Nickel-Catalyzed Umpolung Arylation of Ambiphilic α-Bromoalkyl Boronic Esters

Dual Nickel- and Photoredox-Catalyzed Asymmetric Reductive Cross-Couplings: Just a Change of the Reduction System?

ConspectusIn recent years, nickel-catalyzed asymmetric coupling reactions have emerged as efficient methods for constructing chiral C(sp3) carbon centers. Numerous novel approaches have been reported to rapidly construct chiral carbon-carbon bonds through nickel-catalyzed asymmetric couplings between electrophiles and nucleophiles or asymmetric reductive cross-couplings of two different electrophiles. Building upon these advances, our group has been devoted to interrogating dual nickel- and photoredox-catalyzed asymmetric reductive cross-coupling reactions.In our endeavors over the past few years, we have successfully developed several dual Ni-/photoredox-catalyzed asymmetric reductive cross-coupling reactions involving organohalides. While some probably think that this system is just a change of the reduction system from traditional metal reductants to a photocatalysis system, a question that we also pondered at the beginning of our studies, both the achievable reaction types and mechanisms suggest a different conclusion: that this dual catalysis system has its own advantages in the chiral carbon-carbon bond formation. Even in certain asymmetric reactions where the photocatalysis regime functions only as a reducing system, the robust reducing capability of photocatalysts can effectively accelerate the regeneration of low-valent nickel species, thus expanding the selectable scope of chiral ligands. More importantly, in many transformations, besides reducing nickel catalysts, the photocatalysis system can also undertake the responsibility of alkyl radical formation, thereby establishing two coordinated, yet independent catalytic cycles. This catalytic mode has been proven to play a crucial role in achieving diverse asymmetric coupling reactions with great challenges.In this Account, we elucidate our understanding of this system based on our experience and findings. In the Introduction, we provide an overview of the main distinctions between this system and traditional Ni-catalyzed asymmetric reductive cross-couplings with metal reductants and the potential opportunities arising from these differences. Subsequently, we outline various chiral carbon-carbon bond-forming types obtained by this dual Ni/photoredox catalysis system and their mechanisms. In terms of chiral C(sp3)-C(sp2) bond formation, extensive discussion focuses on the asymmetric arylations of α-chloroboronates, α-trifluoromethyl alkyl bromides, α-bromophosphonates, and so on. In the realm of chiral C(sp3)-C(sp) bond formation, asymmetric alkynylations of α-bromophosphonates and α-trifluoromethyl alkyl bromides have been presented herein. Regarding C(sp3)-C(sp3) bond formation, we take the asymmetric alkylation of α-chloroboronates as a compelling example to illustrate the great efficiency of this dual catalysis system. This summary would enable a better grasp of the advantages of this dual catalysis system and clarify how the photocatalysis regime facilitates enantioselective transformations. We anticipate that this Account will offer valuable insights and contribute to the development of new methodologies in this field.

Mechanochemical Synthesis of α-halo Alkylboronic Esters

α-halo alkylboronic esters, acting as ambiphilic synthons, play a pivotal role as versatile intermediates in fields like pharmaceutical science and organic chemistry. The sequential transformation of carbon-boron and carbon-halogen bonds into a broad range of carbon-X bonds allows for programmable bond formation, facilitating the incorporation of multiple substituents at a single position and streamlining the synthesis of complex molecules. Nevertheless, the synthetic potential of these compounds is constrained by limited reaction patterns. Additionally, the conventional methods often necessitate the use of bulk toxic solvents, exhibit sensitivity to air/moisture, rely on expensive metal catalysts, and involve extended reaction times. In this report, a ball milling technique is introduced that overcomes these limitations, enabling the external catalyst-free multicomponent coupling of aryl diazonium salts, alkenes, and simple metal halides. This approach offers a general and straightforward method for obtaining a diverse array of α-halo alkylboronic esters, thereby paving the way for the extensive utilization of these synthons in the synthesis of fine chemicals.

Palladium-Catalyzed Cascade Heck Coupling and Allylboration of Iododiboron Compounds via Diboryl Radicals

Geminal bis(boronates) are versatile synthetic building blocks in organic chemistry. The fact that they predominantly serve as nucleophiles in the previous reports, however, has restrained their synthetic potential. Herein we disclose the ambiphilic reactivity of α-halogenated geminal bis(boronates), of which the first catalytic utilization was accomplished by merging a formal Heck cross-coupling with a highly diastereoselective allylboration of aldehydes or imines, providing a new avenue for rapid assembly of polyfunctionalized boron-containing compounds. We demonstrated that this cascade reaction is highly efficient and compatible with various functional groups, and a wide range of heterocycles. In contrast to a classical Pd(0/II) scenario, mechanistic experiments and DFT calculations have provided strong evidence for a catalytic cycle involving Pd(I)/diboryl carbon radical intermediates.

Triply Selective & Sequential Diversification at Csp 3: Expansion of Alkyl Germane Reactivity for C-C & C-Heteroatom Bond Formation

We report the triply selective and sequential diversification of a single Csp 3 carbon carrying Cl, Bpin and GeEt3 for the modular and programmable construction of sp3-rich molecules. Various functionalizations of Csp 3-Cl and Csp 3-BPin (e.g. alkylation, arylation, homologation, amination, hydroxylation) were tolerated by the Csp 3-GeEt3 group. Moreover, the methodological repertoire of alkyl germane functionalization was significantly expanded beyond the hitherto known Giese addition and arylation to alkynylation, alkenylation, cyanation, halogenation, azidation, C-S bond formation as well as the first demonstration of stereo-selective functionalization of a Csp 3-[Ge] bond.

α-Boryl Carbanions: The Influence of Geminal Heteroatoms in C-C Bond Formation

The wide applications of alpha-boryl carbanions in selective coupling with organohalides, imines/carbonyls and conjugated unsaturated substrates has become an interesting tool for organic synthesis. Strategically, the inclusion of heteroatoms, such as Si, S, N, F, Cl, Br and I in the alpha position opens a new venue towards multifunctionalities in molecular design. Here, a conceptual and practical view on powerful carbanions, containing α-silicoboron, α-thioboron, α-haloboron and α-aminoboron is given, as well as a prespective on their efficient application for selective electrophilic trapping.

Ni-Catalyzed 1,2-Alkyl Borylation and Silylation of Allenes En Route to [1,3]-Bis-Organometallic Reagents

A nickel-catalyzed multicomponent reaction that rapidly and reliably accesses [1,3]-bis-organometallic reagents from allenes is reported. The protocol exhibits a predictable regioselectivity pattern that enables the incorporation of B,B(Si) fragments across the allene backbone under mild conditions, thus offering a complementary platform for accessing polyorganometallic reagents possessing both sp2 and sp3 hybridization from readily available precursors.

Efficient Synthesis of α-Haloboronic Esters via Cu-Catalyzed Atom Transfer Radical Addition

The synthesis of α-haloboronic esters via atom transfer radical addition (ATRA) is constrained due to its limited range of compatible substrates or the need to manipulate the olefin coupling partners. Herein, we present a novel approach for their synthesis via Cu-catalyzed ATRA to vinyl boronic esters. The catalyst is proposed to mediate a traditionally inefficient halogen atom transfer of the α-boryl radical intermediate, thus significantly expanding the range of participating substrates relative to established methods. The forty-eight examples illustrate that a wide range of radical precursors, including primary, secondary, and tertiary alkyl halides, readily add across both unsubstituted and α-substituted vinyl pinacol boronic esters. Further, a one-pot, two-step protocol is presented for direct access to an array of α-functionalized products. Finally, the synthetic utility of this methodology is demonstrated in the synthesis of an ixazomib analogue.

Stereospecific and Regioselective Synthesis of E-Allylic Alcohols through Reductive Cross Coupling of Terminal Alkynes

We have developed a convergent method for the synthesis of allylic alcohols that involves a reductive coupling of terminal alkynes with α-chloro boronic esters. The new method affords allylic alcohols with excellent regioselectivity (anti-Markovnikov) and an E/Z ratio greater than 200:1. The reaction can be performed in the presence of a wide range of functional groups and has a substrate scope that complements the stoichiometric alkenylation of α-chloro boronic esters performed using alkenyl lithium and Grignard reagents. The transformation is stereospecific and allows for the robust and highly selective synthesis of chiral allylic alcohols. Our studies support a mechanism that involves hydrocupration of the alkyne and cross-coupling of the alkenyl copper intermediate with α-chloro boronic esters. Experimental evidence excludes a radical mechanism of the cross-coupling step and is consistent with the formation of a boron-ate intermediate and a 1,2-metalate shift.

Construction of α-Halogenated Boronic Esters via Visible Light-Induced C-H Bromination

α-Halogenated boronic esters are versatile building blocks that can be diversified into a wide variety of polyfunctionalized molecules. However, their synthetic potential has been hampered by limited preparation methods. Herein, we report a visible light-induced C-H bromination reaction of readily available benzyl boronic esters. This method features high yields, mild conditions, simple operation, and good functional group tolerance. The analogous chlorides and iodides can be accessed via Finkelstein reaction. Synthesis of halogenated geminal diborons has also been demonstrated.

Direct Light-Enabled Access to α-Boryl Radicals: Application in the Stereodivergent Synthesis of Allyl Boronic Esters

Operationally simple strategies to assemble boron containing organic frameworks are highly enabling in organic synthesis. While conventional retrosynthetic logic has engendered many platforms focusing on the direct formation of C-B bonds, α-boryl radicals have recently reemerged as versatile open-shell alternatives to access organoborons via adjacent C-C bond formation. Direct light-enabled α-activation is currently contingent on photo- or transition metal-catalysis activation to efficiently generate radical species. Here, we disclose a facile activation of α-halo boronic esters using only visible light and a simple Lewis base to enable homolytic scission. Intermolecular addition to styrenes facilitates the rapid construction of highly versatile E-allylic boronic esters. The simplicity of activation permits the strategic merger of this construct with selective energy transfer catalysis to enable the complimentary stereodivergent synthesis of Z-allylic boronic esters.

Understanding Formation and Roles of NiII Aryl Amido and NiIII Aryl Amido Intermediates in Ni-Catalyzed Electrochemical Aryl Amination Reactions

Ni-catalyzed electrochemical aryl amination (e-amination) is an attractive, emerging approach to building C-N bonds. Here, we report in-depth experimental and computational studies that examined the mechanism of Ni-catalyzed e-amination reactions. Key NiII-amine dibromide and NiII aryl amido intermediates were chemically synthesized and characterized. The combination of experiments and DFT calculations suggest (1) there is coordination of an amine to the NiII catalyst before the cathodic reduction and oxidative addition steps, (2) a stable NiII aryl amido intermediate is produced from the cathodic half-reaction, a critical step in controlling the selectivity between cross-coupling and undesired homo-coupling reaction pathways, (3) the diazabicycloundecene additive shifts the aryl halide oxidative addition mechanism from a NiI-based pathway to a Ni0-based pathway, and (4) redox-active bromide in the supporting electrolyte functions as a redox mediator to promote the oxidation of the stable NiII aryl amido intermediate to a NiIII aryl amido intermediate. Subsequently, the NiIII aryl amido intermediate undergoes facile reductive elimination to provide a C-N cross-coupling product at room temperature. Overall, our results provide new fundamental understandings about this e-amination reaction and guidance for further development of other Ni-catalyzed electrosynthetic reactions such as C-C and C-O cross-couplings.

Pd-Catalyzed Organometallic-Free Homologation of Arylboronic Acids Enabled by Chemoselective Transmetalation

A Pd-catalyzed homologation of arylboronic acids is reported. Halomethylboronic acid pinacol esters (Bpin) undergo a remarkably facile, yet rare, oxidative addition enabled by an α-boryl effect. Simultaneous chemoselective transmetalation allows use of these metalloid reagents for formal C₁ insertion to deliver benzyl Bpin products without the requirement for stoichiometric organometallic reagents. The utility of the process is demonstrated by stepwise C(sp³)-C(sp²) cross-coupling of the boronic ester products into diarylmethane pharmacophores and electrophile/nucleophile chemoselective cross-coupling. Control experiments that demonstrate the reactivity enhancement provided by the α-boryl effect are provided, along with a description of the limitations of the formal homologation process.

Directed Photochemically Mediated Nickel-Catalyzed (Hetero)arylation of Aliphatic C-H Bonds

Site-selective functionalization of unactivated C(sp3)-H centers is challenging because of the ubiquity and strength of alkyl C-H bonds. Herein, we disclose a position-selective C(sp3)-C(sp2) cross-coupling reaction. This process engages C(sp3)-H bonds and aryl bromides, utilizing catalytic quantities of a photoredox-capable molecule and a nickel precatalyst. Using this technology, selective C-H functionalization arises owing to a 1,6-hydrogen atom transfer (HAT) process that is guided by a pendant alcohol-anchored sulfamate ester. These transformations proceed directly from N-H bonds, in contrast to previous directed, radical-mediated, C-H arylation processes, which have relied on prior oxidation of the reactive nitrogen center in reactions with nucleophilic arenes. Moreover, these conditions promote arylation at secondary centers in good yields with excellent selectivity.

Enantioselective C(sp3)-C(sp3) Reductive Cross-Electrophile Coupling of Unactivated Alkyl Halides with α-Chloroboronates via Dual Nickel/Photoredox Catalysis

Substantial advances in enantioconvergent C(sp³)-C(sp³) bond formations have been made with nickel-catalyzed cross-coupling of racemic alkyl electrophiles with organometallic reagents or nickel-hydride-catalyzed hydrocarbonation of alkenes. Herein, we report an unprecedented enantioselective C(sp³)-C(sp³) reductive cross-coupling by the direct utilization of two different alkyl halides with dual nickel/photoredox catalysis system. This highly selective coupling of racemic α-chloroboronates and unactivated alkyl iodides furnishes chiral secondary alkyl boronic esters, which serve as useful and important intermediates in the realm of organic synthesis and enable a desirable protocol to fast construction of enantioenriched complex molecules.

Enantioselective catalytic 1,2-boronate rearrangements

[Figure: see text].

Dealkenylative Ni-Catalyzed Cross-Coupling Enabled by Tetrazine and Photoexcitation

A new and general method to functionalize the C(sp³)-C(sp²) bond of alkyl and alkene linkages has been developed, leading to the dealkenylative generation of carbon-centered radicals that can be intercepted to undergo Ni-catalyzed C(sp³)-C(sp²) cross-coupling. This one-pot protocol leverages the easily procured alkene feedstocks for organic synthesis with excellent functional group compatibility without the need for a photoredox catalyst.

Ni-Catalyzed Reductive Allylation of α-Chloroboronates to Access Homoallylic Boronates

The transition-metal-catalyzed allylation reaction is an efficient strategy for the construction of new carbon-carbon bonds alongside allyl or homoallylic functionalization. Herein we describe a Ni-catalyzed reductive allylation of α-chloroboronates to efficiently render the corresponding homoallylic boronates, which could be readily converted into valuable homoallylic alcohols or amines or 1,4-diboronates. This reaction features a broad substrate scope with good functional group compatibility that is complementary to the existing methods for the preparation of homoallylic boronates.

Dual Ni/photoredox-catalyzed asymmetric cross-coupling to access chiral benzylic boronic esters

The flourishing Ni/photoredox-catalyzed asymmetric couplings typically rely on redox-neutral reactions. In this work, we report a reductive cross-coupling of aryl iodides and α-chloroboranes under a dual catalytic regime to further enrich the metallaphotoredox chemistry. This approach proceeds under mild conditions (visible light, ambient temperature, no strong base) to access the versatile benzylic boronic esters with good functional group tolerance and excellent enantioselectivities.

Metallaphototoredox catalysis has been rarely applied to reductive cross-couplings, in contrast to typical redox-neutral methods. Here, the authors report a mild Ni/photoredox-catalyzed reductive cross-coupling of aryl iodides and α-chloroboranes, further enriching the metallaphotoredox chemistry.

Photo-induced weak base-catalyzed synthesis of α-haloboronates from vinylboronates and polyfluoroalkyl halides

An unprecedented strategy for the synthesis of sp²-α-haloboronates has been developed. Unique KOAc catalytic system, high synthetic application value of the product and no participation of metal constitute the notable features of this reaction.

Recent Advances in Radical Addition to Alkenylboron Compounds

The addition of carbon-centered radicals to alkenylboron compounds provides a useful method for the construction of organoboron reagents which are versatile reagents in chemical synthesis. While the first examples of this type or process appeared 70 years ago, until recently, attention to this type of reaction has been limited. A number of examples of this reactivity have been demonstrated recently, and strategies are emerging that allow for tuning the reagent structure in order to modulate reactivity. This review surveys recent advances in this class of reaction.

Decarboxylative Conjunctive Cross-coupling of Vinyl Boronic Esters using Metallaphotoredox Catalysis

The synthesis of complex alkyl boronic esters through conjunctive cross-coupling of vinyl boronic esters with carboxylic acids and aryl iodides is described. The reaction proceeds under mild metallaphotoredox conditions and involves an unprecedented decarboxylative radical addition/cross-coupling cascade of vinyl boronic esters. Excellent functional-group tolerance is displayed, and application of a range of carboxylic acids, including secondary α-amino acids, and aryl iodides provides efficient access to highly functionalized alkyl boronic esters. The decarboxylative conjunctive cross-coupling was also applied to the synthesis of sedum alkaloids.

Site-Selective 1,2-Dicarbofunctionalization of Vinyl Boronates through Dual Catalysis

A modular, site-selective 1,2-dicarbofunctionalization of vinyl boronates with organic halides through dual catalysis is described. This reaction proceeds under mild conditions and is characterized by excellent chemo- and regioselectivity. It thus represents a complementary new technique for preparing densely functionalized alkyl boron architectures from simple and accessible precursors.

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 Regioselective Hydroalkylation and Hydroarylation of Alkenyl Boronic Esters

Metal hydride catalyzed hydrocarbonation reactions of alkenes are an efficient approach to construct new carbon-carbon bonds from readily available alkenes. However, the regioselectivity of hydrocarbonation remains challenging to be controlled. In nickel hydride (NiH) catalyzed hydrocarbonation, linear selectivity is most often obtained because of the relative stability of the linear Ni-alkyl intermediate over its branched counterpart. Herein, we show that the boronic pinacol ester (Bpin) group directs a Ni-catalyzed hydrocarbonation to occur at its adjacent carbon center, resulting in formal branch selectivity. Both alkyl and aryl halides can be used as electrophiles in this hydrocarbonation, providing access to a wide range of secondary alkyl Bpin derivatives, which are valuable building blocks in synthetic chemistry. The utility of the method is demonstrated by the late-stage functionalization of natural products and drug molecules, the synthesis of an anticancer agent, and iterative syntheses.

Rapid Access to Highly Functionalized Alkyl Boronates by NiH-Catalyzed Remote Hydroarylation of Boron-Containing Alkenes

The direct and selective functionalization of relatively simple and readily accessible precursors to produce highly functionalized alkyl boronates is a synthetically useful process. Herein we report a NiH-catalyzed remote hydroarylation process that can, through a synergistic combination of chain walking and subsequent cross-coupling, introduce an aryl group at the adjacent carbon atom of alkyl boronates under mild conditions. By means of a preliminary experiment with moderate enantioselectivity, it was shown that an asymmetric version could also be realized.

Selective, Intermolecular Alkylarylation of Alkenes via Photoredox/Nickel Dual Catalysis

A regioselective, intermolecular 1,2-alkylarylation of alkenes with aryl halides and alkyl oxalates has been developed via photoredox/nickel dual catalysis. This dual-catalytic protocol involves a radical relay process, where radical addition is followed by a nickel-assisted coupling, forging two consecutive C-C bonds in a single operation. The mild and redox-neutral conditions allow for good compatibility in the scope of olefins, (hetero)aryl halides, as well as alkyl oxalates.

Deaminative Reductive Cross-Electrophile Couplings of Alkylpyridinium Salts and Aryl Bromides

A nickel-catalyzed reductive cross-coupling of alkylpyridinium salts and aryl bromides has been developed using Mn as the reductant. Both primary and secondary alkylpyridinium salts can be used, and high functional group and heterocycle tolerance is observed, including for protic groups. Mechanistic studies indicate the formation of an alkyl radical, and controlling its fate was key to the success of this reaction.

Harnessing Alkylpyridinium Salts as Electrophiles in Deaminative Alkyl-Alkyl Cross-Couplings

A Negishi cross-coupling of alkylpyridinium salts and alkylzinc halides has been developed. This is the first example of alkyl-alkyl bond formation via cross-coupling of an alkyl amine derivative with an unactivated alkyl group, and allows both primary and secondary alkylpyridinium salts to react with primary alkylzinc halides with high functional group tolerance. When combined with formation of the pyridinium salts from primary amines, this method enables the noncanonical transformation of NH2 groups into a wide range of alkyl substituents with broad functional group tolerance.