Divergent Aminocarbonylations of Alkynes Enabled by Photoredox/Nickel Dual Catalysis

Photoinduced Nickel-Catalyzed Homolytic C(sp3)-N Bond Activation of Isonitriles for Selective Carbo- and Hydro-Cyanation of Alkynes

The exploration of strong chemical bonds as synthetic handles offers new disconnection strategies for the synthesis of functionalized molecules via transition metal catalysis. However, the slow oxidative addition rate of these covalent bonds to a transition metal center hampers their synthetic utility. Here, we report a C(sp3)-N bond activation strategy that bypasses thermodynamically challenging 2e- or 1e- oxidative addition via a distinct pathway in nickel catalysis. This strategy leverages a previously unknown activation pathway of photoinduced inner-sphere charge transfer of low-valent nickel(isonitriles), triggering a C(sp3)-N bond cleavage distal to the metal-ligand interaction to deliver nickel(cyanide) and versatile alkyl radicals. Utilizing this catalytic strategy, the selective intermolecular 1,2-carbocyanation reaction of alkynes with alkyl isonitriles as both alkylating and cyanating agents can be achieved, delivering a wide array of trisubstituted alkenyl nitriles with excellent atom-economy, regio-, and stereoselectivity under mild conditions. Furthermore, Markovnikov-selective hydrocyanation of aliphatic alkynes can be accomplished through the synergistic action of a photocatalyst utilizing isonitriles as the cyanation agents. Mechanistic investigations support the photogeneration of low-valent Ni(isonitrile) complexes that undergo photochemical homolysis of the C(sp3)-N bond to engage catalytic cyanation with alkynes.

Nickel-Catalyzed Narasaka-Heck Cyclization Carbonylation of Unsaturated Oxime Esters with Arylboronic Acids

The Narasaka-Heck reaction is one of the most straightforward methods for constructing pyrroline derivatives. Herein, we report a novel nickel-catalyzed three-component carbonylation reaction, which cleverly realizes the continuous construction of C(sp3)-N bonds and C(sp3)-C(sp2) bonds and effectively promotes the synthesis of acyl-substituted pyrroline derivatives. Furthermore, this strategy not only expands the conversion pathway of γ,δ-unsaturated oxime esters but also provides a new method for the synthesis of nitrogen-containing heterocyclic compounds.

Probing the Mechanism of Ni-Catalyzed Asymmetric Reppe Carbonylation of Cyclopropenes with CO and ROH

There is ongoing intense interest in catalysis with the Earth-abundant metal nickel. This DFT study reveals a plausible mechanism for the first Ni-catalyzed asymmetric Reppe carbonylation of cyclopropenes with carbon monoxide and phenols/alcohols. The RO-H bond undergoes a distinct heterolytic cleavage rather than the proposed oxidative addition, transferring a proton to a nickel-bound anionic carbon atom in a stereoselective manner. This and other novel insights gained can have implications for developing new asymmetric Reppe reactions.

Synthesis of Hexafluoroisopropyl Aryl Ethers via Dual Photoredox/Nickel-Catalyzed C-O Coupling

Herein we report a photoredox/nickel-catalyzed cross-coupling of aryl bromides with 1,1,1,3,3,3-hexafluoroisopropanol for the construction of hexafluoroisopropyl aryl ethers. The mild reaction conditions employed allow for the applicability of a wide range of aryl and heteroaryl bromides. Late-stage functionalization and preliminary mechanistic studies have been demonstrated.

Metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes

Metal-catalyzed highly Markovnikov-type selective hydrofunctionalization of terminal alkynes provides a straightforward and atom-economical route to access 1,1-disubstituted alkenes, which have a wide range of applications in organic synthesis. However, the highly Markovnikov-type selective transformations are challenging due to the electronic and steric effects during the addition process. With the development of metal-catalyzed organic synthesis, different metal catalysts have been developed to solve this challenge, especially for platinum group metal catalysts. In this perspective, we review homogeneous metal-catalyzed Markovnikov-type selective hydrofunctionalization of terminal alkynes according to the classified element types as well as reaction mechanisms. Future avenues for investigation are also presented to help expand this exciting field.

Visible light driven photoredox/nickel-catalyzed stereoselective synthesis of Z- or E-vinyl thioethers from thiosilane and terminal alkynes

A new method for the synthesis of anti-Markovnikov Z- or E-vinyl thioethers from thiosilane and terminal alkynes under visible-light-induced photoredox/nickel dual catalysis conditions is described. With a judicious choice of a simple nickel catalyst and a ligand, this strategy enables efficient and divergent access to both Z- or E-vinyl thioethers from the same set of simple starting materials. Notably, the approach is free of odorous thiol and has excellent compatibility with functional groups and substrate scope.

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.

Nickel-Catalyzed Carbonylative Negishi Cross-Coupling of Unactivated Secondary Alkyl Electrophiles with 1 atm CO Gas

We describe a nickel-catalyzed carbonylative cross-coupling of unactivated secondary alkyl electrophiles with the organozinc reagent at atmospheric CO gas, thus allowing the expedient construction of unsymmetric dialkyl ketones with broad functional group tolerance. The leverage of a newly developed NN2-pincer type ligand enables the chemoselective three-component carbonylation by overcoming the competing Negishi coupling, the undesired β-hydride elimination, and dehalogenation of alkyl iodides side pathways. Both alkyl iodides and alkyl tosylates are compatible in the single electron transfer involved mechanism.

Nickel-catalyzed acylzincation of allenes with organozincs and CO

Transition metal-catalyzed carbonylative reaction with CO gas are among the central task in organic synthesis, enabling the construction of highly valuable carbonyl compound. Here, we show an earth-abundant nickel-catalyzed three-component tandem acylzincation/cyclization sequence of allene and alkylzinc reagent with 1 atm of CO under mild conditions. This protocol is featured by broad functional group tolerance with high reaction selectivity, providing a rapid and convenient synthetic method for the construction of diverse fully substituted benzotropone derivatives. Mechanistic studies reveal that the installation of a cyano group tethered to allene moiety enables the high regio- and stereoselectivity of this acylzincation of allene, allowing the selective formation of three consecutive C-C bonds in a highly efficient manner.

Electrochemical Oxidative Carbonylation of NH-Sulfoximines

The electrochemical synthesis of N-aroylsulfoximines features the use of tetra-n-butylammonium iodide (TBAI) as the medium and a broad substrate scope, thus affording a wide range of N-aroylated sulfoximines in moderate to good yields. The advantages of this electrochemical strategy are augmented by mild reaction conditions that are external oxidant-free, ligand-free, and easy to scale up to gram scale. Both the control experiments and the mechanistic studies revealed that the whole electrochemical process proceeded through a palladium (II/IV/II) catalytic cycle.

Condition-Controlled O-Acylation and N-O Bond Reduction of Hydroximic Acids with Thioacetic Acid

Condition-dependent transformations between hydroximic acids and thioacetic acid were achieved. Using NH₄HCO₃ in the ethanol solvent, efficient N-O bond cleavage of hydroxamic acids occurred to afford primary amides with high functional group compatibility. The reaction was switched to O-acylation when NEt₃ and H₂O were used as the base and solvent, respectively. These facile transformations could be scaled up to the gram level smoothly. Preliminary mechanistic studies suggested that the N-O bond cleavage involves a cascade process of acylation/reduction.

Site- and Stereoselective Synthesis of Alkenyl Chlorides by Dual Functionalization of Internal Alkynes via Photoredox/Nickel Catalysis

Herein, we report a redox-neutral and atom-economical protocol to synthesize valuable alkenyl chlorides from unactivated internal alkynes and abundant organochlorides via photoredox and nickel catalysis. This protocol enables the site- and stereoselective addition of organochlorides to alkynes via chlorine photoelimination-initiated sequential hydrochlorination/remote C-H functionalization. The protocol is compatible with a wide range of medicinally relevant heteroaryl, aryl, acid, and alkyl chlorides for efficiently producing γ-functionalized alkenyl chlorides, exhibiting excellent regioselectivities and stereoselectivities. Late-stage modifications and synthetic manipulations of the products and preliminary mechanistic studies are also presented.

Regioselective C-H Active Carbonylation via 1,4-Palladium Migration

We report a highly regioselective three-component coupling reaction of styrene, CO gas, and an amine compound to synthesize multisubstituted α,β-unsaturated amides, which involves a palladium-catalyzed sequential 1,4-palladium migration, C(sp²)-H activation, carbonylation, and amination. Salient features of this strategy include the use of 1 atm of CO, excellent stereochemistry, and good functional group tolerance. Further, a series of control experiments and density functional theory calculations were performed to afford some insights for the transfer mechanism.

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.

Nickel-Catalyzed Aminocarbonylation of Aryl Iodides with 1 atm CO

Reported here is a nickel-catalyzed aminocarbonylation of aromatic iodides with (hetero)aryl anilines and alkyl amines under atmospheric CO pressure. The reaction features with broad substrate scope with excellent functional group tolerance, providing an expedient method for the construction of amide analogues. Notably, amino alcohols can be selectively transformed into the corresponding amides successfully without interfering the hydroxyl group under the current standard conditions.

Switchable Radical Carbonylation by Philicity Regulation

Carbonylation reactions involving CO as readily available C1 synthons have become one of the most important tools for the construction of carbonyl compounds from feedstock chemicals. Despite numerous catalytic methods for carbonylation reactions proceeding via ionic or radical pathways, an inherent limitation to these methods is the need to control switchable single and double carbonylative formation of value-added products from the same and simple starting materials. Here, we describe a new strategy that exploits photoredox catalysis to regulate the philicity of amine coupling partners to drive switchable radical carbonylation reactions. In double carbonylation, amines were first transformed into nitrogen radical cations by single-electron transfer-oxidation and coupled with CO to form carbamoyl radicals, which further underwent radical cross-coupling with the incipient cyanoalkyl acyl radicals to afford the double carbonylation products. Upon the addition of stoichiometric 4-dimethylaminopyridine (DMAP), DMAP competitively traps the initially formed cyanoalkyl acyl radical to form the relatively stabilized cyanoalkyl acyl-DMAP salts that engaged in the subsequent substitution with the nucleophilic amines to produce the single carbonylation products. The reaction proceeded smoothly with excellent selectivity in the presence of various amine nucleophiles at room temperature, generating valuable amides and α-ketoamides in a versatile and controlled fashion. Combined experimental and computational studies provided mechanistic insights into the possible pathways.

Nickel-Catalyzed Carbonylative Synthesis of α,β-Unsaturated Thioesters from Vinyl Triflates and Arylsulfonyl Chlorides

A straightforward and efficient synthesis of α,β-unsaturated thioesters has been developed via a nickel-catalyzed thiocarbonylation reaction of vinyl triflates with arylsulfonyl chlorides. With Mo(CO)₆ as both CO source and reductant, a variety of α,β-unsaturated thioesters were obtained in moderate to good yields with very good functional group compatibility. It is noteworthy that the present method is the first example on nickel-catalyzed carbonylative synthesis of α,β-unsaturated thioesters by using arylsulfonyl chlorides as the coupling partner.

Nickel-catalyzed alkoxycarbonylation of aryl iodides with 1 atm CO

A nickel-catalyzed alkoxycarbonylation of aromatic iodides with alcohols under atmospheric pressure of carbon monoxide is presented here. This operationally simple protocol allows the facile synthesis of (hetero)aromatic esters, exhibiting broad substrate scope with excellent functional group tolerance. Various primary and secondary aliphatic alcohols as well as phenols are suitable for this transformation.

Nickel-Catalyzed Carbonylation of Cyclopropanol with Benzyl Bromide for Multisubstituted Cyclopentenone Synthesis

Herein, we reported a Ni-catalyzed carbonylation of cyclopropanol with benzyl bromide to afford multisubstituted cyclopentenone under 1 atm of CO. The reaction proceeds through cascade carbonylation of benzyl bromides, followed by generation of nickel homoenolate from cyclopropanols via β-C elimination to afford 1,4-diketones, which undergoes intramolecular Aldol condensation to furnish highly substituted cyclopentenone derivatives in moderate to good yields. The reaction exhibits high functional group tolerance with broad substrate scope.