Brønsted Base Assisted Photoredox Catalysis: Proton Coupled Electron Transfer for Remote C−C Bond Formation via Amidyl Radicals

Abstract

The synthesis of alkyne- and alkene-decorated lactams has been achieved through a photoredox-initiated radical cascade reaction. The developed Brønsted base assisted, photoredox-catalyzed, intramolecular 5-exo-trig cyclization/intermolecular radical addition/elimination reaction provides facile access to functionalized γ-lactams, with good functional group tolerance and high yields.

Chemistry With N-Centered Radicals Generated by Single-Electron Transfer-Oxidation Using Photoredox Catalysis

“…The Rueping group disclosed the cyclizing amidoalkynylation of alkenes 17 for the preparation of diverse lactams 19 (Scheme 5). 42 As with the examples discussed above, a PCET strategy was chosen to generate the reactive N-radicals from amides, carbamates, and ureas 17. The aryl alkynyl sulfones 18, known as efficient C-radical alkynylating reagents, 43 were selected as the radical acceptors to trap the C-radicals obtained after 5-exo cyclization to eventually furnish the products 19 in moderate to good yields.…”

Alkynylation of radicals: spotlight on the “Third Way” to transfer triple bonds

“…Arylalkynylsulfones were also used in a radical cyclization/alkynylation cascade by Rueping and coworkers (Scheme 21). 67 Upon a PCET event realized by fine tuning the photocatalyst and base ({Ir(dFCF 3 ppy) 2 bpy}PF 6 and NBu 4 (OMe) 2 PO 2 is the best combination), a nitrogen-centered radical is generated and can easily add onto alkenes to form a 5-membered ring along with an alkyl radical, which reacts with alkynylsulfones. Another example of photoredox catalyzed difunctionalization of alkenes using arylalkynylsulfones is the three components alkynyl difluoroalkylation reported by Zhu 68…”

Intermolecular radical carboamination of alkenes

“…For example, carboamination comprising an N-radical cyclization followed by an intermolecular C-C bond formation to provide pyrrolidine or pyrrolidin-2-one scaffolds has been intensively explored using redox catalysis. [26][27][28][29][30][31][32][33] As compared to these cyclizing carboaminations, intermolecular vicinal radical alkene carboamination is more challenging and accordingly less well explored. Based on the nature of the initially attacking species, intermolecular alkene radical carboamination can be categorized into three different classes: (1) initial formation of the C-N bond through intermolecular addition of an N-centered radical to an alkene followed by C-radical trapping; (2) initial formation of the C-C bond through addition of a C-radical to an alkene followed by C-amination; (3) single-electron-transfer (SET) oxidation of the alkene to generate a radical cation that gets trapped with an N-nucleophile resulting in a C-radical that eventually engages in a C-C bond forming step (Scheme 1).…”