Ni-Catalyzed Reductive Dicarbofunctionalization of Nonactivated Alkenes: Scope and Mechanistic Insights

Ni-Catalyzed Reductive 1,2-Alkylarylation of Alkenes for the Synthesis of Spirocyclic γ-Lactams

An intermolecular nickel-catalyzed reductive 1,2-alkylarylation of acrylates with cyclopropylamine NHP esters and aryl iodides is reported. This operationally simple protocol provides direct access to 1-alkylcyclopropylamine scaffolds. The mild conditions are compatible with four-membered α-amino strained rings as well as five- and six-membered ring systems. The products undergo cyclization to access α-arylated spirocyclic γ-lactams─a motif present in several pharmaceuticals.

Triple Radical Sorting: Aryl-Alkylation of Alkenes

The cross-coupling of aryl bromides with alkenes can provide access to diverse combinatorial chemical space. Two-component couplings between these partners are well-known, but three-component aryl-functionalizations of unactivated alkenes remain underdeveloped. In particular, the aryl-alkylation of unactivated alkenes would allow for rapid construction of molecular complexity and the expedient exploration of a pharmaceutically relevant and C(sp3)-rich structural landscape. Herein, we report a general approach toward the aryl-alkylation of alkenes through a triple radical sorting mechanism. Over the course of the reaction, a high energy aryl radical, a primary radical, and a hindered alkyl radical are simultaneously formed. Through mediation by a nickel-based catalyst, the three radicals are sorted into productive bond-forming pathways toward the efficient aryl-alkylation of alkenes. A wide range of electronically and sterically differentiated alkenes and aryl radical precursors can be used to access complex scaffolds. This method was further applied to the synthesis of highly substituted semisaturated fused heterocycles.

Ni(2,2':6',2''-terpyridine)2: a high-spin octahedral formal Ni(0) complex

We have synthesised and characterised the complex Ni(tpy)2 (tpy = 2,2':6',2''-terpyridine). This formally Ni(0) complex is paramagnetic both in the solid state and in solution (S = 2). The crystal structure shows an octahedral geometry, with molecules arranged in independent dimers involving π-stacking between pairs of complexes. Magnetic measurementes and DFT calculations suggest the existence of temperature-dependent intermolecular antiferromagnetic coupling in the solid state.

Effect of 6,6'-Substituents on Bipyridine-Ligated Ni Catalysts for Cross-Electrophile Coupling

A family of 4,4'-tBu2-2,2'-bipyridine (tBubpy) ligands with substituents in either the 6-position, 4,4'-tBu2-6-Me-bpy (tBubpyMe), or 6 and 6'-positions, 4,4'-tBu2-6,6'-R2-bpy (tBubpyR2; R = Me, iPr, sBu, Ph, or Mes), was synthesized. These ligands were used to prepare Ni complexes in the 0, I, and II oxidation states. We observed that the substituents in the 6 and 6'-positions of the tBubpy ligand impact the properties of the Ni complexes. For example, bulkier substituents in the 6,6'-positions of tBubpy better stabilized (tBubpyR2)NiICl species and resulted in cleaner reduction from (tBubpyR2)NiIICl2. However, bulkier substituents hindered or prevented coordination of tBubpyR2 ligands to Ni0(cod)2. In addition, by using complexes of the type (tBubpyMe)NiCl2 and (tBubpyR2)NiCl2 as precatalysts for different XEC reactions, we demonstrated that the 6 or 6,6' substituents lead to major differences in catalytic performance. Specifically, while (tBubpyMe)NiIICl2 is one of the most active catalysts reported to date for XEC and can facilitate XEC reactions at room temperature, lower turnover frequencies were observed for catalysts containing tBubpyR2 ligands. A detailed study on the catalytic intermediates (tBubpy)Ni(Ar)I and (tBubpyMe2)Ni(Ar)I revealed several factors that likely contributed to the differences in catalytic activity. For example, whereas complexes of the type (tBubpy)Ni(Ar)I are low spin and relatively stable, complexes of the type (tBubpyMe2)Ni(Ar)I are high-spin and less stable. Further, (tBubpyMe2)Ni(Ar)I captures primary and benzylic alkyl radicals more slowly than (tBubpy)Ni(Ar)I, consistent with the lower activity of the former in catalysis. Our findings will assist in the design of tailor-made ligands for Ni-catalyzed transformations.

A Bimolecular Homolytic Substitution-Enabled Platform for Multicomponent Cross-Coupling of Unactivated Alkenes

The construction of C(sp3)-C(sp3) bonds remains one of the most difficult challenges in cross-coupling chemistry. Here, we report a photoredox/nickel dual catalytic approach that enables the simultaneous formation of two C(sp3)-C(sp3) linkages via trimolecular cross-coupling of alkenes with alkyl halides and hypervalent iodine-based reagents. The reaction harnesses a bimolecular homolytic substitution (SH2) mechanism and chemoselective halogen-atom transfer (XAT) to orchestrate the regioselective addition of electrophilic and nucleophilic alkyl radicals across unactivated alkenes without the need for a directing auxiliary. Utility is highlighted through late-stage (fluoro)alkylation and (trideutero)methylation of C═C bonds bearing different substitution patterns, offering straightforward access to drug-like molecules comprising sp3-hybridized carbon scaffolds.

Iron-Mediated Dialkylation of Alkenylarenes with Benzyl Bromides

We disclose a method for the dibenzylation of alkenylarenes with benzyl bromides using iron powder. This reaction generates branched alkyl scaffolds adorned with functionalized aryl rings through the formation of two new C(sp3)-C(sp3) bonds at the vicinal carbons of alkenes. This protocol tolerates electron-rich, electron-neutral, and electron-poor benzyl bromides and alkenylarenes. Mechanistic studies suggest the formation of benzylic radical intermediates as a result of single-electron transfer from the iron, which is intercepted by alkenylarenes.

Synthesis of Interlocked and Non-Interlocked Deca(para-phenylene) Derivatives by Ni-mediated Biaryl Coupling

Oligo(para-phenylene) (PPn) is a compound composed of directly connected 1,4-phenylene moieties. The synthesis of PPn composed of six or more phenylene moieties with no substituent at the internal phenylene moiety has been challenging because of its low solubility. Herein we synthesized oligo(para-phenylene)[2]rotaxanes, including a deca(para-phenylene)[2]rotaxane, with a defined number of phenylene moieties. Biaryl coupling of iodoarenes mediated by macrocyclic dibenzodihydrophenanthroline-Ni complex was utilized for the first time to synthesize the [2]rotaxanes. Compared to the non-interlocked deca(para-phenylene), the deca(para-phenylene)[2]rotaxane showed higher solubility. The properties of the oligo(para-phenylene)[2]rotaxanes and non-interlocked oligo(para-phenylene)s were analyzed by spectroscopic methods.

A free-radical design featuring an intramolecular migration for a synthetically versatile alkyl-(hetero)arylation of simple olefins

A free-radical approach has enabled the development of a synthetically versatile alkyl-(hetero)arylation of olefins. Alkyl and (hetero)aryl groups were added concurrently to a full suite of mono- to tetrasubstituted simple alkenes (i.e., without requiring directing or electronically activating groups) for the first time. Key advances also included the introduction of synthetically diversifiable alkyl groups featuring different degrees of substitution, good diastereocontrol in both cyclic and acyclic settings, the addition of biologically valuable heteroarenes featuring Lewis basic nitrogen atoms as well as simple benzenes, and the generation of either tertiary or quaternary benzylic centers. The synthetic potential of this transformation was demonstrated by leveraging it as the key step in a concise synthesis of oliceridine, a new painkiller that received FDA approval in 2020.

Iron-catalyzed fluoroalkylative alkylsulfonylation of alkenes via radical-anion relay

Transition metal-catalyzed reductive difunctionalization of alkenes with alkyl halides is a powerful method for upgrading commodity chemicals into densely functionalized molecules. However, super stoichiometric amounts of metal reductant and the requirement of installing a directing group into alkenes to suppress the inherent β-H elimination bring great limitations to this type of reaction. We demonstrate herein that the difunctionalization of alkenes with two different alkyl halides is accessible via a radical-anion relay with Na2S2O4 as both reductant and sulfone-source. The Na2S2O4 together with the electron-shuttle catalyst is crucial to divert the mechanistic pathway toward the formation of alkyl sulfone anion instead of the previously reported alkylmetal intermediates. Mechanistic studies allow the identification of carbon-centered alkyl radical and sulfur-centered alkyl sulfone radical, which are in equilibrium via capture or extrusion of SO2 and could be converted to alkyl sulfone anion accelerated by iron electron-shuttle catalysis, leading to the observed high chemoselectivity.

Nickel-Catalyzed Reductive Alkene Cross-Dialkylation with Unactivated Alkyl Electrophiles

A Ni-catalyzed reductive dialkylation of 8-aminoquinoline-tethered aliphatic alkenes with two unactivated alkyl electrophiles is disclosed here. Key to the development of this transformation is the combination of primary alkyl (pseudo)halides and secondary alkyl iodides that produce products in a single regioselective manner. The reaction exhibits good functional group compatibility, and its synthetic utility was demonstrated by the concise synthesis of the precursors of biologically relevant molecules.

Visible-Light-Induced Photoredox 1,2-Dialkylation of Styrenes with α-Carbonyl Alkyl Bromides and Pyridin-1-ium Salts

A visible-light-driven photoredox dialkylation of styrenes with α-carbonyl alkyl bromides and pyridin-1-ium salts for the synthesis of polysubstituted 1,4-dihydropyridines is reported. This reaction enables the formation of two new C(sp3)-C(sp3) bonds in a single reaction step and provides a strategy that employs pyridin-1-ium salts as the functionalized alkylating reagents via dearomatization to directly trap the resulting alkyl radicals from radical addition of alkenes and then terminate the alkene dialkylation.

Nickel/photoredox dual catalyzed arylalkylation of nonactivated alkenes

Alkene dicarbofunctionalization is an efficient strategy and operation-economic fashion for introducing complexity in molecules. A nickel/photoredox dual catalyzed arylalkylation of nonactivated alkenes for the simultaneous construction of one C(sp3)-C(sp3) bond and one C(sp3)-C(sp2) bond has been developed. The mild catalytic method provided valuable indanethylamine derivatives with wide substrate scope and good functional group compatibility. An enantioselective dicarbofunctionalization was also achieved with pyridine-oxazoline as a ligand. The efficiency of metallaphotoredox dicarbofunctionalization was demonstrated for the concise synthesis of pharmaceutically active compounds.

Nickel-Catalyzed Three-Component Alkylarylation of Alkenyl N-Heteroarenes

A nickel-catalyzed three-component alkylarylation of alkenyl N-heteroarenes with α-bromocarboxylates and aryl boronic acids is reported. The protocol provides a new method to access a variety of N-heteroarene substituted diarylalkanes in moderate to good yields. It features mild reaction conditions, cheap nickel catalyst, readily available substrates, and broad substrate scope.

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.

Enantioselective Reductive Cross-Couplings of Olefins by Merging Electrochemistry with Nickel Catalysis

The merger of electrochemistry and transition metal catalysis has emerged as a powerful tool to join two electrophiles in an enantioselective manner. However, the development of enantioselective electroreductive cross-couplings of olefins remains a challenge. Inspired by the advantages of the synergistic use of electrochemistry with nickel catalysis, we present here a Ni-catalyzed enantioselective electroreductive cross-coupling of acrylates with aryl halides and alkyl bromides, which affords chiral α-aryl carbonyls in good to excellent enantioselectivity. Additionally, this catalytic reaction can be applied to (hetero)aryl chlorides, which is difficult to achieve by other methods. The combination of cyclic voltammetry analysis with electrode potential studies suggests that the NiI species activates aryl halides by oxidative addition and alkyl bromides by single-electron transfer.

Synthesis of α-Vinyltrialkoxysilanes via Nickel-Mediated Cross-Electrophile Coupling Reactions

Vinyltrialkoxysilanes are indispensable for organic synthesis, particularly cross-coupling reactions. Hydrosilylation of alkynes inevitably yields α- and β-isomers of vinyltrialkoxysilanes even with complex ligands and catalysts, limiting its usage in organic synthesis. We report the synthesis of α-vinyltrialkoxysilanes via cross-electrophile C(sp2)-C(sp2) coupling of bromoalkenes. The method is quite compatible with functional groups under milder reaction conditions. The gram-scale synthesis of most substrates is impressive. The intermediacy of vinyl iodide and radical escape rebound path are supported by mechanistic studies.

Palladium(I)-Iodide-Catalyzed Deoxygenative Heck Reaction of Vinyl Triflates: A Formate-Mediated Cross-Electrophile Reductive Coupling with cine-Substitution

The first deoxygenative Heck reactions are described, as illustrated by formate-mediated cine-substitutions of vinyl triflates with aryl iodides. The collective data corroborate a mechanism in which Pd(OAc)2 and Bu4NI form the dianionic iodide-bridged dimer [Pd2I6][NBu4]2, which, under reducing conditions, serves as a precursor to the palladium(I) complex [Pd2I4][NBu4]2. Dinculear oxidative addition of aryl iodide forms [Pd2I5(Ar)][NBu4]2, which dissociates to the monometallic complex [PdI2(Ar)][NBu4]. Vinyl triflate migratory insertion-sulfonate elimination delivers a palladium(IV) carbene, which upon β-hydride elimination/C-H reductive elimination gives the product of cine-substitution. These processes are the first efficient formate-mediated cross-electrophile reductive couplings beyond carbonyl addition.

Comproportionation and disproportionation in nickel and copper complexes

Disproportionation and comproportionation reactions have become increasingly important electron transfer events in organometallic chemistry and catalysis. The renewed interest in these reactions is in part attributed to the improved understanding of first-row metals and their ability to occupy odd and even oxidation states. Disproportionation and comproportionation reactions enable metal complexes to shuttle between various oxidation states, a matter of utmost relevance for controlling the speciation and catalytic turnover. In addition, these reactions have a direct impact in the thermodynamic and kinetic stability of the corresponding metal complexes. This review covers the relevance and impact of these processes in electron transfer reactions and provides valuable information about their non-negligible influence in Ni- and Cu-catalysed transformations.

Nickel-Catalyzed Regioselective Intermolecular Dialkylation of Alkenylarenes: Generation of Two Vicinal C(sp3 )-C(sp3 ) Bonds Across Alkenes

We disclose a Ni-catalyzed regioselective dialkylation reaction of alkenylarenes with α-halocarbonyls and alkylzinc reagents. The reaction produces γ-arylated alkanecarbonyl compounds with the generation of two new C(sp3 )-C(sp3 ) bonds at the vicinal carbons of alkenes. This reaction is effective for the use of primary, secondary and tertiary α-halocarboxylic esters, amides and ketones in conjunction with primary and secondary alkylzinc reagents as the sources of two C(sp3 ) carbons for the dialkylation of terminal and cyclic internal alkenes.

Development of Photoredox Cross-Electrophile Coupling of Strained Heterocycles with Aryl Bromides Using High-Throughput Experimentation for Library Construction

Microscale high-throughput experimentation was used to develop a photoredox-assisted reductive cross-coupling reaction of aryl halides with strained aliphatic heterocycles facilitated via a ring-opening reaction. This methodology was found to be applicable to medicinally relevant substrates including Boc-protected strained aliphatic heterocycles and (hetero)aryl bromides and was used for compound library construction via parallel medicinal chemistry. Furthermore, the coupling reactions were shown to be scalable to the gram scale by continuous flow reaction. A possible reaction mechanism is also discussed.

Machine Learning-Guided Development of Trialkylphosphine Ni(I) Dimers and Applications in Site-Selective Catalysis

Owing to the unknown correlation of a metal's ligand and its resulting preferred speciation in terms of oxidation state, geometry, and nuclearity, a rational design of multinuclear catalysts remains challenging. With the goal to accelerate the identification of suitable ligands that form trialkylphosphine-derived dihalogen-bridged Ni^(I) dimers, we herein employed an assumption-based machine learning approach. The workflow offers guidance in ligand space for a desired speciation without (or only minimal) prior experimental data points. We experimentally verified the predictions and synthesized numerous novel Ni^(I) dimers as well as explored their potential in catalysis. We demonstrate C-I selective arylations of polyhalogenated arenes bearing competing C-Br and C-Cl sites in under 5 min at room temperature using 0.2 mol % of the newly developed dimer, [Ni^(I)(μ-Br)PAd₂(n-Bu)]₂, which is so far unmet with alternative dinuclear or mononuclear Ni or Pd catalysts.

Three-component reductive conjugate addition/aldol tandem reaction enabled by nickel/photoredox dual catalysis

A three-component reductive cross-coupling of aryl halides, aldehydes, and alkenes by nickel/photoredox dual catalysis is disclosed. The key to success for this tandem transformation is to identify α-silylamine as a unique organic reductant, which releases silylium ions instead of protons to prevent unwanted protonation processes, and meanwhile serves as Lewis acid to activate aldehydes in situ. This dual catalytic protocol completes a traditional conjugate addition/aldol sequence that eliminates the requirement of organometallic reagents and metal-based reductants, thus providing a mild synthetic route to highly valuable β-hydroxyl carbonyl compounds with contiguous 1,2-stereocenters.

Revised Mechanism of C(sp3)-C(sp3) Reductive Elimination from Ni(II) with the Assistance of a Z-Type Metalloligand

Reductive elimination is a key step in Ni-catalyzed cross-couplings. Compared with processes that proceed from Ni(III) or Ni(IV) intermediates, C(sp³)-C(sp³) reductive eliminations from Ni(II) centers are challenging due to the weak oxidizing ability of Ni(II) species. In this report, we present computational evidence that supports a mechanism in which Zn coordination to the nickel center as a Z-type ligand accelerates reductive elimination. This Zn-assisted pathway is found to be lower in energy compared with direct reductive elimination from a σ-coordinated Ni(II) intermediate, providing new insights into the mechanism of Ni-catalyzed cross-coupling with organozinc nucleophiles. Mayer bond order, Hirshfield charge, Laplacian of the electron density, orbital, and interaction region indicator analyses were conducted to elucidate details of the reductive elimination process and characterize the key intermediates. Theoretical calculations indicate a significant Z-type Ni-Zn interaction that reduces the electron density around the Ni center and accelerates reductive elimination. This mechanistic study of reductive elimination in Ni(0)-catalyzed conjunctive cross-couplings of aryl iodides, organozinc reagents, and alkenes is an important case study of the involvement of Zn-assisted reductive elimination in Ni catalysis. We anticipate that the novel Zn-assisted reductive elimination mode may extend to other cross-coupling processes and explain the unique effectiveness of organozinc nucleophiles in many instances.

Ni-Catalyzed Regioselective Reductive 1,3-Dialkenylation of Alkenes

Dicarbofunctionalization is an important efficient synthetic technique for adding two chemical moieties across an alkene. Here, a novel method of reductive dicarbofunctionalization has been developed using a single alkenyl triflate as the electrophile, combined with an unactivated alkene. The reaction does not require an external auxiliary and proceeds with complete regioselectivity.

Mechanistic views and computational studies on transition-metal-catalyzed reductive coupling reactions

Transition-metal-catalyzed reductive coupling reactions have been considered as a powerful tool to convert two electrophiles into value-added products. Numerous related reports have shown the fascinating potential. Mechanistic studies, especially theoretical studies, can provide important implications for the design of novel reductive coupling reactions. In this review, we summarize the representative advancements in theoretical studies on transition-metal-catalyzed reductive coupling reactions and systematically elaborate the mechanisms for the key steps of reductive coupling reactions. The activation modes of electrophiles and the deep insights of selectivity generation are mechanistically discussed. In addition, the mechanism of the reduction of high-oxidation-state catalysts and further construction of new chemical bonds are also described in detail.

Regioselective Ni-Catalyzed reductive alkylsilylation of acrylonitrile with unactivated alkyl bromides and chlorosilanes

Transition-metal catalyzed carbosilylation of alkenes using carbon electrophiles and silylmetal (-B, -Zn) reagents as the nucleophiles offers a powerful strategy for synthesizing organosilicones, by incorporating carbon and silyl groups across on C-C double bonds in one step. However, to the best of our knowledge, the study of silylative alkenes difunctionalization based on carbon and silyl electrophiles remains underdeveloped. Herein, we present an example of silylative alkylation of activated olefins with unactivated alkyl bromides and chlorosilanes as electrophiles under nickel catalysis. The main feature of this protocol is employing more easily accessible substrates including primary, secondary and tertiary alkyl bromides, as well as various chlorosilanes without using pre-generated organometallics. A wide range of alkylsilanes with diverse structures can be efficiently assembled in a single step, highlighting the good functionality tolerance of this approach. Furthermore, successful functionalization of bioactive molecules and synthetic applications using this method demonstrate its practicability.

Nickel-Catalyzed Reductive Dicarbofunctionalization of Vinylarenes Enabled by Electrochemical Process

An intermolecular alkene dicarbofunctionalization via electrochemical reduction that combines alkyl and aryl iodides with styrene derivatives was herein reported. The multicomponent reaction exhibited several synthetic advantages including simple operation, wide substrate scope, and convenience of amplification. Mechanistic investigations, including cyclic voltammetry (CV), electron paramagnetic resonance (EPR), and radical trapping reactions, support the electrochemical nickel catalytic cycle and formation of alkyl radical species from alkyl iodides.

The Merger of Benzophenone HAT Photocatalysis and Silyl Radical-Induced XAT Enables Both Nickel-Catalyzed Cross-Electrophile Coupling and 1,2-Dicarbofunctionalization of Olefins

A strategy for both cross-electrophile coupling and 1,2-dicarbofunctionalization of olefins has been developed. Carbon-centered radicals are generated from alkyl bromides by merging benzophenone hydrogen atom transfer (HAT) photocatalysis and silyl radical-induced halogen atom transfer (XAT) and are subsequently intercepted by a nickel catalyst to forge the targeted C(sp³)–C(sp²) and C(sp³)–C(sp³) bonds. The mild protocol is fast and scalable using flow technology, displays broad functional group tolerance, and is amenable to a wide variety of medicinally relevant moieties. Mechanistic investigations reveal that the ketone catalyst, upon photoexcitation, is responsible for the direct activation of the silicon-based XAT reagent (HAT-mediated XAT) that furnishes the targeted alkyl radical and is ultimately involved in the turnover of the nickel catalytic cycle.

Synthesis of Vicinal Carbocycles by Intramolecular Nickel-Catalyzed Conjunctive Cross-Electrophile Coupling Reaction

A nickel-catalyzed intramolecular conjunctive cross-electrophile coupling reaction has been established. This method enables the synthesis of 3,5-vicinal carbocyclic rings found in numerous biologically active compounds and natural products. We provide mechanistic experiments that indicate this reaction proceeds through alkyl iodides formed in situ, initiates at the secondary electrophilic center, and proceeds through radical intermediates.

Nickel-Catalyzed Tandem Ueno-Stork Cyclization: Stereoselective 1,2-Dicarbofunctionalization of Cyclic Alkenes

A nickel-catalyzed tandem Ueno-Stork cyclization is developed to enable stereoselective 1,2-dicarbofunctionalization of cyclic alkenes and efficiently build various bicyclic products. This new protocol does not involve any toxic or difficult-to-remove tin reagent and is scalable and amenable to build all-carbon quaternary centers.

Interception of Nickel Hydride Species and Its Application in Multicomponent Reactions

The hydrogen borrowing strategy is an economical method for the α-functionalization of ketones. While this strategy is extremely advantageous, it does not lend itself to the synthesis of β,β-disubstituted ketones. This can be achieved, if the in situ generated metal hydride can be intercepted with a nucleophilic coupling partner. We present a multicomponent strategy for the coupling of alcohols, ketones, and boronic acids using only 1 mol % nickel catalyst and without the need for added ligands.

Modular Difunctionalization of Unactivated Alkenes through Bio-Inspired Radical Ligand Transfer Catalysis

Development of visible light-mediated atom transfer radical addition of haloalkanes onto unsaturated hydrocarbons has seen rapid growth in recent years. However, due to its radical chain propagation mechanism, diverse functionality other than the pre-existing (pseudo-)halide on the alkyl halide source cannot be incorporated into target molecules in a one-step, economic fashion. Inspired by the prominent reactivities shown by cytochrome P450 hydroxylase and non-heme iron-dependent oxygenases, we herein report the first modular, dual catalytic difunctionalization of unactivated alkenes via manganese-catalyzed radical ligand transfer (RLT). This RLT elementary step involves a coordinated nucleophile rebounding to a carbon-centered radical to form a new C-X bond in analogy to the radical rebound step in metalloenzymes. The protocol leverages the synergetic cooperation of both a photocatalyst and earth-abundant manganese complex to deliver two radical species in succession to minimally functionalized alkenes, enabling modular diversification of the radical intermediate by a high-valent manganese species capable of delivering various external nucleophiles. A broad scope (97 examples, including drugs/natural product motifs), mild conditions, and excellent chemoselectivity were shown for a variety of substrates and fluoroalkyl fragments. Mechanistic and kinetics studies provide insights into the radical nature of the dual catalytic transformation and support radical ligand transfer (RLT) as a new strategy to deliver diverse functionality selectively to carbon-centered radicals.

Ni-Catalyzed Regio- and Stereoselective Alkylarylation of Unactivated Alkenes in γ,δ-Alkenylketimines

We disclose a Ni-catalyzed vicinal alkylarylation of unactivated alkenes in γ,δ-alkenylketimines with aryl halides and alkylzinc reagents. The reaction produces γ-C(sp3)-branched δ-arylketones with the construction of two new C(sp3)-C(sp3) and C(sp3)-C(sp2) bonds. Electron-deficient alkenes play crucial dual roles as ligands to stabilize reaction intermediates and to increase catalytic rates for the formation of C(sp3)-C(sp3) bonds. This alkene alkylarylation reaction is also effective for secondary alkylzinc reagents and internal alkenes, and proceeds with a complete regio- and stereocontrol, affording products with up to three contiguous all-carbon all-cis secondary stereocenters.

Homogeneous Organic Electron Donors in Nickel-Catalyzed Reductive Transformations

Many contemporary organic transformations, such as Ni-catalyzed cross-electrophile coupling (XEC), require a reductant. Typically, heterogeneous reductants, such as Zn⁰ or Mn⁰, are used as the electron source in these reactions. Although heterogeneous reductants are highly practical for preparative-scale batch reactions, they can lead to complications in performing reactions on process scale and are not easily compatible with modern applications, such as flow chemistry. In principle, homogeneous organic reductants can address some of the challenges associated with heterogeneous reductants and also provide greater control of the reductant strength, which can lead to new reactivity. Nevertheless, homogeneous organic reductants have rarely been used in XEC. In this Perspective, we summarize recent progress in the use of homogeneous organic electron donors in Ni-catalyzed XEC and related reactions, discuss potential synthetic and mechanistic benefits, describe the limitations that inhibit their implementation, and outline challenges that need to be solved in order for homogeneous organic reductants to be widely utilized in synthetic chemistry. Although our focus is on XEC, our discussion of the strengths and weaknesses of different methods for introducing electrons is general to other reductive transformations.

Nickel-Catalyzed Three-Component Alkylacylation of Alkenes Enabled by a Photoactive Electron Donor-Acceptor Complex

An electron donor-acceptor complex-enabled, nickel-catalyzed three-component net-reductive 1,2-alkylacylation of alkenes is developed. This conjunctive reductive acyl cross-coupling process obviates the use of an exogenous photocatalyst and a stoichiometric metal-based reductant, affording various synthetically useful 1,3-dicarbonyl compounds in good yields with a broad substrate scope and excellent functional group tolerance. Both alkyl and acyl electrophiles are derived from the highly abundant and readily accessible carboxylic acids, making the catalytic 1,2-dicarbofunctionalization more synthetically general and sustainable.

Ni-Catalyzed Reductive 1,2-Cross-Dialkylation of Unactivated Alkenes with Two Alkyl Bromides

Cross-dialkylation of unactivated alkenes represents a significant challenge due to competitive β-hydride elimination and associated selectivity issues. Herein, a Ni-catalyzed reductive 1,2-dialkylation of unactivated aliphatic alkenes has been developed using two different alkyl bromides. The reaction proceeds smoothly under mild conditions to install two C_sp3-C_sp3 bonds onto directed aliphatic alkenes, demonstrating excellent chemo- and regioselectivity with good functional group tolerance.

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.