Enantioselective Nickel-Catalyzed anti-Carbometallative Cyclizations of Alkynyl Electrophiles Enabled by Reversible Alkenylnickel E/Z Isomerization

Transition metal-catalyzed transformations of 2-formylarylboronic acids

2-Formylarylboronic acids are easily available precursors in organic chemistry. Different types of transition metal catalysts, such as Pd(0), Pd(II), Rh(I), Ir(I), Ni(II), Cu(I), Cu(II), and Co(II), can efficiently catalyze coupling reactions of 2-formylarylboronic acids with other organic reactants. In this review, we describe the synthesis of a diverse range of carbocyclic and heterocyclic compounds, as well as acyclic compounds, via transition metal-catalyzed reactions of 2-formylarylboronic acids over the past two decades.

Ni-Catalyzed Kumada-Corriu Cross-Coupling Reactions of Tertiary Grignard Reagents and Bromostyrenes

The development of protocols for the construction of congested quaternary centers is highly sought-after. Herein, we report a method for the cross-coupling of C(sp3) tertiary Grignard reagents with C(sp2) styrenyl bromides using readily available nickel precatalysts. We identified conditions that afford the products in practical yield for several combinations of electrophiles and nucleophiles, including sensitive α-magnesiated Grignard reagents. Dependent upon the nature of their substituents, regiodivergency was observed when α-vinyl bromides were employed.

Ruthenium and Iodine Anion Cocatalyzed Cascade Dihalogenation and Cyclization of Internal Alkyne-Tethered Cyclohexadienones with 1,2-Dihaloethanes

We have established an efficient ruthenium(II) and iodine anion cocatalyzed dihalogenation and cascade cyclization of internal alkyne-tethered cyclohexadienones, which stereoselectively afforded numerous dihalogenation products with a bioactive hydrobenzofuran skeleton in high yields under mild conditions. In this transformation, the reaction pathway was determined by the concentration of electrophilic iodine reagent, which also provided a strategy for control of the reaction selectivity. Furthermore, this method features the use of 1,2-dihaloroethane as the halogen source via iodine anion catalyst.

Rhodium(I)/Chiral Diene Complexes Catalyzed Asymmetric Desymmetrization of Alkynyl-Tethered 2,5-Cyclohexadienones Through an Arylative Cyclization Cascade

Cis-hydrobenzofurans, cis-hydroindoles, and cis-hydrindanes, privileged structural motifs found in numerous biologically active natural and synthetic compounds, are efficiently prepared by a Rh(I)-catalyzed cascade syn-arylation/1,4-addition protocol. This approach starts with the regioselective syn-arylation of the alkyne tethered to 2,5-hexadienone moieties, using a chiral Rh(I) catalyst generated in situ from a chiral bicyclo[2.2.1]hepatadiene ligand L4f. By forging two new carbon-carbon bonds and introducing two chiral centers, the resulting alkenylrhodium species undergoes desymmetrization via an intramolecular 1,4-addition reaction, delivering annulated products with high yields and enantioselectivities.

Nickel-catalyzed switchable arylative/endo-cyclization of 1,6-enynes

Carbo- and heterocycles are frequently used as crucial scaffolds in natural products, fine chemicals, and biologically and pharmaceutically active compounds. Transition-metal-catalyzed cyclization of 1,6-enynes has emerged as a powerful strategy for constructing functionalized carbo- and heterocycles. Despite significant progress, the regioselectivity of alkyne functionalization is entirely substrate-dependent. And only exo-cyclization/cross-coupling products can be obtained, while endo-selective cyclization/cross-coupling remains elusive and still poses a formidable challenge. In this study, we disclose a nickel-catalyzed switchable arylation/cyclization of 1,6-enynes in which the nature of the ligand dictates the regioselectivity of alkyne arylation, while the electrophilic trapping reagents determine the selectivity of the cyclization mode. Specifically, using a commercially available 1,10-phenanthroline as a ligand facilitates trans-arylation/cyclization to obtain seven-membered ring products, while a 2-naphthyl-substituted bisbox ligand promotes cis-arylation/cyclization to access six-membered ring products. Diastereoselective cyclizations have also been developed for the synthesis of enantioenriched piperidines and azepanes, which are core structural elements of pharmaceuticals and natural products possessing important biological activities. Furthermore, experimental and density functional theory studies reveal that the regioselectivity of the alkyne arylation process is entirely controlled by the steric hindrance of the ligand; the reaction mechanism involves exo-cyclization followed by Dowd-Beckwith-type ring expansion to form endo-cyclization products.

Pd(0)-Catalyzed Asymmetric Cyclization/Coupling Cascade of Alkyne-Tethered Unsaturated Carbonyls: Development and Mechanism Elucidation

While the Pd(0)-catalyzed cyclization of alkyne-tethered unsaturated carbonyl substrates has been reported, the mechanism has not been well elucidated, and the potential asymmetric version remains to be developed. Here, we disclose that a chiral Pd(0) complex can efficiently promote the desymmetrizative cyclization of alkyne-tethered cyclohexadienones in CH3OH, and the resultant Pd(II) intermediates further undergo an array of tandem coupling reactions, including Suzuki, Sonogashira, and even chemoselective reduction by CH3OH in the absence of additional coupling partners. As a result, a broad spectrum of hydrobenzofuran derivatives, having a tetra- or trisubstituted exo-alkene motif, is constructed with moderate to outstanding enantioselectivity in an exclusive cis-difunctionalization pattern. In addition, this enantioselective protocol can be well expanded to linear alkyne-tethered unsaturated carbonyls, and a new desymmetrizative and asymmetric cyclization/coupling cascade of bis-alkyne-tethered enones is further realized efficiently, furnishing diversely structured frameworks with high stereoselectivity. Moreover, kinetic transformation for various racemic alkyne-tethered enones can be accomplished under similar catalytic conditions, and unusual kinetic reactions by chemoselectively undertaking Suzuki or Sonogashira coupling, or reduction by CH3OH, occur sequentially, finally yielding two types of chiral products, both with high enantioselectivity via either ligand- or substrate-based control. The experimental results demonstrate that the current Pd(0)-based strategy is superior to the classical Pd(II)-catalyzed carbopalladation/cyclization process of the identical substrates with regard to enantioselectivity and synthetic versatility. Moreover, density functional theory calculations are conducted to rationalize the Pd(0)-catalyzed oxidative cyclometalation pathway in the key cyclization step, which leads to the observed cis-difunctionalized products exclusively.

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.

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.

DFT Mechanistic Insights into the Ni(II)-Catalyzed Enantioselective Arylative Cyclization of Tethered Allene-Ketones

Density functional theory (DFT) has provided a detailed mechanistic picture for the redox neutral nickel(II)-catalyzed arylative cyclization reactions of a tethered allene-ketone with arylboronic acids. A mechanistic rationale for the high diastereo- and enantioselectivity achieved experimentally at high reaction temperature was uncovered through modeling the reaction with a chiral ligand and the predicted stereochemical outcome corroborates with experimental results. An unprecedented mechanism for the base-free organoboron transmetalation was revealed and the regioselectivity of migratory insertion of tethered allene-ketones as well as the stability of the possible allylnickel isomers (σ-allyl vs π-allyl) were clarified. The multifaceted nature of the reaction is revealed with certain elementary steps preferring cationic compared to the neutral state.

Asymmetric anti-Selective Borylalkylation of Terminal Alkynes by Nickel Catalysis

Selective transformation of alkyne triple bonds to double bonds serves as an efficient platform to construct substituted alkenes. While significant advances have been made in its spatiotemporal regulation, achieving a multicomponent enantioselective reaction that requires multifaceted selectivity issues to be overcome is still uncommon. Here, we report an unprecedented asymmetric anti-stereoselective borylcarbofunctionalization of terminal alkynes by nickel catalysis. The utilization of an inexpensive chiral diamine ligand enables the three-component cross-coupling of terminal alkynes, a diboron reagent, and prochiral alkyl electrophiles with high levels of regio-, stereo-, and enantioselectivities. This reaction provides an efficient protocol to access enantioenriched alkenyl esters bearing an α-stereogenic center, is remarkably practical, and has a broad scope and an outstanding functional group compatibility. In addition, the value of this method has been highlighted in a diversity of follow-up stereoretentive derivatizations and the stereoselective concise synthesis of complex drug molecules.

Cobalt-Catalyzed Chemo- and Stereoselective Arylative Carbocyclization of 1,6-Allenynes

Different from the well-investigated enynes, transition-metal-catalyzed carbocyclization reactions of allenynes are more attractive as a result of the unique structure and versatile reactivity of allenes. Herein, we report the first cobalt-catalyzed highly chemo- and stereoselective arylative carbocyclization of 1,6-allenynes with arylboronic acids, affording five-membered carbocycles and heterocycles with moderate to high yields, broad substrate scope, and wide functional group compatibility. Moreover, several mechanistic experiments were conducted to gain insight into the reaction process.

Nickel-Catalyzed Markovnikov-Selective Hydrodifluoromethylation of Alkynes Using BrCF2 H

A Markovnikov-selective hydrodifluoromethylation of alkynes with BrCF2 H via nickel catalysis is described. This protocol proceeds via a migratory insertion of nickel hydride to alkyne followed by a CF2 H-coupling, enabling straightforward access to diverse branched CF2 H-alkenes with high efficiency and exclusive regioselectivity. The mild condition applies to a wide array of aliphatic and aryl alkynes with good functional group compatibility. Mechanistic studies are presented to support the proposed pathway.

Catalyst-Controlled Diastereoselective Synthesis of Bridged [3.3.1] Bis(Indolyl)-Oxanes and Oxepanes via Desymmetrization of Bis(Indolyl)-Cyclohexadienones

A catalyst-controlled divergent synthesis of bridged [3.3.1] bis(indolyl)-oxanes and cis-[6.7] fused bis(indolyl) oxepanes via diastereoselective desymmetrization of bis(indolyl)-cyclohexadienones is presented for the first time. The reaction is highly atom- and step-economic, furnishing sp³-rich functionalized bis(indolyl) derivatives in good to excellent yields with wide substrate scope. The reaction proceeds through Friedel-Crafts alkylation followed by catalyst-controlled selective C-C bond formation/rearrangement. Gram scale synthesis and synthetic utility to generate bis(indolyl) alkaloid-like molecular diversity were also illustrated.

Rhodium-Catalyzed Asymmetric Annulation of Unactivated Alkynes with 3-(ortho-Boronated Aryl) Conjugated Enones: Enantioselective Synthesis of 2,3-Disubstituted Indenes

A rhodium-catalyzed tandem arylation/cyclization reaction of 3-(ortho-boronated aryl) conjugated enones with unactivated alkynes is reported. By using a rhodium(I)/chiral-diene complex as the catalyst, the protocol was processed smoothly to provide various 2,3-disubstituted indene compounds in high yields with excellent regioselectivities and enantioselectivities. The approach outlined herein is appealing, as simple diarylalkynes, diakylalkynes, and alkyl(aryl)alkynes are the starting materials.

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.

Modulation of metal species as control point for Ni-catalyzed stereodivergent semihydrogenation of alkynes with water

A base-assisted metal species modulation mechanism enables Ni-catalyzed stereodivergent transfer semihydrogenation of alkynes with water, delivering both olefinic isomers smoothly using cheap and nontoxic catalysts and additives. Different from most precedents, in which E-alkenes derive from the isomerization of Z-alkene products, the isomers were formed in orthogonal catalytic pathways. Mechanistic studies suggest base as a key early element in modulation of the reaction pathways: by adding different bases, nickel species with disparate valence states could be accessed to initiate two catalytic cycles toward different stereoisomers. The practicability of the method is showcased with nearly 70 examples, including internal and terminal triple bonds, enynes and diynes, affording semi-hydrogenated products in high yields and selectivity.

Synthesis of gem-Difluorinated 1,4-Dienes via Nickel-Catalyzed Three-Component Coupling of (Trifluoromethyl)alkenes, Alkynes, and Organoboronic Acids

Herein, a nickel-catalyzed defluorinative three-component coupling of trifluoromethyl alkenes, internal alkynes, and organoboronic acids is presented. The protocol provides a highly efficient and selective route for the synthesis of structurally diverse gem-difluorinated 1,4-dienes under mild conditions. Mechanistic studies suggest that C-F bond activation proceeds probably through the oxidative cyclization of trifluoromethyl alkenes with Ni(0) species, sequential addition to alkynes, and β-fluorine elimination.

Enantioselective Desymmetrization Triggered by Iminium-Enamine Activation: Access to Complex Cyclohepta[b]indoles

The expeditious construction of complex molecules having multiple stereocentres is highly desirable in organic chemistry. In the present communication, we report the development of an organocatalytic asymmetric desymmetrization of prochiral enal-tethered cyclohexadienones via the C3-selective Friedel-Crafts alkylation of indoles triggered by LUMO-lowering iminium activation/HOMO-raising enamine activation. The reaction provides access to bicyclic enones, which further undergo acid-mediated intramolecular annulation from C2-position to afford highly strained cyclohepta[b]indoles with five contiguous stereocentres and three new C-C bonds in excellent enantioselectivity and diastereoselectivity.

Diphenyl Ditelluride: An Unconventional Reducing Agent in the Sulfonylative Cascade of Alkynyl Cyclohexadienones

Herein, we report a reductive hydrazo-sulfonylative difunctionalization cascade of alkynyl cyclohexadienones employing PhTeTePh as an uncommon reducing agent. Diphenyl ditelluride is a commercially available solid with a good solubility profile in most organic solvents, and this is the first report illustrating it as a reducing agent. The protocol afforded a variety of difunctionalized dihydrochromenones and dihydrobenzofuranones in good yields under relatively mild conditions. The reactions were scalable, and mechanistic studies were conducted to probe the reaction mechanism. Additionally, photophysical studies of the products were carried out, which revealed that they had significant absorption (400-450 nm) and emission (520-570 nm) in the visible region.

Palladium(0)-Catalyzed Anti-Selective Addition-Cyclizations of Alkynyl Electrophiles

Palladium(0)/monophosphine complexes catalyze anti-selective alkylative, arylative, and alkynylative cyclizations of alkynyl electrophiles with organometallic reagents. The remarkable anti-selectivity results from novel oxidative addition, that is, the nucleophilic attack of electron-rich palladium(0) on the electrophile across the alkyne followed by transmetalation and reductive elimination ("anti-Wacker"-type cyclization). With regard to 5-alkynals, triphenylphosphine (PPh3 )-ligated palladium(0) catalyzes the cyclization of terminal alkynes and conjugated alkenyl- or alkynyl-substituted ones to afford 2-cyclohexen-1-ol and 2-alkylidene-cyclopentanol derivatives, respectively. For 6-alkyl- or 6-aryl-5-alkynals, the cyclization does not proceed with the palladium/PPh3 catalyst; however, it does proceed with palladium/tricyclohexylphosphine (PCy3 ), to yield the former products predominantly. Remarkably, the latter catalyst completely switches the regioselectivity in the cyclization of the conjugated diyne-aldehydes. Notably, palladium/PPh3 -catalyzed cyclizations also proceed with other organometallics or even without them.

Sequential Cu(II)-Catalyzed Multicomponent C-N Coupling, Nucleophilic Addition, and Cyclization Cascade: A Diastereoselective Approach to Carboxamide-Embedded Hexahydrobenzofuran Core

Cascade or domino reactions serve as a powerful technique for the synthesis of complex organic scaffolds in one pot. Herein, a Cu(II)-catalyzed and silica gel-assisted multicomponent reaction (MCR) between bromoalkyne-tethered cyclohexadienones, amides, and water for the construction of hexahydrobenzofuran-3-carboxamide is developed. The reaction proceeds via a C-N coupling reaction followed by hydrative cyclization of ynamide intermediates. Notably, good to excellent diastereoselectivity is complementary of this reaction.

Ligand-controlled stereodivergent alkenylation of alkynes to access functionalized trans- and cis-1,3-dienes

Precise stereocontrol of functionalized alkenes represents a long-standing research topic in organic synthesis. Nevertheless, the development of a catalytic, easily tunable synthetic approach for the stereodivergent synthesis of both E-selective and even more challenging Z-selective highly substituted 1,3-dienes from common substrates remains underexploited. Here, we report a photoredox and nickel dual catalytic strategy for the stereodivergent sulfonylalkenylation of terminal alkynes with vinyl triflates and sodium sulfinates under mild conditions. With a judicious choice of simple nickel catalyst and ligand, this method enables efficient and divergent access to both Z- and E-sulfonyl-1,3-dienes from the same set of simple starting materials. This method features broad substrate scope, good functional compatibility, and excellent chemo-, regio-, and stereoselectivity. Experimental and DFT mechanistic studies offer insights into the observed divergent stereoselectivity controlled by ligands.

Enantioselective Metal-Catalyzed Reductive Coupling of Alkynes with Carbonyl Compounds and Imines: Convergent Construction of Allylic Alcohols and Amines

The use of alkynes as vinylmetal pronucleophiles in intermolecular enantioselective metal-catalyzed carbonyl and imine reductive couplings to form allylic alcohols and amines is surveyed. Related hydrogen auto-transfer processes, wherein alcohols or amines serve dually as reductants and carbonyl or imine proelectrophiles, also are cataloged, as are applications in target-oriented synthesis. These processes represent an emerging alternative to the use of stoichiometric vinylmetal reagents or Nozaki-Hiyama-Kishi (NHK) reactions in carbonyl and imine alkenylation.

Palladium(0)-Catalyzed Remote Decarboxylative Allylation and Base-Mediated 1,3-Migration

Herein we have reported the palladium(0)-catalyzed decarboxylative oxa-Michael addition/remote α-allylation/1,3- migration of prochiral allyl carbonate-tethered cyclohexadienones in good yields. This unconventional intramolecular rearrangement is triggered by the base-mediated retro-Michael ring-opening reaction (β-elimination) and subsequent syn-selective oxa-Michael addition on the less substituted enone functionality. The generality of tandem decarboxylative allylation was examined with various substrates and in the gram-scale reaction.

A metal-free four-component sulfonylation, Giese cyclization, selenylation cascade via insertion of sulfur dioxide

We hereby report a highly regio- and diastereoselective arylsulfonylation-radical cyclization-selenylation cascade of alkynyl cyclohexadienones for the facile synthesis of highly functionalized dihydrochromenones. The protocol utilizes aryldiazonium salts as aryl partners and DABSO as a benign SO₂ source and also as a redox mediator. Additionally, we also developed a visible light mediated protocol wherein diaryliodonium salts were used as the aryl partners at room temperature.

Endo-Selective Intramolecular Alkyne Hydroamidation Enabled by NiH Catalysis Incorporating Alkenylnickel Isomerization

Intramolecular alkyne hydroamidation represents a straightforward approach for the access to synthetically valuable cyclic enamides. Despite some advances made in this realm, the ability to attain a precise regiocontrol still remains challenging, especially for endo cyclization that leads to six-membered and larger azacyclic rings. Herein, we report a NiH-catalyzed intramolecular hydroamidation of alkynyl dioxazolones that allows for an excellent endo selectivity, thus affording a range of six- to eight-membered endocyclic enamides with a broad scope. Mechanistic investigations revealed that Ni(I) catalysis is operative in the current system, proceeding via regioselective syn-hydronickelation, alkenylnickel E/Z isomerization, and Ni-centered inner-sphere nitrenoid transfer. In particular, the key alkenylnickel isomerization step, which previously lacked mechanistic understandings, was found to take place through the η²-vinyl transition state. The synthetic value of this protocol was demonstrated by diastereoselective modifications of the obtained endocyclic enamides to highly functionalized δ-lactam scaffolds.

Rapid access to t-butylalkylated olefins enabled by Ni-catalyzed intermolecular regio- and trans-selective cross-electrophile t-butylalkylation of alkynes

Among the carbo-difunctionalization of alkynes, the stereoselective dialkylation of alkynes is the most challenging transformation due to associated competitive side reactions and thus remains underdeveloped. Herein, we report the first Ni-catalyzed regio- and trans-selective cross-dialkylation of alkynes with two distinct alkyl bromides to afford olefins with two aliphatic substituents. The reductive conditions circumvent the use of organometallic reagents, enabling the cross-dialkylation process to occur at room temperature from two different alkyl bromides. This operationally simple protocol provides a straightforward and practical access to a wide range of stereodefined dialkylated olefins with broad functional group tolerance from easily available starting materials.

A direct reductive cross-dialkylation of alkynes is achieved to afford trans-dialkylated olefins using two distinct alkyl bromides. The reaction undergoes with exclusive chemo-, regio- and stereoselectivity without the use of organometallic reagents.

Mechanism and Origin of Stereoselectivity of Ni-Catalyzed Cyclization/Carboxylation of Bromoalkynes with CO2

Bromoalkynes play important roles in coupling reactions because they can show obvious stereoselectivity to form E- and Z-isomers when substituents are different. However, the origin of the stereoselectivity in the bromoalkynes reaction is still unclear. Density functional theory (DFT) calculations were performed to provide an in-depth study of the reaction mechanism, clarifying the mechanistic details of the main reaction and the origin of the stereoselectivity. By comparing the syn-insertion mechanism of alkynes and the radical pathway, it is indicated that the electrostatic effect caused by the different charge distributions of the reactants is the main reason that Ni(I) species are more prone to syn-insertion of alkynes than Ni(II) species. In addition, the lower reaction energy barrier in the radical pathway suggests that it is more advantageous in terms of kinetics. The bond between Ni(I) species and alkenylation products has two directions to generate products of different configurations, which are the direct stereoselectivity-determining stages. The distortion/interaction analysis shows that the distortion energy mainly affects the product configuration, and the steric hindrance is the main factor controlling the stereoselectivity.

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.

Steric- and Electronic-Controlled Intramolecular [2 + 2]-Cycloaddition of Cyclohexadienone-Containing 1,7-Enynes

Herein we have developed the silver-catalyzed electronic- and steric-controlled intramolecular formal [2 + 2]-cycloaddition of alkyne-tethered cyclohexadienones. Substrates with electron-rich alkynes and a less hindered quaternary carbon center afford tricyclic fused cyclobutenes through 1,7-enyne cyclization. In contrast, the formation of dihydrofurans was observed from electron-deficient alkynes via proton abstraction/C-O bond cleavage. The synthetic potential of this method was also broadened with a gram-scale reaction and various transformations on cyclobutene.

Precatalyst-Enabled Selectivity: Enantioselective NiH-Catalyzed anti-Hydrometalative Cyclization of Alkynones to Endo- and Heterocyclic Allylic Alcohols

A highly enantioselective NiH-catalyzed hydrocyclization of alkynones with unparalleled anti- and endocyclic selectivities is described. The choice of the precatalysts has significant influence in tuning the regio- and enantioselectivity. Using Ni(OTs)₂ /Phox as a precatalyst and (EtO)₂ MeSiH as a hydride source, an array of enantioenriched O-, N-, and S-containing heterocyclic tertiary allylic alcohols are obtained in 24-81 % yields with 80:20-99:1 er. Mechanistic investigations and synthetic application are also carried out. This study represents an efficient access to a set of allylic alcohols that are unable to access by the state-of-the-art coupling reactions.

Divergent Aminocarbonylations of Alkynes Enabled by Photoredox/Nickel Dual Catalysis

A metallaphotoredox-catalyzed strategy for the selective and divergent aminocarbonylation of alkynes with amines and 1 atm of CO is reported. This synergistic protocol not only enables the Markovnikov-selective hydroaminocarbonylation of alkynes to afford α,β-unsaturated amides, but also facilitates a sequential four-component hydroaminocarbonylation/radical alkylation in the presence of tertiary and secondary alkyl boronate esters, which allows for straightforward conversion of alkynes into corresponding amides. Preliminary mechanistic studies disclose that a photoinduced oxidative insertion of aniline and CO into nickel followed by a migratory insertion of (carbamoyl)nickel species could be involved.

Nickel-Catalyzed Ipso/Ortho Difunctionalization of Aryl Bromides with Alkynes and Alkyl Bromides via a Vinyl-to-Aryl 1,4-Hydride Shift

Polysubstituted arenes are ubiquitous structures in a myriad of medicinal agents and complex molecules. Herein, we report a new catalytic blueprint that merges the modularity of nickel catalysis for bond formation with the ability to enable a rather elusive 1,4-hydride shift at arene sp² C-H sites, thus allowing access to ipso/ortho-difunctionalized arenes from readily available aryl halides under mild conditions and exquisite selectivity profile.

Dual C(sp3)-H Functionalization of Cyclic Ethers via Singlet Oxygen-Mediated Ring Opening and Ring Closing

A metal-free dual C(sp³)-H bond functionalization of saturated cyclic ethers via photooxidative singlet oxygen-mediated ring opening and ring closing has been developed, providing a method for generating hydrobenzofurans/pyrans/dioxins. Mechanistic studies have confirmed that ring-opening intermediates were effectively generated by singlet oxygen-mediated C(sp³)-H activation and efficiently reacted with aldehydes and activated methylene compounds to form a wide array of products with high diastereoselectivities (up to >95:5 dr). This study is a rare example of α,β-dual C(sp³)-H bond functionalization of ethers.

Palladium-Catalyzed Modular and Enantioselective cis-Difunctionalization of 1,3-Enynes with Imines and Boronic Reagents

Here we report that a palladium(0) complex can mediate the unprecedented intermolecular coupling reaction of 1,3-enynes and N-sulfonylimines regio- and stereoselectively, and the resultant palladium(II) species undergo a cascade Suzuki reaction with organoboronic reagents. The substrate scope is substantial for the asymmetric three-component process, and the enantioenriched all-carbon tetra-substituted alkene derivatives are efficiently constructed in a modular and cis-difunctionalized manner. Control experiments and density functional theory (DFT) calculations support the idea that the palladium(0) acts as a π-Lewis base catalyst by chemoselectively forming η²-complexes with the alkene moiety of 1,3-enynes, thus increasing the nucleophilicity of the alkyne group based on the principle of vinylogy, to attack imines enantioselectively. The preferable formation of aza-palladacyclopentene intermediates, via a 90° single bond rotation from the resultant π-allyl complex, guarantees the formal cis-carbopalladation of alkyne group. In addition, a palladium(0)-catalyzed enantioselective reductive coupling of 1,3-enyne and imine is realized by using formic acid as hydrogen transfer reagent.