Access to Multifunctionalized Benzofurans by Aryl Nickelation of Alkynes: Efficient Synthesis of the Anti‐Arrhythmic Drug Amiodarone

Copper-Catalyzed Three-Component Tandem Cyclization for One-Pot Synthesis of Indole-Benzofuran Bis-Heterocycles

A one-pot, three-component synthesis of indole-benzofuran bis-heterocycles from terminal alkynes, salicylaldehydes, and indoles has been developed via copper-catalyzed tandem annulation. This catalytic system utilizes readily available starting materials, enabling predictable synthesis of indole-benzofuran bis-heterocycles with broad substrate versatility, excellent regiocontrol, and gram-scale amenability. The reaction proceeds via a sequential pathway involving A3 coupling, 1,4-conjugate addition, and 5-exo-dig cyclization.

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.

Nickel-catalyzed electrophiles-controlled enantioselective reductive arylative cyclization and enantiospecific reductive alkylative cyclization of 1,6-enynes

Transition metal-catalyzed asymmetric cyclization of 1,6-enynes is a powerful tool for the construction of chiral nitrogen-containing heterocycles. Despite notable achievements, these transformations have been largely limited to the use of aryl or alkenyl metal reagents, and stereoselective or stereospecific alkylative cyclization of 1,6-enynes remains unexploited. Herein, we report Ni-catalyzed enantioselective reductive anti-arylative cyclization of 1,6-enynes with aryl iodides, providing enantioenriched six-membered carbo- and heterocycles in good yields with excellent enantioselectivities. Additionally, we have realized Ni-catalyzed enantiospecific reductive cis-alkylative cyclization of 1,6-enynes with alkyl bromides, furnishing chiral five-membered heterocycles with high regioselectivity and stereochemical fidelity. Mechanistic studies reveal that the arylative cyclization of 1,6-enynes is initiated by the oxidative addition of Ni(0) to aryl halides and the alkylative cyclization is triggered by the oxidative addition of Ni(0) to allylic acetates. The utility of this strategy is further demonstrated in the enantioselective synthesis of the antiepileptic drug Brivaracetam.

Nickel-Catalyzed Reductive anti-Arylative Cyclization of Alkynyl Enones with Aryl Halides

A nickel-catalyzed reductive anti-arylative cyclization of alkynyl enones with aryl halides has been developed. The reaction avoids the use of stoichiometric organometallic reagents and has a broad reaction scope and high functional group tolerance. This method offers an efficient way to access a variety of synthetically useful carbocycles that are widely found in many natural products and biologically active molecules.

Facile access to benzofuran derivatives through radical reactions with heteroatom-centered super-electron-donors

The development of suitable electron donors is critical to single-electron-transfer (SET) processes. The use of heteroatom-centered anions as super-electron-donors (SEDs) for direct SET reactions has rarely been studied. Here we show that heteroatom anions can be applied as SEDs to initiate radical reactions for facile synthesis of 3-substituted benzofurans. Phosphines, thiols and anilines bearing different substitution patterns work well in this inter-molecular radical coupling reaction and the 3-functionalized benzofuran products bearing heteroatomic functionalities are given in moderate to excellent yields. The reaction mechanism is elucidated via control experiments and computational methods. The afforded products show promising applications in both organic synthesis and pesticide development.

Atroposelective Synthesis of C-N Vinylindole Atropisomers by Palladium-Catalyzed Asymmetric Hydroarylation of 1-Alkynylindoles

Transition-metal-catalyzed hydroarylation of unsymmetrical internal alkynes remains challenging because of the difficulty in controlling regioselectivity and stereoselectivity. Moreover, the enantioselective hydroarylation of alkynes using organoboron reagents has not been reported. Herein, we report for the first time that palladium compounds can catalyze the hydroarylation of 1-alkynylindoles with organoborons for the synthesis of chiral C-N atropisomers. A series of rarely reported vinylindole atropisomers was synthesized with excellent regio-, stereo- (Z-selectivity), and enantioselectivity under mild reaction conditions. The ready availability of organoborons and alkynes and the simplicity, high stereoselectivity, and good functional group tolerance of this catalytic system make it highly attractive.

Visible Light-Induced Metal-Free Fluoroalkylations

Fluoroalkylation is a crucial synthetic process that enables the modification of molecules with fluoroalkyl groups, which can enhance the properties of compounds and have potential applications in medicine and materials science. The utilization of visible light-induced, metal-free methods is of particular importance as it provides an environmentally friendly alternative to traditional methods and eliminates the potential risks associated with metal-catalyst toxicity. This Account describes our studies on visible light-induced, metal-free fluoroalkylation processes, which include the use of organic photocatalysts or EDA complexes. We have utilized organophotocatalysts such as Nile red, tri(9-anthryl)borane, and an indole-based tetracyclic complex, as well as catalyst-free EDA chemistry through photoactive halogen bond formation or an unconventional transient ternary complex formation with nucleophilic fluoroalkyl source. A variety of π-systems including arenes/heteroarenes, alkenes, and alkynes have been successfully fluoroalkylated under the developed reaction conditions.

Enantioselective Nickel-Catalyzed Reductive anti-Arylative Annulation of Alkyne-Tethered Malononitriles to Construct Quaternary Stereocenters

A nickel-catalyzed reductive desymmetrizing annulation of alkyne-tethered malononitriles and (hetero)aryl iodides is reported for the access of cyclohexenones containing an α-all-carbon quaternary stereocenter. The use of a nickel catalyst derived from an electron-rich phosphinooxazoline ligand combined with iron powder as a reductant is crucial to the success of this transformation.

Construction of a Pentacyclic Framework Enabled by Nickel Catalysis

We present a novel nickel-catalyzed reaction of indole-tethered 2-alkynylphenol esters with various (hetero)aryl boronic acids, resulting in the synthesis of diversely functionalized pentacyclic benzofurocyclohepta[b]indole derivatives. This unprecedented cascade reaction involves the arylative cyclization of alkynes, nucleophilic attack of the indole moiety on the oxonium ion intermediate, 1,2-alkyl group migration, and aromatization. The synthesized molecules exhibit exceptional cytotoxicity against multiple cancer cell lines while maintaining biocompatibility toward healthy cells.

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.

Intramolecular activation of strong Si-O bonds by gold(I): regioselective synthesis of 3-bromo-2-silylbenzofurans

The high reactivity of gold-vinylidene complexes, generated in situ by [1,2]-bromine shift from the corresponding 1-bromoalkynes, allows the activation of one of the strongest bonds in organic chemistry (Si-O), strategically placed at the ortho-position. In this way, the synthesis of 3-bromo-2-silylbenzofuranes is achieved in good yields. Several substituents with different electronic properties and substitution patterns are well tolerated on the tethering aromatic ring as well as a number of silyl groups on the O-atom. A preliminary mechanistic study is compatible with the participation of gold vinylidene intermediate species. The synthetic applicability of the obtained scaffolds was preliminarily showcased by orthogonal C-C bond forming transformations.

Nickel-Catalyzed Arylative Cyclizations of Alkyne- and Allene-Tethered Electrophiles using Arylboron Reagents

The use of arylboron reagents in metal‐catalyzed domino addition–cyclization reactions is a well‐established strategy for the preparation of diverse, highly functionalized carbo‐ and heterocyclic products. Although rhodium‐ and palladium‐based catalysts have been commonly used for these reactions, more recent work has demonstrated nickel catalysis is also highly effective, in many cases offering unique reactivity and access to products that might otherwise not be readily available. This review gives an overview of nickel‐catalyzed arylative cyclizations of alkyne‐ and allene‐tethered electrophiles using arylboron reagents. The scope of the reactions is discussed in detail, and general mechanistic concepts underpinning the processes are described.

Synthesis of α,β-unsaturated ketones through nickel-catalysed aldehyde-free hydroacylation of alkynes

α,β-Unsaturated ketones are common feedstocks for the synthesis of fine chemicals, pharmaceuticals, and natural products. Transition metal-catalysed hydroacylation reactions of alkynes using aldehydes have been recognised as an atom-economical route to access α,β-unsaturated ketones through chemoselective aldehydic C-H activation. However, the previously reported hydroacylation reactions using rhodium, cobalt, or ruthenium catalysts require chelating moiety-bearing aldehydes to prevent decarbonylation of acyl-metal-hydride complexes. Herein, we report a nickel-catalysed anti-Markovnikov selective coupling process to afford non-tethered E-enones from terminal alkynes and S-2-pyridyl thioesters in the presence of zinc metal as a reducing agent. Utilization of a readily available thioester as an acylating agent and water as a proton donor enables the mechanistically distinctive and aldehyde-free hydroacylation of terminal alkynes. This non-chelation-controlled approach features mild reaction conditions, high step economy, and excellent regio- and stereoselectivity.

A Convenient Procedure for Sonogashira Reactions Using Propyne

A modified Sonogashira coupling of aryl iodides and propyne­ was achieved using only two equivalents of propyne in THF from –78 °C to room temperature.

Ni-Catalyzed Reductive Coupling of Alkynes and Amides to Access Multi-Functionalized Indoles

A nickel-catalyzed reductive coupling of alkynes and amides, followed by base-free transmetalation, proceeded selectively in the presence of an uncommon bidentate primary aminophosphine ligand to access highly functionalized indoles comprising biologically important trifluoromethyl groups and challenging electron-rich alkenyl groups at the 2- and 3-positions, respectively. Indole molecules were installed within natural products or drug molecules under mild conditions, and a trifluoromethylated analogue of a drug molecule (pravadoline) was also synthesized.

Reductive Ni-catalysis for stereoselective carboarylation of terminal aryl alkynes

Stereoselective dicarbofunctionalization of terminal aryl alkynes has been achieved through reductive Ni-catalysis. The exclusive regioselective and anti-addition selective alkylarylation of terminal alkynes is accomplished using alkyl iodide and aryl iodide as electrophilic coupling partners in the presence of NiBr₂ as the catalyst and Mn as an inexpensive reductant.

Synthesis of ortho-Diamino-Functionalized 1-Arylnaphthalenes through Nickel-Catalyzed Cyclization of Ynamide-Benzylnitriles with Organoboronic Acids

A nickel-catalyzed highly regioselective addition/cyclization of ynamide-benzylnitriles with organoboronic acids has been developed. The reaction offers an attractive route for the construction of diamino-functionalized 1-arylnaphthalenes, which is difficult to synthesize by other methods. In addition, a wide range of functional groups are tolerated in this reaction.

Enantioselective nickel-catalyzed anti-arylmetallative cyclizations onto acyclic electron-deficient alkenes

Enantioselective nickel-catalyzed reactions of (hetero)arylboronic acids or alkenylboronic acids with substrates containing an alkyne tethered to various acyclic electron-deficient alkenes are described.

Au-Ag Bimetallic Catalysis: 3-Alkynyl Benzofurans from Phenols via Tandem C-H Alkynylation/Oxy-Alkynylation

The development of new methodologies enabling a facile access to valuable heterocyclic frameworks still is an important subject of research. In this context, we describe a dual catalytic cycle merging C-H alkynylation of phenols and oxy-alkynylation of the newly introduced triple bond by using a unique redox property and the carbophilic π acidity of gold. Mechanistic studies support the participation of a bimetallic gold-silver species. The one-pot protocol offers a direct, simple, and regio-specific approach to 3-alkynyl benzofurans from readily available phenols. A broad range of substrates, including heterocycles, is transferred with excellent functional group tolerance. Thus, this methodology can be used for the late-stage incorporation of benzofurans.

Phosphine-Catalyzed Chemoselective Reduction/Elimination/Wittig Sequence for Synthesis of Functionalized 3-Alkenyl Benzofurans

An efficient protocol for the construction of functionalized 3-alkenyl benzofurans is demonstrated under metal-free conditions using catalytic amount of phosphine proceeding an intramolecular Wittig reaction. This one-pot reaction initiated by the phospha-Michael addition of phosphine to O-acylated nitrostyrene, in which phosphine was in-situ-generated from the chemoselective reduction of phosphine oxide with PhSiH₃, would provide the phosphorus ylide to result in the aforementioned multifunctionalized benzofuran via O-acylation/nitrous acid elimination/Wittig reaction.

One-pot synthesis of benzofurans via heteroannulation of benzoquinones

Three different reactions were explored leading to the synthesis of various benzofurans. All reactions took place under AcOH catalysis in a one-pot manner. As a result, benzoquinone derivatives underwent heteroannulation with either itself or cyclohexanones to produce furanylidene-benzofuran or benzofuran structures, respectively.

Enantioselective Nickel-Catalyzed anti-Arylmetallative Cyclizations onto Acyclic Ketones

Domino reactions involving nickel-catalyzed additions of (hetero)arylboronic acids to alkynes, followed by cyclization of the alkenylnickel intermediates onto tethered acyclic ketones to give chiral tertiary-alcohol-containing products in high enantioselectivities, are described. The reversible E/Z isomerization of the alkenylnickel intermediates enables overall anti-arylmetallative cyclization to occur. The ring system of the products are substructures of certain diarylindolizidine alkaloids.

Synthesis of Sulfonylated Heterocycles via Copper-Catalyzed Heteroaromatization/Sulfonyl Transfer of Propargylic Alcohols

An unprecedented copper-catalyzed heteroaromatization/sulfonyl transfer of propargylic alcohols with isocyanide has been developed. 3-Sulfonyl benzofurans and indoles were produced under Cu(I) catalysis in good to high yields. The developed catalytic methodology provides controlled, modular, and facile access to sulfonyl benzoheterocycle scaffolds.

Nickel-Catalyzed trans-Carboamination across Internal Alkynes to Access Multifunctionalized Indoles

A Ni-catalyzed reaction was developed for the synthesis of multifunctionalized indoles. The reaction proceeded through oxidative cyclization of the Ni(0)/P^N complex with an enyne system, 2-alkynyl anilinoacrylate, to provide a nickelacycle intermediate. The trans-carboamination around the internal alkyne was achieved by syn/anti-rotation of the Ni-carbenoid intermediate formed by C-N bond cleavage of the nickelacycle, and 3-alkenylated indoles were formed by C-N bond-forming reductive elimination. Notably, the synthesized indoles could be successfully transformed to functionalized carbazoles.

Acylation of oxindoles using methyl/phenyl esters via the mixed Claisen condensation - an access to 3-alkylideneoxindoles

Predominantly, aggressive acid chlorides and stoichiometric coupling reagents are employed in the acylating process for synthesizing carbonyl tethered heterocycles. Herein, we report simple acyl sources, viz. methyl and phenyl esters, which acylate oxindoles via the mixed Claisen condensation. This straightforward protocol is mediated by LiHMDS and KOtBu and successfully applied to a wide range of substrates. It is a noteworthy transformation that skips the stepwise generation of enolates and acylation, and the reaction is performed at a moderate temperature with no side reactions. This protocol produces the first examples of ortho-substituents in an aryl ring flanked with electron-donating and electron-withdrawing substrates. Interestingly, robust organometallic ferrocenyl methyl ester cleaved under these conditions with ease. Furthermore, biologically important Tenidap's analog was synthesized by this protocol.

Copper-catalyzed tandem cis-carbometallation/cyclization of imine-ynamides with arylboronic acids

An efficient copper-catalyzed tandem regioselective cis-carbometallation/cyclization of imine-ynamides with arylboronic acids has been developed. This method leads to a facile and practical synthesis of valuable 2,3-disubstituted indolines in moderate to excellent yields and features a broad substrate scope and wide functional group tolerance. Other significant features of this protocol include the use of readily available starting materials, high flexibility, simple procedure and mild reaction conditions.

Enantioselective nickel-catalyzed arylative and alkenylative intramolecular 1,2-allylations of tethered allene-ketones

The enantioselective nickel-catalyzed reaction of tethered allene-ketones with (hetero)arylboronic acids or potassium vinyltrifluoroborate is described. Carbonickelation of the allene gives allylnickel species, which undergo cyclization by 1,2-allylation to produce chiral tertiary-alcohol-containing aza- and carbocycles in high diastereo- and enantioselectivities.