Alkyl Bromides as Mild Hydride Sources in Ni-Catalyzed Hydroamidation of Alkynes with Isocyanates

Multicomponent Construction of Tertiary Alkylamines by Photoredox/Nickel-Catalyzed Aminoalkylation of Organohalides

Tertiary alkylamines are privileged structural motifs widely present in natural products, pharmaceutical agents, and bioactive molecules, and their efficient synthesis has been a longstanding goal in organic chemistry. The functionalization of α-amino radicals derived from abundant precursors represents an emerging approach to accessing alkylamines, but application of this strategy to obtain tertiary alkylamines remains challenging. Here, we show that dual photoredox/nickel catalysis enables aminoalkylation of organohalides (sp2- and sp3-hybridized) in combination with secondary alkylamines and aldehydes. The multicomponent process proceeds through selective generation of α-amino radicals from the reduction of in situ-generated iminium ions by photoredox catalysis, followed by nickel-catalyzed cross-coupling to build a wide array of functionally diverse tertiary alkylamines. This strategy could also be extended to unprecedented four-component reactions and their asymmetric variants to deliver enantioenriched α-aryl-substituted γ-amino acid derivatives. Taken together, this work offers a streamlined synthetic route to aliphatic tertiary amines.

Regiodivergent Radical-Relay Alkene Dicarbofunctionalization

Herein, we report a regiodivergent 1,n-dicarbofunctionalization of unactivated olefins enabled by a Ni-catalyzed radical relay that forges both C(sp3)-C(sp3) and C(sp2)-C(sp3) linkages, even at long-range. Initial studies support an intertwined scenario resulting from the merger of an atom-transfer radical addition (ATRA) and a chain-walking event, with site-selectivity being dictated by a judicious choice of the ligand backbone.

Ni-catalysed remote C(sp3)-H functionalization using chain-walking strategies

The dynamic translocation of a metal catalyst along an alkyl side chain - often coined as 'chain-walking' - has opened new retrosynthetic possibilities that enable functionalization at unactivated C(sp3)-H sites. The use of nickel complexes in chain-walking strategies has recently gained considerable momentum owing to their versatility for forging sp3 architectures and their redox promiscuity that facilitates both one-electron or two-electron reaction manifolds. This Review discusses the relevance and impact that these processes might have in synthetic endeavours, including mechanistic considerations when appropriate. Particular emphasis is given to the latest discoveries that leverage the potential of Ni-catalysed chain-walking scenarios for tackling transformations that would otherwise be difficult to accomplish, including the merger of chain-walking with other new approaches such as photoredox catalysis or electrochemical activation.

Nickel-Catalyzed Reductive Amidation of Aryl Fluorosulfates with Isocyanates: Synthesis of Amides via C-O Bond Cleavage

With the assistance of nickel as catalyst, 2,2'-bipyridine (bpy) as ligand, and manganese as reducing metal, the reductive amidation of isocyanates with readily accessible aryl fluorosulfates could be successfully accomplished. The reactions proceeded effectively via C-O bond activation in DMF at room temperature, enabling the facile synthesis of a range of structurally diverse amides in moderate to high yields with broad functionality compatibility. In addition, the synthetic usefulness of the method was further demonstrated by applying the reaction in scale-up synthesis and the late-stage functionalization of complex molecules with biological activities.

Synthesis of Stereodefined Enones from the Cross-Electrophile Coupling of Activated Acrylic Acids with Alkyl Bromides

We report a one-pot synthesis of (E)-trisubstituted enones from acrylic acids through the in situ generation of a 2-pyridyl ester and subsequent cross-electrophile coupling with a nickel catalyst under reducing conditions. The scope of trisubstituted enones is broad and compatible with functionality that can be challenging in established olefination techniques. We highlight conditions necessary to suppress undesired side reactions from the α,β-unsaturated carbonyl and improve cross-electrophile coupling approaches to prepare enones.

Ni-Catalyzed Csp2 and Csp3 Coupling for Divergent Bisboronic Ester Synthesis

Bisboronic esters are critical compounds in various research fields, including drug discovery, chemical biology, and material sciences. Currently, the bisboronic esters with reactive functional groups are difficult to synthesize; this is partially due to the lack of a robust method to produce these products with diverse structures and various functional groups at specific locations. To overcome this issue, this study introduced a Ni-catalysis approach to produce bisboronic esters efficiently via cross-coupling and homocoupling using readily available halogenated boronic esters as the starting material under mild reaction conditions. This newly developed strategy enables Csp2-Csp2, Csp3-Csp3, and Csp2-Csp3 couplings, demonstrating a broad substrate scope and excellent compatibility with various functional groups.

Diphosphine Ligand-Enabled Nickel-Catalyzed Chelate-Assisted Inner-Selective Migratory Hydroarylation of Alkenes

The precise control of the regioselectivity in the transition metal-catalyzed migratory hydrofunctionalization of alkenes remains a big challenge. With a transient ketimine directing group, the nickel-catalyzed migratory β-selective hydroarylation and hydroalkenylation of alkenyl ketones has been realized with aryl boronic acids using alkyl halide as the mild hydride source for the first time. The key to this success is the use of a diphosphine ligand, which is capable of the generation of a Ni(II)-H species in the presence of alkyl bromide, and enabling the efficient migratory insertion of alkene into Ni(II)-H species and the sequent rapid chain walking process. The present approach diminishes organosilanes reductant, tolerates a wide array of complex functionalities with excellent regioselective control. Moreover, this catalytic system could also be applied to the migratory hydroarylation of alkenyl azahetereoarenes, thus providing a general approach for the preparation of 1,2-aryl heteroaryl motifs with wide potential applications in pharmaceutical discovery.

Nickel-Catalyzed Migratory Cross-Coupling Reactions: New Opportunities for Selective C-H Functionalization

ConspectusMigratory cross-coupling via metal migration is a process of significant academic and industrial interest. It provides an attractive alternative for the selective installation of a functional group at remote C-H positions from simple precursors, thus enabling the direct synthesis of challenging structures not accessible with traditional cross-coupling. In particular, with the merger of 1,n-Ni/H shift and cross-coupling of nickel, the Ni-catalyzed migratory functionalization of simple precursors has undergone particularly intense development and emerged as a valuable field of research in the past few years. This Account will outline the recent progress made in this arena in terms of migration-functionalization modes, diverse functionalizations, and strategies for regio- and stereocontrol. Mechanistic studies and synthetic applications are also discussed.In detail, we systematically categorize our work into two parts based on the migration modes. In the first part, a platform is created for Ni-catalyzed migratory sp3 C-H functionalization of alkenes or alkyl halides via iterative 1,2-Ni/H shift-selective cross-coupling. The key reactive Ni(II)H species for chain-walking could be generated in situ either in a polarity-reversed fashion relying on stoichiometric reductants (X-Ni(II)-H) or in a redox-neutral fashion with the participation of nucleophilic coupling partners (FG-Ni(II)-H). One significant advantage associated with the polarity-reversed NiH system is the use of relatively stable, abundant, and safe olefin surrogates or alkyl halides instead of the sensitive organometallics required in traditional cross-coupling reactions. Another advantage is that diverse functionalizations, including carbonation and more challenging amination and thiolation could be smoothly achieved with suitable electrophiles or their precursors. Finally, to address the challenging multifaceted selectivity and reactivity issues in asymmetric migratory cross-coupling reactions, we have developed a feasible ligand relay catalytic strategy. In this dynamic ligand exchange process, one ligand promotes rapid migration while the other promotes highly regio- and stereoselective coupling. This innovative strategy overcomes the formidable challenge stemming from the difficulty of designing a single ligand to efficiently promote both steps of chain-walking and asymmetric coupling. In the second part, a new platform for Ni-catalyzed migratory sp2 C-H functionalization via 1,4-Ni/H shift-selective cross-coupling has been reported. Starting from readily available aryl or vinyl coupling partners, the in situ-generated aryl- or vinylnickel(II) species could undergo a rapid and reversible 1,4-Ni/H shift along an sp2 backbone, and subsequent selective coupling with various coupling partners would allow regio- and stereoselective access to diverse 1,4-migratory functionalization products. The key to success was the discovery of an appropriate ligand to efficiently promote both migration and subsequent selective cross-coupling. A vinyl-to-aryl 1,4-Ni/H shift successfully enables the modular ipso/ortho difunctionalization of aryl coupling partners, while an aryl-to-vinyl 1,4-Ni/H shift enables regio- and stereoselective access to functionalized trisubstituted alkenes.We hope that this Account will inspire broad interest and future development of migratory cross-coupling reactions. We strongly believe that continued efforts in this fascinating field will overcome many of the remaining challenges, including cutting-edge ligand/catalyst design to enhance reactivity and selectivity, conceptually new migration modes for additional transformations, and in-depth mechanistic studies for rational reaction design.

Recent Advances in First-Row Transition Metal-Catalyzed Reductive Coupling Reactions for π-Bond Functionalization and C-Glycosylation

ConspectusEfficient construction of ubiquitous carbon-carbon bonds between two electrophiles has garnered interest in recent decades, particularly if it is mediated by nonprecious, first-row transition metals. Reductive coupling has advantages over traditional cross-coupling by obviating the need for stoichiometric air- and moisture-sensitive organometallic reagents. By harnessing transition metal-catalyzed reductive coupling as a powerful tool, intricate molecular architectures can be readily assembled through the installation of two C-C bonds across π systems (alkenes/alkynes) via reaction with two appropriate electrophiles. Despite advances in reductive alkene difunctionalization, there remains significant potential for the discovery of novel reaction pathways. In this regard, development of reductive protocols that enable the union of challenging alkyl/alkynyl electrophiles in high regio- and chemoselectivity remains a highly sought-after goal.Apart from π-bond functionalization, reductive coupling has found application in carbohydrate chemistry, particularly in the synthesis of valuable C-glycosyl compounds. In this vein, suitable glycosyl donors can be used to generate reactive glycosyl radical intermediates under reductive conditions. Through elaborately designed reactions, these intermediates can be trapped to furnish pharmaceutically relevant glycoconjugates. Consequently, diversification in C-glycosyl compound synthesis using first-row transition metal catalysis holds strong appeal.In this Account, we summarize our efforts in the development of first-row transition metal-catalyzed reductive coupling reactions for applications in alkene/alkyne functionalization and C-glycosylation. We will first discuss the nickel (Ni)-catalyzed reductive difunctionalization of alkenes, aided by an 8-aminoquinoline (AQ) directing auxiliary. Next, we highlight the Ni-catalyzed hydroalkylation of alkenyl amides tethered with a similar AQ-derived directing auxiliary. Lastly, we discuss an efficient synthesis of 1,3-enynes involving site- and stereoselective reductive coupling of terminal alkynes with alkynyl halides and NHPI esters.Beyond alkene dicarbofunctionalization, we extended the paradigm of transition metal-catalyzed reductive coupling toward the construction of C-glycosidic linkages in carbohydrates. By employing an earth-abundant iron (Fe)-based catalyst, we show that useful glycosyl radicals can be generated from glycosyl chlorides under reductive conditions. These intermediates can be captured in C-C bond formation to furnish valuable C-aryl, C-alkenyl, and C-alkynyl glycosyl compounds with high diastereoselectivity. Our Ni-catalyzed multicomponent union of glycosyl chlorides, aryl/alkyl iodides, and isobutyl chloroformate under reductive conditions led to the stereoselective synthesis of C-acyl glycosides. In addition to Fe and Ni, we discovered a Ti-catalyzed/Mn-promoted synthetic route to access C-alkyl and C-alkenyl glycosyl compounds, through the reaction of glycosyl chlorides with electron-deficient alkenes/alkynes. We further developed an electron donor-acceptor (EDA) photoactivation system leveraging decarboxylative and deaminative strategies for C-glycosylation under Ni catalysis. This approach has been demonstrated to selectively activate carboxyl and amino motifs to furnish glycopeptide conjugates. Finally, through two distinct catalytic transformations of bench-stable heteroaryl glycosyl sulfones, we achieved stereodivergent access to both α- and β-anomers of C-aryl glycosides, one of which involves a Ni-catalyzed reductive coupling with aryl iodides.The findings presented in this Account are anticipated to have far-reaching implications beyond our research. We foresee that these results will pave the way for new transformations founded on the concept of reductive coupling, leading to the discovery of novel applications in the future.

Understanding the Reaction Mechanism of Ni-Catalyzed Regio- and Enantioselective Hydroalkylation of Enamines: Chemoselectivity of (Bi-oxazoline)NiH

This density functional theory study explores the detailed mechanism of nickel-catalyzed hydroalkylation of the C═C bond of N-Cbz-protected enamines (Cbz = benzyloxycarbonyl) with alkyl iodides to give chiral α-alkyl amines. The active catalyst (biOx)NiH, a chiral bioxazoline (biOx)-chelated Ni(I) hydride, exhibits chemoselectivity that favors single electron transfer to the alkyl iodide over C═C hydrometalation with the enamine. This generates an alkyl radical and a Ni(II) intermediate, which takes up the enamine substrate CbzNHCH═CH2CH3 via a regio- and enantioselective C═C insertion into the NiII-H bond. The resulting Ni(II) alkyl complex combines with the alkyl radical, forming a Ni(III) intermediate, from which the alkyl-alkyl reductive elimination delivers the chiral amine product. The regioselectivity arises from a combination of orbital and noncovalent interactions, both of which are induced by the Cbz group. Thus, Cbz plays an additional role in controlling regioselectivity. The enantioselectivity stems from the differing distortion energies of CbzNHCH═CH2CH3. The reductive elimination is the rate-determining step (ΔG⧧ = 18.7 kcal/mol). In addition, the calculations show a noninnocent behavior of the biOx ligand induced by the insertion of CbzNHCH═CH2CH3 into the Ni-H bond of (biOx)NiH. These computationally gained insights can have implications for developing new Ni(I)-catalyzed reactions.

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.

Nickel-Catalyzed Hydroamination and Hydroalkoxylation of Enelactams with Unactivated Amines and Alcohols

Nickel-catalyzed hydroamination and hydroalkoxylation of enelactams with unactivated amines and alcohols are reported. This method showed good functional group tolerance and delivered the corresponding hydrofunctionalized products in good to excellent yields (≤98%). Furthermore, an intramolecular hydroalkoxylation of an enelactam was also realized, giving a cyclization product in a good yield. Mechanistic studies indicated that tBuI acts as a hydride donor and radical precursor, which is crucial for the success of the reaction.

NiH-Catalyzed Functionalization of Remote and Proximal Olefins: New Reactions and Innovative Strategies

Transition metal hydride catalyzed functionalization of remote and proximal olefins has many advantages over conventional cross-coupling reactions. It avoids the separate, prior generation of stoichiometric amounts of organometallic reagents and the use of preformed organometallic reagents, which are sometimes hard to access and may compromise functional group compatibility. The migratory insertion of metal hydride complexes generated in situ into readily available alkene starting materials, the hydrometalation process, provides an attractive and straightforward route to alkyl metal intermediates, which can undergo a variety of sequential cross-coupling reactions. In particular, with the synergistic combination of chain-walking and cross-coupling chemistry of nickel, NiH-catalyzed functionalization of remote and proximal olefins has undergone particularly intense development in the past few years. This Account aims to chronicle the progress made in this arena in terms of activation modes, diverse functionalizations, and chemo-, regio-, and enantioselectivity.We first provide a brief introduction to the general reaction mechanisms. Taking remote hydroarylation as an example, the four oxidation states of Ni have allowed us to develop two different reaction strategies to form the final product: a Ni(I)-H/X-Ni(II)-H platform that relies on stoichiometric reductants and a Ni(I/II/III) cycle and a redox-neutral functional group or FG-Ni(II)-H platform that reacts with an alkene substrate and forms the migratory products via a Ni(0/II) pathway. We also demonstrate that diverse functionalization, including general C-C bond-forming reactions and the more challenging C-N/C-S bond-forming reactions could be realized. Moreover, the employment of appropriate chiral ligands has allowed us to successfully realize the corresponding asymmetric hydrofunctionalization reactions of olefins, including hydroalkylation, hydroarylation, hydroalkenylation, hydroalkynylation, and hydroamination. Interestingly, the enantio-determining step could be enantioselective hydronickelation, selective oxidative addition, or selective reductive elimination. To realize more challenging asymmetric migratory hydrofunctionalization, we have developed a general ligand relay catalytic strategy with a combination of two simple ligands, the first for chain-walking and the second for asymmetric coupling. This novel strategy avoids the design of a single, possibly structurally complex chiral ligand to promote both steps of chain-walking and asymmetric coupling. In addition, the success of multicomponent hydrofunctionalization provides a convenient approach to gain simple access to complex molecules. Finally, alkyl halides could be used as olefin precursors to undergo a variety of reductive migratory cross-electrophile coupling reactions. Applications of these remote hydrofunctionalization reactions are also discussed. We hope this Account will inspire future development in the field to overcome key challenges, including conceptually new catalytic strategies, development of high-performance systems with enhanced reactivity and selectivity, cutting-edge catalyst design, and further mechanistic studies.

Reductive Elimination from Sterically Encumbered Ni–Polypyridine Complexes

Herein we disclose the synthesis of sterically encumbered dialkylnickel(II) complexes bearing 2,9-dimethyl-1,10-phenanthroline ligands. A comparison with their unsubstituted analogues by both X-ray crystallography and theoretical calculations revealed significant distortions in their molecular structures. Eyring plots along with stoichiometric and photoexcitation studies revealed that sterically encumbered dialkylnickel(II) complexes enable facile C(sp³)–C(sp³) reductive elimination, thus offering an improved understanding of Ni catalysis.

Synthesis of (Z)-β-Chloro-enamides via a Base-Promoted trans-Hydroamidation of Alkynyl Chlorides Using 1,1-Dichloroalkenes as Precursor

An efficient and general base-promoted reaction of 1,1-dichloroalkenes with secondary sulfonamides and amides for the synthesis of (Z)-β-chloro-enamides has been described. This reaction exhibits functional group tolerance under simple and mild conditions. Mechanistic study indicated that a stereoselective trans-hydroamidation of alkynyl chlorides generated in situ from 1,1-dichloroalkenes was the key step.

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.

Highly Chemoselective Ni-Catalyzed Protecting-Group-Free 2,2'-Biphenol Synthesis and Mechanistic Insights

The utilization of readily available starting materials to produce useful molecules is often challenged by selectivity issues. In this study, a Ni-catalyzed protecting-group-free C-C coupling protocol is described for the efficient synthesis of 2,2'-biphenol derivatives. Its remarkable chemoselectivity control ability, wide substrate scope, and excellent functional group tolerance highlight this newly developed strategy. Detailed mechanistic studies have demonstrated that potassium tert-butoxide acts as a critical agent to prevent the occurrence of protonation events.

Streamlined Alkylation via Nickel-Hydride-Catalyzed Hydrocarbonation of Alkenes

Compounds rich in sp³-hybridized carbons are desirable in drug discovery. Nickel-catalyzed hydrocarbonation of alkenes is a potentially efficient method to synthesize these compounds. By using abundant, readily available, and stable alkenes as pro-nucleophiles, these reactions can have broad scope and high functional group tolerance. However, this methodology is still in an early stage of development, as the first efficient examples were reported only in 2016. Herein, we summarize the progress of this emerging field, with an emphasis on enantioselective reactions. We highlight major developments, critically discuss a wide range of possible mechanisms, and offer our perspective of the state and challenges of the field. We hope this Perspective will stimulate future works in this area, making the methodology widely applicable in organic synthesis.

NiH-catalyzed C(sp3)–Si coupling of alkenes with vinyl chlorosilanes

A novel NiH-catalyzed highly selective cross-coupling of alkenes with vinyl chlorosilanes is developed. Using this practical chemistry, various benzyl organosilanes could be produced with good functional group tolerance.

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 Reductive Carboxylation and Amidation Reactions

The ubiquity and importance of carboxylic acids and amides in peptides, pharmaceuticals, agrochemicals, and synthetic materials has challenged chemists to design de novo catalytic carboxylation and amidation protocols. They represent a powerful alternative to canonical oxidation of alcohols and aldehydes, hydrolysis of nitriles, transamidation reactions, or condensation techniques for the synthesis of these functional groups. Among various scenarios, the recent years have witnessed considerable advances in Ni-catalyzed reductive carboxylation and amidation reactions utilizing carbon dioxide and isocyanate counterparts. This Account aims to highlight the progress made in this arena with a historical perspective, with particular emphasis on the methodologies that have emanated from our laboratories without losing sight of the underlying principles by which these reactions operate, with the ultimate goal of allowing the transition from comprehension to prediction in this exciting field.Unlike the utilization of conventional polar yet highly reactive organometallic reagents in carboxylation or amidation reactions, the utilization of nickel catalysts has allowed the use of carbon dioxide and isocyanates with less reactive and less-polarized counterparts for the formations of carboxylic acids and amides. These less reactive groups include organic halides and pseudohalides (i.e., alkyl bromides and chlorides, esters, alcohols, and ammonium salts), unsaturated hydrocarbons (i.e., alkynes, styrenes, unactivated alkenes, and dienes) or even C-H bonds, where forging the targeted C-C bond at previously unfunctionalized C-H linkages was possible, thus giving access to densely functionalized compounds that would be difficult to access otherwise. The C-H functionalization includes chain-walking scenarios, where subtle changes in the ligand and reaction conditions marked the selectivity of the transformations, and reactions via a [1,4]-Ni shift, where selective carboxylation in aromatic rings could be achieved. Conceptuality and practicality aside, these transformations have even offered the possibility of modulating and dictating the site-selectivity pattern, thus providing not only new vistas when controlling the selectivity of bond-forming reactions at specific sites within the side chain but also new knowledge in retrosynthetic analysis when accessing carboxylic acids and amide backbones. Importantly, these techniques have shown to be particularly suited for the preparation of isotopically labeled molecules when using ¹³CO₂ or even ¹⁴CO₂, thus becoming a useful endeavor in the drug discovery pipeline. Although mechanistic understanding at the molecular level still constitutes the "Achilles heel" of these transformations, the recent empirical discoveries and the rapid adoption of these protocols by the community augurs well for the widespread utilization of reductive carboxylation and amidation reactions in both academic and industrial laboratories.

Catalytic Carbochlorocarbonylation of Unsaturated Hydrocarbons via C-COCl Bond Cleavage*

Here we report a palladium‐catalysed difunctionalisation of unsaturated C−C bonds with acid chlorides. Formally, the C−COCl bond of an acid chloride is cleaved and added, with complete atom economy, across either strained alkenes or a tethered alkyne to generate new acid chlorides. The transformation does not require exogenous carbon monoxide, operates under mild conditions, shows a good functional group tolerance, and gives the isolated products with excellent stereoselectivity. The intermolecular reaction tolerates both aryl‐ and alkenyl‐substituted acid chlorides and is successful when carboxylic acids are transformed to the acid chloride in situ. The reaction also shows an example of temperature‐dependent stereodivergence which, together with plausible mechanistic pathways, is investigated by DFT calculations. Moreover, we show that benzofurans can be formed in an intramolecular variant of the reaction. Finally, derivatisation of the products from the intermolecular reaction provides a highly stereoselective approach for the synthesis of tetrasubstituted cyclopentanes.

Reductive hydrobenzylation of terminal alkynes via photoredox and nickel dual catalysis

A photoredox/nickel dual catalyzed reductive hydrobenzylation of alkynes and benzyl chlorides by employing alkyl amines as a stoichiometric reductant is described. This synergistic protocol proceeds via Markovnikov-selective migratory insertion of an alkyne into nickel hydride, followed by cross-coupling with benzyl chloride, providing facile access to important 1,1-disubstituted olefins. This reaction enables the generation of nickel hydride by utilizing readily available alkyl amines as the hydrogen source. The mild conditions are compatible with a wide range of aryl and alkyl alkynes as well as chlorides.

Selective Synthesis of Z-Silyl Enol Ethers via Ni-Catalyzed Remote Functionalization of Ketones

We report a remote functionalization strategy, which allows the Z-selective synthesis of silyl enol ethers of (hetero)aromatic and aliphatic ketones via Ni-catalyzed chain walking from a distant olefin site. The positional selectivity is controlled by the directionality of the chain walk and is independent of thermodynamic preferences of the resulting silyl enol ether. Our mechanistic data indicate that a Ni^(I) dimer is formed under these conditions, which serves as a catalyst resting state and, upon reaction with an alkyl bromide, is converted to [Ni^(II)-H] as an active chain-walking/functionalization catalyst, ultimately generating a stabilized η³-bound Ni^(II) enolate as the key selectivity-controlling intermediate.

Alkyl halides as both hydride and alkyl sources in catalytic regioselective reductive olefin hydroalkylation

Among the plethora of catalytic methods developed for hydrocarbofunctionalization of olefins to date, reactions that regioselectively install a functionalized alkyl unit at the 2-position of a terminal unactivated C=C bond to afford branched products are scarce. Here, we show that a Ni-based catalyst in conjunction with a stoichiometric reducing agent promote Markovnikov-selective hydroalkylation of unactivated alkenes tethered to a recyclable 8-aminoquinaldine directing auxiliary. These mild reductive processes employ readily available primary and secondary haloalkanes as both the hydride and alkyl donor. Reactions of alkenyl amides with ≥ five-carbon chain length regioselectively afforded β-alkylated products through remote hydroalkylation, underscoring the fidelity of the catalytic process and the directing group’s capability in stabilizing five-membered nickelacycle intermediates. The operationally simple protocol exhibits exceptional functional group tolerance and is amenable to the synthesis of bioactive molecules as well as regioconvergent transformations.

Methods that regioselectively install a functionalized alkyl unit at the 2-position of a terminal unactivated C=C bond are scarce. Here, the authors report a Markovnikov-selective hydroalkylation of unactivated amide-tethered alkenes catalyzed by nickel in conjunction with a stoichiometric reductant.

Stereoselective Formal Hydroamidation of Si-Substituted Arylacetylenes with DIBAL-H and Isocyanates: Synthesis of (E)- and (Z)-α-Silyl-α,β-unsaturated Amides

An efficient and stereoselective method for the synthesis of (E)- and (Z)-α-silyl-α,β-unsaturated amides and its synthetic applications are presented herein. The solvent-controlled hydroaluminations of Si-substituted alkynes with DIBAL-H generate diastereomerically enriched alkenylaluminum reagents that are directly reacted with isocyanates at ambient temperature to afford α-silyl-α,β-unsaturated amides in high yields with retained stereoselectivity. In particular, this process enables the synthesis of a broad range of (E)-α-silyl-α,β-unsaturated amides, which are the less studied isomers. The synthetic utility of this method is highlighted by its short reaction time, ease of purification, easily accessible substrates and reagents, gram-scale synthesis, and the further transformations of C-Si bonds into C-H, C-X, and C-C bonds.

Iridium-Catalyzed Hydrochlorination and Hydrobromination of Alkynes by Shuttle Catalysis

Described herein are two different methods for the synthesis of vinyl halides by a shuttle catalysis based iridium-catalyzed transfer hydrohalogenation of unactivated alkynes. The use of 4-chlorobutan-2-one or tert-butyl halide as donors of hydrogen halides allows this transformation in the absence of corrosive reagents, such as hydrogen halides or acid chlorides, thus largely improving the functional-group tolerance and safety profile of these reactions compared to the state-of-the-art. This method has granted access to alkenyl halide compounds containing acid-sensitive groups, such as tertiary alcohols, silyl ethers, and acetals. The synthetic value of those methodologies has been demonstrated by gram-scale synthesis where low catalyst loading was achieved.

Highly chemoselective difluoromethylative homologation of iso(thio)cyanates: expeditious access to unprecedented α,α-difluoro(thio)amides

The new motif - α,α-difluoromethyl thioamide - has been assembled starting from isothiocyanate (as thioamide precursor) and a formal difluoromethyl-carbanion generated from commercially available TMSCHF2. Upon proper activation of this reagent with potassium tert-amylate, the high-yielding transfer of the difluorinated nucleophile takes place under high chemocontrol. Various sensitive functionalities (e.g. ester, nitrile, nitro, azido groups) can be accommodated across the isothiocyanate core, thus allowing a wide scope. The methodology is highly flexible and adaptable to prepare analogous α,α-difluoromethyl oxoamides by conveniently using isocyanates as the electrophilic building-blocks.

Site-Selective Catalytic Deaminative Alkylation of Unactivated Olefins

A catalytic deaminative alkylation of unactivated olefins is described. The protocol is characterized by its mild conditions, wide scope, including the use of ethylene as substrate, and exquisite site-selectivity pattern for both α-olefins and internal olefins, thus unlocking a new catalytic platform to forge sp³-sp³ linkages, even in the context of late-stage functionalization.

Nickel-Catalyzed Hydroarylation of Alkynes under Reductive Conditions with Aryl Bromides and Water

An operationally simple nickel-catalyzed hydroarylation reaction for alkynes is described. This three-component coupling reaction utilizes commercially available alkynes and aryl bromides, along with water and Zn. An air-stable and easily synthesized Ni(II) precatalyst is the only entity used in the reaction that is not commercially available. This reductive cross-coupling reaction displays a fairly unusual anti selectivity when aryl bromides with ortho substituents are used. In addition to optimization data and a preliminary substrate scope, complementary experiments including deuterium labeling studies are used to provide a tentative catalytic mechanism. We believe this report should inspire and inform other Ni-catalyzed carbofunctionalization reactions.

Direct Transformation of Aryl 2-Pyridyl Esters to Secondary Benzylic Alcohols by Nickel Relay Catalysis

A direct transformation of aryl esters to secondary benzylic alcohols via tandem Ni-catalyzed cross-coupling reactions of aromatic 2-pyridyl esters with alkyl zinc reagents and carbonyl group reduction by Ni-H species is achieved. Preliminary mechanistic studies reveal that the Ni-H species is generated in situ via β-hydride elimination of the Negishi reagents. The reaction is catalyzed by bench-stable nickel salts under mild conditions with wide functional group tolerance.

Ni-catalyzed Reductive Deaminative Arylation at sp3 Carbon Centers

A Ni-catalyzed reductive deaminative arylation at unactivated sp³ carbon centers is described. This operationally simple and user-friendly protocol exhibits excellent chemoselectivity profile and broad substrate scope, thus complementing existing metal-catalyzed cross-coupling reactions to forge sp³ C-C linkages. These virtues have been assessed in the context of late-stage functionalization, hence providing a strategic advantage to reliably generate structure diversity with amine-containing drugs.

Ni-catalyzed migratory fluoro-alkenylation of unactivated alkyl bromides with gem-difluoroalkenes

We describe a nickel-catalyzed highly regio- and stereoselective migratory fluoro-alkenylation of unactivated alkyl bromides. A unique catalytic cycle merging alkyl nickel chain-walking and defluorinative coupling enables the introduction of a broad array of fluoroalkenyl moieties into carbon chains. Control experiments with other halogenated alkenes demonstrated the essential role of fluorine atoms in this reaction. Notably, the reaction proceeds under mild conditions and allows for the synthesis of a variety of valuable monofluoroalkenes.