Reprint of: Recent advances in the functionalization of allenes via radical process

Radical functionalization of allenes

Allenes exhibit comparatively lower stability compared to alkenes and alkynes, which confers heightened reactivity to these compounds. Recently, the radical functionalization of allenes has progressed considerably, leading to a renaissance in the synthesis of functional natural products, drugs and their analogues, but summary work addressing this aspect has not been reported. This review systematically summarizes recent advancements in the field of radical functionalization of allenes reported within the past five years. It encompasses the difunctionalization and trifunctionalization of the three carbon atoms in allenes, as well as the functionalization of C-Y bonds (Y = H, Br). The representative studies are categorized based on the type of radicals generated, including C-, N-, O-, S-, and Se-centered radicals. For individual more complex reactions, the mechanisms are explored and briefly discussed.

Lewis Base-Catalyzed Interhalogenation of Terminal Allenes: Selective Strategy for Accessing Vicinal Vinylic, Allylic Heterodihalides

Though precedent remains limited, the selective interhalogenation of allenes offers a valuable synthetic strategy to access products where each halide exhibits orthogonal reactivity. Here, we describe a Lewis base-catalyzed approach for the dihalogenation (bromochlorination, iodochlorination, iodobromination, and dibromination) of terminal allenes. By employing just 1 mol % of triphenylphosphine oxide or hexamethylphosphoramide to activate thionyl halides in the presence of electrophilic halogenation reagents, we achieve the conversion of monosubstituted allenes to vicinal allylic, vinylic dihalides with up to 93% yield and >20:1 regioisomeric ratio, favoring the branched dihalogenated product. A range of functional groups is tolerated, including nitrile, ester, phosphate, sulfonamide, and silyl groups, and the reaction proved to be scalable. The utility of various dihalide products was investigated in substitution and cross-coupling chemistry, highlighting the distinct reactivity among the different classes.

Dual Nickel/Photoredox-Catalyzed Arylsulfonylation of Allenes

The nickel/photoredox dual catalysis system is an efficient conversion platform for the difunctionalization of unsaturated hydrocarbons. Herein, we disclose the first dual nickel/photoredox-catalyzed intramolecular 1,2-arylsulfonylation of allenes, which can accurately construct a C(sp2)-C(sp2) bond and a C(sp3)-S bond. The reaction exhibits excellent chemoselectivity and regioselectivity, allowing modular conformations of a diverse series of 3-sulfonylmethylbenzofuran derivatives. Control experiments showed that the bipyridine ligand is crucial for the formation of a stable σ-alkyl nickel intermediate, providing the possibility for sulfonyl radical insertion. Meanwhile, the electrophilic sulfonyl radical facilitates further oxidative addition of the σ-alkyl nickel intermediate and inhibits addition with allenes. In addition, control experiments, cyclic voltammetry tests, Stern-Volmer experiments, and density functional theory calculations afford evidence for the Ni(0)/Ni(I)/Ni(II)/Ni(III) pathway in this 1,2-arylsulfonylation.