When Light Meets Nitrogen-Centered Radicals: From Reagents to Catalysts

Photoredox-Catalyzed Alkene Acylesterification with Acyloxime Esters via C-C and Tertiary C-O Bond Formation

We describe an efficient acyl esterification method for alkenes utilizing acyloxime esters as bifunctional reagents featuring radical acylation and congested C-O bond formation. This approach is characterized by mild photoredox conditions, high step and atom economy, a broad substrate scope, and excellent regioselectivity. A variety of valuable α-acyl hindered alcohol esters, including those obtained via gram-scale synthesis and late-stage functionalization of pharmaceutical molecules, were presented, demonstrating its synthetic potential and practicability.

Photoinduced Radical Relay Reaction of 2-Methylthiolated Phenylacetylenes/Alkynones Initiated by Electron Donor-Acceptor Complexes

A method was found to construct sulfur-containing five- and six-membered heterocyclic alkyl sulfonyl compounds by using visible light and free radicals activated and/or generated by EDA complexes/homolytic cleavage as initiators to stimulate the relay reaction of alkynes/alkynones. This method puts forward a new strategy to initiate alkyl sulfonation of alkynes/alkynones with only a catalytic amount of the initiator. This strategy of generating the initiator by EDA complex activation/homolytic cleavage provides a new idea for the following substances that must be excited.

Photoinduced Autopromoted Ni-Catalyzed Three-Component Arylsulfonation Inspired by Density Functional Theory/Time-Dependent Density Functional Theory-Simulated Photoactive Nickel Species

The structure of the novel photoactive nickel species was simulated by density functional theory (DFT)/time-dependent density functional theory (TD-DFT) calculations. The application of the simplified photoactive nickel catalyst was demonstrated in a photoinduced nickel-catalyzed three-component arylsulfonation of 1,6-enynes. This reaction was autopromoted and proceeded in the absence of an additional photocatalyst. This methodology exhibited mild conditions, a broad substrate scope, and high efficiency.

Biomimetic Dehydrogenative Intermolecular Formal Allylic Amidation of Branched α-Olefins

Allylic amide moieties are commonly encountered in natural products and are privileged structures in pharmaceuticals and agrochemicals. Moreover, because allylic amide can be to converted into an array of high-value motifs, they have been widely employed in organic synthesis. However, the development of catalytic systems for intermolecular allylic amidation of olefins, particularly branched α-olefins, has proven to be challenging. Here, a biomimetic, synergistic catalytic method is reported that combines photoredox, cobalt, and Brønsted base catalysis for the synthesis of substituted allylic amides from branched α-olefins and simple imides without using oxidants. This low-cost, operationally simple method features a broad substrate scope and excellent functional group compatibility. Moreover, it is successfully used for the functionalization of several structurally complex molecules demonstrating the method's potential utility for medicinal chemistry applications. Mechanistic studies revealed that C(sp3)─N bond formation is mediated by a nitrogen-centered radical intermediate, which is generated via a sequence involving deprotonation and single-electron oxidation.

Regio- and Stereoselective β-Sulfonylamination of Alkynes via Photosensitized Bifunctional N-S Bond Homolysis

Nitrogen central radicals (NCRs) are versatile synthetic intermediates for creating functional nitrogen-containing molecules. Herein, a photosensitized β-sulfonylamination of terminal alkynes as well as acetylene has been established by employing N-sulfonyl heteroaromatics as bifunctional reagents (BFRs) to efficiently deliver versatile (E)-β-sulfonylvinylamines with excellent regio- and stereoselectivities. Mechanistic studies suggest a base-accelerated energy transfer (EnT) photocatalysis involving aromatic NCR formation, radical addition to alkynes, and sulfonylation processes.

Visible light-induced cascade sulfonylation/annulation of ortho-allyloxy chalcones with sodium sulfinates for the synthesis of sulfonated chromane derivatives

A visible-light-induced radical cascade reaction for the synthesis of structurally diverse sulfonated chromanes is described. The protocol involves the addition of sulfonyl radicals to ortho-allyloxy chalcones and intramolecular Michael addition reactions in the presence of eosin Y as a photocatalyst. Additionally, this protocol shows that it is also an effective method to construct seven-membered oxygen-containing heterocycles. The method features a wide substrate scope, the use of easily accessible materials and excellent functional group tolerance with high to excellent yields. Control experiments and mechanistic studies indicate that a visible light-induced radical cascade process is involved in the transformation.

N-Carbazolyl π-Radical and Its Antiaromatic Nitrenium Ion: A Threshold Photoelectron Spectroscopic Study

Understanding the structure and properties of heterocyclic radicals and their cations is crucial for elucidating reaction mechanisms as they serve as versatile synthetic intermediates. In this work, the N-carbazolyl radical 1 was generated via pyrolysis and characterized using photoion mass-selected threshold photoelectron spectroscopy coupled with tunable vacuum-ultraviolet synchrotron radiation. The N-centered radical 1 is classified as a π-radical (2B1), with the unpaired electron found to be delocalized over the central five-membered ring of the carbazole. Adiabatic ionization energies corresponding to the transition from radical 1 to its singlet 1+(1A1) and triplet 1+(3B2) cations were determined to be 7.70 ± 0.03 and 8.14 ± 0.03 eV, respectively. The antiaromatic nitrenium ion 1+ exhibits a singlet ground state with an experimental singlet-triplet energy gap (ΔES-T) of -0.44 eV (10.1 kcal/mol), in very good agreement with theory. N-centered radicals are found to have a higher ionization energy than their C-centered analogues due to stabilization of the singly occupied molecular orbital.

Photocatalysis Meets Copper Catalysis: A New Opportunity for Asymmetric Multicomponent Radical Cross-Coupling Reactions

ConspectusIn recent years, radical-mediated cross-coupling reactions have emerged as a compelling strategy for achieving a rich diversity in molecular topologies under benign conditions. However, the inherent high reactivity of radicals presents considerable challenges in controlling reaction pathways and selectivity, which often results in a limited range of substrates and a constrained reaction profile. Given the capacity of visible-light photoredox catalysis to generate a wide variety of reactive radicals and radical ions in a controlled manner and the propensity of copper complexes toward radical species, we envisaged that the synergy between chiral copper catalysts and photoactive catalysts would pave the way for developing innovative strategies. This integration is poised to unlock a broad spectrum of enantioselective multicomponent radical cross-coupling reactions.In this Account, we describe our insights and recent efforts in the realm of enantioselective multicomponent radical cross-coupling reactions. These advancements have been achieved through the innovative application of dual photoredox/copper catalysis or bifunctional copper catalysis under visible light irradiation. Our work is systematically divided into two sections based on the activation modes. The first section focuses on photoinduced copper-catalyzed chiral C-C and C-O bond formation through a radical addition/nucleophilic trap sequence. Our discussion of chiral C-C bond formation is particularly concentrated on the asymmetric carbocyanation and carboarylation of vinylarenes, 1,3-enynes, and 1,3-dienes. Our findings underscore that irradiation with visible light can adeptly modulate the pace of radical generation, thus orchestrating consecutive reaction stages and ensuring the attainment of both chemo- and stereoselectivity. In the domain of chiral C-O bond formation, leveraging carboxylic acids as a nucleophilic oxygen source, we introduce a suite of esterification reactions of benzylic, allylic, and propargylic radicals. These radicals are derived from a variety of radical precursors, showcasing the versatility of our approach. The following section highlights our innovative discovery in the field of dual photoredox/copper catalysis, which enables enantioselective three-component radical transformations via the direct activation of aromatic alkenes. This methodology begins with the generation of formal distonic radical anions through the photocatalytic single-electron reduction of aromatic alkenes, thus, enabling orthogonal reactivity. Employing H2O, D2O, and CO2 as external electrophile agents, we have developed three types of radical cyanofunctionalization reactions: hydrocyanation, deuteriocyanation, and cyanocarboxylation. These reactions provide practical access to diversely functionalized chiral nitriles with high enantiomeric excess.Collectively, these synthetic methodologies highlight the immense potential inherent in the synergistic integration of photocatalysis and asymmetric copper catalysis. This Account aspires to deepen our comprehension of the advantages conferred by these catalytic systems, elucidating the crucial role of photocatalysis in facilitating enantioselective multicomponent radical cross-couplings. We anticipate that this Account will provide valuable insights and stimulate the evolution of innovative methodologies within this rapidly expanding field.

Dearomative hydroamination of heteroarenes catalyzed by the phenolate photocatalyst

Dearomative functionalization of heteroarenes offers an attractive and sustainable approach for the rapid construction of complex 3D heterocyclic scaffolds from planar structures. Despite progress in this field, dearomative amination of heteroarenes via a radical anion intermediate remains a challenge. Here, we report a photoredox-catalyzed dearomative hydroamination of heteroarenes with hydrazodiformates under mild and transition-metal-free reaction conditions. Various benzofurans and benzothiophenes can efficiently participate in this transformation. A series of mechanistic experiments revealed that heteroaryl radical anions are the crucial intermediates, generated through photo-induced electron transfer between the excited phenolate photocatalyst and heteroarenes.

Light-Driven N-Heterocyclic Carbene-Catalyzed Multi-Component Reaction for the Synthesis of β-Amino Ketones

A N-heterocyclic carbene-catalyzed (NHC) three-component reaction involving N-aminopyridinium salts, alkenes, and aldehydes for the synthesis of β-amino ketones is described. In this reaction, N-aminopyridinium salts and the Breslow intermediate, which is generated from NHC and aldehydes, are utilized to undergo a single-electron transfer process, forming a ketyl radical intermediate and amidyl radicals. Subsequent to the formation of the amidyl radical, it undergoes selective capture by alkenes, followed by radical cross-coupling to yield the desired β-amino ketones.

Photoredox/Copper-Cocatalyzed Domino Annulation of Oxime Esters and NH4SCN: Access to Fully Substituted 2-Aminothiazoles

Domino cyclization of oxime esters and NH4SCN facilitated by photoredox and copper cocatalysis has been established. Various structurally diverse fully substituted 2-aminothiazoles have been obtained in good yields at room temperature. It is featured by mild conditions, favorable functional group tolerance, and wide substrate scope. The present reaction is amenable to gram-scale synthesis, which is expected to find potential applications in organic synthesis and drug discovery. A plausible reaction mechanism is proposed.

Visible-Light-Induced Stereoselective Radical trans-Iodoalkylation of Terminal Alkyne with Iodoform

We describe herein a novel stereoselective trans-iodoalkylation protocol by using three components of nucleophilic dicarbonyl compounds, iodoform and terminal alkynes. The generation of tertiary carbon radical species under mild reaction conditions allows this radical addition and stereoselective iodine atom transfer sequence with terminal alkyne to access highly synthetic applicable disubstituted vinyl iodide. The synthetic application of the present three-component photochemical protocol was demonstrated by the gram-scale reaction and product derivatization.

Cooperative NHC and Photoredox Catalyzed Radical Aminoacylation of Alkenes to Tetrahydropyridazines

Tetrahydropyridazines constitute an important structural motif found in numerous natural products and pharmaceutical compounds. Herein, we report an aminoacylation reaction of alkenes that enables the synthesis of 1,4,5,6-tetrahydropyridazines through cooperative N-heterocyclic carbene (NHC) and photoredox catalysis. This approach involves the 6-endo-trig cyclization of N-centered hydrazonyl radicals, generated via single-electron oxidation of hydrazones, followed by a radical-radical coupling step. The mild process tolerates a wide range of common functional groups and affords a variety of tetrahydropyridazines in moderate to high yields. Preliminary investigations using chiral NHC catalysts demonstrate the potential of this protocol for asymmetric radical reactions.

Nickel-Catalyzed Narasaka-Heck Cyclization Carbonylation of Unsaturated Oxime Esters with Arylboronic Acids

The Narasaka-Heck reaction is one of the most straightforward methods for constructing pyrroline derivatives. Herein, we report a novel nickel-catalyzed three-component carbonylation reaction, which cleverly realizes the continuous construction of C(sp3)-N bonds and C(sp3)-C(sp2) bonds and effectively promotes the synthesis of acyl-substituted pyrroline derivatives. Furthermore, this strategy not only expands the conversion pathway of γ,δ-unsaturated oxime esters but also provides a new method for the synthesis of nitrogen-containing heterocyclic compounds.

Catalytic Aerobic Carbooxygenation for the Construction of Vicinal Tetrasubstituted Centers: Application to the Synthesis of Hexasubstituted γ-Lactones

Strategic design for the construction of contiguous tetrasubstituted carbon centers represents a daunting challenge in synthetic organic chemistry. Herein, we report a combined experimental and computational investigation aimed at developing catalytic aerobic carbooxygenation, involving the intramolecular addition of tertiary radicals to geminally disubstituted alkenes, followed by aerobic oxygenation. This reaction provides a straightforward route to various α,α,β,β-tetrasubstituted γ-lactones, which can be readily transformed into hexasubstituted γ-lactones through allylation/translactonization. Computational analysis reveals that the key mechanistic foundation for achieving the developed aerobic carbooxygenation involves the design of endothermic (energetically uphill) C-C bond formation followed by exothermic (energetically downhill) oxygenation. Furthermore, we highlight a unique fluorine-induced stereoelectronic effect that stabilizes the endothermic stereodetermining transition state.

Harnessing the versatility of hydrazones through electrosynthetic oxidative transformations

Hydrazones are important structural motifs in organic synthesis, providing a useful molecular platform for the construction of valuable compounds. Electrooxidative transformations of hydrazones constitute an attractive opportunity to take advantage of the versatility of these reagents. By directly harnessing the electrical current to perform the oxidative process, a large panel of organic molecules can be accessed from readily available hydrazones under mild, safe and oxidant-free reaction conditions. This review presents a comprehensive overview of oxidative electrosynthetic transformations of hydrazones. It includes the construction of azacycles, the C(sp2)-H functionalization of aldehyde-derived hydrazones and the access to diazo compounds as either synthetic intermediates or products. A special attention is paid to the reaction mechanism with the aim to encourage further development in this field.

Deboronative functionalization of alkylboron species via a radical-transfer strategy

With advances in organoboron chemistry, boron-centered functional groups have become increasingly attractive. In particular, alkylboron species are highly versatile reagents for organic synthesis, but the direct generation of alkyl radicals from commonly used, bench-stable boron species has not been thoroughly investigated. Herein, we describe a method for activating C-B bonds by nitrogen- or oxygen-radical transfer that is applicable to alkylboronic acids and esters and can be used for both Michael addition reactions and Minisci reactions to generate alkyl or arylated products.

A review on oxime functionality: an ordinary functional group with significant impacts in supramolecular chemistry

The oxime functional group is pivotal in chemistry, finding extensive applications in medical science, catalysis, organic functional group transformations, and the recognition of essential and toxic analytes. While the coordination chemistry of oxime derivatives has been thoroughly explored and several reviews have been published on this topic in reputable journals, a comprehensive review encompassing various aspects such as crystal engineering, cation and anion recognition, as well as coordination chemistry activities, is still in demand. This feature article highlights the diverse applications of oxime derivatives across multiple domains of chemistry, including medicine, agriculture, crystal engineering, coordination chemistry, and molecular recognition studies. Each of the oxime derivatives in this feature article are meticulously described in terms of their medicinal applications, crop protection, crystal engineering attributes, analyte recognition capabilities, and coordination chemistry aspects. By providing a comprehensive overview of their versatile applications, this article aims to inspire researchers to explore and develop novel oxime-based derivatives for future applications.

Recent advances in C(sp3)-N bond formation via metallaphoto-redox catalysis

The C(sp3)-N bond is ubiquitous in natural products, pharmaceuticals, biologically active molecules and functional materials. Consequently, the development of practical and efficient methods for C(sp3)-N bond formation has attracted more and more attention. Compared to the conventional ionic pathway-based thermal methods, photochemical processes that proceed through radical mechanisms by merging photoredox and transition-metal catalyses have emerged as powerful and alternative tools for C(sp3)-N bond formation. In this review, recent advances in the burgeoning field of C(sp3)-N bond formation via metallaphotoredox catalysis have been highlighted. The contents of this review are categorized according to the transition metals used (copper, nickel, cobalt, palladium, and iron) together with photocatalysis. Emphasis is placed on methodology achievements and mechanistic insight, aiming to inspire chemists to invent more efficient radical-involved C(sp3)-N bond-forming reactions.

Aromatization-driven deconstructive functionalization of spiro dihydroquinazolinones via dual photoredox/nickel catalysis

Aromatization-driven deconstruction and functionalization of spiro dihydroquinazolinones via dual photoredox/nickel catalysis is developed. The aromatization effect was introduced to synergistically drive unstrained cyclic C-C bond cleavage, with the aim of overcoming the ring-size limitation of nitrogen-centered radical induced deconstruction of carbocycles. Herein, we demonstrate the synergistic photoredox/nickel catalyzed deconstructive cross-coupling of spiro dihydroquinazolinones with organic halides. Remarkably, structurally diverse organic halides including aryl, alkenyl, alkynyl, and alkyl bromides were compatible for the coupling. In addition, this protocol is also characterized by its mild and redox-neutral conditions, excellent functional group compatibility, high atom economy, and easy scalability. A telescoped procedure involving condensation and ring-opening/coupling was found to be accessible. This work provides a complementary strategy to the existing radical-mediated C-C bond cleavage of unstrained carbocycles.

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.

Visible-light-induced three-component reactions of α-diazoesters, quinazolinones and cyclic ethers toward quinazoline-based hybrids

An effective approach for the construction of 4-short-chain ether attached carbonyl group-substituted quinazolines was developed. Visible-light-induced three-component reactions of α-diazoesters, quinazolinones, and cyclic ethers, with a broad substrate scope and excellent functional group tolerance, under extremely mild conditions without the need for any additional additives and catalysts, selectively led to quinazoline-based hybrids in good to excellent yields. The synthesized hybrids, which are a conglomeration of a quinazoline, a short-chain ether, and a carbonyl group in one molecular skeleton, have potential for application in the development of new drugs or drug candidates.

Nitrogen-Centered Radicals Derived from Azidonucleosides

Azido-modified nucleosides have been extensively explored as substrates for click chemistry and the metabolic labeling of DNA and RNA. These compounds are also of interest as precursors for further synthetic elaboration and as therapeutic agents. This review discusses the chemistry of azidonucleosides related to the generation of nitrogen-centered radicals (NCRs) from the azido groups that are selectively inserted into the nucleoside frame along with the subsequent chemistry and biological implications of NCRs. For instance, the critical role of the sulfinylimine radical generated during inhibition of ribonucleotide reductases by 2'-azido-2'-deoxy pyrimidine nucleotides as well as the NCRs generated from azidonucleosides by radiation-produced (prehydrated and aqueous) electrons are discussed. Regio and stereoselectivity of incorporation of an azido group ("radical arm") into the frame of nucleoside and selective generation of NCRs under reductive conditions, which often produce the same radical species that are observed upon ionization events due to radiation and/or other oxidative conditions that are emphasized. NCRs generated from nucleoside-modified precursors other than azidonucleosides are also discussed but only with the direct relation to the same/similar NCRs derived from azidonucleosides.

I2 Catalyzed and TBHP/Ammonium-Promoted Conversion of Arylethanone to Nitriles via β-Scission of Iminyl Radicals

Herein, we report a general I2-catalyzed and TBHP/ammonium-promoted conversion of arylethanone to aromatic nitriles under air. This procedure proceeded with the β-scission of iminyl radical, which was facilitated via quenching the released alkyl radical by tert-butyl peroxyl radical leading to peroxide followed with Kornblum-DeLaMare rearrangement. A series of aryl methyl ketone and alkyl aryl ketone worked well with good functional group tolerance in high yields. As such, this metal-free procedure represents a facile, safe, green, and practical procedure in conversion of arylethanone to aromatic nitriles.

Remote Radical Azidation of Unactivated C(sp3)-H Bonds in Sulfamoyl Azides

An efficient method for remote radical C(sp3)-H azidation at unactivated sites is described. C-H functionalization proceeds via intramolecular 1,5-hydrogen atom transfer to N-centered radicals that are generated via azido group transfer and/or fragmentation. The readily installed sulfamoyl azide serves as both an amidyl radical precursor and an azido source. This reaction features excellent site selectivity for tertiary, secondary, primary, and benzylic C(sp3)-H bonds and exhibits broad functional group compatibility.

Cyclic Amine Synthesis via Catalytic Radical-Polar Crossover Cycloadditions

The rapid assembly of valuable cyclic amine architectures in a single step from simple precursors has been recognized as an ideal platform in term of efficiency and sustainability. Although a vast number of studies regarding cyclic amine synthesis has been reported, new synthetic disconnection approaches are still high in demand. Herein, we report a catalytic radical-polar crossover cycloaddition to cyclic amine synthesis triggered from primary sulfonamide under photoredox condition. This newly developed disconnection, comparable to established synthetic approaches, will allow to construct β, β-disubstituted cyclic amine and β-monosubstituted cyclic amine derivatives efficiently. This study highlights the unique utility of primary sulfonamide as a bifunctional reagent, which acts as a radical precursor and a nucleophile. The open-shell methodology demonstrates broad tolerance to various functional groups, drug derivatives and natural products in an economically and sustainable fashion.

Integrating Olefin Carboamination and Hofmann-Löffler-Freytag Reaction by Radical Deconstruction of Hydrazonyl N-N Bond

As a type of elementary organic compounds containing N-N single bond, hydrazone involved chemical conversions are extremely extensive, but they are mainly limited to N2-retention and N2-removal modes. We report herein an unprecedented protocol for the realization of division utilization of the N2-moiety of hydrazone by a radical facilitated N-N bond deconstruction strategy. This new conversion mode enables the successful combination of alkene carboamination and Hofmann-Löffler-Freytag reaction by the reaction of N-homoallyl mesitylenesulfonyl hydrazones with ethyl difluoroiodoacetate under photocatalytic redox neutral conditions. Mechanism studies reveal that the reaction undergoes a radical relay involving addition, crucial remote imino-N migration and H-atom transfer. Consequently, a series of structurally significant ϵ-N-sulphonamide-α,α-difluoro-γ-amino acid esters are efficiently produced via continuous C-C bond and dual C-N bonds forging.

Programmed Heterocycle Synthesis Using Halomucononitriles as Pyridinimine Precursors

Herein we report a method to convert primary amines, ubiquitous motifs found in pharmaceutical libraries, to either imidazo[1,2-a]pyridines or 7-alkyl azaindoles in two steps from known compounds. Using halomucononitrile reagents, we can directly access 5-bromo-6-imino-1-alkyl-1,6-dihydropyridine-2-carbonitriles (pyridinimines) in a single step from primary amines (25-93% yield) through the cyclization of transient aminomucononitrile intermediates. We then demonstrate that these compounds can be readily converted to 7-alkylazaindoles using Sonogashira cross-coupling conditions (13 examples, up to 91% yield). Under oxidative conditions, the pyridinimines serve as directing groups for C-H functionalization reactions to afford imidazo[1,2-a]pyridines. We also studied the mechanism of the cyclization event using DFT calculations and propose that this takes place via sequential base-mediated E/Z isomerization and cyclization steps.

Photoredox-Enabled Deconstructive [5 + 1] Annulation Approach to Isoquinolones from Indanones in Water

We disclose a deconstructive [5 + 1] annulation protocol for the synthesis of isoquinolones through a nitrogen insertion into abundant indanones. This method exploits photoredox-catalyzed ring-opening of oxime esters. The reaction proceeds smoothly with water as the reaction medium and tolerates a range of functional groups on diverse thiophenols, amines, or indanones. Moreover, the representative isoquinolones exhibit promising antifungal activities.

γ-Amino Alcohols via Energy Transfer Enabled Brook Rearrangement

In the long-standing quest to synthesize fundamental building blocks with key functional group motifs, photochemistry in the recent past has comprehensively established its attractiveness. Amino alcohols are not only functionally diverse but are ubiquitous in the biologically active realm of compounds. We developed bench-stable bifunctional reagents that could then access the sparsely reported γ-amino alcohols directly from feedstock alkenes through energy transfer (EnT) photocatalysis. A designed 1,3-linkage across alkenes is made possible by the intervention of a radical Brook rearrangement that takes place downstream to the EnT-mediated homolysis of our reagent(s). A combination of experimental mechanistic investigations and detailed computational studies (DFT) indicates a radical chain propagated reaction pathway.

Catalytic Light-Driven Strategy for Transforming Oximes to Carbonyls

Oxime and carbonyl functional groups serve as powerful chemical hubs for constructing complex synthetic targets and valuable molecular scaffolds. In furthering this value, we report a photopromoted catalytic deoximation protocol for converting oximes and their derivatives to carbonyl functional groups. This strategic approach benefits from the use of renewable light energy input and ambient air conditions, in addition to demonstrating good substrate scope, functional group tolerance, and product yields. In offering, insights into these reactivity mechanistic studies are communicated, and the value of this protocol is further shown through one-pot operations.

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.

Unveiling Mechanistic Insights and Photocatalytic Advancements in Intramolecular Photo-(3 + 2)-Cycloaddition: A Comparative Assessment of Two Paradigmatic Single-Electron-Transfer Models

The synthesis of 1-aminonorbornane (1-aminoNB), a potential aniline bioisostere, through photochemistry or photoredox catalysis signifies a remarkable breakthrough with implications in organic chemistry, pharmaceutical chemistry, and sustainable chemistry. However, an understanding of the underlying mechanisms involved in these reactions remains limited and ambiguous. Herein, we employ high-precision CASPT2//CASSCF calculations to elucidate the intricate mechanisms regulating the intramolecular photo-(3 + 2)-cycloaddition reactions for the synthesis of 1-aminoNB in the presence or absence of the Ir-complex-based photocatalyst. Our investigations delve into radical cascades, stereoselectivity, particularly single-electron-transfer (SET) events, etc. Furthermore, we innovatively introduce and compare two SET models integrating Marcus electron-transfer theory and transition-state theory. These models combined with kinetic data contribute to recognizing the critical control factors in diverse photocatalysis, thereby guiding the design and manipulation of photoredox catalysis as well as the improvement and modification of photocatalysts.

Electrochemical cascade migratory versus ortho-cyclization of 2-alkynylbenzenesulfonamides

Efficient control over several possible reaction pathways of free radicals is the chemical basis of their highly selective transformations. Among various competing reaction pathways, sulfonimidyl radicals generated from the electrolysis of 2-alkynylbenzenesulfonamides undergo cascade migratory or ortho-cyclization cyclization selectively. It is found that the incorporation of an extra 2-methyl substituent biases the selective migration of the acyl- over vinyl-linker of the key spirocyclic cation intermediate and thus serves as an enabling handle to achieve the synthetically interesting yet under-investigated cascade migratory cyclization of spirocyclic cations.

Photocatalytic Multicomponent Annulation of Amide-Anchored 1,7-Diynes Enabled by Deconstruction of Bromotrichloromethane

We present the first example of visible-light-mediated multicomponent annulation of 1,7-diynes by taking advantage of quadruple cleavage olf carbon-halogen bonds of BrCCl3 to generate a C1 synthon, which was adeptly applied to the preparation of skeletally diverse 3-benzoyl-quinolin-2(1H)-one acetates in moderate to good yields. Controlled experiments demonstrated that H2O acted as both oxygen and hydrogen sources, and gem-dichlorovinyl carbonyl compound exhibited as a critical intermediate in this process. The mechanistic pathway involves Kharasch-type addition/6-exo-dig cyclization/1,5-(SN")-substitution/elimination/binucleophilic 1,6-addition/proton transfer/tautomerization sequence.

Visible-Light-Driven Multicomponent Diamination and Oxyamination of Alkene

Herein, we describe an effective method for the synthesis of 2-alkoxyamides and 1,2-diamines through visible-light-mediated difunctionalization of alkenes. N-Aminopyridinium salts were employed as appropriate precursors to generate key amidyl radical intermediates via a photoinduced single-electron transfer (SET) process. The amidyl radicals would react with alkenes, followed by oxidation and nucleophilic addition. Excellent functional group tolerance and good yields demonstrate the synthetic potential of this transformation.

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.

Photoredox radical cyclization reaction of o-vinylaryl isocyanides with acyl chlorides to access 2,4-disubstituted quinolines

We herein report an efficient photoredox radical cyclization reaction of o-vinylaryl isocyanides with acyl chlorides to access a wide range of 2,4-disubstituted quinolines. Preliminary mechanism experiment results suggested that this reaction was initiated by an acyl radical generated from acyl chlorides through a single-electron-transfer (SET) process. This transformation showed good substrate suitability and functional group compatibility at room temperature.

Visible-Light-Enabled Catalytic Approach to N,O-Spirocycles through Amidyl Radical Addition/Cyclization

A rational combination of photoredox catalyst anthraquinone and hydrogen atom transfer (HAT) catalyst methyl thioglycolate allows for the rapid and straightforward conversion of a range of 2-amidated acetylenic alcohols to multifunctional N,O-spirocycles under visible light irradiation. With oxygen as the sole terminal oxidant, these reactions can be carried out efficiently at room temperature without the involvement of transition metals or strong oxidants. The successful application of this mild catalytic strategy in the late-stage functionalization of bioactive skeletons further highlights its practical value.

New Mode of Asymmetric Induction for Enantioselective Radical N-Heterobicyclization via Kinetically Stable Chiral Radical Center

Enantioselective radical N-heterobicyclization of N-allylsulfamoyl azides have been developed via metalloradical catalysis (MRC). The Co(II)-based catalytic system can homolytically activate the organic azides with varied electronic and steric properties for asymmetric radical N-heterobicyclization under mild conditions without the need of oxidants, allowing for stereoselective construction of chiral [3.1.0]-bicyclic sulfamoyl aziridines in excellent yields with high diastereoselectivities and enantioselectivities. The key to achieving the enantioselective radical process relies on catalyst development through ligand design. We demonstrate that the use of new-generation D2-symmetric chiral bridged amidoporphyrin ligand HuPhyrin with judicious variation of the alkyl bridge length can dictate both reactivity and selectivity of Co(II)-based MRC. We present both experimental and computational studies that shed light on the working details of the unprecedented mode of asymmetric induction consisting of enantioface-selective radical addition and stereospecific radical substitution. We showcase the synthetic applications of the resulting enantioenriched bicyclic aziridines through a number of stereospecific transformations.

Photocatalytic Unsymmetrical Diamination of Styrenes, Indoles, and Benzofurans Facilitated by Benzotriazolyl and Iminyl Radicals

Utilizing energy transfer catalysis, this research employed the bifunctional reagents benzotriazole carboxylic acid oxime esters to simultaneously generate benzotriazole and imine radicals. The synthesis of two distinct C-N bonds in a single conversion is showcased through radical addition and radical-radical cross-coupling processes between benzotriazole carboxylic acid oxime ester and olefins. This process facilitates the intermolecular two-component unsymmetrical diamination reaction of olefins. Using this approach, more than 40 benzotriazole-containing molecules were successfully synthesized using styrene, indole, and benzofuran as acceptors, with yields ranging from moderate to excellent.

Photoredox Catalytic Phosphine-Mediated Deoxygenative Hydroacylation of Azobenzenes with Carboxylic Acids

The convenient and precise preparation of N,N'-diarylhydrazides, especially from readily available raw materials, remains highly challenging. Here, a photoredox catalytic phosphine-mediated deoxygenative hydroacylation of azobenzenes with abundant and readily available carboxylic acids has been developed. With Ir[dF(CF3)ppy]2(dtbbpy)PF6 as the photocatalyst, the reactions proceeded smoothly in the presence of PPh3 under visible light irradiation, delivering various N,N'-diarylhydrazides in up to 92% yields. Mechanistic studies revealed that the reaction proceeds via photoredox catalysis and phosphoranyl-radical-mediated C-O bond cleavage of carboxylic acids.

Photoredox-Catalyzed Tandem Cyclization of Enaminones with N-Sulfonylaminopyridinium Salts toward the Synthesis of 3-Sulfonaminated Chromones

A photoredox-catalyzed intermolecular tandem sulfonamination/cyclization of enaminones was realized by using N-aminopyridinium salts as the sulfonaminated reagents without transition-metal catalysts or bases. The reaction exhibits a broad scope and good functional group tolerance, good yields, and regioselectivity. Preliminary mechanistic studies support the radical property of the reaction and the involvement of N-centered radical intermediates.

Synthesis of Pyrroloindolines via N-Heterocyclic Carbene Catalyzed Dearomative Amidoacylation of Indole Derivatives

Pyrroloindoline is a privileged heterocyclic motif that is widely present in many natural products and pharmaceutical compounds. Herein, we report an amidyl radical-mediated dearomatization for synthesizing a series of pyrroloindolines via N-heterocyclic carbene catalysis. In this organocatalytic process, the Breslow enolate served as both a single electron donor and an acyl radical equivalent to assemble C3a-acyl pyrroloindolines with a broad substrate scope. Sequential reduction of the indole derivatives provided the analogues of (±)-desoxyeseroline, which exhibited potential anticancer activity.

Modular access to alkylgermanes via reductive germylative alkylation of activated olefins under nickel catalysis

Carbon-introducing difunctionalization of C-C double bonds enabled by transition-metal catalysis is one of most straightforward and efficient strategies to construct C-C and C-X bonds concurrently from readily available feedstocks towards structurally diverse molecules in one step; however, analogous difunctionalization for introducing germanium group and other functionalities remains elusive. Herein, we describe a nickel-catalyzed germylative alkylation of activated olefins with easily accessible primary, secondary and tertiary alkyl bromides and chlorogermanes as the electrophiles to form C-Ge and C-Calkyl bonds simultaneously. This method provides a modular and facile approach for the synthesis of a broad range of alkylgermanes with good functional group compatibility, and can be further applied to the late-stage modification of natural products and pharmaceuticals, as well as ligation of drug fragments. More importantly, this platform enables the expedient synthesis of germanium substituted ospemifene-Ge-OH, which shows improved properties compared to ospemifene in the treatment of breast cancer cells, demonstrating high potential of our protocol in drug development.

A three-component reaction of cyclobutanone oxime esters, sulfur dioxide and N-alkyl-N-methacryloyl benzamides

A three-component reaction of cyclobutanone oxime esters, DABCO·(SO2)2 and N-alkyl-N-methacryloyl benzamides is described. This reaction proceeds without the addition of any oxidant or transition metal, affording sulfonyl-containing isoquinoline-1,3-(2H,4H)-diones in moderate to good yields. Various functional groups are tolerated well in this transformation. Mechanistic studies suggest that a radical pathway is involved, including β-scission, sulfur dioxide insertion, and intramolecular cyclization processes.

Visible-Light-Driven C,N-Selective Heteroarylation of N-Fluoroalkyl Hydroxylamine Reagents with Quinoxalin-2(1H)-ones

Herein, we disclose a direct and powerful strategy for the synthesis of highly valuable α-trifluoromethylamine and N-trifluoroethylamine derivatives from a visible-light-promoted C,N-selective heteroarylation of N-trifluoroethyl hydroxylamine reagents with quinoxalin-2(1H)-ones under ambient conditions. The chemoselectivity of the process (trifluoroalkylation or N-trifluoroethylamination) can easily be dictated and modulated by a selection of N-trifluoroethyl hydroxylamine substrates. The key to success is the protecting group on the N atom of hydroxylamine reagents, which can control the process of 1,2-H shift of the in situ-generated N-trifluoroethyl radical. Remarkable features of this method include mild conditions, easy operation, high selectivity, and excellent functional group tolerability. More importantly, the trifluoroalkylated products can be readily derivatized into other interesting imidazo-fused heterocycles that would be of great potential for the exploitation of pharmaceutically relevant molecules.

Three-Component Photochemical 1,2,5-Trifunctionalizations of Alkenes toward Densely Functionalized Lynchpins

Radical remote 1,n-difunctionalization reactions (n > 2) of alkenes are powerful tools to efficiently introduce functional groups with selected distances into target molecules. Among these reactions, 1,5-difunctionalizations are an important subclass, leading to sought-after scaffolds, but typically suffer from tailored starting materials and strict limitations for the formed functional group in 2-position. Seeking to address these issues and to make radical 1,5-difunctionalizations of alkenes more applicable, we report a novel three-component 1,2,5-trifunctionalization reaction between imine-based bifunctional reagents and two distinct alkenes, driven by visible light energy transfer-catalysis. Key to achieving this selective one-step installation of three different functional groups via the choreographed formation of four bonds was the utilization of a 1,2-boron shift and the rigorous capitalization of radical polarities and stabilities. Thorough mechanistic studies were carried out, and the synthetic utility of the obtained products was demonstrated by various downstream modifications. Notably, in addition to the functionalization of individual functional groups, their interplay gave rise to a unique array of cyclic products.

Visible-Light-Promoted Cascade Cyclization of 3-Ethynyl-[1,1'-biphenyl]-2-Carbonitriles with Unsaturated α-Bromocarbonyls

A visible-light-promoted cascade cyclization of 3-ethynyl-[1,1'-biphenyl]-2-carbonitriles with unsaturated α-bromocarbonyls for the synthesis of tetrahydrobenzo[mn]cyclopenta[b]acridines is described. Three C(sp3)-C(sp2) bonds, one C(sp2)-N bond, and three cycles can be formed in a single reaction through the addition of a C-centered radical to the carbon-carbon triple bond and three radical cyclizations. This reaction features mild conditions, wide substrate scope, and high bond-forming efficiency.

Recent Advances in Photochemical Asymmetric Three-Component Reactions

Over the past decades, asymmetric photochemical synthesis has garnered significant attention for its sustainability and unique ability to generate enantio-enriched molecules through distinct reaction pathways. Photochemical asymmetric three-component reactions have demonstrated significant potential for the rapid construction of chiral compounds with molecular diversity and complexity. However, noteworthy challenges persist, including the participation of high-energy intermediates such as radical species, difficulties in precise control of stereoselectivity, and the presence of competing background and side reactions. Recent breakthroughs have led to the development of sophisticated strategies in this field. This review explores the intricate mechanisms, synthetic applications, and limitations of these methods. We anticipate that it will contribute towards advancing asymmetric catalysis, photochemical synthesis, and green chemistry.