Visible-Light-Mediated Generation of Nitrogen-Centered Radicals: Metal-Free Hydroimination and Iminohydroxylation Cyclization Reactions

Hypervalent Iodine Promoted Selective [2 + 2 + 1] Cycloaddition of Aromatic Ketones and Methylamines: A One-Pot Access to 1-Pyrrolines

Herein, a versatile highly regioselective three-component annulation of simple aromatic ketones and methylamines using a hypervalent iodine reagent for polyarylated 1-pyrrolines has been described in good to excellent yields. Meanwhile, unsymmetrical 1-pyrroline isomers could be realized and synthesized. Such an intriguing one-pot two-step tandem assembly strategy with green conditions and high regioselectivity shows predictable inspiration in related annulation reactions.

Cu(II)-Catalyzed Hydroboration Reactions of 1,1-Disubstituted α,β-Unsaturated Ketones, Esters, and Amides in Pure Water

Here, a Cu2(OH)2CO3-catalyzed hydroboration reaction of 1,1-disubstituted α,β-unsaturated compounds has been developed. The reaction was carried out using water as a solvent at room temperature except for N-monosubstituted α,β-unsaturated amides. This method is applicable to diverse 1,1-disubstituted α,β-unsaturated ketones, esters, and amides, showing excellent reactivity (up to 98% yield). Gram-scale experiments and functional group transformations further demonstrated the practicality of this method.

Electron Donor-Acceptor Complex Driven Photocatalyst-Free Trifluoromethylation of Heterocycles

Heterocyclic trifluoromethylation is efficiently initiated through a photochemical reaction utilizing an electron donor-acceptor (EDA) complex, proceeding smoothly without the use of photocatalysts, transition-metal catalysts, or additional oxidants. This method has been optimized through extensive experimentation, demonstrating its versatility and efficacy across various substrates, including quinoxalinones, coumarins, and indolones. Notably, this approach enables the practical synthesis of trifluoromethylated quinoxalinones on a gram scale. Mechanistic investigations that incorporate radical trapping and ultraviolet/visible spectroscopy, confirmed the formation of the an EDA complex and elucidated the reaction pathways. This study highlights the crucial role of EDA photoactivation in trifluoromethylation, significantly expanding the application scope of EDA complexes in chemical synthesis.

Intramolecular Radical Amidooxygenation of Alkenes for the Construction of Pyrrolidinones

An intramolecular 1,2-amidooxygenation of unactivated alkenes for the construction of the pyrrolidinone scaffold containing a masked 5-hydroxymethyl functionality is reported. γ,δ-Unsaturated N-aryloxyamides react with sodium 2,2,6,6-tetramethylpiperidin-1-olate (TEMPONa) to afford alkoxyaminated pyrrolidinones. The cascade proceeds through reductive amidyl radical generation with TEMPONa, 5-exo cyclization, and TEMPO trapping. No transition metal is required to perform these transformations, and complex (fused, bridged) pyrrolidinones are accessible in moderate to good yields. The product alkoxyamines are readily further converted into ketones and alcohols through either oxidation or reduction.

Nickel-catalyzed C(sp2)-C(sp3) coupling via photoactive electron donor-acceptor complexes

We have developed a novel Ni-catalyzed reductive cross-coupling reaction of aryl bromides and alkyl iodides via a photoactive electron donor-acceptor (EDA) complex. This photo-induced process enables the efficient construction of C(sp2)-C(sp3) bonds in the absence of an external photocatalyst. Electronically and structurally diverse aryl bromides, as well as secondary and primary alkyl iodides could undergo this transformation smoothly. Natural product derivatives were employed successfully, and UV-vis spectroscopy was utilized to gain mechanistic insight.

Radical reactions enabled by polyfluoroaryl fragments: photocatalysis and beyond

Polyfluoroarenes have been known for a long time, but they are most often used as fluorinated building blocks for the synthesis of aromatic compounds. At the same time, due to peculiar fluorine effect, they have unique properties that provide applications in various fields ranging from synthesis to materials science. This review summarizes advances in the radical chemistry of polyfluoroarenes, which have become possible mainly with the advent of photocatalysis. Transformations of the fluorinated ring via the C-F bond activation, as well as use of fluoroaryl fragments as activating groups and hydrogen atom transfer agents are discussed. The ability of fluoroarenes to serve as catalysts is also considred.

Metal- and photocatalyst-free approach to visible-light-induced acylation of quinoxalinones

A transition-metal- and photocatalyst-free photochemical reaction was successfully developed for the direct acylation of quinoxalin-2(1H)-ones, which was enabled by the formation of electron donor-acceptor (EDA) complexes. The use of 1,8-diazabicyclo[5.4.0]undec-7-ene (DBU) as the electron donor allows efficient and operationally simple access to a series of C3-aroylated and acylated quinoxalin-2(1H)-ones with moderate to good yields.

Synthesis of γ-Oxo-phosphonates via N-Heterocyclic Carbene-Catalyzed Acylphosphorylation of Alkenes

In this study, we present an N-heterocyclic carbene-catalyzed method for the radical acylphosphorylation of alkenes. Electrochemical investigations were employed to identify an appropriate class of oxime phosphonates capable of undergoing a single-electron transfer (SET) with Breslow enolates. The resulting phosphoryl radicals were effectively coupled with diverse styrenes and aldehydes to yield a variety of γ-oxo-phosphonates. Both radical clock experiments and electrochemical studies support our reaction design, and a plausible mechanism for the organocatalytic transformation is proposed.

N-Iminopyridinium Compounds in Giese Reaction: Photoinduced Homolytic N-N and C-C Bond Cleavage for Cyanoalkyl Radical Generation

We present an innovative photoinduced cyanoalkyl radical addition methodology using N-iminopyridinium reagents derived from cyclic ketones. Mechanistic investigations reveal the association of the excited Hantzsch ester and iminopyridinium with pyridyl radical generation. The ensuing cascade involving homolytic N-N bond and C-C bond cleavage of the pyridyl radical ultimately leads to the formation of cyanoalkyl radical species, leading to diverse Giese-type products. The method showcases versatility and synthetic utility in late-stage functionalization.

Visible-light-driven EDA complex-promoted cascade cyclization to construct 4-cyanoalkyl isoquinoline-1,3-diones

Visible-light-induced EDA complex-promoted ring-opening of cycloketone oxime esters to synthesise various cyanoalkylated products with N-methacryloyl benzamides was developed. Various radical receptors were compatible with the current reaction system to furnish diverse heterocyclic compounds. Mechanistic analysis shows that the formation of an EDA complex was crucial to the photocatalytic strategy. Importantly, 4-cyanoalkyl isoquinoline-1,3-diones were obtained in high yields by using a catalytic amount of 1,4-diazabicyclo[2.2.2]octane (DABCO) through prolonging the reaction time, which provided a practical approach to give a variety of isoquinoline-1,3-dione derivatives.

Harnessing the Potential of Electron Donor-Acceptor Complexes and N-Centered Radicals: Expanding the Frontiers of Isoquinoline Derivative Synthesis

Research on synthesizing nitrogen-containing heterocyclic scaffolds is important because these structures are commonly found in Nature, such as in the alkaloids' family. In our study, we propose a new method to synthesize the isoquinoline core using an electron donor-acceptor (EDA) complex strategy. Our mechanistic investigations have confirmed that our synthesis process operates through an EDA mechanism, which is not extensively discussed in the literature, particularly regarding its applications on alkynyl substrates. This EDA strategy has proven to be a simple and straightforward way to produce isoquinoline scaffolds and their derivatives without the need for metal catalysts.

Dioxygen compatible electron donor-acceptor catalytic system and its enabled aerobic oxygenation

The photochemical properties of Electron Donor-Acceptor (EDA) complexes present exciting opportunities for synthetic chemistry. However, these strategies often require an inert atmosphere to maintain high efficiency. Herein, we develop an EDA complex photocatalytic system through rational design, which overcomes the oxygen-sensitive limitation of traditional EDA photocatalytic systems and enables aerobic oxygenation reactions through dioxygen activation. The mild oxidation system transfers electrons from the donor to the effective catalytic acceptor upon visible light irradiation, which are subsequently captured by molecular oxygen to form the superoxide radical ion, as demonstrated by the specific fluorescent probe, dihydroethidine (DHE). Furthermore, this visible-light mediated oxidative EDA protocol is successfully applied in the aerobic oxygenation of boronic acids. We believe that this photochemical dioxygen activation strategy enabled by EDA complex not only provides a practical approach to aerobic oxygenation but also promotes the design and application of EDA photocatalysis under ambient conditions.

Regiodivergent Radical Termination for Intermolecular Biocatalytic C-C Bond Formation

Radical hydrofunctionalizations of electronically unbiased dienes are challenging to render regioselective, because the products are nearly identical in energy. Here, we report two engineered FMN-dependent "ene"-reductases (EREDs) that catalyze regiodivergent hydroalkylations of cyclic and linear dienes. While previous studies focused exclusively on the stereoselectivity of alkene hydroalkylation, this work highlights that EREDs can control the regioselectivity of hydrogen atom transfer, providing a method for selectively preparing constitutional isomers that would be challenging to prepare using traditional synthetic methods. Engineering the ERED from Gluconabacter sp. (GluER) furnished a variant that favors the γ,δ-unsaturated ketone, while an engineered variant from a commercial ERED panel favors the δ,ε-unsaturated ketone. The effect of beneficial mutations has been investigated using substrate docking studies and the mechanism probed by isotope labeling experiments. A variety of α-bromo ketones can be coupled with cyclic and linear dienes. These interesting building blocks can also be further modified to generate difficult-to-access heterocyclic compounds.

Ground State Generation and Cyclization of Aminium Radicals in the Formation of Tetrahydroquinolines

This paper reports the first examples of ground state radical-mediated intramolecular C-H amination to afford 1-methyl-1,2,3,4-tetrahydroquinolines from N-2,4-dinitrophenoxy derivatives of arylpropylamines. Whereas the photoactivation of N-2,4-dinitrophenoxyamines for intermolecular reactions has been established, ground state chemistry provides the desired cyclization products in moderate to excellent yields using Ru(bpy)3Cl2 (42-95% yields) under acidic conditions under an air atmosphere.

Access to pyrrolines and fused diaziridines by selective radical addition to homoallylic diazirines

Pyrroline derivatives are common in bioactive natural products and therapeutic agents. We report here a synthesis of pyrrolines and fused diaziridines by divergent radical cyclization of homoallylic diazirines, which can serve as an internal radical trap and a nitrogen source. This reaction proceeds by selective radical addition to C[double bond, length as m-dash]C or N[double bond, length as m-dash]N bonds followed by intramolecular cyclization. Frontier molecular orbital analysis provides a deep insight into the origin of the selectivity. The reaction demonstrates a new cyclization mode, broad functional group compatibility and high product diversity, and reveals a much broader chemical space for diazirine studies.

Photoexcitation of (diarylmethylene)amino benziodoxolones for alkylamination of styrene derivatives with carboxylic acids

The alkylamination of alkenes using pristine carboxylic acids was achieved by the photoexcitation of (diarylmethylene)amino benziodoxolones (DABXs), which serve as both an oxidant and an aminating reagent (an iminyl radical precursor). The developed method is a simple photochemical reaction without the need for external photosensitizers and shows a broad substrate scope for aliphatic carboxylic acids leading to the formation of primary, secondary, and tertiary alkyl radicals, thus enabling the facile synthesis of various structurally complex amines. Mechanistic investigations including transient absorption spectroscopy measurements using a laser flash photolysis (LFP) method disclosed the unique photochemical reactivity of DABXs, which undergoes homolysis of their I-N bonds to give an iminyl radical and ortho-iodobenzoyloxy radical, the latter of which participates in the single-electron oxidation of carboxylates.

Catalytic π-π Interactions Triggered Photoinduced Synthesis of Biaryls

A highly regioselective photocatalytic method to access a variety of biaryl motifs under metal-free conditions has been developed. The organophotocatalyst is involved in π-π stacking interactions with the alkyne species, which promotes this photocatalytic process with thiophene. Mechanistic studies have shed light on these interactions and the overall process. Along with a broad functional-group tolerance and excellent regioselectivity, this protocol has been utilized in the late-stage functionalization of pharmaceuticals and other natural products.

Metal-organic framework boosts heterogeneous electron donor-acceptor catalysis

Metal-organic framework (MOF) is a class of porous materials providing an excellent platform for engineering heterogeneous catalysis. We herein report the design of MOF Zr-PZDB consisting of Zr6-clusters and PZDB (PZDB = 4,4'-(phenazine-5,10-diyl)dibenzoate) linkers, which served as the heterogeneous donor catalyst for enhanced electron donor-acceptor (EDA) photoactivation. The high local concentration of dihydrophenazine active centers in Zr-PZDB can promote the EDA interaction, therefore resulting in superior catalytic performance over homogeneous counterparts. The crowded environment of Zr-PZDB can protect the dihydrophenazine active center from being attacked by radical species. Zr-PZDB efficiently catalyzes the Minisci-type reaction of N-heterocycles with a series of C-H coupling partners, including ethers, alcohols, non-activated alkanes, amides, and aldehydes. Zr-PZDB also enables the coupling reaction of aryl sulfonium salts with heterocycles. The catalytic activity of Zr-PZDB extends to late-stage functionalization of bioactive and drug molecules, including Nikethamide, Admiral, and Myristyl Nicotinate. Systematical spectroscopy study and analysis support the EDA interaction between Zr-PZDB and pyridinium salt or aryl sulfonium salt, respectively. Photoactivation of the MOF-based EDA adduct triggers an intra-complex single electron transfer from donor to acceptor, giving open-shell radical species for cross-coupling reactions. This research represents the first example of MOF-enabled heterogeneous EDA photoactivation.

Synthesis of Isoquinolines and Related Heterocycles under Visible-Light Conditions

Visible-light catalysis is now widely acknowledged as a potent approach for efficiently constructing various types of frameworks, particularly bioactive compounds, with high yields. However, there is still a strong need for further advancements in these methods because certain delicate compounds cannot be synthesized due to structural sensitivity. In this context, a seamless reaction pathway toward 6-membered ring nitrogen-containing heterocycles is presented here, offering access to isoquinoline derivatives and related heterocycles.

Photochemical Oximesulfonylation of Alkenes Using Sulfonyl-Oxime-Ethers as Bifunctional Reagents

Utilization of oxime ethers as bifunctional reagents remains unknown. Herein, we present a mechanistically distinct strategy that enables oximesulfonylation of olefins using sulfonyl-oxime-ethers as bifunctional reagents under metal-free photochemical conditions. Via concomitant C-S and C-C bond formation, the process permits incorporation of oxime and sulfonyl groups into olefins in a complete atom-economic fashion, providing rapid access to multi-functionalized β-sulfonyl oxime ethers with good yields and stereoselectivity. The method is amenable to functionalization of complex bioactive molecules and is shown to be scalable. A radical chain mechanism initiated via photochemical Hydrogen Atom Transfer (HAT) mediated N-O bond cleavage is suggested for the process, based on our results on mechanistic investigations.

Access to Chromenopyrrolidines Enabled by Organophotocatalyzed [2 + 2 + 1] Annulation of Chromones with N-Arylglycines

A facile approach toward chromenopyrrolidines was achieved under mild conditions via organophotocatalyzed aerobic decarboxylative [2 + 2 + 1] annulation of chromones with N-arylglycines, in which N-arylglycines perform dual roles (i.e., radical precursor and methylene provider). Mechanistic studies suggested that a Giese-type radical addition and consequent Mannich pathway were likely responsible for the annulation reaction.

Photo-induced imino functionalizations of alkenes via intermolecular charge transfer

A catalyst-free photosensitized strategy has been developed for regioselective imino functionalizations of alkenes via the formation of an EDA complex. This photo-induced protocol facilitates the construction of structurally diverse β-imino sulfones and vinyl sulfones in moderate to high yields. Mechanistic studies reveal that the reaction is initiated with an intermolecular charge transfer between oximes and sulfinates, followed by fragmentation to generate a persistent iminyl radical and transient sulfonyl radical. This catalyst-free protocol also features excellent regioselectivity, broad functional group tolerance and mild reaction conditions. The late stage functionalization of natural product derived compounds and total synthesis of some bioactive molecules have been demonstrated to highlight the utility of this protocol. Meanwhile, the compatibility of different donors has proved the generality of this strategy.

Photocatalytic Intramolecular Alkene Hydroamination of N-Alkoxy Ureas: An Approach to Imidazolinones

Imidazolinones were obtained in good yields by intramolecular hydroamination of N-alkoxy ureas in the presence of an organic photocatalyst and an inorganic base. In this reaction, the N-alkoxy urea anion generated by deprotonation undergoes photocatalyzed single-electron-transfer oxidation to generate the corresponding radical, which cyclizes to afford the imidazolinone ring. This new protocol grants access to an array of complex molecules containing a privileged imidazolinone core.

Net-1,2-Hydrogen Atom Transfer of Amidyl Radicals: Toward the Synthesis of 1,2-Diamine Derivatives

Hydrogen atom transfer (HAT) processes are among the most useful approaches for the selective construction of C(sp3)-C(sp3) bonds. 1,5-HAT with heteroatom-centered radicals (O•, N•) have been well established and are favored relative to other 1,n-HAT processes. In comparison, net 1,2-HAT processes have been observed infrequently. Herein, the first amidyl radicalls are reported that preferentially undergo a net 1,2-HAT over 1,5-HAT. Beginning with single electron transfer from 2-azaallyl anions to N-alkyl N-aryloxy amides, the latter generate amidyl radicals. The amidyl radical undergoes a net-1,2-HAT to generate a C-centered radical that participates in an intermolecular radical-radical coupling with the 2-azaallyl radical to generate 1,2-diamine derivatives. Mechanistic and EPR experiments point to radical intermediates. Density functional theory calculations provide support for a base-assisted, stepwise-1,2-HAT process. It is proposed that the generation of amidyl radicals under basic conditions can be greatly expanded to access α-amino C-centered radicals that will serve as valuable synthetic intermediates.

Sulfonium Salts as Acceptors in Electron Donor-Acceptor Complexes

The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. Electron donor-acceptor (EDA) complexation, however, imposes electronic constraints on the donor and acceptor components and this can limit the range of radicals that can be generated using the approach. New EDA complexation strategies exploiting sulfonium salts allow radicals to be generated from native functionality. For example, aryl sulfonium salts, formed by the activation of arenes, can serve as the acceptor components in EDA complexes due to their electron-deficient nature. This "sulfonium tag" approach relaxes the electronic constraints on the parent substrate and dramatically expands the range of radicals that can be generated using EDA complexation. In this review, these new applications of sulfonium salts will be introduced and the areas of chemical space rendered accessible through this innovation will be highlighted.

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.

Strategies Toward the Catalyst-Free α-C-H Functionalizations of Tertiary Amines

α-C-H functionalization of tertiary amines has been a highly studied field for the past two decades because several important nitrogen containing heterocycles or compounds can be synthesized through this strategy. Though transition metal catalysts and some metal-free catalysts are mainly used for these reactions, a few catalyst-free reactions have recently been efficiently performed. Catalyst-free reactions are cost-effective, less sensitive to air/moisture, easier to operate, have a simple purification process, and are relatively environment-friendly. In this article, we have summarized all the α-C-H functionalization reactions of tertiary amines performed without using any external catalysts. The content of this article will undoubtedly encourage readers to do more work in this area.

Modular synthesis of unsaturated aza-heterocycles via copper catalyzed multicomponent cascade reaction

The unsaturated aza-heterocycles such as tetrahydropyridines pose significant applications in both drug discovery and development. However, the methods to construct polyfunctionalized tetrahydropyridines are still limited. Herein, we report a modular synthesis of tetrahydropyridines via copper catalyzed multicomponent radical cascade reaction. The reaction features mild conditions and broad substrate scope. In addition, the reaction could scale up to gram scale with similar yield. A variety of 1,2,5,6-tetrahydropyridines with C3 and C5 substituents could be assembled from simple starting materials. More importantly, the products could serve as versatile intermediate to access various functionalized aza-heterocycles which further demonstrates its utility.

Recent Advances in Molecule Synthesis Involving C-C Bond Cleavage of Ketoxime Esters

The synthetic strategies of oxime derivatives participating in radical-type reactions have been rapidly developed in the last few decades. Among them, the N–O bond cleavage of oxime esters leading to formation of nitrogen-centered radicals triggers adjacent C–C bond cleavage to produce carbon-centered free radicals, which has been virtually used in organic synthesis in recent years. Herein, we summarized the radical reactions involving oxime N–O bond and C–C bond cleavage through this special reaction form, including those from acyl oxime ester derivatives and cyclic ketoxime ester derivatives. These contents were systematically classified according to different reaction types. In this review, the free radical reactions involving acyl oxime esters and cyclic ketoxime esters after 2021 were included, with emphasis on the substrate scope and reaction mechanism.

Nickel-Catalyzed Reductive Iminoarylation of Oxime Ester-Tethered Alkenes: Rapid Entrance to Diverse Functionalized Pyrrolines

Herein, we disclose a general and practical iminoarylation of alkenes by nickel-catalyzed reductive cross-coupling of unsaturated oxime esters with readily available aryl halides, providing an expedient approach for constructing pyrroline derivatives. The absence of organometallic reagents enables the reaction to occur under mild conditions with a broad substrate scope and good functional group tolerance. Moreover, other C-based electrophiles, including alkenyl, alkynyl and alkyl halides, or pseudohalides, were also competent substrates for this reaction.

Visible Light-Mediated Cyclisation Reaction for the Synthesis of Highly-Substituted Tetrahydroquinolines and Quinolines

Condensation of 2-vinylanilines and conjugated aldehydes followed by an efficient light-mediated cyclisation selectively yields either substituted tetrahydroquinolines with typically high dr, or in the presence of an iridium photocatalyst the synthesis of quinoline derivatives is demonstrated. These atom economical processes require mild conditions, with the substrate scope demonstrating excellent site selectivity and functional group tolerance, including azaarene-bearing substrates. A thorough experimental mechanistic investigation explores multiple pathways and the key role that imine and iminium intermediates play in the absorption of visible light to generate reactive excited states. The synthetic utility of the reactions is demonstrated on gram scale quantities in both batch and flow, alongside further manipulation of the medicinally relevant products.

A general arene C-H functionalization strategy via electron donor-acceptor complex photoactivation

The photoactivation of electron donor-acceptor complexes has emerged as a sustainable, selective and versatile strategy for the generation of radical species. However, when it comes to aryl radical formation, this strategy remains hamstrung by the electronic properties of the aromatic radical precursors, and electron-deficient aryl halide acceptors are required. This has prevented the implementation of a general synthetic platform for aryl radical formation. Our study introduces triarylsulfonium salts as acceptors in photoactive electron donor-acceptor complexes, used in combination with catalytic amounts of newly designed amine donors. The sulfonium salt label renders inconsequential the electronic features of the aryl radical precursor and, more importantly, it is installed regioselectively in native aromatic compounds by C-H sulfenylation. Using this general, site-selective aromatic C-H functionalization approach, we developed metal-free protocols for the alkylation and cyanation of arenes, and showcased their application in both the synthesis and the late-stage modification of pharmaceuticals and agrochemicals.

Microwave- and Thermally Promoted Iminyl Radical Cyclizations: A Versatile Method for the Synthesis of Functionalized Pyrrolines

A detailed study of iminyl radical cyclizations of O-aryloximes tethered to alkenes is reported. The reactions can be triggered by either microwave irradiation or conventional heating in an oil bath. A variety of radical traps can be employed, enabling C-C, C-N, C-O, C-S, or C-X bond formation and producing a diverse array of functionalized pyrrolines. Substrates containing an allylic sulfide furnish terminal alkenes by a tandem cyclization-thiyl radical β-elimination pathway. Cyclizations of hydroxylated substrates exhibit moderate diastereoselectivity that in some cases can partially be attributed to intramolecular hydrogen bonding. Computational studies suggested a possible role for thermodynamics in controlling the stereochemistry of cyclizations. The reaction temperature can be lowered from 120 to 100 °C by employing O-(p-tert-butylphenyl)oximes instead of O-phenyloximes as substrates, and these second-generation iminyl radical precursors can be used in a one-pot oxime ether formation-cyclization that is promoted by conventional heating. The functionalized pyrrolines obtained from these reactions can be conveniently transformed in several different ways.

Diastereoselective Radical Aminoacylation of Olefins through N-Heterocyclic Carbene Catalysis

There have been significant advancements in radical-mediated reactions through covalent-based organocatalysis. Here, we present the generation of iminyl and amidyl radicals via N-heterocyclic carbene (NHC) catalysis, enabling diastereoselective aminoacylation of trisubstituted alkenes. Different from photoredox catalysis, single electron transfer from the deprotonated Breslow intermediate to O-aryl hydroxylamine generates an NHC-bound ketyl radical, which undergoes diastereocontrolled cross-coupling with the prochiral C-centered radical. This operationally simple method provides a straightforward access to a variety of pyrroline and oxazolidinone heterocycles with vicinal stereocenters (77 examples, up to >19:1 d.r.). Electrochemical studies of the acyl thiazolium salts support our reaction design and highlight the reducing ability of Breslow-type derivatives. A detailed computational analysis of this organocatalytic system suggests that radical-radical coupling is the rate-determining step, in which π-π stacking interaction between the radical intermediates subtly controls the diastereoselectivity.

Chemodivergent photocatalytic access to 1-pyrrolines and 1-tetralones involving switchable C(sp3)–H functionalization

A chemodivergent photocatalytic approach to 1-pyrrolines and 1-tetralones from alkyl bromides and vinyl azides has been developed through chemoselectively controllable intermolecular [3 + 2] and [4 + 2] cyclization. This photoredox-neutral two-component protocol involves intermolecular radical addition and switchable distal C(sp³)–H functionalization enabled by iminyl radical-mediated 1,5-hydrogen atom transfer. Meanwhile, chemoselectivity between C(sp³)–N bond formation and C(sp³)–C(sp²) bond formation is precisely switched by photocatalysts (Ru(bpy)₃(PF₆)₂ vs. fac-Ir(ppy)₃) and additives (base vs. acid).

N-O Bond Activation by Energy Transfer Photocatalysis

A radical shift toward energy transfer photocatalysis from electron transfer photocatalysis under visible-light photoirradiation is often due to the greener prospects of atom and process economy. Recent advances in energy transfer photocatalysis embrace unique strategies for direct small-molecule activation and sometimes extraordinary chemical bond formation in the absence of additional/sacrificial reagents. Selective energy transfer photocatalysis requires careful selection of substrates and photocatalysts for a perfect match with respect to their triplet energies while having incompatible redox potentials to prevent competitive electron transfer pathways. Substrates containing labile N-O bonds are potential targets for generating reactive key intermediates via photocatalysis to access a variety of functionalized molecules. Typically, the differential electron densities of N and O heteroatoms have been exploited for generation of either N- or O-centered radical intermediates from the functionalized substrates by the electron transfer pathway. However, the latest developments involve direct N-O bond homolysis via energy transfer to generate both N- and O-centered radicals for their subsequent utilization in diverse organic transformations, also in the absence of sacrificial redox reagents. In this Account, we highlight our key contributions in the field of N-O bond activation via energy transfer photocatalysis to generate reactive radical intermediates, with coverage of useful mechanistic insights. More specifically, well-designed N-O bond-containing substrates such as 1,2,4-oxadiazolines, oxime esters, N-indolyl carbonates, and N-enoxybenzotriazoles were successfully utilized in versatile transformations involving selective energy transfer over electron transfer from photocatalysts with high triplet state energy. Direct access to reactive N-, O-, and C-centered (if decarboxylation follows) radical intermediates was achieved for diverse cross-couplings and rearrangement processes. In particular, a variety of open-shell nitrogen reactive intermediates, including N(sp²) and N(sp³) radicals and nitrenes, have been utilized. Notably, diversified transformations of identical substrates have been achieved through careful control of the reaction conditions. 1,2,4-Oxadiazolines were converted into spiro-azolactams through iminyl intermediates in the presence of ¹O₂, benzimidazoles, or sulfoximines with external sulfoxide reagent through triplet nitrene intermediates under inert conditions. Besides, oxime esters underwent either intramolecular C(sp³)-N radical-radical coupling or intermolecular C(sp³)-N radical-radical coupling by a combined energy transfer-hydrogen atom transfer strategy. Furthermore, a series of electrochemical and photophysical experiments as well as computational studies were performed to substantiate the proposed selective energy-transfer-driven reaction pathways. We hope that this Account will serve as a guide for the rational design of selective energy transfer processes through the activation of further labile chemical bonds.

Recent advances in visible light-induced C(sp3)–N bond formation

Synthetic chemists have long focused on selective C(sp ³)-N bond-forming approaches in response to the high value of this motif in natural products, pharmaceutical agents and functional materials. In recent years, visible light-induced protocols have become an important synthetic platform to promote this transformation under mild reaction conditions. These photo-driven methods rely on converting visible light into chemical energy to generate reactive but controllable radical species. This Review highlights recent advances in this area, mostly after 2014, with an emphasis placed on C(sp ³)-H bond activations, including amination of olefins and carbonyl compounds, and cross-coupling reactions.

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.

Visible-light-driven C(sp2)-H arylation of phenols with arylbromides enabled by electron donor-acceptor excitation

We have developed a catalyst-free visible-light-driven C(sp²)-H arylation of unprotected phenols with arylbromides to give 2-arylated phenols. This reaction proceeds through the excitation of an electron donor-acceptor complex between a phenolate and an arylbromide, electron transfer, and debrominative C(sp²)-C(sp²) coupling.

Photoinduced remote regioselective radical alkynylation of unactivated C-H bonds

A method for the remote regioselective alkynylation of unactivated C(sp³)-H bonds in diverse aliphatic amides by photogenerated amidyl radicals has been developed. The site-selectivity is dominated via a 1,5-hydrogen atom transfer (HAT) process of the amide. Mild reaction conditions and high regioselectivity are demonstrated in this methodology.

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

For more than 70 years, nitrogen-centered radicals have been recognized as potent synthetic intermediates. This review is a survey designed for use by chemists engaged in target-oriented synthesis. This review summarizes the recent paradigm shift in access to and application of N-centered radicals enabled by visible-light photocatalysis. This shift broadens and streamlines approaches to many small molecules because visible-light photocatalysis conditions are mild. Explicit attention is paid to innovative advances in N-X bonds as radical precursors, where X = Cl, N, S, O, and H. For clarity, key mechanistic data is noted, where available. Synthetic applications and limitations are summarized to illuminate the tremendous utility of photocatalytically generated nitrogen-centered radicals.

Iron-Catalyzed Ring Expansion of Cyclobutanols for the Synthesis of 1-Pyrrolines by Using MsONH3OTf

The synthesis of 1-pyrrolines from cyclobutanol derivatives and an aminating reagent (MsONH₃OTf) has been developed. This one-pot procedure achieves C-N bond/C═N bond formation via ring expansion. A series of 1-pyrroline derivatives are synthesized in moderate to good yields under mild conditions.

Electron donor–acceptor complex-catalyzed photoredox reactions mediated by DIPEA and inorganic carbonates

A DIPEA–NHPI ester–inorganic carbonate catalytic EDA complex is reported as an efficient and sustainable radical generation platform for developing photocatalytic reactions.

Metal-free photocleavage of C(non-acyl)-S bond of thioesters for regioselective pyridylthioesterification of styrenes

Transformation of thioesters via transition-metal-mediated C(acyl)−S bond cleavage is an emerging method to forge C-C and C-heteroatom bonds. Herein, we report the first activation of stronger C(non-acyl)–S bond of thioesters...

Visible-light-promoted radical amidoarylation of arylacrylamides towards amidated oxindoles

A visible-light-promoted intermolecular radical amidation/cyclization of arylacrylamides was realized by using N-aminopyridinium salts as the source of primary amidyl radicals. The reaction exhibits a broad scope and good functionality tolerance,...