Photoinduced nickel-catalyzed enantioselective coupling reactions

Ni/Photoredox-Catalyzed Enantioselective Acylation of α-Bromobenzoates with Aldehydes: A Formal Approach to Aldehyde-Aldehyde Cross-Coupling

The catalytic cross-coupling of identical or similar functional groups is a cornerstone strategy for carbon-carbon bond formation, as exemplified by renowned methods, such as olefin cross-metathesis, Kolbe electrolysis, and various cross-electrophile couplings. However, similar methodologies for coupling aldehydes─fundamental building blocks in organic synthesis─remain underdeveloped. While the benzoin-type condensation, first reported in 1832, offers a reliable route for aldehyde dimerization, the chemo- and enantioselective cross-coupling of nonidentical yet similar aldehydes remains an unsolved challenge. Herein, we report a unified platform enabling highly chemo- and enantioselective cross-coupling of aldehydes. By leveraging nickel photoredox catalysis in tandem with discrete activation strategies for each aldehyde, this mechanistically distinct approach facilitates the enantioselective union of an aldehyde-derived α-oxy radical with an acyl radical, photocatalytically generated from a distinct aldehyde. This novel strategy enables modular access to enantioenriched α-oxygenated ketones with two minimally differentiated aliphatic substituents, a feat not achievable with existing chemocatalytic or biocatalytic techniques. The synthetic utility of this method is demonstrated by its application in the streamlined asymmetric synthesis of various medicinally relevant molecules. Additionally, mechanistic investigations rationalize the versatility of nickel photoredox catalysis to exploit new pathways for addressing long-standing synthetic challenges.

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.

Mechanisms of Photoredox Catalysis Featuring Nickel-Bipyridine Complexes

Metallaphotoredox catalysis can unlock useful pathways for transforming organic reactants into desirable products, largely due to the conversion of photon energy into chemical potential to drive redox and bond transformation processes. Despite the importance of these processes for cross-coupling reactions and other transformations, their mechanistic details are only superficially understood. In this review, we have provided a detailed summary of various photoredox mechanisms that have been proposed to date for Ni-bipyridine (bpy) complexes, focusing separately on photosensitized and direct excitation reaction processes. By highlighting multiple bond transformation pathways and key findings, we depict how photoredox reaction mechanisms, which ultimately define substrate scope, are themselves defined by the ground- and excited-state geometric and electronic structures of key Ni-based intermediates. We further identify knowledge gaps to motivate future mechanistic studies and the development of synergistic research approaches spanning the physical, organic, and inorganic chemistry communities.

Ligand Relay for Nickel-Catalyzed Decarbonylative Alkylation of Aroyl Chlorides

Transition metal-catalyzed direct decarboxylative transformations of aromatic carboxylic acids usually require high temperatures, which limit the substrate's scope, especially for late-stage applications. The development of the selective decarbonylative of carboxylic acid derivatives, especially the most fundamental aroyl chlorides, with stable and cheap electrophiles under mild conditions is highly desirable and meaningful, but remains challenging. Herein, a strategy of nickel-catalyzed decarbonylative alkylation of aroyl chlorides via phosphine/nitrogen ligand relay is reported. The simple phosphine ligand is found essential for the decarbonylation step, while the nitrogen ligand promotes the cross-electrophile coupling. Such a ligand relay system can effectively and orderly carry out the catalytic process at room temperature, utilizing easily available aroyl chlorides as an aryl electrophile for reductive alkylation. This discovery provides a new strategy for direct decarbonylative coupling, features operationally simple, mild conditions, and excellent functional group tolerance. The mild approach is applied to the late-stage methylation of various pharmaceuticals. Extensive experiments are carried out to provide insights into the reaction pathway and support the ligand relay process.

Dual Nickel- and Photoredox-Catalyzed Asymmetric Reductive Cross-Coupling To Access Chiral Secondary Benzylic Alcohols

Transition-metal-catalyzed asymmetric cross-coupling represents a powerful strategy for C-C bond formation and the synthesis of enantiomerically pure molecules. Here, we report a dual nickel/photoredox-catalyzed enantioselective reductive cross-coupling of aryl halides with α-bromobenzoates, readily generated from aliphatic aldehydes, to provide diverse chiral secondary benzylic alcohols that are important motifs in bioactive natural products and pharmaceuticals. This dual catalytic system features mild conditions, good functional group tolerance, broad substrate scope, excellent enantiocontrol, and avoidance of stoichiometric metal reductants, presenting great potential for late-stage functionalization of complex molecules.

Asymmetric imino-acylation of alkenes enabled by HAT-photo/nickel cocatalysis

By merging nickel-mediated facially selective aza-Heck cyclization and radical acyl C-H activation promoted by tetrabutylammonium decatungstate (TBADT) as a hydrogen atom transfer (HAT) photocatalyst, we accomplish an asymmetric imino-acylation of oxime ester-tethered alkenes with readily available aldehydes as the acyl source, enabling the synthesis of highly enantioenriched pyrrolines bearing an acyl-substituted stereogenic center under mild conditions. Preliminary mechanistic studies support a Ni(i)/Ni(ii)/Ni(iii) catalytic sequence involving the intramolecular migratory insertion of a tethered olefinic unit into the Ni(iii)-N bond as the enantiodiscriminating step.

Rapid Synthesis of β-Chiral Sulfones by Ni-Organophotocatalyzed Enantioselective Sulfonylalkenylation of Alkenes

β-Chiral sulfones are substructures widespread in drug molecules and bioactive targets and serve as important chiral synthons in organic synthesis yet are challenging to access. Herein, a three-component strategy enabled by visible-light- and Ni-catalyzed sulfonylalkenylation of styrenes for the synthesis of enantioenriched β-chiral sulfones has been developed. This dual-catalysis strategy allows for one-step skeletal assembly along with the control of enantioselectivity in the presence of a chiral ligand, providing an efficient and straightforward access to enantioenriched β-alkenyl sulfones from easily available and simple starting materials. Mechanistic investigations reveal that the reaction undergoes a chemoselective radical addition over two alkenes followed by a Ni-intercepted asymmetric C_sp³-C_sp² coupling with alkenyl halides.

Photo/Ni dual-catalyzed radical defluorinative sulfonylation to synthesize gem-difluoro allylsulfones

Radical defluorinative functionalization of α-trifluoromethyl styrenes represents an effective way toward gem-difluoroalkenes. There are general interests in developing novel synthetic protocols for defluorinative functionalization with various types of radicals. However, reports on the preparation of gem-difluoro allylsulfones via an S-centered radical pathway are limited. Herein, we developed a photo/nickel dual-catalyzed defluorinative sulfonylation that rapidly and reliably synthesizes gem-difluoro allylsulfones. The merit of this protocol is exhibited by its mild conditions and wide scope, thus providing a novel strategy for the sulfonyl radical participating in radical defluorinative coupling.

Site- and enantioselective cross-coupling of saturated N-heterocycles with carboxylic acids by cooperative Ni/photoredox catalysis

Site- and enantioselective cross-coupling of saturated N-heterocycles and carboxylic acids-two of the most abundant and versatile functionalities-to form pharmaceutically relevant α-acylated amine derivatives remains a major challenge in organic synthesis. Here, we report a general strategy for the highly site- and enantioselective α-acylation of saturated N-heterocycles with in situ-activated carboxylic acids. This modular approach exploits the hydrogen-atom-transfer reactivity of photocatalytically generated chlorine radicals in combination with asymmetric nickel catalysis to selectively functionalize cyclic α-amino C-H bonds in the presence of benzylic, allylic, acyclic α-amino, and α-oxy methylene groups. The mild and scalable protocol requires no organometallic reagents, displays excellent chemo-, site- and enantioselectivity, and is amenable to late-stage diversification, including a modular synthesis of previously inaccessible Taxol derivatives. Mechanistic studies highlight the exceptional versatility of the chiral nickel catalyst in orchestrating (i) catalytic chlorine elimination, (ii) alkyl radical capture, (iii) cross-coupling, and (iv) asymmetric induction.

Cooperative photocatalysis and l-/d-proline catalysis enables enantioselective oxidative cross-dehydrogenative coupling of acyclic benzylic secondary amines with ketones

We developed an enantioselective cross-dehydrogenative coupling of acyclic benzylic secondary amines with ketones by combining photocatalysis and l-/d-proline catalysis.

Enantioselective Radical Addition to Ketones through Lewis Acid-Enabled Photoredox Catalysis

Photocatalysis opens up a new window for carbonyl chemistry. Despite a multitude of photochemical reactions of carbonyl compounds, visible light-induced catalytic asymmetric transformations remain elusive and pose a formidable challenge. Accordingly, the development of simple, efficient, and economic catalytic systems is the ideal pursuit for chemists. Herein, we report an enantioselective radical photoaddition to ketones through a Lewis acid-enabled photoredox catalysis wherein the in situ formed chiral N,N'-dioxide/Sc(III)-ketone complex serves as a temporary photocatalyst to trigger single-electron transfer oxidation of silanes for the generation of nucleophilic radical species, including primary, secondary, and tertiary alkyl radicals, giving various enantioenriched aza-heterocycle-based tertiary alcohols in good to excellent yields and enantioselectivities. The results of electron paramagnetic resonance (EPR) and high-resolution mass spectrum (HRMS) measurements provided favorable evidence for the stereocontrolled radical addition process involved in this reaction.

The Nickel Age in Synthetic Dual Photocatalysis: A Bright Trip Toward Materials Science

Recently, the field of dual photocatalysis has grown rapidly, to become one of the most powerful tools for the functionalization of organic molecules under mild conditions. In particular, the merging of Earth-abundant nickel-based catalytic systems with visible-light-activated photoredox catalysts has allowed the development of a number of unique green synthetic approaches. This goes in the direction of ensuring an effective and sustainable chemical production, while safeguarding human health and environment. Importantly, this relatively new branch of catalysis has inspired an interdisciplinary stream of research that spans from inorganic and organic chemistry to materials science, thus establishing itself as one dominant trend in modern organic synthesis. This Review aims at illustrating the milestones on the timeline evolution of the photocatalytic systems used, with a critical analysis toward novel applications based on the use of photoactive two-dimensional carbon-based nanostructures. Lastly, forward-looking opportunities within this intriguing research field are discussed.