Highly Diastereoselective Synthesis of γ-Lactams Enabled by Photoinduced Deaminative [3 + 2] Annulation Reaction

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.

Photoredox-Catalyzed Three-Component Sulfonaminoalkynylation of Alkenes via a Radial/Polar Crossover

We report a photoredox-catalyzed three-component sulfonaminoalkynylation of alkenes with N-aminopyridine salts and potassium alkynyltrifluoroborate salts. This aminoalkylation reaction underwent a radial/polar crossover mechanism, which was distinguished from the previous reports. A variety of β-alkynylated sulfonamides were obtained in moderate to excellent yields. The versatility of this method was further evidenced by its successful application in modifying biological molecules in advanced stages of development.

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

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

Photoredox-catalyzed unsymmetrical diamination of alkenes for access to vicinal diamines

A photoredox-catalyzed unsymmetrical diamination of alkenes by using N-aminopyridinium salts and nitriles as the amination reagents has been developed. Various vicinal diamines were obtained in moderate to excellent yields under mild reaction conditions. Furthermore, this protocol could be applied in the late-stage modification of pharmaceuticals and natural products. Preliminary mechanistic studies suggested that this methodology may undergo a radical pathway followed by a Ritter-type reaction.

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

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

Visible-Light-Driven Multicomponent Diamination and Oxyamination of Alkene

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

Three-Component Aminoheteroarylation of Alkenes via Photoinduced EDA Complex Activation

A catalyst-free approach for the multicomponent aminoheteroarylation reaction of alkenes with N-aminopyridinium salts and heteroarenes is herein described. The reaction shows good functional group tolerance and allows the generation of valuable β-heteroarylethylamines in satisfying yields. In this transformation, N-aminopyridinium salts and heteroarenes are utilized to generate electron donor-acceptor complexes, which undergo a single-electron transfer process upon light irradiation to form key amidyl radicals and heteroaryl radical cations. The amidyl radical is subsequently captured by alkenes, followed by a Minisci-type reaction to yield the desired β-heteroarylamines as products.

Isostructural Nanocluster Manipulation Reveals Pivotal Role of One Surface Atom in Click Chemistry

Elucidating single-atom effects on the fundamental properties of nanoparticles is challenging because single-atom modifications are typically accompanied by appreciable changes to the overall particle's structure. Herein, we report the synthesis of a [Cu58 H20 PET36 (PPh3 )4 ]2+ (Cu58 ; PET: phenylethanethiolate; PPh3 : triphenylphosphine) nanocluster-an atomically precise nanoparticle-that can be transformed into the surface-defective analog [Cu57 H20 PET36 (PPh3 )4 ]+ (Cu57 ). Both nanoclusters are virtually identical, with five concentric metal shells, save for one missing surface copper atom in Cu57 . Remarkably, the loss of this single surface atom drastically alters the reactivity of the nanocluster. In contrast to Cu58 , Cu57 shows promising activity for click chemistry, particularly photoinduced [3+2] azide-alkyne cycloaddition (AAC), which is attributed to the active catalytic site in Cu57 after the removal of one surface copper atom. Our study not only presents a unique system for uncovering the effect of a single-surface atom modification on nanoparticle properties but also showcases single-atom surface modification as a powerful means for designing nanoparticle catalysts.

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.

Copper-Catalyzed Sulfonylation/Cyclization of Pent-4-ynamides toward Sulfonyl-Functionalized Pyrrol-2-ones

A domino sulfonylation/intramolecular C-N coupling/dehydrogenation reaction was realized between pent-4-ynamides and sulfonyl chlorides by catalysis of Cu(acac)₂ and 2,2'-bis(diphenylphosphanyl)-1,1'-binaphthalene. The reaction provides a convenient approach to sulfonyl-functionalized pyrrol-2-ones. This method can also be applied to the synthesis of 3-alkylidene isoindolinones from 2-ethynyl-benzamides.

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.

Synthesis of Trifluoromethylated γ-Lactams through Radical Cascades of N-Cyano Alkenes with CF3SO2Na

We report herein a facile strategy to synthesize trifluoromethylated γ-lactams through trifluoromethylcarbonylation of N-cyano alkenes using readily available CF₃SO₂Na as the CF₃ radical source. A range of trifluoromethyl-containing γ-lactams was obtained in good yields. This transition-metal-free protocol is demonstrated with mild conditions, broad substrate scope, good functional group tolerance, convenient reagents, and an easy-to-handle operating system.

Photoinduced Remote C(sp3)-H Cyanation and Oxidation Enabled by a Vinyl Radical-Mediated 1,5-HAT Strategy

We report a vinyl radical-mediated 1,5-hydrogen atom transfer (1,5-HAT) strategy for the remote C(sp³)-H functionalization reaction, which includes cyanation, oxidation, and etherification under visible-light-induced photochemical conditions. This reaction is achieved using readily available alkyl N-hydroxyphthalimide esters as radical precursors, which can efficiently react with diverse alkynes to form key vinyl radical intermediates followed by a 1,5-HAT process. A series of structurally diverse γ-cyano, γ-carbonyl, and γ-oxygenated alkenes with excellent stereoselectivity can be efficiently constructed by this synthetic protocol.

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.

Recent advances of aminoazanium salts as amination reagents in organic synthesis

This review summarizes the utilization of aminoazaniums as amination reagents in organic synthesis, including the amination of aldehydes, boronic esters, olefins, etc.