Practical and Regioselective Synthesis of C-4-Alkylated Pyridines

Reaction strategies for the meta-selective functionalization of pyridine through dearomatization

The pyridine moiety is a crucial structural component in various pharmaceuticals. While the direct ortho- and para-functionalization of pyridines is relatively straightforward, the meta-selective C-H functionalization remains a significant challenge. This review highlights dearomatization strategies as a key area of interest in expanding the application of meta-C-H functionalization of pyridines. Dearomatization enables the meta-functionalization through various catalytic methods that directly generate dearomatization products, and some products can be rearomatized back to pyridine derivatives. Furthermore, this article also covers the dearomatization of multiple positions of pyridine in the synthesis of polycyclic compounds. It offers a comprehensive overview of the latest advancements in dearomatization at different positions of pyridine, aiming to provide a valuable resource for researchers in this field. It also highlights the advantages and limitations of existing technologies, aiming to inform a broader audience about this important field and foster its future development.

Visible Light-Induced Divergent Deoxygenation/Hydroxymethylation of Pyridine N-Oxides

This study explores the deoxygenation of pyridine N-oxides and presents a one-step photoredox method for the direct synthesis of 2-hydroxymethylated pyridines from pyridine N-oxides. Mechanism studies elucidate the role of the catalyst and provide evidence of the possible electron transfer process and the formation of key radicals. A range of pyridine derivatives, particularly 2-hydroxymethyl-substituted pyridines, which may be difficult to obtain, can be synthesized in a single step.

Phosphite mediated molecular editing via switch to meta-C-H alkylation of isoquinolines: emergence of a distinct photochemical [1,3] N to C rearrangement

The isoquinoline core is present in one of the largest subsets of bioactive natural products. The multifunctional isoquinoline core exerts diverse bioactivity, resulting in the development of numerous isoquinoline-based drugs and molecules that are currently under clinical trials. We developed a new approach for phosphite-mediated [1,2] alkyl migration for an overall ortho-C-H alkylation via N-alkylation of isoquinoline. Tuning the phosphite-mediated protocol to switch the site selectivity would expedite direct and diverse multi-C-H bond functionalization. We report a new approach starting with a simple N-alkylation of isoquinoline with sterically and electronically diverse alkyl bromides for their phosphite-mediated photochemical [1,3] N to C rearrangement followed by a rearomatization sequence that leads to meta-C-H (C4) alkylation. Combined experimental and computational studies unveiled the emergence of an unprecedented C-N bond cleavage pathway from the singlet excited state of the enamine-type intermediate. Our radical bond-cleavage pathway favors substituted alkyl group migration that complements the recently successful meta-alkylation methods with smaller and more reactive electrophiles. This switch in site selectivity via tuning the phosphite-mediated protocol resulted in sequential C-H difunctionalization of isoquinoline including regiodivergent ortho, meta-dialkylations of isoquinolines.

Trinuclear Borane (B3H7)-Mediated Selective C4-H Alkylation and Phosphonation of Quinolines and Tetrahydroquinolines

Herein, we report a method for the regioselective alkylation and phosphonation of quinoline C4-H via a B3H7-mediated nucleophilic addition of Turbo Grignard reagents and phosphine oxide anions to quinolines bearing different substituents, affording the 4-alkyl and 4-phosphoryl quinolines and tetrahydroquinolines after one-pot oxidation or reduction. The results indicate that coordination of the B3H7 group can activate substrates toward a potential 1,4-dearomative addition and subtly control the regioselectivity by preventing the 1,2-dearomative addition.

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.

Functionalization of Pyridines at the C4 Position via Metalation and Capture

The functionalization of pyridines at positions remote to the N-atom remains an outstanding problem in organic synthesis. The inherent challenges associated with overriding the influence of the embedded N-atom within pyridines was overcome using n-butylsodium, which provided an avenue to deprotonate and functionalize the C4-position over traditionally observed addition products that are formed with organolithium bases. In this work, we show that freshly generated 4-sodiopyrdines could undergo transition metal free alkylation reactions directly with a variety of primary alkyl halides bearing diverse functional groups. In addition, after transmetalation to zinc chloride a simple and efficient Negishi cross-coupling protocol was formulated for a variety of aromatic and heteroaromatic halides. The robustness of this protocol was demonstrated through the late-stage installation of 4-pyridyl fragments into a variety of complex active pharmaceutical ingredients including loratadine and prochlorperazine. Furthermore, through rapid injection NMR investigations, we are able to directly observe the evolution of anionic intermediates and determined that two distinct mechanistic pathways lead to the observed site selectivity: (1) the C4-H within 2,6-disubstituted pyridines could be removed directly and (2) the C4 selectivity of unsubstituted pyridine originates from the intermolecular exchange of metalation sites via a thermodynamic pathway.

Ligand Assisted Co(II)-Catalyzed Multicomponent Synthesis of Substituted Pyrroles and Pyridines

Herein, we describe a sustainable Co(II)-catalyzed synthesis of pyrroles and pyridines. Using a Co(II)-catalyst [CoII 2(La)2Cl2] (1 a) bearing redox-active 2-(phenyldiazenyl)-1,10-phenanthroline) (La) scaffold, various substituted pyrroles and pyridines were synthesized in good yields, taking alcohol as one of the primary feedstock. Pyrroles were synthesized by the equimolar reaction of 2-amino and secondary alcohols. A series of 2,4,6-substituted symmetrical pyridines were prepared via a three-component reaction of NH4OAc with 1 : 2.2 molar primary and secondary alcohols, respectively. Unsymmetrically substituted 2,4,6-trisubstituted, 2,4,5,6-tetrasubstituted, and 2,3,4,5,6-pentasubstituted pyridines were achieved via a multi-component coupling reaction of alcohols and NH4OAc. Catalyst 1 a showed encouraging results during the gram-scale synthesis of these N-heterocycles. Mechanistic investigation revealed synergistic involvement of cobalt metal and the ligand during the catalytic reactions.

Nucleophilic Dearomatization of Activated Pyridines Using Vinyl Sulfoxonium Ylides: Application to the Synthesis of Bis-Heterocycles

A highly efficient method has been developed for synthesizing 4-dienyl dihydropyridines through the nucleophilic dearomatization of activated pyridines using vinyl sulfoxonium ylides. This reaction follows the sequence involving ylide addition to activated pyridine, [2,3]-sigmatropic rearrangement, and subsequent sulfenic acid elimination. The resulting 4-dienyl dihydropyridines are then used in the synthesis of highly substituted bis-heterocyles. Control experiments and quantum chemical calculations were conducted to elucidate the selectivity and the mechanistic pathway.

Highly Regioselective Modular Assembly of 3-Phosphonyl Polysubstituted Pyridines through Radical Cascade Cyclization of 1,5-Enynes with Phosphine Oxide by Photoinitiation

A series of 3-phosphonyl polysubstituted pyridine were first synthesized by photocatalysis, combining a phosphonyl radical cascade reaction, Boc deprotection, and aromatization. This strategy can avoid the difficulties of activating the C3-H bond on pyridine to synthesize 3-phosphonylpyridine under mild conditions. Furthermore, by constructing different enynes, we can achieve the metal-free modular synthesis of 3-phosphonyl polysubstituted pyridine, which will be transferred into a new type of phosphine ligand. This is of significance for organometallic catalysis. The regioselective control and detailed reaction mechanism of the cascade reaction are explained by DFT calculations.

Polysubstituted Pyridines from 1,4-Oxazinone Precursors

This study describes a general method for the preparation of 1,4-oxazin-2-one intermediates from acetylene dicarboxylate and β-amino alcohol precursors. Oxazinones prepared in this manner were employed in a tandem cycloaddition/cycloreversion reaction sequence with a model alkyne (phenyl acetylene) to give substituted pyridine products. Fundamental reactivity and selectivity studies are complemented by the synthesis of the polycyclic ergot alkaloid natural product xylanigripone A.

Visible-Light-Mediated Three-Component Decarboxylative Coupling Reactions for the Synthesis of 4-Alkylpyridines

4-Alkylation of pyridines has been achieved by a photocatalyzed three-component reaction of an olefin, an aryloxyacetic acid, and cyanopyridine using Ir[dF(CF3)ppy]2(dtbbpy)PF6 as a photocatalyst. In this way, a new type of 4-alkylpyridines bearing aryl and aryloxy substituents on the phenyl was accessed with potential pharmaceutical value. In addition to mild reaction conditions, the method has several other advantages, including a multicomponent one-pot protocol, good functional group tolerance, and easy amplification of the reaction scale. The reaction mechanism has been clarified by control experiments and computation of the reaction energy profile.

Reductive deaminative cross-coupling of alkyl bistriflimides enabled by electrocatalysis

We present a versatile nickel-electrocatalytic deaminative cross-coupling platform for the efficient construction of C(sp3)-C(sp3) and C(sp3)-C(sp2) bonds from readily available alkyl bistriflimides. This methodology involves the assembly of two leaving groups on alkyl amines to form alkyl bistriflimides, followed by their effective coupling with a wide range of alkyl halides, alkyl pseudohalides, aryl halides, and alkenyl halides under electrochemical reductive conditions. Moreover, the successful application of electrochemical reductive relay cross-coupling and transition metal-free cross-electrophile coupling further demonstrates the versatility of alkyl bistriflimides as valuable building blocks in organic synthesis. Combined control experiments and density functional theory calculations provide insights into the reaction pathway and the crucial role of iodide in the catalytic process.

A Catalytic Version of the Knorr Pyrrole Synthesis Permits Access to Pyrroles and Pyridines

Aromatic N-heterocycles, such as pyrroles and pyridines, are important natural products and bulk and fine chemicals with numerous applications as active ingredients of pharmaceuticals and agrochemicals, as catalysts, and in materials sciences. We report here a catalytic version of the Knorr pyrrole synthesis in which simple and diversely available starting materials, such as 1,2-amino alcohols or 1,3-amino alcohols and keto esters, undergo a dehydrogenative coupling to form pyrroles and pyridines, respectively. Our reaction forms hydrogen as a collectible (and usable) byproduct and is mediated by a well-defined Mn catalyst. The synthesis of highly functionalized heterocycles and applications was demonstrated, and 35 compounds, not yet reported in the literature, were introduced.

Unified ionic and radical C-4 alkylation and arylation of pyridines

C-H Functionalization of pyridines is an efficient strategy to access pyridine derivatives occurring in pharmaceuticals, agrochemicals, and materials. Nucleophilic additions to pyridiniums via both ionic and radical species have proven particularly useful. However, these reactions suffer from poor regioselectivity. By identifying an enzyme-mimic pocket-type urea activation reagent, we report a general platform for pyridine C-4 functionalization. Both ionic and radical nucleophiles can be incorporated to achieve the alkylation and arylation. Notably, the highly regioselective C-4 radical arylation is disclosed for the first time. The broad scope of nucleophiles and pyridines renders this platform applicable to the late-stage functionalization of drug-like molecules and the preparation of complex biologically important molecules.

Construction of the 4-Azafluorenone Core in a Single Operation and Synthesis of Onychine

This study describes the synthesis of the 4-azafluorenone core in a single operation using readily available starting materials. Condensation of an amidrazone with ninhydrin intercepts an intermediate 1,2,4-triazine derivative, which engages norbornadiene in a merged [4 + 2]/bis-retro[4 + 2] sequence to deliver the azafluorenone core. The tricyclic core established in this manner was elaborated to onychine, the simplest natural product in the 4-azafluorenone alkaloid family.

Visible light-induced chemoselective 1,2-diheteroarylation of alkenes

Visible-light photocatalysis has evolved as a powerful technique to enable controllable radical reactions. Exploring unique photocatalytic mode for obtaining new chemoselectivity and product diversity is of great significance. Herein, we present a photo-induced chemoselective 1,2-diheteroarylation of unactivated alkenes utilizing halopyridines and quinolines. The ring-fused azaarenes serve as not only substrate, but also potential precursors for halogen-atom abstraction for pyridyl radical generation in this photocatalysis. As a complement to metal catalysis, this photo-induced radical process with mild and redox neutral conditions assembles two different heteroaryl groups into alkenes regioselectively and contribute to broad substrates scope. The obtained products containing aza-arene units permit various further diversifications, demonstrating the synthetic utility of this protocol. We anticipate that this protocol will trigger the further advancement of photo-induced alkyl/aryl halides activation.

Purple Light-Promoted Coupling of Bromopyridines with Grignard Reagents via SET

Alkyl- and arylpyridines and 2,2'-bipyridines are conventionally prepared by Minisci reactions of pyridines and transition metal-catalyzed coupling reactions of halopyridines. Herein, purple light-promoted radical coupling reactions of 2- or 4-bromopyridines with Grignard reagents in Et2O or a mixture of Et2O and tetrahydrofuran in regular glassware without the need for a transition metal catalyst were disclosed for the first time. Methyl, primary and secondary alkyl, cycloalkyl, aryl, heteroaryl, pyridyl, and alkynyl Grignard reagents were compatible with the protocol. As a result, alkyl- and arylpyridines and 2,2'-bipyridines were easily prepared. Single electron transfer from the Grignard reagent to bromopyridine was stimulated by purple light. An electron extruded from the dimerization of the Grignard reagent worked as the catalyst. Light on/off experiments indicated that constant irradiation was required for product formation. Studies of radical clock substrates verified the involvement of a pyridyl radical from bromopyridine and the noninvolvement of an alkyl or aryl radical from the Grignard reagent. The available proof supports a photoinduced SRN mechanism for the new coupling reactions.

Visible-Light-Induced PhI(OAc)2-Mediated Alkylation of Heteroarenes with Simple Alkanes and Ethers

The direct alkylation of heteroarenes with alkanes has been successfully achieved through visible-light-induced hypervalent iodine-mediated C-H functionalization of both coupling partners at ambient temperatures. This reaction proceeds via the in situ generation of nucleophilic alkyl radicals from alkanes through hydrogen atom transfer (HAT), followed by a Minisci-type reaction with heteroarenes. These mild reaction conditions have demonstrated their suitability for the alkylation of a wide range of heterocycles, including azoles, pyridines, quinolines, isoquinolines, and quinoxalinones.

Nickel-Electrocatalytic Decarboxylative Arylation to Access Quaternary Centers

There is a pressing need, particularly in the field of drug discovery, for general methods that will enable direct coupling of tertiary alkyl fragments to (hetero)aryl halides. Herein a uniquely powerful and simple set of conditions for achieving this transformation with unparalleled generality and chemoselectivity is disclosed. This new protocol is placed in context with other recently reported methods, applied to simplify the routes of known bioactive building blocks molecules, and scaled up in both batch and flow. The role of pyridine additive as well as the mechanism of this reaction are interrogated through Cyclic Voltammetry studies, titration experiments, control reactions with Ni(0) and Ni(II)-complexes, and ligand optimization data. Those studies indicate that the formation of a BINAPNi(0) is minimized and the formation of an active pyridine-stabilized Ni(I) species is sustained during the reaction. Our preliminary mechanistic studies ruled out the involvement of Ni(0) species in this electrochemical cross-coupling, which is mediated by Ni(I) species via a Ni(I)-Ni(II)-Ni(III)-Ni(I) catalytic cycle.

14N to 15N Isotopic Exchange of Nitrogen Heteroaromatics through Skeletal Editing

The selective modification of nitrogen heteroaromatics enables the development of new chemical tools and accelerates drug discovery. While methods that focus on expanding or contracting the skeletal structures of heteroaromatics are emerging, methods for the direct exchange of single core atoms remain limited. Here, we present a method for 14N → 15N isotopic exchange for several aromatic nitrogen heterocycles. This nitrogen isotope transmutation occurs through activation of the heteroaromatic substrate by triflylation of a nitrogen atom, followed by a ring-opening/ring-closure sequence mediated by 15N-aspartate to effect the isotopic exchange of the nitrogen atom. Key to the success of this transformation is the formation of an isolable 15N-succinyl intermediate, which undergoes elimination to give the isotopically labeled heterocycle. These transformations occur under mild conditions in high chemical and isotopic yields.

Electrochemically Driven C4-Selective Decyanoalkylation of Cyanopyridines with Unactivated Alkyl Bromides Enabling C(sp3)-C(sp2) Coupling

With cyanopyridines and alkyl bromides as coupling partners, an electrochemically driven C4-selective decyanoalkylation has been established to access diverse 4-alkylpyridines in one step. The reaction proceeds through the single electron reduction/radical-radical coupling tandem process under mild electrolytic conditions, achieving the cleavage of the C(sp2)-CN bond and the formation of C(sp3)-C(sp2). The practicality of this protocol is illustrated by no sacrificial anodes, a broad substrate scope, and gram-scale synthesis.

C-H Functionalization of Pyridines via Oxazino Pyridine Intermediates: Switching to para-Selectivity under Acidic Conditions

para-Selective C-H functionalization of pyridines holds a significant value but remains underdeveloped. Site-switchable C-H functionalization of pyridines under easily tunable conditions expedites drug development. We recently reported a redox-neutral dearomatization-rearomatization strategy for meta-C-H functionalization of pyridines via oxazino pyridine intermediates. Here, we demonstrate that these oxazino pyridine intermediates undergo highly para-selective functionalization simply by switching to acidic conditions. A broad scope of para-alkylated and arylated pyridines is prepared through radical as well as ionic pathways. These mild and catalyst-free methods are applied to the late-stage para-functionalization of drugs using pyridines as the limiting reagents. Consecutive meta,para-difunctionalization of pyridines is also achieved with complete regiocontrol relying on the pH-dependent reactivity of oxazino pyridines.

Zn(II)-Catalyzed Multicomponent Sustainable Synthesis of Pyridines in Air

Herein, we report a Zn(II)-catalyzed solvent-free sustainable synthesis of tri- and tetra-substituted pyridines using alcohols as the primary feedstock and NH₄OAc as the nitrogen source. Using a well-defined air-stable Zn(II)-catalyst, 1a, featuring a redox-active tridentate azo-aromatic pincer, 2-((4-chlorophenyl)diazenyl)-1,10-phenanthroline (L^a), a wide variety of unsymmetrical 2,4,6-substituted pyridines were prepared by three-component coupling of primary and secondary alcohols with NH₄OAc. Catalyst 1a is equally compatible with the four-component coupling. Unsymmetrical 2,4,6-substituted pyridines were also prepared via a four-component coupling of a primary alcohol with two different secondary alcohols and NH₄OAc. A series of tetra-substituted pyridines were prepared up to 67% yield by coupling primary and secondary alcohols with 1-phenylpropan-1-one or 1,2-diphenylethan-1-one and NH₄OAc. The 1a-catalyzed reactions also proceeded efficiently upon replacing the secondary alcohols with the corresponding ketones, producing the desired tri- and tetra-substituted pyridines in higher yields in a shorter reaction time. A few control experiments were performed to unveil the mechanistic aspects, which indicates that the active participation of the aryl-azo ligand during catalysis enables the Zn(II)-complex to act as an efficient catalyst for the present multicomponent reactions. Aerial oxygen acts as an oxidant during the Zn(II)-catalyzed dehydrogenation of alcohols, producing H₂O and H₂O₂ as byproducts.

Bridge Heteroarylation of Bicyclo[1.1.1]pentane Derivatives

Herein, we report the decarboxylative Minisci heteroarylation of bicyclo[1.1.1]pentane (BCP) and 2-oxabicyclo[2.1.1]hexane (oBCH) derivatives at the bridge positions. In an operationally simple, photocatalyst-free process, free bridge carboxylic acids are directly coupled with nonprefunctionalized heteroarenes to provide rare examples of polysubstituted BCP and oBCH derivatives in synthetically useful yields. Additionally, the impact of the BCP core on the physicochemical properties of a representative example compared to those of its all-aromatic ortho- and meta-substituted analogues is evaluated.

Generation and Use of Glycosyl Radicals under Acidic Conditions: Glycosyl Sulfinates as Precursors

We herein report a method that enables the generation of glycosyl radicals under highly acidic conditions. Key to the success is the design and use of glycosyl sulfinates as radical precursors, which are bench-stable solids and can be readily prepared from commercial starting materials. This development allows the installation of glycosyl units onto pyridine rings directly by the Minisci reaction. We further demonstrate the utility of this method in the late-stage modification of complex drug molecules, including the anticancer agent camptothecin. Experimental studies provide insight into the reaction mechanism.

Recent Advances in Catalytic and Technology-Driven Radical Addition to N,N-Disubstituted Iminium Species

Recently, radical chemistry has grown exponentially in the toolbox of organic synthetic chemists. Upon the (re)introduction of modern catalytic and technology-driven strategies, the implementation of highly reactive radical species is currently facilitated while expanding the scope of numerous synthetic methodologies. In this context, this review intends to cover the recent advances in radical-based transformations of N,N-disubstituted iminium substrates that encompass unique reactivities with respect to imines or protonated iminium salts. In particular, we have focused on the literature concerning the dipole type substrates, such as nitrones or azomethine imines, together with the chemistry of N⁺-X^- (X = O, NR) azaarenium dipoles, which proved to be very versatile platforms in that field of research. The N-alkylazaarenium salts were been considered, which demonstrated specific reactivity profiles in radical chemistry.

Photochemical Organocatalytic Functionalization of Pyridines via Pyridinyl Radicals

We report a photochemical method for the functionalization of pyridines with radicals derived from allylic C-H bonds. Overall, two substrates undergo C-H functionalization to form a new C(sp²)-C(sp³) bond. The chemistry harnesses the unique reactivity of pyridinyl radicals, generated upon single-electron reduction of pyridinium ions, which undergo effective coupling with allylic radicals. This novel mechanism enables distinct positional selectivity for pyridine functionalization that diverges from classical Minisci chemistry. Crucial was the identification of a dithiophosphoric acid that masters three catalytic tasks, sequentially acting as a Brønsted acid for pyridine protonation, a single electron transfer (SET) reductant for pyridinium ion reduction, and a hydrogen atom abstractor for the activation of allylic C(sp³)-H bonds. The resulting pyridinyl and allylic radicals then couple with high regioselectivity.

Diboron(4)-Catalyzed Remote [3+2] Cycloaddition of Cyclopropanes via Dearomative/Rearomative Radical Transmission through Pyridine

Ring structures such as pyridine, cyclopentane or their combinations are important motifs in bioactive molecules. In contrast to previous cycloaddition reactions that necessitated a directly bonded initiating functional group, this work demonstrated a novel through-(hetero)arene radical transmission concept for selective activation of a remote bond. An efficient, metal-free and atom-economical [3+2] cycloaddition between 4-pyridinyl cyclopropanes and alkenes or alkynes has been developed for modular synthesis of pyridine-substituted cyclopentanes, cyclopentenes and bicyclo[2.1.1]hexanes that are difficult to access using known methods. This complexity-building reaction was catalyzed by a very simple and inexpensive diboron(4) compound and took place via dearomative/rearomative processes. The substrate scope was broad and more than 100 new compounds were prepared in generally high yields. Mechanistic experiments and density function theory (DFT) investigation supported a radical relay catalytic cycle involving alkylidene dihydropyridine radical intermediates and boronyl radical transfer.

Regiodivergent Asymmetric Pyridinium Additions: Mechanistic Insight and Synthetic Applications

A practical protocol for the first regiodivergent asymmetric addition of aryl and alkenyl organometallic reagents to substituted N-alkyl pyridinium heterocycles is described. The couplings proceed with high regiochemical and stereochemical selectivities, and provide access to chiral 1,2,3- and 1,3,4-trisubstituted dihydropyridine products, controlled by judicious choice of nitrogen activating agent. To this end, a correlation was found between the parameterized size of the activating group and the C2/C4 regioselectivity in the couplings. The utility of the described chemistry was demonstrated in two concise asymmetric syntheses of (+)-N-methylaspidospermidine and (-)-paroxetine.

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.

Alkyl/Glycosyl Sulfoxides as Radical Precursors and Their Use in the Synthesis of Pyridine Derivatives

We report here the use of simple and readily available alkyl sulfoxides as precursors to radicals and their application in the preparation of pyridine derivatives. We show that alkyl sulfoxides, N-methoxy pyridinium salts and fluoride anions form electron donor-acceptor (EDA) complexes in solution, which, upon visible light irradiation, undergo a radical chain process to afford various pyridine derivatives smoothly. This reaction displays broad scope with respect to both sulfoxides and N-methoxy pyridiniums. The synthetic versatility of sulfoxides as a handle in chemistry adds to their power as radical precursors. Glycosyl sulfoxides are converted to the corresponding pyridyl C-glycosides with high stereoselectivities. Computational and experimental studies provide insights into the reaction mechanism.

1,2-Silylpyridylation Reaction of Aryl Alkenes with Silylboronate

A metal-free silyl-pyridylation of alkenes using silyl boronates and B₂pin₂ through a pyridine-mediated B-interelement activation has been demonstrated, which provides a practical strategy for a variety of C₄-silylalkylated pyridines. DFT calculations and control experiments show that the reaction proceeds through a silyl radical addition/radical-radical coupling sequence. This protocol features a broad substrate scope and excellent functional group compatibility, and thus it showcases great potential in the late-stage modification of bioactive molecules.

Divergent Reaction of Activated Pyridines with α,α-Difluorinated gem-Diols: Regioselective Synthesis of gem-Difluorinated Dihydropyridines and Dihydropyridones

The nucleophilic reactivity of α,α-difluorinated gem-diols toward activated pyridinium salts has been capitalized in a highly regioselective fashion, offering biologically relevant 1,4-dihydropyridines and 3,4-dihydro-2-pyridones adorned with the valuable gem-difluoromethylene motif. The protocol is scalable as well as high yielding and accommodates a broad range of substrates and functional groups. Additionally, the synthesis of difluorinated oxa-azabicyclo[3.3.1]nonane frameworks has been showcased through product diversification.

Electrochemical-Promoted Nickel-Catalyzed Reductive Allylation of Aryl Halides

Compared with conventional reductive coupling, reductive coupling under electrochemical conditions without external reductants is greener, milder, and more efficient and is of increasing interest to organic chemists. In this work, we report the sacrificial anode, nickel-catalyzed electrochemical allylation reaction of aryl and alkyl halides. The reaction can be applied to a range of allylation reagents such as trifluoroalkenes, oxalates, and acetates.

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

A catalytic method for the enantioselective and C4-selective functionalization of pyridine derivatives is yet to be developed. Herein, we report an efficient method for the asymmetric β-pyridylations of enals that involve N-heterocyclic carbene (NHC) catalysis with excellent control over enantioselectivity and pyridyl C4-selectivity. The key strategy for precise stereocontrol involves enhancing interactions between the chiral NHC-bound homoenolate and pyridinium salt in the presence of hexafluorobenzene, which effectively differentiates the two faces of the homoenolate radical. Room temperature is sufficient for this transformation, and reaction efficiency is further accelerated by photo-mediation. This methodology exhibits broad functional group tolerance and enables facile access to a diverse range of enantioenriched β-pyridyl carbonyl compounds under mild and metal-free conditions.

Visible-Light-Induced C4-Selective Functionalization of Pyridinium Salts with Cyclopropanols

The site-selective C-H functionalization of heteroarenes is of considerable importance for streamlining the rapid modification of bioactive molecules. Herein, we report a general strategy for visible-light-induced β-carbonyl alkylation at the C4 position of pyridines with high site selectivity using various cyclopropanols and N-amidopyridinium salts. In this process, hydrogen-atom transfer between the generated sulfonamidyl radicals and O-H bonds of cyclopropanols generates β-carbonyl radicals, providing efficient access to synthetically valuable β-pyridylated (aryl)ketones, aldehydes, and esters with broad functional-group tolerance. In addition, the mild method serves as an effective tool for the site-selective late-stage functionalization of complex and medicinally relevant molecules.

Photoredox-Catalysed Regioselective Synthesis of C-4-Alkylated Pyridines with N -(Acyloxy)phthalimides

A method of direct C-4 selective alkylation of pyridine under visible light irradiation at room temperature was reported, using simple maleate-derived pyridinium salts as pyridine precursors, and the readily available...

Photocatalytic redox-neutral arylation of cyclopropanols with cyanoarenes via radical-mediated C–C and C–CN bond cleavage

β-arylated ketones widely exist in many biologically active molecules and natural products. Herein, we disrcibled a photocatalytic redox-neutral arylation of cyclopropanols with cyanoarenes via radical-mediated C–C and C–CN bond cleavage...

Switchable synthesis of 1,4-bridged dihydroisoquinoline-3-ones and isoquinoline-1,3,4-triones through radical oxidation of isoquinolinium salts with phenyliodine(iii) diacetate

A switchable synthesis of 1,4-bridged dihydroisoquinoline-3-ones and isoquinoline-1,3,4-triones is developed via radical oxidation of isoquinolinium salts with PhI(OAc)2.

Remote C-H Pyridylation of Hydroxamates through Direct Photoexcitation of O-Aryl Oxime Pyridinium Intermediates

Herein, we report an efficient strategy for the remote C-H pyridylation of hydroxamates with excellent ortho-selectivity by designing a new class of photon-absorbing O-aryl oxime pyridinium salts generated in situ from the corresponding pyridines and hydroxamates. When irradiated by visible light, the photoexcitation of oxime pyridinium intermediates generates iminyl radicals via the photolytic N-O bond cleavage, which does not require an external photocatalyst. The efficiency of light absorption and N-O bond cleavage of the oxime pyridinium salts can be modulated through the electronic effect of substitution on the O-aryl ring. The resultant iminyl radicals enable the installation of pyridyl rings at the γ-CN position, which yields synthetically valuable C2-substituted pyridyl derivatives. This novel synthetic approach provides significant advantages in terms of both efficiency and simplicity and exhibits broad functional group tolerance in complex settings under mild and metal-free conditions.