Exploring visible light for carbon–nitrogen and carbon–oxygen bond formation via nickel catalysis

Abstract

Heteroatom-containing motifs are one of the most privileged scaffolds for pharmaceuticals, agrochemicals, and functional materials. Transition-metal-catalyzed carbon–heteroatom bond-forming reactions have emerged as an indispensable synthetic tool for the rapid assembly of these valuable skeletons. Despite impressive progress, the development of general and efficient methods for the catalytic construction of carbon–heteroatom bonds with Earth-abundant catalysts under mild conditions is still highly desirable. Utilizing the new and unique reactivity uncovered by photoexcitation, recently, exciting progress has been made in the area of visible-light-driven nickel-catalyzed carbon–heteroatom bond-forming reactions, enabling facile access to diverse carbon–heteroatom bonds under exceptionally mild conditions. In this review, we highlight the recent synthetic methodology development for the formation of C–N and C–O bonds via visible-light-driven high-valent nickel complexes or photoexcited nickel complexes, with in-depth discussions with reaction designs and mechanistic scenarios.

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Shengqing Zhu

Shengqing Zhu was born in Jiangxi, China. He received his BS degree from the University of Jinan in 2012 and his Ph.D. degree from Shanghai Institute of Organic Chemistry (SIOC) under the supervision of Professor Feng-Ling Qing in 2017. He then joined Donghua University working with Professor Lingling Chu as a Junior Researcher. His research interest focuses on the fields of transition metal catalysis and fluorine chemistry.

Huan Li

Huan Li was born in 1992 in Anhui Province (China). She obtained her MS in Chemistry from Donghua University in 2018 and then joined the group of Prof. Lingling Chu at Donghua University. Her research focused on the development of highly efficient Ni-catalyzed radical tandem reactions of alkynes. She received her Ph.D. degree in 2021.

Yingying Li

Yingying Li was born in 1998 in Shandong Province (China). She obtained her BSc in chemistry from Dezhou University. In 2021, she started her Ph.D. studies at Donghua University under the guidance of Prof. Lingling Chu. Her research focuses on the development of efficient Ni-catalyzed radical asymmetric coupling reactions.

Zhonghou Huang

Zhonghou Huang was born in 1998 in Jiangxi Province (China). He obtained his BSc in chemistry from Jiujiang University. In 2021, he joined the group of Prof. Lingling Chu at Donghua University, where he is currently pursuing his MSc degree. His research focuses on the development of highly efficient Cu-catalyzed radical difunctionalization of alkenes.

Lingling Chu

Lingling Chu was born in Anhui Province (China). She obtained her BSc in Engineering from Hefei University of Technology in 2007. That same year, she began her Ph.D. studies at the lab of Professor Feng-Ling Qing at Shanghai Institute of Organic Chemistry (SIOC), where she focused on the development of new Cu-mediated oxidative trifluoromethylation reactions. In 2013, she joined the lab of Professor David MacMillan at Princeton University as a Postdoctoral Research Associate, working on photoredox catalysis. In 2016, she began her independent career at Donghua University. Her research interest focuses on the fields of transition metal catalysis and radical chemistry.

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1. Introduction

The heart of chemical synthesis relies on constructing chemical bonds. Due to their remarkable capability to forge carbon–carbon or carbon–heteroatom bonds thus enabling the streamlined synthesis of valuable complex molecules with simple starting materials, transition-metal catalyzed cross-coupling reactions have emerged as an indispensable synthetic tool in modern chemical synthesis.^1–3 Significant achievements have been made via palladium catalysis, as notably exemplified by the 2010 Nobel Prize.^4–6 Nevertheless, increasing interest has been devoted to searching more Earth-abundant and less expensive catalysts. Nickel stands out as one of the most prominent complementary catalysts to palladium.^7–15 Besides the advantageous cost and sustainability, more importantly, nickel catalysts exhibit unique properties compared with palladium. For instance, nickel is more electropositive, thus tending to undergo facile oxidative addition with electrophiles. Furthermore, the readily accessible ability of nickel to various oxidation states [0, +1, +2, +3, +4] facilitates its engagement with various types of substrates, via either a two-electron or a single-electron pathway. These unique features of nickel have offered opportunities to innovate retrosynthetic strategies for valuable functionalized molecules. A tremendous achievement is merging nickel catalysis with visible-light photocatalysis to uncover new reactivity with clean and sustainable light energy.^16–27

During the last decade, visible-light-induced photocatalysis has rapidly emerged as a powerful synthetic platform for the development of valuable yet challenging transformations, by harvesting photonic energy.^16,28–33 Upon light excitation, a photoexcited catalyst or sensitizer is capable of facilitating a single-electron transfer (SET) event with organic molecules to generate highly reactive open-shell radical species under exceptionally mild conditions. Notably, such a photochemical process can engage with transition metal catalysis to modulate the oxidation state of the metal complex via the SET process or the excitation state of the metal complex via the energy transfer (EnT) process,^34–36 thereby facilitating the construction of carbon–carbon or carbon–heteroatom bonds which is typically challenging under the traditional thermal conditions. Not surprisingly, notable progress has been made in the area of dual photoredox and transition metal catalysis (i.e. metallaphotoredox),^{16–27,37–39} with a remarkable portion focusing on nickel catalysis owing to the unique properties of nickel mentioned above.^16–27 Led by the seminal work reported by Molander,⁴⁰ MacMillan and Doyle,⁴¹ photo/nickel dual catalysis has offered elegant solutions to the significantly challenging carbon–heteroatom reductive elimination step from nickel complexes, thereby enabling the development of valuable carbon–heteroatom bond-forming protocols to deliver complex heteroatom-containing motifs from simple starting materials under exceptionally mild and operationally simple conditions.^{19,23,42–44}

This review aims to highlight the recent synthetic methodology development for the formation of C–N and C–O bonds enabled by visible-light driven nickel catalysis (Fig. 1). Synthetic applications and mechanistic discussions of these important studies are included. This review is first organized according to the bond-forming types. In each section, reactions are then classified based on mechanistic pathways: high-valent nickel species via the SET process and photoexcited nickel species via the EnT process.

Fig. 1 Nickel catalyzed C–heteroatom coupling reactions.

2. C–N bond-forming reactions

2.1 Significance and challenges

Nitrogen atoms are not only critical to the molecules of life, but also present in numerous biologically active natural products, pharmaceuticals, and functional materials, as well as common ligands for organic synthesis.^45–47 Therefore, the development of efficient methods for the selective construction of C–N bonds is highly sought-after. Transition-metal-catalyzed C–N cross-couplings represent one of the most widely employed protocols for forging C–N bonds.^48–52 Following the landmark discovery of Ullmann C–N coupling with stoichiometric copper salts,^53,54 impressive achievements have been made in the C–N couplings via copper catalysis^55–57 and palladium catalysis.^48,58–61 Due to the inherent properties of palladium and copper catalysts, these reactions generally require the use of high-temperature or well-designed ligands. Recently, Earth-abundant nickel catalysis has emerged as a powerful synthetic platform for C–C bond-forming reactions. However, parallel catalytic C–N couplings via nickel catalysis remain challenging, due to the thermodynamically unfavorable C–N reductive elimination step from nickel complexes.⁶² The development of photoredox catalysis has provided an elegant solution to this challenge: (i) SET-enabled access to Ni(III) species and (ii) energy transfer-facilitated photoexcited nickel species (Scheme 1).

Scheme 1 Photoinduced C–N reductive elimination via nickel complexes.

2.2 C–N bond formation from Ni(III) enabled by SET

In 2015, the Jamison group elegantly disclosed the first example of C–N bond-forming reactions via dual photoredox and nickel catalysis (Scheme 2).⁶³ With a catalytic combination of Ru(bpy)₃(PF₆)₂ and Ni(COD)₂/N-heterocyclic carbene (NHC), irradiation of the reaction mixture of 2-iodoacetanilides and mono-substituted alkenes in the presence of triethylamine with visible light furnished a series of 3-substituted indolines with high efficiency and excellent regioselectivity under mild conditions. The use of N-acetyl-protected iodoanilines and triethylamine as a base was found to be important for the high efficiency of this reaction. Both mono-substituted aliphatic and aryl alkenes were applicable, offering a one-step synthetic strategy for the construction of biologically active motifs from simple starting materials; nevertheless, internal and 1,1-disubstituted alkenes were not suitable coupling partners. The utilization of synergistic photoredox and Ni(0)/Ipr catalysis is mechanistically advantageous: facilitating the challenging C(sp³)–N reductive elimination over the undesired β-hydride elimination. Control reactions with stoichiometric amounts of Ni(cod)₂/Ipr and exogenous oxidants suggested that high-valent Ni(III) species were likely involved. Mechanistically, the authors proposed that the oxidative addition of 2-iodoacetanilide with Ni(0) gave Ar–Ni(II) species, followed by migratory insertion with an alkene to form alkyl-Ni(II) species 2-III. Then, single-electron oxidation of 2-III by photoexcited *Ru(bpy)₃²⁺ [E_p = 0.77 V vs. SCE], together with intramolecular ligand exchange, delivered Ni(III) amido species 2-IV. Subsequently, 2-IV underwent a facile reductive elimination to furnish the indoline product and Ni(I) species 2-V. Finally, SET reduction of Ni(I) 2-V by the reducing state of Ru(I) regenerated Ni(0) and the ground-state Ru(II) to close the two catalytic cycles. An interesting aspect of this transformation is that the photocatalyst is responsible for the oxidation and reduction of organonickel species, other than generating organic radical species, and the two critical SET events have been further supported by the cyclic voltammetry (CV) studies of nickel species and redox potential data of the photocatalyst.

Scheme 2 Jamison's indoline synthesis via 2-iodoacetanilide and alkene coupling.

A general protocol for the construction of aryl amines is direct cross-coupling of aryl halides with amines via dual photoredox and nickel catalysis. In 2016, MacMillan and Buchwald's groups reported a light-facilitated nickel-catalyzed amination of aryl halides with amines at ambient temperature (Scheme 3).⁶⁴ In the presence of catalytic Ir[dF(CF₃)ppy]₂(dtbbpy)PF₆ and NiBr₂·glyme with diazabicyclo[2.2.2]octane (DABCO) as a base, both primary and secondary alkyl amines were efficiently coupled with a wide range of aryl bromides, including structurally complex and druglike molecules, to construct of a wide array of valuable C–N bonds. In contrast to the Pd-catalyzed amination reactions, this photochemical protocol features no use of high temperature, strong bases, or exogenous well-designed structurally complex ligands, offering a ligand-free and synthetically advantageous alternative. Mechanistically, oxidative addition of aryl bromide with in situ generated Ni(0) afforded Ar–Ni(II)–Br 3-II, followed by subsequent ligand exchange with an amine to give Ni(II) aryl amido species 3-III. Then, SET oxidation of 3-III by photoexcited *Ir(III) (E^red_1/2 [*Ir^III/Ir^II] = +1.21 V vs. SCE) produced the critical Ni(III) amido species IV, which underwent a facile reductive elimination to furnish the desired anilines as well as Ni(I) species 3-V. Finally, a SET between Ni(I) 3-V and the reduced Ir(II) (E^ox_1/2 [Ir^III/Ir^II] = −1.37 V vs. SCE) regenerated Ni(0) and the ground state Ir(III) catalyst to close the two cycles.

Scheme 3 MacMillan and Buchwald's amination with aryl bromides.

Later, Johannes and coworkers reported the amination of aryl halides with primary aryl amines via dual photoredox and nickel catalysis (Scheme 4).⁶⁵ This photochemical protocol employed Ir[dF(CF₃)ppy]₂ (dtbbpy)PF₆ as a photocatalyst, NiBr₂(dtbbpy) as a nickel catalyst, and Et₃N as a base, enabling the straightforward synthesis of a diverse range of pharmaceutically valuable secondary diaryl amines under mild and operationally simple conditions. Electronically and sterically diverse heteroaryl and aryl iodides were applicable with moderate to high efficiency, whilst aryl bromides gave lower yields. Nevertheless, neither secondary aryl amines nor aliphatic amines were suitable coupling partners. Considering the distinct reactivity, the authors proposed that SET oxidation of aniline (E^ox_1/2 = +0.8 V vs. SCE) by photoexcited *Ir(III) (E^red_1/2 [*Ir^III/Ir^II] = +1.21 V vs. SCE) afforded the aniline radical cation 4-VIII, followed by subsequent deprotonation by Et₃N to release the anilino radical 4-IX. Then, the anilino radical 4-IX was captured by the Ni(I)X complex I to give the Ni(II) amido species 4-II. A subsequent SET event between the Ni(II) amido species 4-II and the reducing Ir(II) afforded the Ni(I) amido species 4-III and regenerated ground-state Ir(III). Then, oxidative addition of aryl iodide to Ni(I) amido 4-III yielded Ni(III) aryl amido, which subsequently underwent a facile reductive elimination to afford the diaryl amine product as well as Ni(I)X to close the nickel cycle (path A). Another step-wise oxidative addition pathway starting with Ni(0) was also proposed, considering the observed high efficiency with a Ni(0) precatalyst (path B). Presumably, a radical trap of the anilino radical by Ni(0) produced the Ni(I) amido species, which might undergo excitation to form a photoexcited, highly reducing *Ni(I) amido species 4-VI. Subsequent SET reduction of aryl iodide by 4-VI yielded the ionic Ni(II) amido species 4-VII and aryl radical. Radical recombination followed by reductive elimination finally yielded the amination products. Based on the observed reductive dehalogenation of aryl halides and low efficiency of alkyl amines, the step-wise oxidative addition pathway seemed to be more plausible in this specific system.

Scheme 4 Johannes's C–N coupling of aryl halides with primary aryl amines.

In 2018, Vannucci and coworkers described a similar photochemical amination of aryl iodides and anilines in the presence of Ru(phen)₃²⁺ as a photocatalyst, [Ni(tpy)(py)](PF₃)₂ ⁶⁶ as a nickel catalyst, and K₃PO₄ as a base (Scheme 5).⁶⁷ Compared to Johannes's reaction,⁶⁵ this protocol demonstrated a broader scope with regard to aryl amines. Both primary and secondary aryl amines were suitable coupling partners, affording a series of aryl amines with synthetically useful yields. Mechanistically, the authors proposed a partially different reaction pathway: SET oxidation of aniline by photoexcited *Ru(phen)₃²⁺ followed by deprotonation afforded the anilino radical, which was then trapped by the (aryl)Ni(II) complex, generated via oxidative addition of aryl iodide to Ni(0), to afford the critical Ni(III) aryl amino species. The final SET event between Ni(I) and reducing Ru(I) closed the two catalytic cycles.

Scheme 5 Vannucci's diaryl amine synthesis via aryl iodides and anilines.

Besides aryl and alkyl amines, efforts have been devoted to exploring other aminating agents for light-driven nickel-catalyzed amination reactions. In 2018, Johannes and coworkers reported a reductive amination of aryl electrophiles with aryl azides as amine sources in the presence of a stoichiometric amount of reductant (Scheme 6).⁶⁸ With a catalytic combination of Ru(bpy)₃Cl₂·6H₂O, NiBr₂·glyme, and bathophenanthroline (BPhen), this protocol employed N,N-diisopropylethylamine (DIPEA) as a sacrificial electron donor and Hantzsch ester (HE) as an additive. Compared to previous protocols, this reductive protocol utilized less strongly oxidizing photocatalysts, thus demonstrating more broad substrate compatibility, particularly those containing heterocycle motifs. A wide range of functional (hetero)aryl iodides, bromides, chlorides, and triflates underwent selective C–N couplings with (hetero)aryl azides to forge the corresponding diarylamines with high efficiency and functional group compatibility. With regard to the reaction pathway, photoexcited *Ru(II) was SET reduced by DIPEA to generate Ru(I) 6-III. The highly reducing Ru(I) was capable of reducing the aryl azide into a related radical anion, yielding the anilino radical after the spontaneous loss of one molecule of N₂. Similar to previous literature, the anilino radical was then trapped by in situ generated aryl-Ni(II) to afford the critical Ni(III) aryl amido complex 6-VI.

Scheme 6 Johannes's aryl amination with aryl azides.

During the last several years, visible-light-induced organic photocatalysis has emerged as a versatile and metal-free platform for numerous synthetic transformations in chemical synthesis.³⁰ Nevertheless, most of the currently known photoredox/nickel dual catalyzed C–N coupling reactions rely on the use of transition metal-based photocatalysts, such as iridium and ruthenium polypyridyl complexes, with only a few exceptions. In 2017, Miyake and coworkers reported a synergistic organic photoredox/nickel-catalyzed C–N coupling with aryl bromides and amines (Scheme 7).⁶⁹ With the highly reducing phenoxazine-based PC-I or dihydrophenazine-based PC-II, which exhibited similar photophysical properties to Ir(ppy)₃, as the organic photocatalyst, the C–N coupling of aryl bromides and alkyl amines proceeded smoothly in the presence of 5 mol% NiBr₂·glyme with DABCO as a base and DMA as a solvent, affording related aryl amines in 68%–98% yields.

Scheme 7 Miyake's C–N coupling of aryl bromides with an organic photoredox catalyst.

In 2022, Zhu and Huang reported a similar organic photoredox/nickel catalyzed C–N coupling reactions of aryl bromides with an organic photocatalyst derived from the anthrazoline framework (Scheme 8).⁷⁰ With similar conditions to Miyake's protocol,⁶⁹ a wide range of linear and cyclic alkyl amines, as well as simple sulfonamides and benzamides, were suitable coupling partners with moderate to high efficiency. Investigations on the photophysical properties showed that the photoexcited anthrazoline catalyst PC-III exhibits strong oxidizing ability, similar to metal-based photocatalysts Ru(bpy)₃²⁺ and Ir[dF(CF₃)₂ppy]₂(dtbbpy)⁺. As such, the authors reasoned that PC-III could undergo reductive quenching rather than oxidative quenching in photochemical reactions.

Scheme 8 Zhu and Huang's anthrazoline-catalyzed amination of aryl halides.

2.3 C–N bond-formation with photoexcited Ni(II) enabled by energy transfer

By utilizing the unique reactivity of triplet states, energy transfer photocatalysis has emerged as a powerful platform for the construction of versatile chemical bonds in photochemistry.^34–36 Generally, photoexcited photocatalysts or sensitizers facilitate energy transfer to the ground-state organic substrates. Recently, similar energy transfer from photosensitizers to metal complexes to afford the excited states of transition metal complexes has been discovered and exploited to develop versatile cross-coupling reactions that are difficult to achieve via the ground state of transition metal complexes. In contrast to indirect excitation which requires a photocatalyst or photosensitizer, metal complexes can undergo direct excitation to generate triplet states without exogenous photosensitizers. As a representative example, Fu and Peters have reported light-driven Cu-catalyzed Ullman couplings at ambient temperature.⁷¹ Notably, the unique photophysical and photochemical properties of the excited states of nickel complexes, generated via direct or indirect photoexcitation, can facilitate many bond-forming transformations including C–N couplings.

2.3.1 Energy transfer via indirect excitation. Indirect excitation relies on the use of an exogenous photocatalyst or photosensitizer for absorbing photons. The first example of a photoinduced nickel-catalyzed amination reaction via energy transfer was disclosed by MacMillan and coworkers in 2018 (Scheme 9).⁷² With the combination of Ir(ppy)₂(bpy)PF₆ as a photocatalyst, NiCl₂·glyme as a nickel catalyst, and tetramethylguanidine (TMG) as a base, aryl sulfonamide underwent smooth couplings with a series of heteroaryl and aryl bromides via direct excitation of blue LEDs, affording a wide range of biologically valuable N-aryl sulfonamide products under mild conditions. Two sets of reaction conditions, with or without ligands, were developed: the ligand-free system required a higher temperature (68 °C) whilst the ligated system with 1 mol% of dtbbpy proceeded with higher yields at room temperature. Control reactions showed that the triplet energy of the photocatalyst was related to the reaction efficiency, and only a small amount of C–N coupling product was observed in the absence of a photocatalyst. Mechanistically, energy transfer between (aryl)Ni(II), generated via oxidation addition of aryl bromide to Ni(0), and the photoexcited catalyst produced the excited state of *(aryl)Ni(II) 9-II. Subsequent C–N reductive elimination from 9-II delivered the desired amination product and regenerated the Ni(0) species. By utilizing high-throughput evaluation, furthermore, the authors demonstrated a facile synthesis of dabrafenib (a selective B-Rafkinase inhibitor)⁷³ in 57% yield from readily accessible starting materials via this photosensitized nickel-catalyzed amination protocol.

Scheme 9 MacMillan's C–N coupling of aryl electrophiles with sulfonamides.

Later, Reddy and coworkers expanded the amine scope to carbamates under similar conditions (Scheme 10).⁷⁴ Control reactions indicated that photocatalyst, nickel catalyst, and visible light were all essential to this transformation, while they didn't mention whether ligand or base was required or not. Under optimal conditions, benzyl carbamates and tert-butyl carbamates underwent efficient couplings with (hetero)aryl bromides to produce a wide range of N-(hetero)aryl carbamates.

Scheme 10 Reddy's N-arylation with carbamates.

In 2019, Miyake and coworkers reported an example of photoredox/nickel dual-catalyzed C–N couplings of aryl halides and amines via an energy transfer process (Scheme 11).⁷⁵ Utilizing [Ru(bpy)₃]Cl₂ as the photocatalyst, the authors performed detailed crystallography and spectroscopic binding studies to support the Förster-type energy transfer from the excited state photocatalyst to the nickel–amine complex. The resulting excited state nickel–amine complex would undergo direct reductive elimination to afford the desired amination products. Together with quantitative Förster theory to calculate the EnT rate constant, interestingly, they were able to predict organic phenoxazine PC-I as a more efficient energy transfer photocatalyst for the photochemical nickel-catalyzed amination reactions of aryl halides. Such a prediction was evaluated through the parallel reactions of 13 substrates with these two photocatalysts, in which organic phenoxazine PC-I generally afforded higher yields than [Ru(bpy)₃]Cl₂.

Scheme 11 Miyake's C–N coupling of aryl halides with amines enabled by energy transfer.

In 2020, Roizen and coworkers disclosed a photoinduced nickel-catalyzed C–N bond-forming reaction of aryl halides with sulfamides (Scheme 12).⁷⁶ With a combination of [Ir(ppy)₂(dtbbpy)]PF₆ and NiBr₂·glyme as catalysts, DBU as the base, diverse electron-deficient (hetero)aryl halides are smoothly coupled with sulfamide without exogenous ligands, enabling mild synthesis of N-(hetero)aryl sulfamides with high efficiency. The reactions of electron-rich aryl halides needed the use of dtbbpy as the ligand and ethanol as the solvent. The authors found that the reaction efficiency was correlated with the triplet-state energy of the photocatalyst, which further supported the involvement of energy transfer between the excited-state photocatalyst and the nickel complexes. Accordingly, photocatalysts possessing triplet-state energies in the range of 46.2–49.2 kcal mol⁻¹ offered high efficiency in this transformation.

Scheme 12 Roizen's N-arylation of sulfamides.

2.3.2 Energy transfer via direct excitation. In contrast to indirect excitation, direct excitation proceeds via light-induced photosensitization of metal complexes to their excited states without an exogenous photocatalyst or photosensitizer. Considering the light absorbance ability, a wavelength range between 360 and 400 nm is typically required from the direct excitation mode.

In 2018, Miyake and coworkers disclosed a light-driven Ni-catalyzed amination of aryl halides (Scheme 13).⁷⁷ With the simple NiBr₂·3H₂O as the catalyst and 365 nm ultraviolet light as the light source, the solution of aryl halides and amines in DMAc underwent efficient C–N couplings to afford a series of aryl amines under mild conditions. No added photocatalysts, ligands, or bases were required, as the amine substrate served as a potential base and ligand. A wide range of primary and secondary alkyl amines were suitable coupling partners. With regard to aryl halides, electron-poor aryl halides worked well, whilst electron-rich aryl and heteroaryl halides functioned with low efficiency, even with an exogenous quinuclidine base. Presumably, this reaction proceeded via direct excitation of the in situ generated nickel–amine complex [Ni(II)Br₂(morph)_n] 13-I. A photoinduced ligand to metal charge transfer (LMCT) of 13-I led to the formation of the Ni(I) species 13-II and an amine radical cation. Association of 13-III and the amine radical cation, followed by neutralization with a base formed the corresponding ion pair 13-IV. Then, the authors proposed two alternative pathways for the formation of products from 13-IV. On one hand, the addition of reactive amino radicals to an aryl halide, associated with bromine radical rejection, afforded the amination product. The subsequent capture of the bromine radical by Ni(I) gave Ni(II)Br to finish the nickel cycle. On the other hand, oxidative addition of aryl halide to the Ni(I) of the ion pair 13-IV formed the Ni(III) ion pair species 13-VII, followed by reductive elimination, to give the C–N product. Detailed DFT calculations supported the formation of amino radicals and shed light on the potential pathways. Also, kinetic isotope studies suggested that the dissociation of a N–H bond could be the rate-determining step in this photocatalyst-free amination reaction.

Scheme 13 Miyake's amination of aryl halides via photoexcited nickel complexes.

In 2022, Cheng and Jiang groups reported a 420 nm LED facilitated Ni-catalyzed C–N cross-coupling reaction of aryl iodides (Scheme 14).⁷⁸ With NiBr₂·glyme as the catalyst, dtbbpy as the ligand and DBU as the base, this protocol enabled the C–N bond-forming reactions with a wide range of nitrogen-based nucleophiles, including amines, amides, and sulphonamides. Both electron-poor and -rich aryl iodides worked with high efficiency. Interestingly, 420 nm LEDs afforded higher efficiency than 395 nm LEDs. DBU played an important role in this reaction, as switching to other inorganic bases or no bases led to trace products. The roles of DBU were supported by UV-vis and electron paramagnetic resonance (EPR) studies, which suggested that ligation of DBU to Ni(II) afforded the Ni(II)-DBU complex, followed by a photoinduced LMCT process to generate the active Ni(I)Br species. Subsequently, a Ni(I)/Ni(III) cycle involving oxidative addition to Ni(I), ligand exchange, and reductive elimination from Ni(III) was proposed.

Scheme 14 Cheng and Jiang's amination of aryl iodides.

In 2022, Xue and Xiao utilized a light-induced Ni-catalyzed C–N coupling of aryl halides and amines with a Ni(II)-bipyridine complex as precatalyst I in the absence of added photocatalyst (Scheme 15).⁷⁹ With 10 mol% of the Ni(II)-bipyridine complex, stoichiometric DBU as the base, and 390–395 nm purple LED as the light source, this C–N coupling reaction demonstrated excellent compatibility of functional groups and a broad substrate scope with regard to both aryl halides and alkyl amines. Besides aryl bromides, a wide range of electron-rich and -poor (hetero)aryl chlorides, including those derived from biologically active agents, proved to be suitable, undergoing efficient couplings with linear and cyclic primary and secondary alkyl amines. Visible light and elevated reaction temperature were needed for the success of this transformation, and no reactions were observed in the dark with heating. Upon photoexcitation, presumably, precatalyst 15-I underwent homolytic cleavage to generate the ligated Ni(I) species 15-II, followed by oxidative addition with aryl halide to afford aryl-Ni(III) 15-III. Subsequent ligand exchange by the amine, facilitated by the DBU base, formed Ni(III) amido species 15-IV, which then underwent facile reductive elimination to yield the corresponding aryl amine product and regenerate Ni(I). Detailed mechanistic studies, including UV-Vis and EPR studies, were performed to support the proposed Ni(I)/(III) cycle.

Scheme 15 Xue's amination of aryl halides with alkyl amines.

Utilization of chiral alkyl amines and amino acid esters as nitrogen sources is capable of facilitating the straightforward synthesis of pharmaceutically important chiral N-aryl amines and amino acids. However, the stereogenic centers of these chiral amino esters are prone to undergo racemization in the presence of transition metal salts or strong bases, thus leveraging significant synthetic challenges to C–N couplings with chiral amino esters. In 2021, Xue and coworkers reported a light-facilitated nickel-catalyzed C–N coupling of aryl bromides with chiral amines in the absence of added photosensitizers (Scheme 16).⁸⁰ With a catalytic amount of Ni(OAc)₂ and 4,4′-dimethylbipyridine under the irradiation of 395 nm light, chiral amines were selectively coupled with aryl bromides in the presence of DBU, affording the N-arylated products with good efficiency and complete enantiomeric retention. Moreover, mechanistic studies supported a Ni(I)/Ni(III) catalytic cycle, in which oxidative addition of ArBr to a ligated Ni(I) species gave the Ar–Ni(III)–N complex, which then underwent reductive elimination to form the C–N bond and regenerate the Ni(I) species. The role of light is to generate Ni(I) species from the Ni(II) complex, thus maintaining efficient catalytic turnover.

Scheme 16 Xue's C–N coupling of aryl bromides with chiral amines.

Nitroarenes are versatile building blocks in organic synthesis. In 2021, Xue and coworkers demonstrated a photoinduced Ni-catalyzed reductive cross-coupling of aryl halides with nitroarenes as the nitrogen source under the irradiation of purple light (Scheme 17).⁸¹ In the presence of the Ni(II)-aryl complex [Ni]-1 as the catalyst and DIPEA as the sacrificial electron donor, (hetero)aryl bromides and electron-deficient aryl chlorides were efficiently coupled with nitroarenes at 70 °C, furnishing diaryl amines without added photocatalysts or pre-reduction step of nitroarenes. Control reactions confirmed the nitroso compound as a possible intermediate, similar to previous Ni-catalyzed amination work with nitroarenes under thermal conditions.^82,83 Further detailed mechanistic studies suggested that a Ni(I)/N(III) cycle was involved.

Scheme 17 Xue's amination of aryl halides with nitroarenes.

2.4 C–N bond-forming reactions with heterogeneous materials

Although significant progress has been made via the use of transition metal-based photocatalysts or organic photocatalysts, these photocatalysts suffer from incompatibility or catalyst deactivation under some strong basic or acidic conditions. In this context, increasing attention has been devoted to the use of semiconductor materials, which are not only photo- and chemically stable but also capable of adapting two independent redox events on one particle surface, thus facilitating many photochemical transformations.

In 2019, König and coworkers utilized mesoporous graphitic carbon nitride (mpg-CN) as an organic semiconductor photocatalyst to develop a light-induced nickel-catalyzed C–N coupling of aryl halides with amines under mild conditions (Scheme 18).⁸⁴ Upon excitation with a 455 nm LED, alkyl amines, aryl amines, and sulfonamides were efficiently coupled with aryl bromides and chlorides to afford diverse aryl amines. A distinct advantage of this mpg-CN/Ni dual catalytic system was easy to scale up with readily recovered and recycled semiconductor materials via simple centrifugation. More importantly, the photoexcited mpg-CN showed a broad redox window of 2.7 V (span from approximately +1.2 V to −1.5 V versus SCE), compared to common transition metal-based photocatalysts and organic photocatalysts.

Scheme 18 König's amination of aryl halides.

Typically, electron-rich aryl halides demonstrate low efficiency in dual photo/nickel-catalyzed C–N bond-forming reactions, due to their slow oxidation addition to nickel species thus leading to the formation of nickel-black and deactivation of the nickel catalyst. In 2020, Pieber and coworkers utilized a carbon nitride CN-OA-m photocatalyst to avoid nickel catalyst deactivation and developed a 450 nm LED-induced Ni-catalyzed amination of aryl halides with amines (Scheme 19).⁸⁵ A broad range of electron-poor, -neutral, and -rich aryl bromides as well as electron-deficient aryl chlorides worked with moderate to high efficiency, notably with reliable reproducibility. The authors found that irradiation of the carbon nitride CN-OA-m with longer wavelength visible light (450 nm) could decelerate light-mediated reductive elimination. Furthermore, performing the reactions at high concentrations could increase the rate of oxidative addition, taken together, thus avoiding the aggregation and deactivation of the nickel catalyst to improve the coupling efficiency of C–N bond-forming reactions.

Scheme 19 Pieber's C–N coupling reaction with a carbon nitride photocatalyst.

Recently, covalent organic frameworks (COFs) have been reported as efficient visible-light heterogeneous photocatalysts for a plethora of photochemical transformations.⁸⁶ In 2022, Thomas and Pieber reported a dual COFs/nickel catalyzed C–N cross-coupling reactions with 440 nm blue LEDs (Scheme 20).⁸⁷ The authors synthesized a novel acridine-based COFs and studied their light absorption and charge separation properties. Notably, the use of 2 mol% Tp-Acr COF and 5 mol% NiBr₂·3H₂O was capable of facilitating the C–N coupling of pyrrolidine and electron-poor aryl bromides in DMAc with high yields. Further investigations on the reusability of the Tp-Acr COF photocatalyst were also demonstrated.

Scheme 20 Thomas and Pieber's C–N coupling reactions enabled by COFs.

The efficient synthesis of primary anilines remains a challenge in chemical synthesis. In 2022, Reisner and coworkers reported a photocatalytic C–N coupling of aryl halides with sodium azide as the nitrogen source in the presence of stoichiometric Et₃N as an electron donor (Scheme 21).⁸⁸ In contrast to previous work, this reaction employed a heterogeneous mesoporous carbon nitride material with nickel(II) deposited (Ni-mpg-CN_x) as a photocatalyst, which can be easily synthesized and, more importantly, readily recovered and reused several times without a significant loss in the catalytic efficiency. The reaction conditions were compatible with a wide range of aryl halides including aryl iodides and bromides, while aryl chlorides exhibited low yields. Kinetic studies and DFT calculations suggested that Ni-mpg-CN_x played two roles in this transformation: (i) mediating the C–N coupling to form aryl azide via a Ni(I)/Ni(III) cycle; (ii) facilitating the photoreduction of aryl azides to deliver the final primary anilines.

Scheme 21 Reisner's amination of aryl halides with sodium azide.

2.5 Large-scale C–N bond-forming reactions

Although impressive achievement has been made in the area of visible-light-driven nickel catalysis to enable a plethora of valuable transformations under mild and operationally simple conditions, implementation of these reactions at gram- or kilo-gram scales remains a significant challenge, due to the inherent attenuation of light by the Beer–Lambert law. To address this challenge, in 2019, Harper and coworkers designed a continuous stirred tank reactor (CSTR) equipped with higher-intensity light lasers, enabling kg per day throughput of the photochemical Ni-catalyzed amination with 1.85 kg of aryl bromide (Scheme 22).⁸⁹ More importantly, the CSTR design employed a fiber-coupled laser to allow flexible reactor configuration, thus providing modularity in reaction vessel choice.

Scheme 22 Harper's C–N coupling reactions using a continuous stirred tank reactor (CSTR).

In 2020, Buchwald and coworkers developed a 10 mmol scale of photoredox/nickel-catalyzed C–N coupling of aryl halides and amines under mild conditions (Scheme 23).⁹⁰ The reaction was performed with a Vapourtec E-series integrated flow chemistry system equipped with an appended UV-150 photochemical reactor. Based on their previous work, the flow reaction used 0.02 mol% of Ru(bpy)₃(PF₆)₂ and 5 mol% of NiBr₂·DME as catalysts, DABCO as the base, DMSO as the solvent, affording significantly higher yields than the corresponding batch reactions on 10 and 50 mmol scales, respectively. A wide range of (hetero)aryl bromides and chlorides was efficiently coupled with aryl and alkyl amines under optimal flow conditions, demonstrating excellent material throughput and practical utility.

Scheme 23 Buchwald's aryl amination.

In 2020, Kappe and coworkers reported a dual photoredox/nickel catalyzed cross-coupling reaction of aryl halides with tert-butyl carbamate as the nitrogen source in flow (Scheme 24).⁹¹ In the presence of a catalytic amount of Ir(ppy)₂(dtbbpy)PF₆ and NiCl₂·3H₂O, a wide range of aryl hydrazines could be synthesized with high efficiency and selectivity in short residence times. Subsequent deprotection of these arylhydrazine products gave the corresponding HCl salt with high yields. The reaction was conducted by using a glass corning advanced-flow lab photo reactor, which could be adopted with different light wavelengths and high temperatures. The authors found that the reaction rate was enhanced by a high photocatalyst loading and high temperature, thus reasoning that this reaction proceeded via reduction of aryl halides by the photoexcited catalyst. The authors successfully performed a scale-up process with 100 mL of the starting material achieved by reducing the loading of the photocatalyst and extending the residence time to inhibit the formation of nickel black, a significant challenge in large-scale reactions.

Scheme 24 Kappe's arylhydrazine synthesis.

The flow setup with heterogeneous materials such as CN photocatalysts remains challenging. Kappe and coworkers reported the use of a micro-structured plug flow photoreactor together with an oscillatory pump to facilitate a stable suspension of the heterogeneous photocatalyst, thus enabling the execution of a light-induced CN-OA-m/nickel-catalyzed C–N coupling of aryl halides in continuous flow (Scheme 25).⁹² Notably, the mode reaction using this flow reactor afforded over 12 grams (90% yield) of the desired amination product, showing a high productivity of 2.67 g h⁻¹ compared to some homogeneous reactions with a Ru- or Ir-based photocatalyst.

Scheme 25 Kappe's C–N coupling reactions with a CN-OA-m/nickel catalyst.

3 C–O bond-forming reactions

3.1 Significance and challenges

C–O bonds are ubiquitous in natural products, pharmaceuticals, and functionalized materials. Furthermore, many compounds containing C–O bonds, such as ethers, esters, and phenols, are manufactured on industrial scales and function as important building blocks in chemical synthesis. Transition metal-catalyzed C–O bond-forming reactions have emerged as general and efficient protocols for the synthesis of these valuable compounds.^93,94 Significant progress has been made with palladium⁶¹ and copper catalysts,⁹⁵ while efficient and selective C–O bond-forming reactions enabled by nickel remain underdeveloped. A significant challenge lies in the crucial reductive elimination from Ni(II) alkoxide species, the step of which is endothermic and reluctant to proceed at ambient or elevated temperatures based on Hillhouse's findings^96,97 and computational studies.⁹⁸ In 1997, Hartwig and colleagues reported the first example of Ni(COD)₂-catalyzed etherification of electron-deficient aryl bromides with three specific oxides at elevated temperature.⁹⁹ Nevertheless, a general and mild nickel-catalyzed C–O coupling protocol remains elusive. The development of photoredox and nickel dual catalysis has provided a promising platform for addressing this challenge. Similar to the catalytic C–N bond-forming reactions, tuning the oxidation state or excited state of Ni(II) alkoxide via a SET or EnT process would facilitate C–O reductive elimination, thus enabling C–O bond-formation under mild conditions (Scheme 26).

Scheme 26 C–O bond formation via photoinduced nickel catalysis.

3.2 C–O bond-formation enabled by SET

Although Ni(II) alkoxide is reluctant to reductive elimination, previous studies have disclosed that Ni(III) alkoxide, generated with a stoichiometric amount of oxidant, is feasible to undergo reductive elimination to forge C–O bonds.^100,101 In this context, the MacMillan group reported the first example of photoredox/nickel dual-catalyzed etherification of aryl bromides with alcohols in 2015 (Scheme 27, up).¹⁰² With Ir[dF(CF₃)ppy]₂(dtbbpy) PF₆ as the photocatalyst, NiCl₂·glyme as the nickel catalyst, dtbbpy as the ligand, and quinuclidine as the base, a wide range of electron-poor, -neutral, -and -rich aryl bromides underwent selective couplings with primary and secondary alcohols with blue LEDs, affording corresponding aryl ethers with high efficiency under mild conditions. Water was a suitable coupling partner to deliver phenols with moderate yields. Control reactions showed that photocatalyst, nickel catalyst, visible light, and quinuclidine were all essential to this C–O reaction. Mechanistically, the oxidative addition of Ni(0) with aryl bromide gave aryl Ni(II) species, which then underwent transmetallation with alcohol to form aryl Ni(II) alkoxide 27-II. Subsequent SET oxidation of 27-II by the photoexcited *Ir(III) afforded Ni(III) alkoxide 27-III, followed by facile reductive elimination to deliver the ether product as well as Ni(I). The final SET reduction of Ni(I) by the reducing Ir(II) regenerated Ni(0) as well as the ground state photocatalyst to close two catalytic cycles. The role of quinuclidine amine was reasoned to function as an electron shuttle to facilitate the redox events of nickel species. Stoichiometric reactions of isolated aryl Ni(II) alkoxide and Stern–Volmer fluorescence quenching experiments supported the critical oxidation event of Ni(II) alkoxide (E_p = +0.83 V vs. SCE in CH₃CN) by photoexcited *Ir(III) (E^red_1/2[*Ir^III/Ir^II] = +1.21 V vs. SCE).

Scheme 27 MacMillan's C–O coupling reactions.

Later, the Nocera group performed detailed studies of this dual-catalyzed etherification by utilizing photophysics, electrochemistry, and synthetic approaches (Scheme 27, bottom).¹⁰³ A key finding was that this reaction proceeded via a self-sustained Ni(I/III) cycle, other than the off-cycle Ni(II) species. Specifically, SET reduction of the Ni(II)X₂ precursor 27-IV by the reducing photocatalyst Ir(II) formed a putative monomeric Ni(I) species 27-V, which was captured by excess Ni(II) 27-IV to afford a critical bimetallic complex 27-VIII. Then, amine-facilitated dissociation of the bimetallic complex 27-VIII afforded active Ni(I) 27-V, followed by an oxidative addition/reductive elimination sequence, to produce the final aryl ethers.

In 2021, the Nevado group reported a dual photoredox/nickel-catalyzed C–O cross-coupling of aryl halides with phenols (Scheme 28).^104,105 In the presence of 1,2,3,5-tetrakis(carbazol-9-yl)-4,6-dicyanobenzene (4CzIPN) as the photocatalyst and NiBr₂(dtbbpy) as the nickel catalyst, a wide range of aryl iodides and bromides smoothly coupled with phenols, alcohols, and water to forge the corresponding diaryl ethers, aryl alkyl ethers, and phenols with high yields under mild conditions. Aryl chlorides did not work in this protocol. Preliminary mechanistic studies, including Stern–Volmer quenching studies, suggested the involvement of a Ni(III) intermediate, in contrast to Nocera's finding.¹⁰³

Scheme 28 Nevado's diaryl ether synthesis.

3.3 C–O bond-formation enabled by energy transfer

3.3.1 Energy transfer via indirect excitation. By employing carboxylic acids as the coupling reagent, in 2017, the MacMillan group reported photo-induced Ni-catalyzed esterification of aryl halides with carboxylic acids via an energy transfer process (Scheme 29).¹⁰⁶ In contrast to previous studies that typically employed strong oxidizing photocatalysts, this reaction used tris(2-phenylpyridine)iridium(III) [Ir(ppy)₃] as the photocatalyst, the oxidation potentials of which, either at its photoexcited state or high-valent state [Ir(IV)], could not favor SET oxidation of the nickel species. Together with NiBr₂·diglyme as the nickel catalyst, dtbbpy as the ligand, and ^tBuNHⁱPr as the base, a series of electron-deficient aryl bromides proved to be feasible coupling partners to react with alkyl and aryl carboxylic acids, forging synthetically valuable aryl esters under mild conditions. Mechanistic experiments showed that Dexter energy transfer played an important role in the C–O reductive elimination step. Specifically, oxidative addition of Ni(0) I with aryl halide gave the aryl-Ni(II) species 29-II, followed by ligand exchange with carboxylate, to generate the aryl-Ni(II) acetate species 29-III. Then, the energy transfer from the photoexcited *Ir(ppy)₃ to the nickel complex 29-III gave the excited state Ni(II) species 29-IV, which subsequently underwent reductive elimination to deliver the corresponding ester products.

Scheme 29 MacMillan's C–O cross-coupling of aryl halides with carboxylic acids.

Later, the Zhang and Huang group designed and synthesized a new D–A fluorophore, 4DPAPN, that possessed appropriate triplet energy, and utilized 4DPAPN as the photocatalyst to develop a similar photoinduced Ni-catalyzed C–O coupling reaction with aryl bromides and carboxylic acids under mild conditions (Scheme 30).¹⁰⁷ Using Ni(NO₃)₂·6H₂O/dtbbpy as the catalyst, comparable efficiency was obtained for a wide range of electron-poor aryl halides and aryl/alkyl carboxylic acids under otherwise similar conditions.

Scheme 30 Zhang and Huang's C–O coupling reactions with carboxylic acids.

In 2020, Johannes and coworkers developed a dual photoinduced nickel-catalyzed C–O coupling of aryl halides with alcohols via an EnT pathway (Scheme 31).¹⁰⁸ With Ir(ppy)₃/NiBr₂·glyme/di-OMebpy as the catalytic system, a wide array of alcohols, including primary and secondary benzylic, alkyl, and allylic alcohols, underwent efficient couplings with aryl halides upon exposure to blue LED. Electronically diverse aryl and heteroaryl bromides showed high reactivity, while electron-rich or -neutral aryl iodides showed higher efficiency than the corresponding bromides; while aryl chlorides worked with considerably low efficiency. Mechanistically, the authors proposed an energy transfer event between the photoexcited *Ir(ppy)₃ and Ni(II) complex to generate an excited Ni(II) complex. Notably, this reaction protocol was further scaled up via Vaportech flow apparatus.

Scheme 31 Johannes's etherification of aryl halides.

In 2020, the Seeberger and Pieber groups performed comprehensive kinetic studies for photoinduced Ni-catalyzed O-arylation reactions, to understand the role of photocatalysts as well as the current substrate limitations (Scheme 32).¹⁰⁹ The authors found that the two model reactions with the use of a homogeneous photocatalyst Ir(ppy)₃ or a heterogeneous photocatalyst graphitic carbon nitride (g-CN) displayed different kinetics: the reaction with Ir(ppy)₃ showed a turnover-limiting reductive elimination, whilst the reaction with g-CN showed rate dependence on aryl halides. These results indicated that the role of photocatalysts was not limited to promoting reductive elimination, and a Ni(0)/Ni(II) cycle was less likely to be involved in the heterogeneous reaction, based on the lower reduction potentials of g-CN.

Scheme 32 Seeberger and Pieber's kinetic studies.

In 2021, Li and coworkers disclosed a photoinduced Ni-catalyzed O-arylation of phenols with aryl halides in the presence of thioxanthen-9-one (TXO) as the photosensitizer, forging a series of diaryl ethers under mild conditions (Scheme 33).¹¹⁰ Compared to MacMillan's protocol,¹⁰⁶ this reaction used a household 45 W compact fluorescent lamp (CFL) as the light source with a broad scope of electron-deficient aryl bromides and phenols. A Ni(0)/Ni(II) pathway was proposed, i.e. energy transfer from the triplet state of thioxanthen-9-one to the Ni(II) complex gave the excited state of the Ni(II) phenolate intermediate for reductive elimination.

Scheme 33 Li's etherification with phenols.

In 2021, Wang and coworkers reported the first example of photochemical Ni-catalyzed cross-coupling of aryl halides with water with a photosensitive artificial dehalogenase (Scheme 34).¹¹¹ This photosensitizer metalloenzyme [PSP-95C-Ni(bpy)₂] was integrated with benzophenone and Ni(bpy)₂. A wide range of electron-deficient aryl bromides underwent efficient coupling with water in DMF/Tris-HCl (pH 8.8) under 380 nm irradiation, affording the corresponding phenols under mild conditions. Aryl iodide and chlorides were applicable, albeit with lower yields. Application of this protocol in catalytic C–N bond-forming reactions was also shown. Based on the transient absorption studies, the authors proposed a potential reaction pathway. Upon light irradiation, the excited triplet state of the benzophenone core of PSP-95C underwent energy transfer to the adjacent (aryl) Ni(II) oxide, delivering its triplet excited state that would enable the key reductive elimination step. To achieve a maximized energy transfer rate and minimized triplet excited state deactivation, the authors found that the two cores should be well-positioned at the right physical distance.

Scheme 34 Wang's hydroxylation of aryl halides with a photosensitive artificial dehalogenase.

3.3.2 Energy transfer via direct excitation. In 2018, the Doyle group performed detailed computational and ultrafast spectroscopic studies with regard to (aryl)Ni(II)-X complexes, a common intermediate for photoinduced Ni-catalyzed cross-couplings (Scheme 35).¹¹² They found that the long-lived ³MLCT states of (aryl)Ni(II)-X complexes could engage in direct photoinduced electron transfer with the ground-state Ni(II) complex, thus enabling access to the critical Ni(III) species for C–O bond-forming reactions without added photosensitizers. The formation of the long-lived ³MLCT Ni(II) complexes was due to halide ion independence. This study also implies that the Ni(II) complex could function as an economical alternative to Ir- or Ru-based photocatalysts.

Scheme 35 Doyle's mechanistic studies with (aryl)Ni(II)-X complexes.

In 2020, the Xue group reported light-induced nickel-catalyzed etherification of aryl electrophiles with alcohols without added photosensitizers (Scheme 36).¹¹³ With a simple (dtbbpy)Ni(Ar)(Br) (Ni-I) as the precatalyst and DBU as the base, a wide range of aryl halides, mesylates, tosylates, and triflates underwent efficient couplings with primary alcohols to deliver aryl ethers under irradiation of 390–395 nm LEDs. Secondary alcohols worked with decreased yields. Also, intramolecular C–O bond formation was feasible via this protocol. A Ni(I)/Ni(II)/Ni(III) cycle was proposed: the triplet state of the Ni(II) complex Ni-I underwent homolysis to form the active Ni(I) species 36-II with the release of an aryl radical. Then, the oxidative addition of Ni(I) species 36-II with aryl halide gave Ni(III)-Ar 36-III, followed by ligand exchange and reductive elimination to afford the desired ether product. In this case, continuous irradiation was necessary. Later, the Hadt group found that the rate constants of Ni(II)–carbon bond homolyses from the excited-state nickel complexes were temperature- and wavelength-dependent.¹¹⁴

Scheme 36 Xue's etherification of aryl electrophiles with alcohols.

3.4 C–O bond-formation enabled by heterogeneous materials

Several heterogeneous photosensitizers have been reported to promote photochemical Ni-catalyzed C–O bond-formation reactions.

In 2020, Nocera and coworkers employed a surface-modified carbon nitride as the photocatalyst to facilitate Ni-catalyzed C–O coupling reactions of aryl bromides with alcohols (Scheme 37).¹¹⁵ The modified ^NCNCN_x demonstrated broader absorption in the visible region, thus enabling the direct use of solar energy for this reaction. Excellent efficiency was observed for a series of aryl bromides to forge aryl ethers under sunlight.

Scheme 37 Nocera's etherification of aryl bromides with a modified carbon nitride.

In 2019, Lin and coworkers designed a new multifunctional metal–organic layer (MOL), Hf₁₂–Ir–Ni, by linking the photosensitizing iridium core and nickel center together, enabling catalytic C–O bond formation in the presence of quinuclidine as the base and blue LED as the light source (Scheme 38).¹¹⁶ In particular, the Ir and Ni centers in MOL were close, thus significantly increasing the SET efficiency. The etherification of aryl bromides with alcohols catalyzed by Hf₁₂–Ir–Ni showed a broad substrate scope with excellent functional group tolerance. More importantly, Hf₁₂–Ir–Ni demonstrated remarkable stability and recyclability under photochemical conditions.

Scheme 38 Lin's cross-coupling of aryl bromides with alcohols.

Later, various heterogeneous materials incorporating both photosensitizer cores and nickel centers, including C₃N₄-Ni by the Song group,¹¹⁷ Ni-mpg-CN_x by the Reisner group,¹¹⁸ sp²-COF_dpy-Ni by the Chen group,¹¹⁹ and Ni(II)-bpy-COF by the Zhang group,¹²⁰ have been described for the catalytic C–O bond formation reactions of aryl halides with alcohols or water under visible-light irradiation (Scheme 39). Besides the observed broad scope and high efficiency, a remarkable feature of these reactions was that the catalyst could be easily recovered. Similar mechanistic pathways involving Ni(I)/Ni(III) were proposed for these reactions.

Scheme 39 Several C–O couplings with nickel modified heterogeneous materials.

4. Conclusions

In recent years, light-driven nickel catalysis has emerged as a powerful platform for the construction of diverse carbon–carbon and carbon–heteroatom bonds that are typically difficult to access under traditional conditions. By utilizing the distinct reactivity of photoexcited species, particularly, the high-valent state or triplet state of nickel complexes can be accessed via a photoinduced SET or EnT process, thus facilitating the thermodynamically challenging carbon–heteroatom reductive elimination step. Despite this review focusing on the assembly of the most challenging C–N and C–O bonds, significant progress has been made in similar transformations with other types of carbon–heteroatom bonds. Nevertheless, several challenges have not yet been addressed. First, the coupling efficiency of these reactions is affected by the nature of aryl electrophiles, with inexpensive heteroaryl and aryl chlorides showing low efficiency. Second, current successful examples are limited to aryl electrophiles, and rare examples of related reactions with C(sp³)-electrophiles to forge synthetically valuable C(sp³)–heteroatom bonds have not been disclosed yet. Correspondingly, the enantioselective control of these heteroatom-based chiral centres remains under-exploited. Third, studies toward the development of practical and efficient scale-up reactions enabled by simple and economical or recyclable photocatalysts and nickel catalysts remain underdeveloped. Finally, an in-depth mechanistic understanding of these catalytic systems is elusive.

Conflicts of interest

There are no conflicts to declare.

Acknowledgements

We thank the National Natural Science Foundation of China (21971036, 21901036), the National Science Fund for Excellent Young Scholars (22122101), and the Shanghai Rising-Star Program (20QA1400200) for financial support.

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