Enantioselective functionalization at the C4 position of pyridinium salts through NHC catalysis

Construction of Nonadjacent Stereocenters Through Iridium-Catalyzed Desymmetric Hydroheteroarylation of Cyclopentenes

Transition metal-catalyzed direct addition of (hetero)aryl C─H bond to an alkene provides an expedited route to construct benzylic stereocenter from readily available arene and alkene feedstocks with complete atom-economy. However, creation of more than one stereocenter through enantioselective C─H (hetero)arylation remains a challenging goal. Here we report an iridium-catalyzed desymmetric hydroheteroarylation of cyclopentenes to construct 1,3-nonadjacent stereocenters. A series of heteroaryl C─H bonds were cleaved site-selectively and added regio- and enantioselectively to an unactivated alkene containing an amide coordinating group, delivering valuable enantioenriched cyclopentane scaffolds containing 1,3-tertiary-tertiary or 1,3-quaternary-tertiary stereocenters with exclusive diastereoselectivity and excellent enantioselectivity.

Asymmetric copper-catalyzed hydrophosphinylation of ethynylazaarenes to access P-chiral 2-azaaryl-ethylphosphine oxides

We report a cost-effective approach for the enantioselective hydrophosphinylation of ethynylazaarenes utilizing a chiral copper catalytic platform. This strategy efficiently converts racemic secondary phosphine oxides (SPOs) into P-chiral tertiary phosphine oxides (TPOs) bearing functionalized olefin substituents with azaarene moieties, achieving high yields and exceptional enantioselectivities. These adducts serve as crucial intermediates in the development of valuable chiral 1,5-hybrid P,N-ligands. The facile introduction of diverse additional carbon-centered chirality through the transformation of the olefin moiety effectively enhances the enantioselectivity of asymmetric metal catalysis compared to ligands exhibiting solely P-chirality. Mechanistic investigations reveal that the interaction between the chiral Cu(i) complex and azaarenes promotes the kinetic resolution of SPOs. The robustness of this method is further demonstrated by its ability to incorporate deuterium atoms into the olefins, highlighting its potential relevance in pharmaceutical applications.

Kinetic Resolution of Racemic Radicals in Asymmetric Photoredox Minisci Reactions with Azaarenes for Precise Construction of Two Non-adjacent Stereocenters

Despite the significant potential of photocatalysis as a robust synthetic tool, the high reactivity of radicals often presents challenges in achieving optimal chemoselectivity. In this study, we demonstrate that this inherent limitation can be strategically harnessed for asymmetric photoredox catalysis. By utilizing a chiral catalyst to facilitate kinetic resolution between the two enantiomers of racemic radical intermediates, one enantiomer selectively undergoes the desired transformation, while noncatalytic side reactions deplete the other enantiomer. Consequently, an attractive asymmetric photoredox three-component Minisci-type reaction involving bromides, racemic homoallylic tertiary alcohols or amines, and azaarenes has been developed. This approach enables efficient assembly of tertiary alcohols and amines onto the nonadjacent β-position of an azaarene-functionalized tertiary carbon stereogenic center with high levels of enantio- and diastereoselectivity. Therefore, this method not only allows for direct utilization of readily available racemic feedstocks that are challenging to convert into prochiral radicals via redox processes but also provides an efficient strategy for synthesizing complex molecules with multiple stereocenters.

Copper-Catalyzed Regioselective and Enantioselective Hydropyridylation of Dienes for the Synthesis of Chiral Diaryl Compounds via Concerted Nucleophilic Aromatic Substitution

The synthesis of chiral 1,1-diaryl compounds, particularly those containing a pyridine moiety, is of significant interest due to their pharmaceutical applications. Here, we report the development of a copper-catalyzed enantioselective 1,4-hydropyridylation of conjugated dienes. Utilizing 2-fluoropyridine as the electrophile, CuOAc, and the chiral ligand Tol-BINAP, we optimized reaction conditions to achieve the desired chiral 1,1-diaryl products containing both a pyridine and a cis-crotyl group. Mechanistic studies and DFT calculations revealed that the 1,2-hydrocupration step is enantio-determining, and the concerted nucleophilic aromatic substitution proceeds via six-membered cyclic transition states.

Photoinduced stereoselective reactions using pyridinium salts as radical precursors

Pyridinium salts are amine surrogates that are abundant in nature and the redox active nature of the pyridinium salts allows them to serve as precursors for generating radical species under mild conditions that can be initiated by light, heat or metal catalysis. The stereoselective formation of products has always been a topic of interest for synthetic chemists worldwide. In this context, pyridinium salts can readily undergo single electron reduction to form a neutral radical, and the N-X bond's subsequent fragmentation furnishes the X radical without any harsh reaction conditions. As a consequence, the past decade has witnessed an increased effort in utilizing pyridinium salts to photocatalytically generate radicals for the regioselective, diastereoselective as well as enantioselective formation of products that have been summarised in this review.

Asymmetric Photoredox Catalytic Minisci-Type Reactions of α-Bromide Amides

An asymmetric photoredox catalytic Minisci-type reaction between α-bromide amides and imine-containing azaarenes has been successfully developed. This catalyst system employs a chiral phosphoric acid alongside 3DPAFIPN as a photosensitizer. The reaction produces a diverse array of valuable amides, featuring azaarene-substituted tertiary carbon stereocenters at the β-position, in high yields (up to 85%) and good to excellent enantioselectivities (up to >99% enantiomeric excess (ee)). Importantly, this work marks the first example of asymmetric radical addition to simple azaarenes utilizing radicals functionalized with electron-withdrawing carbonyl groups, which are conventionally considered unfavorable for such transformations, especially in an enantioselective manner.

N-Heterocyclic carbene (NHC) organocatalysis: from fundamentals to frontiers

N-Heterocyclic carbenes (NHCs) have been used as organocatalysts for a multitude of C-C and C-heteroatom bond-forming reactions. They enable diverse modalities of activating a wide range of structurally distinct substrate classes and allow access to electronically distinct intermediates. The easy tunability of the NHC scaffold contributes to its versatility. Recent years have witnessed a surge of interest in various organocatalytic reactions of NHCs, leading to the forays of NHC catalysis into the relatively newer domains such as reactions involving radical intermediates, atroposelective synthesis, umpolung of electrophiles other than aldehydes, and the use of NHCs as non-covalent templates for enantioinduction. This tutorial review provides an overview of various important structural features and reactivity modes of NHCs and delves deep into some frontiers of NHC-organocatalysis.

Advances in Pyridine C-H Functionalizations: Beyond C2 Selectivity

The pyridine core is a crucial component in numerous FDA-approved drugs and Environmental Protection Agency (EPA) regulated agrochemicals. It also plays a significant role in ligands for transition metals, alkaloids, catalysts, and various organic materials with diverse properties, making it one of the most important structural frameworks. However, despite its significance, direct and selective functionalization of pyridine is still relatively underdeveloped due to its electron-deficient nature and the strong coordinating ability of nitrogen. Among the variety of synthetic transformation, direct functionalization of C-H bond is straightforward and atom economical approach and it's advantageous for late-stage functionalization of pyridine containing drugs. In recent years, innovative strategies for regioselective C-H functionalization of pyridines and azines have emerged, offering numerous benefits such as high regioselectivity, mild conditions, and enabling transformations that were challenging with traditional methods. This review emphasizes the latest advancements in meta and para-C-H functionalization of pyridines through various approaches, including pyridine phosphonium salts, photocatalytic methods, temporary de-aromatization, Minisci-type reactions, and transition metal-catalyzed C-H activation techniques. We discuss the advantages and limitations of these current methods and aim to inspire further progress in this significant field.

Tunable Thiazolium Carbenes for Enantioselective Radical Three-Component Dicarbofunctionalizations

Asymmetric N-heterocyclic carbene (NHC) organocatalysis is a cornerstone of synthetic organic chemistry. The emerging concept of single-electron NHC catalysis broadened the scope of C-C bond-forming reactions, facilitating the synthesis of a variety of attractive racemic compounds. However, the development of effective and selective chiral NHC catalysts for asymmetric radical-mediated reactions has been challenging. In this report, we introduce a family of highly tunable chiral thiazolium carbenes with three distinct positions for broad electronic and steric modulation featuring bulky chiral flanking groups. We demonstrate the catalytic efficacy of these chiral carbenes in an enantioselective SET-type three-component acyl-difluoroalkylation of olefins using a broad range of aldehydes and difluoroalkyl bromides. This method provides straightforward access to a diverse set of β-difluoroalkylated α-chiral ketones (65 examples) with an up to 87% yield and excellent enantioselectivities of up to >99:1 er. The utility of this methodology is further outlined by enantio- and diastereoselective late-stage modifications of pharmaceutically relevant compounds and selective twofold orthogonal acyl-difluoroalkylations of linchpin reagents.

C-H Functionalization of Heteroarenes via Electron Donor-Acceptor Complex Photoactivation

C-H Functionalization of heteroarenes stands as a potent instrument in organic synthesis, and with the incorporation of visible light, it emerged as a transformative game-changer. In this domain, electron donor-acceptor (EDA) complex, formed through the pairing of an electron-rich substrate with an electron-accepting molecule, has garnered substantial consideration in recent years due to the related avoidance of the requirement of photocatalyst as well as oxidant. EDA complexes can undergo photoactivation under mild conditions and exhibit high functional group tolerance, making them potentially suitable for the functionalization of biologically relevant heteroarenes. This review article provides an overview of recent advancements in the field of C-H functionalization of heteroarenes via EDA complex photoactivation with literature coverage up to April, 2024.

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.

Manganese Complexes with Consecutive Mn(IV) → Mn(III) Excitation for Versatile Photoredox Catalysis

Manganese complexes stand out as promising candidates for photocatalyst design, attributed to their eco- and biocompatibility, versatile valence states, and capability for facilitating multiple electronic excitations. However, several intrinsic constraints, such as inadequate visible light response and short excited-state lifetimes, hinder effective photoinduced electron transfer and impede photoredox activation of substrates. To overcome this obstacle, we have developed a class of manganese complexes featuring boron-incorporated N-heterocyclic carbene ligands. These complexes enable prolonged excited-state durations encapsulating both Mn(IV) and Mn(III) oxidation stages, with lifetimes reaching microseconds for Mn(IV) and nanoseconds for Mn(III), concurrently exhibiting robust redox capabilities. They efficiently catalyze direct, site-selective cross-couplings between diverse arenes and aryl bromides, at a low catalyst loading of 0.5 mol %. Their proficiency spans an extensive array of substrates including both highly electron-rich and electron-deficient molecules, which underscore the superior performance of these manganese complexes in tackling intricate transformations. Furthermore, the versatility of these complexes is further highlighted by their successful applications in various photochemical transformations, encompassing reductive cross-couplings for the formation of C-P, C-B, C-S and C-Se bonds, alongside oxidative couplings for creating C-N bonds. This study sheds light on the distinctive photoredox properties and the remarkable catalytic flexibility of manganese complexes, highlighting their immense potential to drive progress in photochemical synthesis and green chemistry applications.

Enantioselective Dearomatization of Pyridinium Salts by Copper-Catalyzed C4-Selective Addition of Silicon Nucleophiles

A copper-catalyzed C4-selective addition of silicon nucleophiles released from an Si-B reagent to prochiral pyridinium triflates is reported. The dearomatization proceeds with excellent enantioselectivity using Cu(CH3CN)4PF6 as the precatalyst and (R,R)-Ph-BPE (1,2-bis[(2R,5R)-2,5-diphenylphospholan-1-yl]ethane) as the chiral ligand. A carbonyl group at C3 is required for this, likely acting a weak donor group to preorganize and direct the nucleophilic attack towards C4. The resulting 4-silylated 1,4-dihydropyridines can be further converted into functionalized piperidine derivatives.

Enantioselective Relay Coupling of Perfluoroalkyl and Vinylogous Ketyl Radicals

β-Chiral carboxylic acids and their derivatives are highly valuable structural motifs in the fields of asymmetric synthesis and medicinal chemistry. However, the introduction of a sterically demanding sidechain to the β-carbon, such as an all-carbon quaternary center, remains a significant challenge in classical polar processes. Recently, N-heterocyclic carbene (NHC) mediated coupling reactions involving persistent ketyl radicals have emerged as a promising strategy to assemble highly crowded carbon-carbon bonds. Nevertheless, achieving enantioselectivity in these reactions remains highly challenging. In this work, we report our recent progress in controlling enantioselectivity for relay coupling of perfluoroalkyl and persistent vinylogous ketyl radicals. We developed a chiral bifunctional NHC-squaramide catalyst that achieves high facial selectivity in a critical bond-forming event involving the coupling of a congested tertiary carbon radical and vinylogous ketyl radical. Chiral carboxylates bearing an all-carbon quaternary center at the β-position can be prepared in good yield and excellent enantiomeric excess. Results from density functional theory (DFT) calculations and nuclear Overhauser effect (NOE) experiments indicate that the N,N'-diaryl squaramide motif adopts an unusual syn-syn conformation, enabling hydrogen bonding interactions with the enolate oxygen, thereby rigidifying the overall conformation of the transition state.

N-Heterocyclic carbene catalytic 1,2-boron migrative acylation accelerated by photocatalysis

The transformation of organoboron compounds plays an important role in synthetic chemistry, and recent advancements in boron-migration reactions have garnered considerable attention. Here, we report an unprecedented 1,2-boron migrative acylation upon photocatalysis-facilitated N-heterocyclic carbene catalysis. The design of a redox-active boronic ester substrate, serving as an excellent β-boron radical precursor, is the linchpin to the success of this chemistry. With the established protocol, a wide spectrum of β-boryl ketones has been rapidly synthesized, which could further undergo various C─B bond transformations to give multifunctionalized products. The robustness of this catalytic strategy is underscored by its successful application in late-stage modification of drug-derived molecules and natural products. Preliminary mechanistic investigations, including several control experiments, photochemistry measurements, and computational studies, shed light on the catalytic radical reaction mechanism.

1,3-Difunctionalization of [1.1.1]propellane through iron-hydride catalyzed hydropyridylation

Current methodologies for the functionalization of [1.1.1]propellane primarily focus on achieving 1, 3-difunctionalized bicyclo[1.1.1]pentane or ring-opened cyclobutane moiety. Herein, we report an innovative approach for the 1, 3-difunctionalization of [1.1.1]propellane, enabling access to a diverse range of highly functionalized cyclobutanes via nucleophilic attack followed by ring opening and iron-hydride hydrogen atom transfer. To enable this method, we developed an efficient iron-catalyzed hydropyridylation of various alkenes for C - H alkylation of pyridines at the C4 position, eliminating the need for stoichiometric quantities of oxidants or reductants. Mechanistic investigations reveal that the resulting N-centered radical serves as an effective oxidizing agent, facilitating single-electron transfer oxidation of the reduced iron catalyst. This process efficiently sustains the catalytic cycle, offering significant advantages for substrates with oxidatively sensitive functionalities that are generally incompatible with alternative approaches. The strategy presented herein is not only mechanistically compelling but also demonstrates broad versatility, highlighting its potential for late-stage functionalization.

Visible-Light-Induced Alkoxypyridylation of Alkenes Using N-Alkoxypyridinium Salts as Bifunctional Reagents

Considering the ubiquitous presence of pyridine moieties in pharmaceutical compounds, it holds immense value to develop practical and straightforward methodologies for accessing heterocyclic aromatic hydrocarbons. In recent years, N-alkoxypyridinium salts have emerged as convenient radical precursors, enabling the generation of the corresponding alkoxy radicals and pyridine through single-electron transfer. Herein, we present the first report on visible-light-mediated intermolecular alkoxypyridylation of alkenes employing N-alkoxylpyridinium salts as bifunctional reagents with an exceptionally low catalyst loading (0.5 mol %).

Recent advances in combining photo- and N-heterocyclic carbene catalysis

N-Heterocyclic carbenes (NHCs) are unique Lewis basic catalysts that mediate various organic transformations by means of polarity reversal. Although the scope of research on two-electron reactions mediated by NHC catalysts has been expanding, the types of these reactions are limited by the inability of NHCs to engage sp3-electrophiles. However, the revival of photocatalysis has accelerated the development of free-radical chemistry, and combining photoredox catalysis and NHC catalysis to achieve NHC-mediated radical reactions under mild conditions could overcome the above-mentioned limitation. This review summarizes recent advances in combining photoredox and NHC catalysis, focusing on elucidation and exploration of mechanisms, with the aim of identifying challenges and opportunities to develop more types of catalytic models.

Enantioselective SN 2 Alkylation of Homoenolates by N-Heterocyclic Carbene Catalysis

The functionalization of the β-carbon of enals with electrophiles is a signature umpolung reactivity of N-heterocyclic carbene (NHC) derived homoenolates. However, only a limited number of electrophiles are shown to be compatible, with most of them being π-electrophiles. In this study, the successful enantioselective β-alkylation of homoenolates is reported using Csp3 electrophiles through an SN 2 strategy. The protocol shows a broad scope regarding alkyl electrophiles, delivering good yields, and excellent enantioselectivities (up to 99% ee). It enables the installation of drug-like structural motifs in either enals or alkylating agents, demonstrating its potential as a valuable tool for late-stage modification. Furthermore, a concise synthetic route is presented to chiral pyrroloindoline-type skeletons. Preliminary mechanistic studies support a direct SN 2 mechanism.

Enantioselective Chemodivergent Three-Component Radical Tandem Reactions through Asymmetric Photoredox Catalysis

Chemodivergent synthesis has been achieved in asymmetric photocatalysis. Under a dual catalyst system consisting of a chiral phosphoric acid and DPZ as a photosensitizer, different inorganic bases enabled the formation of two sets of valuable products from the three-component radical tandem transformations of 2-bromo-1-arylenthan-1-ones, styrenes, and quinoxalin-2(1H)-ones. The key to success was the distinct pKa environment, in which the radicals that formed on the quinoxalin-2(1H)-one rings after two radical addition processes underwent either single-electron oxidation or single-electron reduction. In addition, this work represents the first use of quinoxalin-2(1H)-ones in asymmetric photoredox catalysis.

Enantioselective C3-Allylation of Pyridines via Tandem Borane and Palladium Catalysis

Herein, we report a one-pot method for enantioselective C-H allylation of pyridines at C3 via tandem borane and palladium catalysis. This method involves borane-catalyzed pyridine hydroboration to generate dihydropyridines, then palladium-catalyzed enantioselective allylation of the dihydropyridines with allylic esters, and finally air oxidation of the allylated dihydropyridines to afford the products. This method enables the introduction of an allylic group at C3 with excellent regio- and enantioselectivities.

Direct C(3)5-H Polyfluoroarylation of 2-Amino/alkoxy Pyridines Enabled by a Transient and Electron-deficient Palladium Intermediate

Herein, we present an unprecedented azine-limited C5-H polyfluoroarylation of 2-aminopyridines enabled by a transient and electron-deficient perfluoroaryl-Pd species via C-H/C-H coupling. The protocol further allows C3(5)-H polyfluoroarylation of 2-alkoxypyridines guided by sterics and electronics for the first time. The late-stage C-H functionalization of drugs, drug derivatives, and natural product derivatives and synthesis of C5-aryl drug derivatives further demonstrated the method's utility. The preliminary mechanistic studies reveal that the synergistic combination of the bulky yet electrophilic perfluoroaryl-Pd species and the partial nucleophilicity of the C5-position of 2-amino/alkoxy-pyridines is the origin of reactivity and selectivity. Importantly, the first experimental evidence for the role of diisopropyl sulfide is provided.

Direct allylic acylation via cross-coupling involving cooperative N‑heterocyclic carbene, hydrogen atom transfer, and photoredox catalysis

Herein, we report a mild, operationally simple, multicatalytic method for the synthesis of β,γ-unsaturated ketones via allylic acylation of alkenes. Specifically, the method combines N‑heterocyclic carbene catalysis, hydrogen atom transfer catalysis, and photoredox catalysis for cross-coupling reactions between a wide range of feedstock carboxylic acids and readily available olefins to afford structurally diverse β,γ-unsaturated ketones without olefin transposition. The method could be used to install acyl groups on highly functionalized natural-product-derived compounds with no need for substrate pre-activation, and C-H functionalization proceed with excellent site selectivity. To demonstrate the potential applications of the method, we convert a representative coupling product into various useful olefin synthons.

N-Amino Pyridinium Salts in Organic Synthesis

C-N bond forming reactions hold immense significance to synthetic organic chemistry. In pursuit of efficient methods for the introduction of nitrogen in organic small molecules, myriad synthetic methods have been developed, and methods based on both nucleophilic and electrophilic aminating reagents have received sustained research effort. In response to continued challenges - the need for substrate prefunctionalization, the requirement for vestigial N-activating groups, and the need to incorporate nitrogen in ever more complex molecular settings - the development of novel aminating reagents remains a central challenge in method development. N-aminopyridinums and their derivatives have recently emerged as a class of bifunctional aminating reagents, which combine N-centered nucleophilicity with latent electrophilic or radical reactivity by virtue of the reducible N-N bond, with broad synthetic potential. Here, we summarize the synthesis and reactivity of N-aminopyridinium salts relevant to organic synthesis. The preparation and application of these reagents in photocatalyzed and metal-catalyzed transformations is discussed, showcasing the reactivity in the context of bifunctional platform and its potential for innovation in the field.

Visible-Light-Active Coumarin- and Quinolinone-Based Photocatalysts and Their Applications in Chemical Transformations

Organic dyes have been actively studied as useful photocatalysts because they allow access to versatile structural flexibility and green synthetic applications. The identification of a new class of robust organic chromophores is, therefore, in high demand to increase structural diversity and variability. Although coumarins and quinolinones have long been acknowledged as organic chromophores, their ability to participate in photoinduced transformations is somewhat less familiar. Fascinated by their chromophoric features and adaptable platform, our group is interested in the identification of fluorescent bioactive molecules and in the development of new photoinduced synthetic methods using coumarins and quinolinones as photocatalysts. This account provides an overview of our recent progress in the discovery and application of light-absorbing coumarin and quinolinone derivatives in photochemistry and medicinal chemistry.

Site-Selective Pyridine C-H Alkylation with Alcohols and Thiols via Single-Electron Transfer of Frustrated Lewis Pairs

A unified strategy for the deoxygenative or desulfurative pyridylation of various alcohols and thiols has been developed through a single-electron transfer (SET) process of frustrated Lewis pairs (FLPs) derived from pyridinium salts and PtBu₃ . Mechanistic studies revealed that N-amidopyridinium salts serve as effective Lewis acids for the formation of FLPs with PtBu₃ , and the generated phosphine radical cation ionically couples with the in situ generated xanthate, eventually affording the alkyl radical through facile β-scission under photocatalyst-free conditions. The reaction efficiency was further accelerated by visible-light irradiation. This method is conceptually appealing by using encounter complexes in FLP chemistry to promote SET, which provides a previously unrecognized opportunity for the selective heteroarylation of a diverse range of alcohols and thiols with various functional groups, even in complex settings under mild reaction conditions.

Dual role of benzophenone enables a fast and scalable C-4 selective alkylation of pyridines in flow

The efficient C-4 selective modification of pyridines is a major challenge for the synthetic community. Current strategies are plagued with at least one drawback regarding functional group-tolerant electronic activation of the heteroarene, mild generation of the required alkyl radicals, regioselectivity, safety and/or scalability. Herein, we describe a fast, safe and scalable flow process which allows preparation of said C-4 alkylated pyridines. The process involves a photochemical hydrogen atom transfer (HAT) event to generate the carbon-centered radicals needed to alkylate the C-2 blocked pyridine. In a two-step streamlined flow process, this light-mediated alkylation step is combined with a nearly instantaneous inline removal of the blocking group. Notably, cheap benzophenone plays a dual role in the pyridine alkylation mechanism by activating the hydrocarbon feedstock reagents via a HAT mechanism, and by acting as a benign, terminal oxidant. The key role of benzophenone in the operative reaction mechanism has also been revealed through a combination of experimental and computational studies.

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.