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

Synthesis of Acyl Cyclopentaquinolinones through Simultaneous Construction of the Heterocyclic Scaffold and Introduction of the Acyl Group

Presented herein is an effective and concise synthesis of acyl cyclopentaquinolinone derivatives via the cascade reactions of N-(o-ethynylaryl)acrylamides with α-diazo carbonyl compounds. The formation of product involves a visible light-induced radical formation from α-diazo carbonyl compound followed by its addition onto the acrylamide moiety to trigger double radical annulation, single-electron oxidation, and β-elimination. To our knowledge, this is the first example in which the cyclopentaquinolinone scaffold was constructed along with the introduction of an acyl group under visible light irradiation conditions. Compared with literature methods for similar purpose, this newly developed protocol has advantages such as readily accessible substrates, mild reaction conditions, valuable products, concise synthetic procedure, and high sustainability. With all these merits, this method is expected to find wide applications in the construction of related acyl heterocyclic skeletons.

Photoredox-Cobaloxime Catalysis for Selective Oxidative Dehydrogenative [4+2] Annulation of Imidazo-Fused Heterocycles with Alkenes

A selective oxidative [4+2] annulation of alkenes with imidazo-fused heterocycles has been developed by using the synergistic combination of photoredox and cobaloxime catalysts. It allows facile access to various imidazole-fused polyaromatic scaffolds accompanied by H2 evolution. This protocol features high regioselectivity as well as a broad substrate scope. Detailed mechanistic studies indicate that twice the electron/H transfer processes facilitated by this catalytic system achieve the annulation π-extension of imidazo-fused heterocycles with alkenes.

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.

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

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

Visible-Light-Driven [2 + 2] Photocycloaddition for Constructing Dimers of N,N'-Diacyl-1,4-dihydropyrazines: Experimental and Theoretical Investigation

In this study, the visible-light-driven [2 + 2] photocycloaddition of 1,4-dihydropyrazines in solution was reported. The N,N'-diacyl-1,4-dihydropyrazines with different substituents showed completely different reactivity under the irradiation of a 430 nm blue light-emitting diode (LED) lamp. N,N'-Diacetyl-1,4-dihydropyrazine and N,N'-dipropionyl-1,4-dihydropyrazine were the only compounds capable of undergoing a [2 + 2] photocycloaddition reaction, yielding syn-dimers and cage-dimers (known as 3,6,9,12-tetraazatetraasteranes) with overall yields of 76 and 83%, correspondingly. The substituent-reactivity effect on [2 + 2] photocycloaddition of N,N'-diacyl-1,4-dihydropyrazines was investigated by density functional theory calculations. The results show that the substituents have little influence on Gibbs free energy for the [2 + 2] photocycloaddition and mainly affect the excited energy, reaction sites, and the triplet excited-state structures of 1,4-dihydropyrazines, which are closely related to whether the reaction occurs. The results offer insights into the photochemical reactivity of 1,4-dihydropyrazines and an approach for constructing dimers of N,N'-diacyl-1,4-dihydropyrazines through a solution-based visible-light-driven [2 + 2] photocycloaddition, especially for the construction of 3,6,9,12-tetraazatetraasteranes. Compared with the solid-state [2 + 2] photocycloaddition of 1,4-dihydropyrazine, this photocycloaddition will be an efficient and environmentally friendly method for synthesizing tetraazatetraasteranes with the advantages of milder reaction conditions, simple operation, adjustable reaction amounts by omitting the cocrystal growth step, etc.

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-Induced Radical Cascade Cross-Coupling via C(sp3)-H Activation and C-N/N-O Cleavage: Feasible Access to Methylenebisamide Derivatives

Here we report facile and manipulable access to methylenebisamide derivatives via visible-light-driven radical cascade processes incorporating C(sp³)-H activation and C-N/N-O cleavage. Mechanistic studies reveal that a traditional Ir-catalyzed photoredox pathway and a novel copper-induced complex-photolysis pathway are both involved, contributing to activating the inert N-methoxyamides and rendering the valuable bisamides. This approach exhibits many advantages, including mild reaction conditions, broad scope and functional group tolerance, and competitive step economy. Given the mechanistic plenitude and operational simplicity, we believe this package deal paves a promising way for the synthesis of valuable nitrogen-containing molecules.