Synthesis of Difluoroalkylated Benzofuran, Benzothiophene, and Indole Derivatives via Palladium-Catalyzed Cascade Difluoroalkylation and Arylation of 1,6-Enynes

K2CO3-mediated annulation of 1,3-acetonedicarboxylates with 2-fluoro-1-nitroarenes: synthesis of indoles

The K2CO3-mediated one-pot reaction of 1,3-acetonedicarboxylates with 2 equiv. of substituted 2-fluoro-1-nitrobenzenes has been developed to synthesize various 2,3-dicarboxylate indoles via a tandem annulation pathway. In the effective reaction, one carbon-carbon double bond, one carbon-carbon single bond and one carbon-nitrogen single bond are formed under open-vessel conditions. DFT calculations are used to rationalize the plausible mechanisms.

Palladium-Catalyzed Three-Component Cascade Carbonylation Reaction to Construct Benzofuran Derivatives

A novel three-component cyclization carbonylation reaction of iodoarene-tethered propargyl ethers with amine and CO is reported. This palladium-catalyzed cascade reaction undergoes a sequence of oxidative addition, unsaturated bond migration, carbonyl insertion, and nucleophilic attack to deliver the benzofuran skeleton. Both aromatic amines and aliphatic amines could proceed smoothly in this transformation under one atm of CO.

Visible-Light-Induced Radical Sulfonylative-Cyclization Cascade of 1,6-Enynol Derivatives with Sulfinic Acids: A Sustainable Approach for the Synthesis of 2,3-Disubstituted Benzoheteroles

Benzoheteroles are promising structural scaffolds in the realm of medicinal chemistry, but sustainable synthesis of 2,3-difunctionalized benzoheterole derivatives is still in high demand. Indeed, we have conceptually rationalized the intrinsic reactivity of propargylic-enyne systems for the flexible construction of 2,3-disubstituted benzoheteroles through radical sulfonylative-cyclization cascade under organophotoredox catalysis. We hereby report an efficient visible-light-induced sulfonyl radical-triggered cyclization of 1,6-enynols with sulfinic acids under the dual catalytic influence of 4CzIPN and NiBr2⋅DME, which led to the formation of 2,3-disubstituted benzoheteroles in good to high yields. Additionally, the Rose Bengal (RB)-catalyzed radical sulfonylative-cycloannulation of acetyl-derived 1,6-enynols with sulfinic acids under blue LED irradiation allowed to access 3-(E-styryl)-derived benzofurans and benzothiophenes in moderate to good yields. The scope and limitations of the present strategies were successfully established using different classes of 1,6-enynols and sulfinic acids bearing various sensitive functional groups, yielding the desired products in a highly stereoselective fashion. Plausible mechanistic pathways were also proposed based on the current experimental and control experiments.

Unified Radical Sulfonylative-Annulation of 1,6-Enynols with Sodium Sulfinates: A Modular Synthesis of 2,3-Disubstituted Benzoheteroles

Benzoheteroles are valuable scaffolds in medicinal chemistry, but the direct synthesis of 3-vinyl benzoheterole analogues remains unexplored. A rationally designed new class of 1,6-enyne-containing propargylic alcohols has been prepared for the modular synthesis of 3-alkenyl benzoheteroles. Ag-catalyzed cascade radical sulfonylative-cycloannulation of 1,6-enynols with sodium sulfinates is realized to access a wide variety of 2,3-disubstituted benzoheteroles in good to high yields. Moreover, a three-component coupling of 1,6-enynols, aryldiazonium salts, and Na₂S₂O₅ (as an SO₂ surrogate) has been achieved to deliver benzoheterole derivatives in moderate to good yields. Of note, a scalable reaction and late-stage synthetic transformations were successfully demonstrated. A plausible mechanism is also presented based on the existing experimental results and control experiments.

Difunctionalization of Dienes, Enynes and Related Compounds via Sequential Radical Addition and Cyclization Reactions

Radical reactions are powerful in creating carbon-carbon and carbon-heteroatom bonds. Designing one-pot radical reactions with cascade transformations to assemble the cyclic skeletons with two new functional groups is both synthetically and operationally efficient. Summarized in this paper is the recent development of reactions involving radical addition and cyclization of dienes, diynes, enynes, as well as arene-bridged and arene-terminated compounds for the preparation of difunctionalization cyclic compounds. Reactions carried out with radical initiators, transition metal-catalysis, photoredox, and electrochemical conditions are included.

Copper-Catalyzed Difluoroalkylation Reaction

This review describes recent advances in copper-catalyzed difluoroalkylation reactions. The RCF₂ radical is generally proposed in the mechanism of these reactions. At present, various types of copper-catalyzed difluoroalkylation reactions have been realized. According to their characteristics, we classify these difluoroalkylation reactions into three types.

Sustainable access to benzothiophene derivatives bearing a trifluoromethyl group via a three-component domino reaction in water

A catalyst-free three-component domino reaction was developed for the synthesis of benzothiophene fused pyrrolidones bearing a CF3 group for the first time. The notable advantages of this strategy over the existing methods include the use of water as a solvent at room temperature, transition metal-free conditions, a broad substrate scope, and easy scale-up synthesis. More importantly, the benzothiophene derivatives have been found to show potent anticancer activities using the Cell Counting Kit-8 (CCK-8) assay.

Visible-Light-Promoted bis(Difluoromethylation)/Cyclization of 2-Vinyloxy Arylalkynes to Prepare Benzofuran Derivatives

A visible-light-promoted difluoromethylation/cyclization of 2-vinyloxy arylalkynes was developed, providing a variety of bis(difluoromethyl)-substituted benzofurans in moderate to good yields. A plausible mechanism involving difluoromethyl radical cascade cyclization and solvent-promoted ionic addition was proposed. This protocol has the advantages of having mild reaction conditions, simple operation, and good functional group tolerance.

Electrochemical selective annulative amino-ketalization and amino-oxygenation of 1,6-enynes

A new electrochemical selective annulative amino-ketalization and amino-oxygenation of 1,6-enynes with disulfonimides and alcohols is reported, producing a series of functionalized benzofurans under catalyst- and oxidant-free conditions. The annulative aminoketalization proceeds with simple short-chain alcohols such as methanol, ethanol and n-propanol as O-nucleophilic reagents, while the reaction occurs in the annulative aminooxygenation direction in the presence of water and large steric sec-butyl alcohol (SBA).

Visible-Light-Induced 1,6-Enynes Triggered C-Br Bond Homolysis of Bromomalonates: Solvent-Controlled Divergent Synthesis of Carbonylated and Hydroxylated Benzofurans

Visible-light-induced 1,6-enyne-triggered C-Br bond homolysis of bromomalonates has been developed. This transition-metal-free, photocatalyst-free, and oxidant- and additive-free protocol affords an efficient approach for divergent synthesis of carbonylated and hydroxylated benzofurans from 1,6-enynes and bromomalonates under mild conditions. Significantly, mechanistic studies reveal that the homolysis of C-Br bonds appears to experience an energy-transfer pathway, and the atom-transfer radical addition products are the key intermediates to generate carbonylated and hydroxylated benzofurans.

Metal-free, Phosphoric Acid-catalyzed Regioselective 1,6-Hydroarylation of para-Quinone Methides with Indoles in Water

An efficient, cheap and green protocol for the highly regioselective 1,6-hydroarylation of para -quinone methides ( p -QMs) with indoles at the C-3 position has been established by phosphoric acid catalysis in water under the transition-metal-free reaction conditions. A wide range of indole derivatives and para -quinone methides ( p -QMs) are compatible for the reaction, affording the corresponding 1,6-hydroarylation products with good to excellent yields. The possible mechanism of the reaction has been explored by step-by-step control experiments. The protocol is convenient for practical application, leading a safe, green and feasible way for the formation of C-3 diarylmethyl functionalized indole derivatives.

Copper-Catalyzed Difluoroalkylation of Alkene/Nitrile Insertion/Cyclization Tandem Sequences: Construction of Difluorinated Bicyclic Amidines

A copper-catalyzed difluoroalkylation of an alkene/nitrile insertion/cyclization tandem sequence of N-cyanamide alkene was described, which provided a convenient synthetic approach for accessing difluorinated bicyclic amidines bearing imine moieties in a sustainable fashion. This protocol is characterized by high yields, a broad substrate scope, and good functional group compatibility. In addition, the desired product can be readily converted into other valuable functionalized fluorinated aza-heterocycles.

Copper and Photocatalytic Radical Relay Enabling Fluoroalkylphosphorothiolation of Alkenes: Modular Synthesis of Fluorine-Containing S-Alkyl Phosphorothioates and Phosphorodithioates

A photoredox and copper-catalyzed fluoroalkylphosphorothiolation of activated and unactivated alkenes via a radical relay mechanism is reported. By employing fluoroalkyl halides as radical precursors and P(O)SH or P(S)SH compounds as coupling partners, a wide range of β-monofluoroalkyl-, -difluoroalkyl-, -trifluoromethyl-, or -perfluoroalkyl-substituted S-alkyl phosphorothioates and phosphorodithioates can be easily constructed under mild conditions with good functional group tolerance. Furthermore, this modular reaction system can be successfully applied to late-stage functionalization of bioactive molecules.

Photocatalyzed Coupling-Cyclization of ortho-Alkynylaryl Vinylethers with Arylsulfonyl Azides

A novel visible-light-induced coupling-cyclization of ortho-alkynylaryl vinylethers with arylsulfonyl azides has been described. This transformation provided a concise approach to access C3-exocyclic C═C bond/C2-alkylsulfone-tethered benzofurans via a solvent-leveraged carbosulfonylation and [2 + 2 + 3] cyclization. Primary mechanistic studies demonstrated that THF belongs to a crucial H atom source.

Palladium-Catalyzed Cascade Difluoroalkylation and Phosphinoylation of 2-Vinyloxy Arylalkynes: Selective Synthesis of Difluoroalkyl-Containing Tetrasubstituted Alkenylphosphine Oxides

A Pd-catalyzed difluoroalkylation/cyclization/phosphinoylation of 2-vinyloxy arylalkynes with ethyl difluoroiodoacetate and diarylphosphine oxides has been successfully developed. This reaction allows the formation of C_sp3-CF₂, C_sp3-C_sp2, and C_sp2-P(O) bonds in one step, providing a straightforward route to difluoroalkyl-containing tetrasubstituted alkenylphosphine oxides with complete stereoselectivities under mild conditions.

Iridium-Catalyzed Isomerization/Cycloisomerization/Aromatization of N-Allyl-N-sulfonyl-o-(λ1-silylethynyl)aniline Derivatives to Give Substituted Indole Derivatives

We have developed a one-iridium-catalyst system that transforms N-allyl-N-sulfonyl-2-(silylalkynyl)aniline derivatives, which are 1,7-enynes in which both multiple bonds have a heteroatom, to the corresponding substituted indole derivatives via isomerization/cycloisomerization/aromatization. This strategy provides an atom-economical and straightforward synthetic approach to a series of valuable indoles having vinyl and silylmethyl groups at the 2- and 3-positions.

Dearomative 1,6-addition of P(O)–H to in situ formed p-QM-like ion pairs from 2-benzofuryl-ols to C3-phosphinoyl hydrobenzofurans

We report a dearomative C3-phosphorylation and a tandem C3-phosphorylation/aromatization of 2-benzofuryl-ols with P(O)–H species to afford C3-phosphinoyl hydrobenzofurans and benzofurans, respectively.

Cu-Catalyzed radical-triggered spirotricyclization of enediynes and enyne-nitriles for the synthesis of pentacyclic spiroindenes

A new copper-catalyzed radical-triggered fluoromethylation-spirotricyclization of enediyne- and enyne-nitrile-containing para-quinone methides (p-QMs) was reported for the first time, and used to produce a series of hitherto unreported pentacyclic spiroindenes.

Fluorination and fluoroalkylation of alkenes/alkynes to construct fluoro-containing heterocycles

This review summarize the established strategies through fluorination and fluoroalkylation of alkenes/alkynes for constructing fluoro-containing heterocycles. Reaction scopes, mechanisms and some shortcomings are also discussed.

Silver-catalyzed decarboxylative radical relay difluoroalkylation–carbocyclization: convenient access to CF2-containing quinolinones

A practical Ag-catalyzed formal decarboxylation and radical difluoroalkylation–carbocyclization–hydrolysis route is established to construct a series of structurally diverse CF2-containing N-heterocycles.

Metal-Catalyzed Spiroannulation-Fluoromethylfunctionaliztions of 1,5-Enynes for the Synthesis of Stereodefined (Z)-Spiroindenes

Two classes of new catalytic spiroannulation-fluormethylfunctionaliztions of para-quinone methide (p-QM)-containing 1,5-enynes have been established under redox-neutral conditions. Palladium-catalyzed spiroannulation-iododifluoromethylation with ethyl difluoroiodoacetate oriented completely stereoselective access to (Z)-spiroindenes and the latter included copper-catalyzed three-component spiroannulation-cyanotrifluoromethylation starting from Togni's reagent and trimethylsilanecarbonitrile (TMSCN). Both reaction pathways involve fluoroalkyl radical-triggered 1,6-addition/5-exo-dig annulation/metal radical cross-coupling/reductive elimination sequence, providing practical and stereoselective protocols for rapidly constructing cyclohexadienone-containing spiroindenes with generally good yields.

Last Decade of Unconventional Methodologies for the Synthesis of Substituted Benzofurans

This review describes the progress of the last decade on the synthesis of substituted benzofurans, which are useful scaffolds for the synthesis of numerous natural products and pharmaceuticals. In particular, new intramolecular and intermolecular C-C and/or C-O bond-forming processes, with transition-metal catalysis or metal-free are summarized. (1) Introduction. (2) Ring generation via intramolecular cyclization. (2.1) C7a-O bond formation: (route a). (2.2) O-C2 bond formation: (route b). (2.3) C2-C3 bond formation: (route c). (2.4) C3-C3a bond formation: (route d). (3) Ring generation via intermolecular cyclization. (3.1) C7a-O and C3-C3a bond formation (route a + d). (3.2) O-C2 and C2-C3 bond formation: (route b + c). (3.3) O-C2 and C3-C3a bond formation: (route b + d). (4) Benzannulation. (5) Conclusion.

Promising reagents for difluoroalkylation

This review describes recent advances in difluoroalkylation reactions using different substrates.

Synthesis of difluoroalkylated 2-azaspiro[4.5]decane derivatives via copper-catalyzed difluoroalkylation/dearomatization of N-benzylacrylamides

A copper-catalyzed synthesis of difluoroalkylated spiro-azacycles from N-benzylacrylamides is presented. The reaction involves the β-difluoroalkylation of acrylamide, 5-exo cyclization, and dearomatization.