Pyridinium Salts as Redox‐Active Functional Group Transfer Reagents

Charge-Enhanced Reactivity of Esters by a Cationic Substituent

In this study, the high electrophilicity of carbonyl carbons attached to cationic heterocycles was observed. Triazolium-substituted esters underwent catalyst-free amidation with aliphatic amines at -50 °C and reduction with NaBH4 at -100 °C. The origin and generality of the high reactivity of these esters were systematically investigated. The findings of this work were utilized for the postmodification of N-heterocyclic carbenes, which are utilized as promising ligands in a wide range of transition-metal-catalyzed reactions.

Exploration of Vitamin B6-Based Redox-Active Pyridinium Salts Towards the Application in Aqueous Organic Flow Batteries

Pyridoxal hydrochloride, a vitamin B6 vitamer, was synthetically converted to a series of diverse redox-active benzoyl pyridinium salts. Cyclic voltammetry studies demonstrated redox reversibility under basic conditions, and two of the most promising salts were subjected to laboratory-scale flow battery tests involving galvanostatic cycling at 10 mM in 0.1 M NaOH. In these tests, the battery was charged completely, corresponding to the transfer of two electrons to the electrolyte, but no discharge was observed. Both CV analysis and electrochemical simulations confirmed that the redox wave observed in the experimental voltammograms corresponds to a two-electron process. To explain the irreversibility in the battery tests, we conducted bulk electrolysis with the benzoyl pyridinium salts, affording the corresponding benzylic secondary alcohols. Computational studies suggest that the reduction proceeds in three consecutive steps: first electron transfer (ET), then proton-coupled electron transfer (PCET) and finally proton transfer (PT) to give the secondary alcohol. 1H NMR deuterium exchange studies indicated that the last PT step is not reversible in 0.1 M NaOH, rendering the entire redox process irreversible. The apparent reversibility observed in CV at the basic media likely arises from the slow rate of the PT step at the timescale of the measurement.

Expedient Access to 18F-Fluoroheteroarenes via Deaminative Radiofluorination of Aniline-Derived Pyridinium Salts

Herein, we disclose that pyridinium salts derived from abundant (hetero)anilines represent a novel precursor class for nucleophilic aromatic substitution reactions with [18F]fluoride. The value of this new 18F-fluorodeamination is demonstrated with the synthesis of over 30 structurally diverse and complex heteroaryl 18F-fluorides, several derived from scaffolds that were yet to be labelled with fluorine-18. The protocol tolerates heteroarenes and functionalities commonly found in drug discovery libraries, and is amenable to scale-up and automation on a commercial radiosynthesiser.

Electrochemical Benzylic C-H Amination via N-Aminopyridinium

An electrochemical protocol for benzylic C(sp3)-H aminopyridylation via direct C-H/N-H cross-coupling of alkylarenes with N-aminopyridinium triflate has been developed. This method features excellent site-selectivity, broad substrate scope, redox reagent-free and facile scalability. The generated benzylaminopyridiniums can be readily converted to benzylamines via electroreductive N-N bond cleavage.

Morita-Baylis-Hillman Spirannulation under Phosphine- and Anion-Binding Catalysis

We introduce an advancement in Morita-Baylis-Hillman (MBH) chemistry that provides access to α-spirannulated enones. The treatment of enone-tethered azaarenium salts with catalytic amounts of organophosphines provides spiroindenyl dihydropyridines. It represents the α-spirannulation of enones via an intramolecular MBH (IMBH) reaction utilizing dual phosphine- and anion-binding catalysis. The IMBH adducts were subjected to several post-synthetic modifications to access highly functionalized molecules.

Review and Theoretical Analysis of Fluorinated Radicals in Direct CAr-H Functionalization of (Hetero)arenes

We highlight key contributions in the field of direct radical CAr- H (hetero)aromatic functionalization involving fluorinated radicals. A compilation of Functional Group Transfer Reagents and their diverse activation mechanisms leading to the release of radicals are discussed. The substrate scope for each radical is analyzed and classified into three categories according to the electronic properties of the substrates. Density functional theory computational analysis provides insights into the chemical reactivity of several fluorinated radicals through their electrophilicity and nucleophilicity parameters. Theoretical analysis of their reduction potentials also highlights the remarkable correlation between electrophilicity and oxidizing ability. It is also established that highly fluorinated radicals (e.g. ⋅OCF3) are capable of engaging in single-electron transfer (SET) processes rather than radical addition, which is in good agreement with experimental literature data. A reactivity scale, based on activation barrier of addition of these radicals to benzene is also elaborated using the high accuracy DLPNO-(U)CCSD(T) method.

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.

Hydroalkoxylation of Enamides Catalyzed by N-Heteroarenium Iodides

A facile and novel N-heteroarenium iodide-catalyzed hydroalkoxylation of enamides has been described. The protocol provides easy access to N,O-acetals, which proved to be a versatile synthetic synthon. The hydrosulfonylation, hydroamination, and hydrophosphorylation products of enamide could be indirectly provided from N,O-acetals. The reaction mechanism was further investigated, which indicated that the hydroalkoxylation of enamides was driven by weak coordination between enamide and the contact ion pair of N-heteroarenium iodide.

Synthesis of Phenol-Pyridinium Salts Enabled by Tandem Electron Donor-Acceptor Complexation and Iridium Photocatalysis

Herein, we describe a dual photocatalytic system to synthesize phenol-pyridinium salts using visible light. Utilizing both electron donor-acceptor (EDA) complex and iridium(III) photocatalytic cycles, the C-N cross-coupling of unprotected phenols and pyridines proceeds in the presence of oxygen to furnish pyridinium salts. Photocatalytic generation of phenoxyl radical cations also enabled a nucleophilic aromatic substitution (SNAr) of a fluorophenol with an electron-poor pyridine. Spectroscopic experiments were conducted to probe the mechanism and reaction selectivity. The unique reactivity of these phenol-pyridinium salts were displayed in several derivatization reactions, providing rapid access to a diverse chemical space.

A review of frustrated Lewis pair enabled monoselective C-F bond activation

Frustrated Lewis pair (FLP) bond activation chemistry has greatly developed over the last two decades since the seminal report of metal-free reversible hydrogen activation. Recently, FLP systems have been utilized to allow monoselective C-F bond activation (at equivalent sites) in polyfluoroalkanes. The problem of 'over-defluorination' in the functionalization of polyfluoroalkanes (where multiple fluoro-positions are uncontrollably functionalized) has been a long-standing chemical problem in fluorocarbon chemistry for over 80 years. FLP mediated monoselective C-F bond activation is complementary to other solutions developed to address 'over-defluorination' and offers several advantages and unique opportunities. This perspective highlights some of these advantages and opportunities and places the development of FLP mediated C-F bond activation into the context of the wider effort to overcome 'over-defluorination'.

Hydroxylation of Aryl Sulfonium Salts for Phenol Synthesis under Mild Reaction Conditions

Hydroxylation of aryl sulfonium salts could be realized by utilizing acetohydroxamic acid and oxime as hydroxylative agents in the presence of cesium carbonate as a base, leading to a variety of structurally diverse hydroxylated arenes in 47-95% yields. In addition, the reaction exhibited broad functionality tolerance, and a range of important functional groups (e.g., cyano, nitro, sulfonyl, formyl, keto, and ester) could be well amenable to the mild reaction conditions.

Iodine-Mediated Heterocyclization for the Synthesis of 6-Alkylthio-1,3,5-triazine-2,4-diamines from N-Alkylpyridinium Salts and NH4SCN

An iodine-mediated method for the synthesis of 6-alkylthio-1,3,5-triazine-2,4-diamines by the reaction of N-alkylpyridinium salts and NH4SCN in air is reported. Twenty-seven compounds were obtained under the standard conditions. Pyridinium salts work as benzyl-group transfer reagents to promote the formation of the CBn-SSCN bond and thereby the construction of the triazine skeleton. A plausible mechanism is proposed based on the experimental results and literature survey.

Visible-Light-Induced Photoredox 1,2-Dialkylation of Styrenes with α-Carbonyl Alkyl Bromides and Pyridin-1-ium Salts

A visible-light-driven photoredox dialkylation of styrenes with α-carbonyl alkyl bromides and pyridin-1-ium salts for the synthesis of polysubstituted 1,4-dihydropyridines is reported. This reaction enables the formation of two new C(sp3)-C(sp3) bonds in a single reaction step and provides a strategy that employs pyridin-1-ium salts as the functionalized alkylating reagents via dearomatization to directly trap the resulting alkyl radicals from radical addition of alkenes and then terminate the alkene dialkylation.

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.

Radical-Based cis-Selective Annulations of N,N'-Cyclic Azomethine Imines with N-Sulfonyl Cyclopropylamines

Azomethine imines, broadly known as 1,3-dipoles, efficiently produce synthetically and biologically significant dinitrogen-fused heterocycles via predominantly concerted or ionic pathways. Herein, we describe a radical-based annulation of azomethine imines utilizing visible-light photoredox catalysis for the first time. This strategy enables the synthesis of dinitrogen-fused saturated six-membered cyclic products that have traditionally been difficult to access. Notably, our process exhibits exceptional cis diastereoselectivity, controlled by the anomeric effect. Initial mechanistic investigations reveal a tandem process comprising intermolecular radical addition and intramolecular 6-exo-trig cyclization. This work illustrates potential within the realm of visible-light-driven radical cyclization reactions involving azomethine imines.

Photo-Triggered, Copper(II) Chloride-Catalyzed Radical Hydroalkylation and Hydrosilylation of Vinylboronic Esters To Access Alkylboronic Esters

Alkyl boronic acids and their derivatives constitute vital building blocks in organic synthesis and are important motifs identified in medicinal chemistry. Herein, we present a phototriggered, CuCl2-catalyzed radical hydroalkylation and hydrosilylation of vinylboronic esters to alkylboronic esters. This approach exhibits mild reaction conditions, utilization of easily accessible reagents, and scalability up to a gram scale. Further synthetic transformations of the hydrosilylation products and mechanistic studies are also demonstrated.

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.

Electron-Transfer-Enabled Concerted Nucleophilic Fluorination of Azaarenes: Selective C-H Fluorination of Quinolines

Direct C-H fluorination is an efficient strategy to construct aromatic C-F bonds, but the cleavage of specific C-H bonds in the presence of other functional groups and the high barrier of C-F bond formation make the transformation challenging. Progress for the electrophilic fluorination of arenes has been reported, but a similar transformation for electron-deficient azaarenes has remained elusive due to the high energy of the corresponding Wheland intermediates. Nucleophilic fluorination of electron-deficient azaarenes is difficult owing to the identity of the Meisenheimer intermediate after fluoride attack, from which fluoride elimination to regenerate the substrate is favored over hydride elimination to form the product. Herein, we report a new concept for C-H nucleophilic fluorination without the formation of azaarene Meisenheimer intermediates through a chain process with an asynchronous concerted F--e--H+ transfer. The concerted nucleophilic aromatic substitution strategy allows for the first successful nucleophilic oxidative fluorination of quinolines.

2,2'-Biquinoline-Based Recyclable Electroauxiliaries for the Generation of Alkyl Radicals via C-C Bond Cleavage

Alkyl radical precursors are essential for a wide variety of photocatalytic and 3d-metal-catalyzed C-C bond forming reactions. Neutral organic heterocycles as electroauxiliaries such as 4-alkyl Hantzsch esters have become reliable tools for alkyl radical formation. Here we show that 2,2'-biquinoline-derived alkyl-substituted dihydroquinolines act as competent radical precursors with the ability to form primary, secondary and tertiary alkyl radicals. Hydroalkylation of benzalmalononitriles and N-Boc protected diazenes has been achieved through copper catalysis under mild conditions of 50 °C with good to very good yields of up to 85 %. Furthermore, the dihydroquinolines' reactivity towards a denitrative alkylation of nitroolefins such as β-nitrostyrene was discovered. Most importantly, the released biquinoline can be recycled, which greatly improves the overall atom-economy of these alkyl radical precursors in comparison to previous N-heterocyclic electroauxiliaries.

Photoredox catalysis harvesting multiple photon or electrochemical energies

Photoredox catalysis (PRC) is a cutting-edge frontier for single electron-transfer (SET) reactions, enabling the generation of reactive intermediates for both oxidative and reductive processes via photon activation of a catalyst. Although this represents a significant step towards chemoselective and, more generally, sustainable chemistry, its efficacy is limited by the energy of visible light photons. Nowadays, excellent alternative conditions are available to overcome these limitations, harvesting two different but correlated concepts: the use of multi-photon processes such as consecutive photoinduced electron transfer (conPET) and the combination of photo- and electrochemistry in synthetic photoelectrochemistry (PEC). Herein, we review the most recent contributions to these fields in both oxidative and reductive activations of organic functional groups. New opportunities for organic chemists are captured, such as selective reactions employing super-oxidants and super-reductants to engage unactivated chemical feedstocks, and scalability up to gram scales in continuous flow. This review provides comparisons between the two techniques (multi-photon photoredox catalysis and PEC) to help the reader to fully understand their similarities, differences and potential applications and to therefore choose which method is the most appropriate for a given reaction, scale and purpose of a project.

An interrupted Corey-Chaykovsky reaction of designed azaarenium salts: synthesis of complex polycyclic spiro- and fused cyclopropanoids

Simultaneous dearomatizing spirannulation of pyridinium salts is still in its infancy. Here, we present an organized skeletal remodeling of designed pyridinium salts by utilizing an interrupted Corey-Chaykovsky reaction to access unprecedented and structurally intriguing molecular architectures such as the vicinal bis-spirocyclic indanones and spirannulated benzocycloheptanones. This hybrid strategy rationally merges the nucleophilic features of sulfur ylides with the electrophilic pyridinium salts to achieve the regio- and stereoselective synthesis of new classes of cyclopropanoids. The plausible mechanistic pathways were derived from experimental results and control experiments.

Formal Cross-Coupling of Amines and Carboxylic Acids to Form sp3-sp2 Carbon-Carbon Bonds

Amines and carboxylic acids are abundant synthetic building blocks that are classically united to form an amide bond. To access new pockets of chemical space, we are interested in the development of amine-acid coupling reactions that complement the amide coupling. In particular, the formation of carbon-carbon bonds by formal deamination and decarboxylation would be an impactful addition to the synthesis toolbox. Here, we report a formal cross-coupling of alkyl amines and aryl carboxylic acids to form C(sp³)-C(sp²) bonds following preactivation of the amine-acid building blocks as a pyridinium salt and N-acyl-glutarimide, respectively. Under nickel-catalyzed reductive cross-coupling conditions, a diversity of simple and complex substrates are united in good to excellent yield, and numerous pharmaceuticals are successfully diversified. High-throughput experimentation was leveraged in the development of the reaction and the discovery of performance-enhancing additives such as phthalimide, RuCl₃, and GaCl₃. Mechanistic investigations suggest phthalimide may play a role in stabilizing productive Ni complexes rather than being involved in oxidative addition of the N-acyl-imide and that RuCl₃ supports the decarbonylation event, thereby improving reaction selectivity.

Formation of C(sp2)-C(sp3) Bonds Instead of Amide C-N Bonds from Carboxylic Acid and Amine Substrate Pools by Decarbonylative Cross-Electrophile Coupling

Carbon-heteroatom bonds, most often amide and ester bonds, are the standard method to link together two complex fragments because carboxylic acids, amines, and alcohols are ubiquitous and the reactions are reliable. However, C-N and C-O linkages are often a metabolic liability because they are prone to hydrolysis. While C(sp2)-C(sp3) linkages are preferable in many cases, methods to make them require different starting materials or are less functional-group-compatible. We show here a new, decarbonylative reaction that forms C(sp2)-C(sp3) bonds from the reaction of activated carboxylic acids (via 2-pyridyl esters) with activated alkyl groups derived from amines (via N-alkyl pyridinium salts) and alcohols (via alkyl halides). Key to this process is a remarkably fast, reversible oxidative addition/decarbonylation sequence enabled by pyridone and bipyridine ligands that, under reaction conditions that purge CO(g), lead to a selective reaction. The conditions are mild enough to allow coupling of more complex fragments, such as those used in drug development, and this is demonstrated in the coupling of a typical Proteolysis Targeting Chimera (PROTAC) anchor with common linkers via C-C linkages.

Visible-light-induced reactions of methylenecyclopropanes (MCPs)

Diverse, visible-light-induced transformations of methylenecyclopropanes (MCPs) have been reported in recent years, attracting significant attention from synthetic chemists. As readily accessible strained molecules, MCPs have sufficient reactivity to selectively generate different target products, through reactions with various radical species upon visible-light irradiation under regulated reaction conditions. These transformations can be classified into three subcategories of reaction pathway, forming ring-opened products, cyclopropane derivatives, and alkynes. These products include pharmaceutical intermediates and polycyclic/heterocyclic compounds that are challenging to obtain using traditional methods. This review summarizes the recent advancements in this field.

Photoinduced Dehydrogenative Borylation via Dihydrogen Bond Bridged Electron Donor and Acceptor Complexes

Air-stable amine- and phosphine-boranes are discovered as donors to integrate with pyridinium acceptor for generating photoactive electron-donor-acceptor (EDA) complexes. Experimental results and DFT calculations suggest a dihydrogen bond bridging the donor and acceptor. Irradiating the EDA complex enables an intra-complex single electron transfer to give a boron-centered radical for dehydrogenative borylation with no need of external photosensitizer and radical initiator. The deprotonation of Wheland-like radical intermediate rather than its generation is believed to determine the good ortho-selectivity based on DFT calculations. A variety of α-borylated pyridine derivatives have been readily synthesized with good functional group tolerance.

Synthesis, antitubercular evaluation, and molecular docking studies of hybrid pyridinium salts derived from isoniazid

In the quest to develop potent inhibitors for Mycobacterium tuberculosis, novel isoniazid-based pyridinium salts were designed, synthesized, and tested for their antimycobacterial activities against the H₃₇ Rv strain of Mycobacterium tuberculosis using rifampicin as a standard. The pyridinium salts 4k, 4l, and 7d showed exceptional antimycobacterial activities with MIC₉₀ at 1 µg/mL. The in vitro cytotoxicity and pharmacokinetics profiles of these compounds were established for the identification of a lead molecule using in vivo efficacy proof-of-concept studies and found that the lead compound 4k possesses LC₅₀ value at 25 µg/mL. The in vitro antimycobacterial activity results were further supported by in silico studies with good binding affinities ranging from -9.8 to -11.6 kcal/mol for 4k, 4l, and 7d with the target oxidoreductase DprE1 enzyme. These results demonstrate that pyridinium salts derived from isoniazid can be a potentially promising pharmacophore for the development of novel antitubercular candidates.

Catalyst- and additive-free syntheses of rhodanine and S-alkyl dithiocarbamate derivatives from sulfoxonium ylides

An efficient catalyst- and additive-free facile access to rhodanine and S-alkyl dithiocarbamate derivatives via multi-component reaction of amines, CS₂ and α-ester sulfoxonium ylides in methanol has been described. The new synthetic methods offer excellent synthetic prospects for several functionalized rhodanines and S-alkyl dithiocarbamates with simple operational procedures.

Substituent-Controlled Regioselective Photoinduced Cyclization of N-Allylbenzamides with N-Sulfonylaminopyridinium Salts

The annulation reactions of N-allylbenzamides with N-sulfonylaminopyridinium salts were developed under metal-free photoinduced mild conditions. Substituent-controlled sulfonaminoarylation and sulfonaminooxylation of benzamides were realized: N-allylbenzamides lead to benzosultams, while N-(2-phenylallyl)benzamides give sulfonamidylated oxazoline derivatives. Control experiments indicated that those reactions undergo a radical pathway with arylsulfonamidyl radicals as the intermediates. The aryl C-H bond functionalization in arylsulfonamidyl was involved for the first time to give benzosultams.

A Photoinduced, Nickel-Catalyzed Reaction for the Stereoselective Assembly of C-Linked Glycosides and Glycopeptides

C-Alkyl glycosides and glycoproteins exist in natural products and are prized for their role as carbohydrate mimics in drug design. However, a practical strategy that merges glycosyl donors with readily accessible reagents, derived from abundant carboxylic acid and amine feedstocks, is yet to be conceived. Herein, we show that a nickel catalyst promotes C-C coupling between glycosyl halides and aliphatic acids or primary amines (converted into redox-active electrophiles in one step), in the presence of Hantzsch ester and LiI (or Et₃ N) under blue LED illumination to deliver C-alkyl glycosides with high diastereoselectivity. Mechanistic studies support the photoinduced formation of alkyl radicals that react with a glycosyl nickel species generated in situ to facilitate cross-coupling. Through this manifold, innate CO₂ H and NH₂ motifs embedded within amino acids and oligopeptides are selectively capped and functionalized to afford glycopeptide conjugates through late-stage glycosylation.

Photocatalytic Late-Stage C-H Functionalization

The emergence of modern photocatalysis, characterized by mildness and selectivity, has significantly spurred innovative late-stage C-H functionalization approaches that make use of low energy photons as a controllable energy source. Compared to traditional late-stage functionalization strategies, photocatalysis paves the way toward complementary and/or previously unattainable regio- and chemoselectivities. Merging the compelling benefits of photocatalysis with the late-stage functionalization workflow offers a potentially unmatched arsenal to tackle drug development campaigns and beyond. This Review highlights the photocatalytic late-stage C-H functionalization strategies of small-molecule drugs, agrochemicals, and natural products, classified according to the targeted C-H bond and the newly formed one. Emphasis is devoted to identifying, describing, and comparing the main mechanistic scenarios. The Review draws a critical comparison between established ionic chemistry and photocatalyzed radical-based manifolds. The Review aims to establish the current state-of-the-art and illustrate the key unsolved challenges to be addressed in the future. The authors aim to introduce the general readership to the main approaches toward photocatalytic late-stage C-H functionalization, and specialist practitioners to the critical evaluation of the current methodologies, potential for improvement, and future uncharted directions.

Catalytic photochemical enantioselective α-alkylation with pyridinium salts

We have developed a chiral amine catalyzed enantioselective α-alkylation of aldehydes with amino acid derived pyridinium salts as alkylating reagents. The reaction proceeds in the presence of visible light and in the absence of a photocatalyst via a light activated charge-transfer complex. We apply this photochemical stereoconvergent process to the total synthesis of the lignan natural products (-)-enterolactone and (-)-enterodiol. Mechanistic studies support the ground-state complexation of the reactive components followed by divergent charge-transfer processes involving catalyst-controlled radical chain and in-cage radical combination steps.

Highly selective γ-alkoxylation, γ-amination and γ-alkylation of unbiased enals by means of photoredox catalysis

We report herein a general and highly selective γ-functionalization protocol under visible light irradiation. This mild radical approach enables the expansion of the scope of application to unbiased enals and the introduction of a wide variety of alkoxy, amino and alkyl functionalities in the γ position with complete regioselectivity.

N-Functionalized Pyridinium Salts: A New Chapter for Site-Selective Pyridine C-H Functionalization via Radical-Based Processes under Visible Light Irradiation

The radical-mediated C-H functionalization of pyridines has attracted considerable attention as a powerful tool in synthetic chemistry for the direct functionalization of the C-H bonds of the pyridine scaffold. Classically, the synthetic methods for functionalized pyridines often involve radical-mediated Minisci-type reactions under strongly acidic conditions. However, the site-selective functionalization of pyridines in unbiased systems has been a long-standing challenge because the pyridine scaffold contains multiple competing reaction sites (C2 vs C4) to intercept free radicals. Therefore, prefunctionalization of the pyridine is required to avoid issues observed with the formation of a mixture of regioisomers and overalkylated side products.Recently, N-functionalized pyridinium salts have been attracting considerable attention in organic chemistry as promising radical precursors and pyridine surrogates. The notable advantage of N-functionalized pyridinium salts lies in their ability to enhance the reactivity and selectivity for synthetically useful reactions under acid-free conditions. This approach enables exquisite regiocontrol for nonclassical Minisci-type reactions at the C2 and C4 positions under mild reaction conditions, which are suitable for the late-stage functionalization of bioactive molecules with greater complexity and diversity. Over the past five years, a variety of fascinating synthetic applications have been developed using various types of pyridinium salts under visible light conditions. In addition, a new platform for alkene difunctionalization using appropriately designed N-substituted pyridinium salts as bifunctional reagents has been reported, offering an innovative assembly process for complex organic architectures. Intriguingly, strategies involving light-absorbing electron donor-acceptor (EDA) complexes between pyridinium salts and suitable electron-rich donors further open up new reactivity under photocatalyst-free conditions. Furthermore, we developed enantioselective reactions using pyridinium salts to afford enantioenriched molecules bearing pyridines through single-electron N-heterocyclic carbene (NHC) catalysis.Herein, we provide a broad overview of our recent contributions to the development of N-functionalized pyridinium salts and summarize the cornerstones of organic reactions that successfully employ these pyridinium salts under visible light conditions. The major advances in the field are systematically categorized on the basis of the pyridines' N-substituent, N-X (X = O, N, C, and SO₂CF₃), and its reactivity patterns. Furthermore, the identification of new activation modes and their mechanistic aspects are discussed by providing representative contributions to each paradigm. We hope that this Account will inspire broad interest in the continued innovation of N-functionalized pyridinium salts in the exploration of new transformations.

Photoredox Activation of Anhydrides for the Solvent-Controlled Switchable Synthesis of gem-Difluoro Compounds

The incorporation of the gem-difluoromethylene (CF₂ ) group into organic frameworks is highly sought due to the influence of this unit on the physicochemical and pharmacological properties of molecules. Herein we report an operationally simple, mild, and switchable protocol to access various gem-difluoro compounds that employs chlorodifloroacetic anhydride (CDFAA) as a low-cost and versatile fluoroalkylating reagent. Detailed mechanistic studies revealed that electron-transfer photocatalysis triggers mesolytic cleavage of a C-Cl bond generating a gem-difluoroalkyl radical. In the presence of alkene, this radical species acts as a unique intermediate that, under solvent-controlled reaction conditions, delivers a wide range of gem-difluorinated γ-lactams, γ-lactones, and promotes oxy-perfluoroalkylation. These protocols are flow- and batch-scalable, possess excellent chemo- and regioselectivity, and can be used for the late-stage diversification of complex molecules.

Selective Three-Component 1,2-Aminoalkoxylation of 1-Aryl-1,3-dienes by Dual Photoredox and Copper Catalysis

A three-component 1,2-aminooxygenation reaction of 1,3-dienes by dual photoredox and copper catalysis is described. This protocol uses N-aminopyridinium salts as N-centered radical precursors and nucleophilic alcohols as oxygen sources, providing modular and practical access to 1,2-aminoalkoxylation products with good yields and regioselectivity. Preliminary mechanistic studies support the radical property of the reaction and the involvement of N-centered radical intermediates.

Reductive Cross-Coupling of Unreactive Electrophiles

Transition-metal-catalyzed reductive coupling of electrophiles has emerged as a powerful tool for the construction of molecules. While major achievements have been made in the field of cross-couplings between organic halides and pseudohalides, an increasing number of reports demonstrates reactions involving more readily available, low-cost, and stable, but unreactive electrophiles. This account summarizes the recent results in our laboratory focusing on this topic. These findings typically include deoxygenative C-C coupling of alcohols, reductive alkylation of alkenyl acetates, reductive C-Si coupling of chlorosilanes, and reductive C-Ge coupling of chlorogermanes.The reductive deoxygenative coupling of alcohols with electrophiles is synthetically appealing, but the potential of this chemistry remains to be disclosed. Our initial study focused on the reaction of allylic alcohols and aryl bromides by the combination of nickel and Lewis acid catalysis. This method offers a selectivity that is opposite to that of the classic Tsuji-Trost reactions. Further investigation on the reaction of benzylic alcohols led to the foundation of a dynamic kinetic cross-coupling strategy with applications in the nickel-catalyzed reductive arylation of benzylic alcohols and cobalt-catalyzed enantiospecific reductive alkenylation of allylic alcohols. The titanium catalysis was later established to produce carbon radicals directly from unactivated tertiary alcohols via C-OH cleavage. The development of their coupling reactions with carbon fragments delivers new methods for the construction of all-carbon quaternary centers. These reactions have shown high selectivity for the functionalization of tertiary alcohols, leaving primary and secondary alcohols intact. Alkenyl acetates are inexpensive, stable, and environmentally friendly and are considered the most attractive alkenyl reagents. The development of reductive alkylation of alkenyl acetates with benzyl ammoniums and alkyl bromides offers mild approaches for the conversion of ketones into aliphatic alkenes.Extensive studies in this field have enabled us to extend the cross-electrophile coupling from carbon to silicon and germanium chemistry. These reactions harness the ready availability of chlorosilanes and chlorogermanes but suffer from the challenge of their low reactivity toward transition metals. Under reductive nickel catalysis, a broad range of alkenyl and aryl electrophiles couple well with vinyl- and hydrochlorosilanes. The use of alkyl halides as coupling partners led to the formation of functionalized alkylsilanes. The C-Ge coupling seems less substrate-dependent, and various common chlorogermanes couple well with aryl, alkenyl, and alkyl electrophiles. In general, functionalities such as Grignard-sensitive groups (e.g., acid, amide, alcohol, ketone, and ester), acid-sensitive groups (e.g., ketal and THP protection), alkyl fluoride and chloride, aryl bromide, alkyl tosylate and mesylate, silyl ether, and amine are tolerated. These methods provide new access to organosilicon and organogermanium compounds, some of which are challenging to obtain otherwise.

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.

Visible-Light-Induced C-F and C-N Bond Cleavage for the Synthesis of gem-Difluoroalkenes

Herein, we describe a novel and efficient photoredox catalytic radical addition/defluoroalkylation coupling reaction between primary amines and trifluoromethyl-substituted alkenes. A series of gem-difluoroalkenes were synthesized via C-N bond cleavage of α-3°, α-2°, and α-1° amines under visible light irradiation. This reaction is characterized by a broad substrate scope and good functional group tolerance.

Visible-Light Photoredox-Catalyzed C(sp2)-H Difluoromethoxylation of (Hetero)arenes Utilizing a Shelf-Stable Pyridinium Reagent

Difluoromethoxyarene moieties have been demonstrated to impart desirable physio-chemical properties to organic molecules. Presented herein is a shelf-stable radical difluoromethoxylating reagent that enables facile and direct C(sp²)-H difluoromethoxylation of (hetero)arenes under blue light photoredox catalysis. 4-Cyano-1-(difluoromethoxy)pyridin-1-ium trifluoromethanesulfonate is prepared in one simple step from the parent pyridine N-oxide. The current protocol tolerates a variety of synthetically and pharmacologically relevant functional groups. Applicability toward the late-stage functionalization of APIs and druglike molecules is also demonstrated.

Nickel-Catalyzed Direct Cross-Coupling of Diaryl Sulfoxide with Aryl Bromide

The direct cross-couplings of diaryl sulfoxides with aryl bromides via C-S bond cleavage could be readily accomplished using nickel(II) as the catalyst, 1,2-bis(diphenylphosphino)ethane (dppe) as the ligand, and magnesium turnings as the reducing metal in THF, leading to the corresponding biaryls in moderate to good yields. The reaction exhibited a broad substrate scope and could be applied to a gram-scale synthesis. The "one-pot" reaction, which avoids the utility of presynthesized and moisture-labile organometallic compounds, is operationally simple and step-economic.

Visible-Light-Driven [3 + 2]/[4 + 2] Annulation Reactions of Alkenes with N-Aminopyridinium Salts

The annulation reactions of benzoamidyl radicals with alkenes were realized under visible light irradiation with fac-Ir(ppy)₃ as catalyst and N-aminopyridinium salts as benzoamidyl radical precursors. The reaction can deliver two distinct types of products: in the case of vinyl arenes, [3 + 2] annulation product dihydrooxazoles were yielded exclusively; when alkyl-substituted alkenes were used, on the other hand, it afforded [4 + 2] annulation product dihydroisoquinolinones. Factors determining the reaction consequence were elucidated by DFT calculations.

Nickel-Catalyzed, Manganese-Assisted Denitrogenative Cross-Electrophile-Coupling of Benzotriazinones with Alkyl Halides for ortho-Alkylated Benzamides

A nickel-catalyzed denitrogenative cross-electrophile-coupling of benzotriazinones with unactivated alkyl halides (X = Cl, Br, I) in the presence of manganese powder as a reductant has been developed. The reaction furnishes ortho-alkylated secondary benzamides in modest to good yields under mild conditions. The scope of the reaction is demonstrated with 25 examples, showing good tolerance of steric hindrance and common functional groups, thus providing an efficient protocol to ortho-alkylated benzamide derivatives without the use of preprepared organometallic reagents.

Redox activated amines in the organophotoinduced alkylation of coumarins

The coumarin core represents the quintessential scaffold of many natural products. While C-3 alkylation is easily achievable, effective greener strategies for C-4 alkylation have been less forthcoming. Herein, we report a metal-free photoinduced deaminative strategy for C-4 alkylation of coumarins using redox activated secondary and benzylic amine derived Katritzky pyridinium salts.

Site-Selective Pyridylic C-H Functionalization by Photocatalytic Radical Cascades

An efficient pyridylic C(sp³ )-H functionalization has been developed through photocatalytic radical-mediated fluoroalkylation or cascade reactions. This method is enabled by the reversible formation of alkylidene dihydropyridine intermediates via the facile enolate formation of C4-alkyl N-amidopyridinium salts in the absence of an external base, thereby establishing the conditions necessary for subsequent intermolecular radical trapping. Rapid structural diversification of the pyridylic site can be achieved through photocatalytic multicomponent cascade reactions involving alkene trifluoromethylation, SO₂ -reincorporation, and sulfonyl radical addition. This operationally simple method features a broad substrate scope and high chemoselectivity and offers a unique approach for the rational modification of the heterobenzylic C-H bonds of pyridines and quinolines with uniform site-selective control. Furthermore, experimental and theoretical studies were performed to elucidate the reaction mechanism.

Traceless Benzylic C-H Amination via Bifunctional N-Aminopyridinium Intermediates

C-H amination reactions provide the opportunity to streamline the synthesis of nitrogen-containing organic small molecules. The impact of intermolecular C-H amination methods, however, is currently limited the frequent requirement for the amine precursors to bear activating groups, such as N-sulfonyl substituents, that are both challenging to remove and not useful synthetic handles for subsequent derivatization. Here, we introduce traceless nitrogen activation for C-H amination-which enables application of selective C-H amination chemistry to the preparation of diverse N-functionalized products-via sequential benzylic C-H N-aminopyridylation followed by Ni-catalyzed C-N cross-coupling with aryl boronic acids. Unlike many C-H amination reactions that provide access to protected amines, the current method installs an easily diversifiable synthetic handle that serves as a lynchpin for C-H amination, deaminative N-N functionalization sequences.