N-Heterocyclic Nitreniums Can Be Employed as Photoredox Catalysts for the Single-Electron Reduction of Aryl Halides

Halogen Bonding Promoted Photoinduced Synthesis of 3,3-Disubstituted Oxindoles

A photoinduced and catalyst-free radical cyclization process for the synthesis of 3,3-disubstituted oxindoles is reported. This method utilizes readily available α-bromoanilides as substrates, showcasing a broad substrate scope. The reaction mechanism is facilitated by a photoactivated charge transfer complex based on the halogen bonding of α-bromoanilide with TMG and alcohol.

Photoinduced N-heterocyclic nitrenium-catalyzed single electron reduction of α-chloro esters for phenanthridine synthesis

Visible-light photoredox catalysis has revolutionized synthetic methodologies by enabling sustainable radical-mediated transformations under mild conditions. Herein, we report a catalytic protocol employing N-heterocyclic nitrenium (NHN) iodide salts to drive the photoreduction of α-chloro esters, generating alkyl radicals that participate in annulation with 2-isocyanobiaryls for the modular synthesis of phenanthridine derivatives. This approach is characterized by easily available NHNs and operational simplicity.

Photocatalytic hydroalkylation of 3-methyleneisoindolin-1-ones with unactivated alkyl iodides

We report herein a simple method for hydroalkylation of 3-methyleneisoindolin-1-ones with unactivated iodoalkanes using visible light photocatalysis and a halogen atom transfer (XAT) process. This operationally simple method exhibits broad substrate scope and allows late-stage modifications of iodoalkanes derived from either active pharmaceutical ingredients or natural products, producing a range of structurally diverse and valuable corresponding hydroalkylation products in decent yields. The generation of alkyl radicals and carbanion intermediates was directly proven in the catalytic cycle through radical trapping/radical clock and isotope labeling studies, respectively.

Iodoarene Activation: Take a Leap Forward toward Green and Sustainable Transformations

Constructing chemical bonds under green sustainable conditions has drawn attention from environmental and economic perspectives. The dissociation of (hetero)aryl-halide bonds is a crucial step of most arylations affording (hetero)arene derivatives. Herein, we summarize the (hetero)aryl halides activation enabling the direct (hetero)arylation of trapping reagents and construction of highly functionalized (hetero)arenes under benign conditions. The strategies for the activation of aryl iodides are classified into (a) hypervalent iodoarene activation followed by functionalization under thermal/photochemical conditions, (b) aryl-I bond dissociation in the presence of bases with/without organic catalysts and promoters, (c) photoinduced aryl-I bond dissociation in the presence/absence of organophotocatalysts, (d) electrochemical activation of aryl iodides by direct/indirect electrolysis mediated by organocatalysts and mediators acting as electron shuttles, and (e) electrophotochemical activation of aryl iodides mediated by redox-active organocatalysts. These activation modes result in aryl iodides exhibiting diverse reactivity as formal aryl cations/radicals/anions and aryne precursors. The coupling of these reactive intermediates with trapping reagents leads to the facile and selective formation of C-C and C-heteroatom bonds. These ecofriendly, inexpensive, and functional group-tolerant activation strategies offer green alternatives to transition metal-based catalysis.

Visible-Light-Mediated Radical α-C(sp3)─H gem-Difluoroallylation of Amides with Trifluoromethyl Alkenes via Halogen Atom Transfer and 1,5-Hydrogen Atom Transfer

Direct gem-difluoroallylation at the α-carbonyl position is a challenging process by conventional methods. Herein we report the photocatalytic radical α-C(sp3)─H gem-difluoroallylation of amides with trifluoromethyl alkenes to access the target compounds with good yields and functional group tolerance. The mild and effective conditions allow gem-difluoroalkene motifs as carbonyl bioisosteres incorporated concisely to some complex molecules, including gemfibrozil and estrone derivatives, presenting great potential for late-stage functionalization of drugs, natural products, and bioactive intermediates. Mechanistic investigations suggest a radical pathway combining XAT and 1,5-HAT.

CO2•- Enabled Synthesis of Phenanthridinones, Oxindoles, Isoindolinones, and Spirolactams

We report herein that photoinduced CO2•- enabled reductive intramolecular radical cyclization of a variety of aryl iodide derivatives to the corresponding phenanthridinone, oxindole, isoindolinone, and spirolactam derivatives in good yields. Preliminary mechanistic studies suggested the generation of CO2•- through homolysis of cesium formate in the presence of light, and the further involvement of CO2•- and the aryl radical was directly proved by trapping with diphenyl styrene and TEMPO.

N-heterocyclic nitrenium-catalyzed photosynthesis of 3,3-disubstituted oxindoles from α-chloroanilides

The radical cascade reaction of α-halogenated anilides represents an efficient approach for synthesizing 3,3-oxindoles. However, most methods have focused on α-bromoanilides, with limited utilization of the more stable and readily available α-chloroanilides. In addition, the transition-metal-free preparation of 3,3-oxindoles has been far less explored. Here, we present a photoreductive N-heterocyclic nitrenium-catalyzed radical cyclization reaction of α-chloroanilides for the construction of 3,3-oxindoles under transition-metal-free conditions.

Triazenolysis of alkenes as an aza version of ozonolysis

Alkenes are broadly used in synthetic applications, thanks to their abundance and versatility. Ozonolysis is one of the most canonical transformations that converts alkenes into molecules bearing carbon-oxygen motifs via C=C bond cleavage. Despite its extensive use in both industrial and laboratory settings, the aza version-cleavage of alkenes to form carbon-nitrogen bonds-remains elusive. Here we report the conversion of alkenes into valuable amines via complete C=C bond disconnection. This process, which we have termed 'triazenolysis', is initiated by a (3 + 2) cycloaddition of triazadienium cation to an alkene. The triazolinium salt formed accepts hydride from borohydride anion and spontaneously decomposes to create new C-N motifs upon further reduction. The developed reaction is applicable to a broad range of cyclic alkenes to produce diamines, while various acyclic C=C bonds may be broken to generate two separate amine units. Computational analysis provides insights into the mechanism, including identification of the key step and elucidating the significance of Lewis acid catalysis.

Radical Replacement Process for Ligated Boryl Radical-Mediated Activation of Unactivated Alkyl Chlorides for C(sp3)-C(sp3) Bond Formation

The ligated boryl radical (LBR) has emerged as a potent tool for activating alkyl halides in radical transformations through halogen-atom transfer (XAT). However, unactivated alkyl chlorides still present an open challenge for this strategy. We herein describe a new activation mode of the LBR for the activation of unactivated alkyl chlorides to construct a C(sp3)-C(sp3) bond. Mechanistic studies reveal that the success of the protocol relies on a radical replacement process between the LBR and unactivated alkyl chloride, forming an alkyl borane intermediate as the alkyl radical precursor. Aided with the additive K3PO4, the alkyl borane then undergoes one-electron oxidation, generating an alkyl radical. The incorporation of the radical replacement activation model to activate unactivated alkyl chlorides significantly enriches LBR chemistry, which has been applied to activate alkyl iodides, alkyl bromides, and activated alkyl chlorides via XAT.

Visible light-induced Mallory reaction of tertiary benzanilides via iminium intermediates

The Mallory reaction, which involves the photocyclization of stilbenes/diarylethenes and their analogues into polycyclic aromatics, is of significant synthetic importance. However, its application to tertiary benzanilides has not been explored to date. Besides, most of the reported Mallory reactions require ultraviolet irradiation. In this study, we show the first Mallory reaction of tertiary benzanilides promoted by visible light via iminium intermediates formed in situ from tertiary benzanilide, Tf2O (triflic anhydride) and pyridine. UV/vis absorption spectroscopy combined with density functional theory (DFT) calculations revealed that the formation of the iminium intermediate decreased the HOMO-LUMO energy gap, thereby enhancing visible light absorption. This study provides a rapid and practical approach for the preparation of the phenanthridinone skeleton and provides a new idea for the design of new visible light photoswitches.

Generation and Application of Aryl Radicals Under Photoinduced Conditions

Photoinduced aryl radical generation is a powerful strategy in organic synthesis that facilitates the formation of diverse carbon-carbon and carbon-heteroatom bonds. The synthetic applications of photoinduced aryl radical formation in the synthesis of complex organic compounds, including natural products, physiologically significant molecules, and functional materials, have received immense attention. An overview of current developments in photoinduced aryl radical production methods and their uses in organic synthesis is given in this article. A generalized idea of how to choose the reagents and approach for the generation of aryl radicals is described, along with photoinduced techniques and associated mechanistic insights. Overall, this article offers a critical assessment of the mechanistic results as well as the selection of reaction parameters for specific reagents in the context of radical cascades, cross-coupling reactions, aryl radical functionalization, and selective C-H functionalization of aryl substrates.

Unveiling N-Fused Nitreniums as Potent Catalytic Photooxidants

The first example of a hitherto-unknown facet of catalytic photooxidant capability of nitrenium cations is reported herein. The fundamental limitation of inability of the traditional and reported nitreniums to achieve the excited-state redox potential beyond +2.0 V (vs Ag/AgCl), the primary requirement for a powerful photooxidant, is addressed in this work by developing a structurally unique class of N-fused nitrenium cations, with the required structural engineering involving extensive π-conjugation through ring fusion at the nitrenium site, which enabled significant lowering of the LUMO energy and easy reduction at the excited state (excited-state redox potential up to +2.5 V vs Ag/AgCl), facilitated by effective delocalization/stabilization of the generated radical. This finding opens a new way to discover novel and useful (photo)catalytic properties of nitrenium cations beyond just Lewis acidity.

Nitrenium ions as new versatile reagents for electrophilic amination

Herein we report the utilization of N-heterocyclic nitrenium ions - easily prepared, bench-stable and non-oxidating nitrogen sources for the efficient electrophilic amination of aliphatic and aromatic organometallic nucleophiles, towards the facile and general preparation of primary amines. To this end, a plethora of abundant organolithium and organomagnesium reagents were combined with nitrenium salts to generate a variety of previously unexplored N-alkyl and N-aryl triazanes. Through the simple hydrogenolysis of these relatively stable triazanes, we have prepared a diverse scope of primary amines, including linear and branched aliphatic as well as (hetero)aromatic amines possessing various stereo-electronic substituents. Furthermore, we present the facile synthesis of valuable 15N-labelled primary amines from easily prepared 15N-labelled nitrenium salts, as well as a one-pot approach to biologically relevant primary amines. Finally, a recyclable variant of the nitrenium precursor was prepared and a simple recovery protocol was developed to improve the atom-economy of this procedure.

N-Heterocyclic Nitrenium-Catalyzed Photohomolysis of CF3SO2Cl for Alkene Trifluoromethylation

N-Heterocyclic nitreniums (NHNs) have been utilized as Lewis acid catalysts to activate substrates with lone pairs. Alternative to their conventional applications, we have discovered that NHNs can also serve as charge transfer complex catalysts. Herein, we present another potential of NHNs by utilizing a weak interaction between NHNs and CF3SO2Cl. The method promotes CF3SO2Cl to undergo photohomolysis, resulting in the CF3 radical. Mechanistic studies suggested that the weak interaction could be due to the π-hole effect of NHNs.

Triarylamine-Substituted Benzimidazoliums as Electron Donor-Acceptor Dyad-Type Photocatalysts for Reductive Organic Transformations

Triarylamine-substituted benzimidazoliums (BI⁺-PhNAr₂), new electron donor-acceptor dyad molecules, were synthesized. Their photocatalytic properties for reductive organic transformations were explored using absorption and fluorescence spectroscopy, redox potential determinations, density functional theory calculations, transient absorption spectroscopy, and reduction reactions of selected substrates. The results show that irradiation of BI⁺-PhNAr₂ promotes photoinduced intramolecular electron transfer to form a long-lived (∼300 μs) charge shifted state (BI^•-PhN^•+Ar₂). In the pathway for photocatalysis of reduction reactions of substrates, BI^•-PhN^•+Ar₂ is subsequently transformed to the neutral benzimidazolyl radical (BI^•-PhNAr₂) by single-electron transfer from the donor 1,3-dimethyl-2-phenylbenzimidazoline (BIH-Ph) serving as a cooperative agent. Among the benzimidazoliums explored, the bromo-substituted analogue BI⁺-PhN(C₆H₄Br-p)₂ in conjunction with BIH-Ph demonstrates the most consistent catalytic performance.

Photoinduced N-Heterocyclic Nitrenium-Catalyzed Single Electron Reduction of Acyl Fluorides for Phenanthridine Synthesis

Acyl fluorides are versatile reagents in organic synthesis. However, there is no precedent to employ acyl fluorides as acyl radical precursors. We herein report an N-heterocyclic nitrenium iodide salt-catalyzed photoreduction of acyl fluorides to produce acyl radicals, which could react with 2-isocyanobiaryls to afford various carbonyl phenanthridines.

Photoinduced halogen-bonding enabled synthesis of oxindoles and isoindolinones from aryl iodides

We report the use of halogen bonding (XB) for the generation of aryl radicals from aryl halides under blue light irradiation and applied it in radical generation/1,5-hydrogen-atom transfer/radical cyclization cascade reactions for the synthesis of oxindoles and isoindolinones. On the basis of experimental studies, we propose that DBU can serve as a suitable XB acceptor with aryl halides for the formation of a photoactive electron donor and acceptor complex.

Multimodal Reactivity of N-H Bonds in Triazanes and Isolation of a Triazinyl Radical

The employment of nitrogen Lewis acids based on nitrenium cations has been increasingly featured in the fields of main group chemistry and catalysis. A formally reduced form of nitrenium D─cyclic triazanes E─are intriguing chemical compounds, the chemistry of which is completely unexplored. In this work, we reveal that N-H-triazanes exhibit unusual N-H bond properties; namely, they can serve as protons, hydrides, or hydrogen atom donors. This unique multimodal reactivity provides an N-cation, N-anion, or N-radical from the same species. It allowed us to isolate, for the first time, a stable naphto[1,2,3]triazinyl radical, which was fully characterized both computationally and experimentally, including its monomeric X-ray structure. Moreover, this radical can be prepared directly from the nitrenium cation by a single electron reduction (E = -0.46 V), and this process is reversible. We envision versatile uses of this radical in synthetic and materials chemistry.

Visible-Light Photocatalytic Reduction of Aryl Halides as a Source of Aryl Radicals

Aryl- and heteroaryl units are present in a wide variety of natural products, pharmaceuticals, and functional materials. The method for reduction of aryl halides with ubiquitous distribution is highly sought after for late-stage construction of various aromatic compounds. The visible-light-driven reduction of aryl halides to aryl radicals by electron transfer provides an efficient, simple, and environmentally friendly method for the construction of aromatic compounds. This review summarizes the recent progress in the generation of aryl radicals by visible-light-driven reduction of aryl halides with metal complexes, organic compounds, semiconductors as catalysts, and alkali-assisted reaction system. The ability and mechanism of reduction of aromatic halides in various visible light induced systems are summarized, intending to illustrate a comprehensive introduction of this research topic to the readers.