Strategies to Generate Nitrogen-centered Radicals That May Rely on Photoredox Catalysis: Development in Reaction Methodology and Applications in Organic Synthesis

Nitrogen-Centered Radicals in Functionalization of sp2 Systems: Generation, Reactivity, and Applications in Synthesis

The chemistry of nitrogen-centered radicals (NCRs) has plentiful applications in organic synthesis, and they continue to expand as our understanding of these reactive species increases. The utility of these reactive intermediates is demonstrated in the recent advances in C-H amination and the (di)amination of alkenes. Synthesis of previously challenging structures can be achieved by efficient functionalization of sp² moieties without prefunctionalization, allowing for faster and more streamlined synthesis. This Review addresses the generation, reactivity, and application of NCRs, including, but not limited to, iminyl, aminyl, amidyl, and aminium species. Contributions from early discovery up to the most recent examples have been highlighted, covering radical initiation, thermolysis, photolysis, and, more recently, photoredox catalysis. Radical-mediated intermolecular amination of (hetero)arenes can occur with a variety of complex amine precursors, generating aniline derivatives, an important class of structures for drug discovery and development. Functionalization of olefins is achievable in high anti-Markovnikov regioselectivity and allows access to difunctionalized structures when the intermediate carbon radicals are trapped. Additionally, the reactivity of NCRs can be harnessed for the rapid construction of N-heterocycles such as pyrrolidines, phenanthridines, quinoxalines, and quinazolinones.

Suzuki-type cross-coupling of alkyl trifluoroborates with acid fluoride enabled by NHC/photoredox dual catalysis

The Suzuki–Miyaura cross-coupling of C(sp³)-hybridised boronic compounds still remains a challenging task, thereby hindering the broad application of alkyl boron substrates in carbon–carbon bond-forming reactions. Herein, we developed an NHC/photoredox dual catalytic cross-coupling of alkyl trifluoroborates with acid fluorides, providing an alternative solution to the classical acylative Suzuki coupling chemistry. With this protocol, various ketones could be rapidly synthesised from readily available materials under mild conditions. Preliminary mechanistic studies shed light on the unique radical reaction mechanism.

An acylative Suzuki-type cross-coupling of alkyl trifluoroborates and acid fluorides was developed by merging NHC organocatalysis with photoredox catalysis. A broad spectrum of ketones could be facilely synthesised under mild reaction conditions.

Employing Visible-Light Photoredox Catalysis in Multicomponent-Multicatalyst Reactions: One-Pot Synthesis of Spiroquinazolin-2-(thi)ones

The multicomponent-multicatalyst reaction ((MC)²R) and visible-light catalysis have emerged as green and powerful strategies for achieving ideal syntheses. Here, we report the first example of a visible-light-induced approach toward spiroquinazolin(thi)ones. This (MC)²R features an eco-friendly energy source and solvent, metal-free catalysts, step- and atom-economy, a relay catalysis strategy, air as green oxidant, mild conditions, and easily accessible starting materials.

Photoredox/Nickel Dual Catalysis Enables the Synthesis of Alkyl Cyclopropanes via C(sp3)-C(sp3) Cross Electrophile Coupling of Unactivated Alkyl Electrophiles

A facile synthesis of mono-, 1,1- and 1,2-disubstituted cyclopropanes via visible light-mediated photoredox/nickel dual catalysis is demonstrated. The challenging intramolecular C(sp³)-C(sp³) cross-electrophile coupling of readily available unactivated 1,3-dialkyl electrophiles was performed under mild conditions that allowed traditionally reactive functional groups to be included. Mechanistic inspection and control experiments revealed the importance of dual catalysis and that the reaction proceeds via a stepwise oxidative addition followed by an intramolecular S_N2 reaction.

Diradical Generation via Relayed Proton-Coupled Electron Transfer

Diradical generation followed by radical-radical cross-coupling is a powerful synthetic tool, but its detailed mechanism has yet to be established. Herein, we proposed and confirmed a new model named relayed proton-coupled electron transfer (relayed-PCET) for diradical generation, which could open a door for new radical-radical cross-coupling reactions. Quantum mechanics calculations were performed on a selected carbene-mediated diradical cross-coupling reaction model and a designed model, and the exact electronic structural changes during the radical processes have been observed for the first time.

Visible-Light-Mediated Late-Stage Sulfonylation of Anilines with Sulfonamides

A visible-light-mediated late-stage sulfonylation of anilines with sulfonamides under simple reaction conditions is presented. Various primary or secondary sulfonamides including several pharmaceuticals were incorporated successfully via N-S bond activation and C-H bond sulfonylation. The synthetic utility of this strategy is highlighted by the construction of complex anilines bearing diverse bioactive groups.

Photoinduced Metal-Free Borylation of Aryl Halides Catalysed by in situ Formed Donor-Acceptor Complex

Organoboron compounds are very important building blocks which can be applied in medicinal, biological and industrial fields. However, direct borylation in a metal free manner has been very rarely reported. Herein, we described the successful direct borylation of haloarenes under mild, operationally simple, catalyst-free conditions, promoted by irradiation with visible light. Mechanistic experiments and computational investigations indicate the formation of an excited donor–acceptor complex with a −3.12 V reduction potential, which is a highly active reductant and can facilitate single-electron-transfer (SET) with aryl halides to produce aryl radical intermediates. A two-step one-pot method was developed for site selective aromatic C–H bond borylation. The protocol's good functional group tolerance enables the functionalization of a variety of biologically relevant compounds, representing a new application of aryl radicals merged with photochemistry.

We reported a facile metal-free conversion of aryl halides to the corresponding boronic esters catalysed by an in situ formed donor–acceptor complex. A two-step one-pot method was also developed for site selective aromatic C–H bond borylation.

Facile synthesis of fully substituted 1H-imidazoles from oxime esters via dual photoredox/copper catalyzed multicomponent reactions

A novel and efficient photoredox/copper cocatalyzed domino cyclization of oxime esters, aldehydes, and amines has been achieved, affording a broad range of fully substituted 1H-imidazoles in good yields.

Ni and Fe Catalyzed Cascade Radical Reactions of Oxime Esters with Diselenides

A radical cyclization and ring-opening of oxime esters with diselenides was developed. Both Ni(0) and Fe(II) catalysts could be employed for the selenylation of olefin-containing and cyclic oxime ester derivatives....

External oxidant-free and selective thiofunctionalization of alkenes enabled by photoredox-neutral catalysis

A photoredox approach was reported to realize a highly selective three-component thiohydroxylation, thioalkoxylation and thioamination of vinylarenes towards valuable vicinal S,O- and S,N-disubstituted molecules under mild conditions.

Rapid synthesis of spirodienones via electrochemical dearomative spirocyclization in flow

An electrochemical dearomative spirocyclization in flow has been developed, featuring the use of electrons as the clean oxidant in a minimum amount of electrolytes to afford diverse spirodienones in a short reaction time.

Radical-mediated alkene carboamination/dearomatization of arylsulfonyl-o-allylanilines via photoredox catalysis

A mild and redox-neutral protocol is developed for the synthesis of 1,4-cyclohexadiene-containing indoline-fused heterocycles via photoredox catalysis.

Visible-Light-Driven Photoredox-Catalyzed C(sp3)-C(sp3) Cross-Coupling of N-arylamines with Cycloketone Oxime Esters

A novel photoredox-catalyzed C(sp3)-C(sp3) cross-coupling between N-arylamines and cycloketone oxime esters under mild conditions has been accomplished. The redox-neutral reaction proceeds good functional group tolerance and excellent regioselectivity without any...

Divergent copper-catalyzed syntheses of 3-carboxylpyrroles and 3-cyanofurans from O-acetyl oximes and β-ketoesters/nitriles

The reaction between O-acetyl oximes and β-ketoesters/nitriles catalyzed by copper generated polysubstituted pyrroles and furans, respectively, under redox–neutral reaction conditions. Using this protocol, pyrroles or furans could be obtained simply by choosing an appropriate active methylene compound. Although both transformations occur essentially under the same reaction conditions, control experiments indicated that they follow different mechanistic pathways.

A copper-catalyzed heteroannulation of O-acetyl oximes and β-ketoesters/nitriles to afford polysubstituted pyrroles and furans was successfully achieved.

Aromatization as an Impetus to Harness Ketones for Metallaphotoredox-Catalyzed Benzoylation/Benzylation of (Hetero)arenes

Herein we report ketones as feedstock materials in radical cross-coupling reactions under Ni/photoredox dual catalysis. In this approach, simple condensation first converts ketones into prearomatic intermediates that then act as activated radical sources for cross-coupling with aryl halides. Our strategy enables the direct benzylation/benzoylation of (hetero)arenes under mild reaction conditions with high functional group tolerance.