions catalyzed by ytterbium triflate efficiently afforded a wide variety of difluoromethylenes avoiding the further C-F bond cleavage, by virtue of the mild conditions without the generation of a Brønsted acid.

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.

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.

Exploring Possible Surrogates for Kobayashi's Aryne Precursors

A class of aryne precursors, that is, 2-(trimethylsilyl)aryl 4-chlorobenzenesulfonates, has been developed through well-established synthetic routes, which allow the formation of arynes under relatively mild conditions. All the aryne precursors were obtained from phenols and 4-chlorobenzenesulfonyl chloride, an inexpensive and easy-to-handle reagent with relatively low toxicity, and subjected to nucleophilic addition reactions, providing addition products in yields of 24 to 92%, and to cycloaddition reactions, affording cycloadducts in yields up to 80%. This work provides interesting insights into the mechanisms of aryne generation. In addition, 2-(trimethylsilyl)phenyl 4-chlorobenzenesulfonate was successfully employed in the total synthesis of (±)-aporphine.

Catalytic 1,2-Rearrangements: Organocatalyzed Michael/Semi-Pinacol-like Rearrangement Cascade of 1,3-Diones and Nitroolefins

New types of organocatalytic 1,2-rearrangements, which resemble the Smiles-like or semi-pinacol-like rearrangement, of Michael adducts of 1,3-dicarbonyl-2-alkyl compounds and nitroalkenes have been realized. Unlike the well-known conjugate addition, the reaction affords the 1-phenyl-1-nitroalkanes via unprecedented rearrangement and cascade reactions. Structures of the appropriate products were unambiguously characterized by X-ray crystallography.

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.

Synthesis of 6-substituted indoloquinazolinones from arynes and 2-acyl-4-quinazolinones: a transition-metal free C–N and C–C bond formation strategy

A versatile transition-metal free synthetic strategy has been developed for the direct synthesis of 6-substituted indoloquinazolinones from 2-acyl-4-quinazolinones and aryne precursors. This cascade strategy proceeds via successive C–N and C–C bond formation in a single reaction vessel.

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.

Aryl Carbamates: Mechanisms of Orthosodiations and Snieckus-Fries Rearrangements

Aryl carbamates are orthometalated by sodium diisopropylamide (NaDA) in tetrahydrofuran. The resulting arylsodiums undergo Snieckus-Fries rearrangement to give orthoacylated phenols in good yield. The intermediate arylsodiums and resulting orthoacylated phenolates are suggested to be monomeric. The rate-limiting step in the two-step sequence depends on the steric demands of the carbamoyl moiety and the substituents in the meta position of the arene. Rate studies reveal a dominant disolvated-monomer-based orthometalation followed by a di- or trisolvated arylsodium monomer-based rearrangement. Kinetic evidence of a NaDA-catalyzed Snieckus-Fries rearrangement suggests the intermediacy of mixed trimers. Competitive halide eliminations to form benzyne are also discussed.