Nickel/Photoredox Dual Catalytic Cross-Coupling of Alkyl and Amidyl Radicals to Construct C(sp³)–N Bonds

Abstract

The construction of C­(sp3)–N bonds via direct radical–radical cross-coupling under benign conditions is a desirable but challenging approach. Herein, the cross-coupling of alkyl and amidyl radicals to build aliphatic C–N bonds in a concise, mild, and oxidant-free manner is implemented by nickel/photoredox dual catalysis. In this protocol, the single electron transfer strategy is successfully employed to generate N- and C-centered radicals from sulfonyl azides/azidoformates and alkyltrifluoroborates, respectively. The photocatalyst-induced triplet–triplet energy-transfer mechanism, however, might not be applicable to this reaction. The oxidative quenching pathway of the excited photocatalyst (RuII/*RuII/RuIII/RuII) combined with a possible NiI/NiII/NiIII/NiI catalytic cycle is proposed to account for the nickel/photoredox dual-catalyzed C­(sp3)–N bond formation based on synergistic experimental and computational studies.

Photocatalytic C(sp3) radical generationviaC–H, C–C, and C–X bond cleavage

“…Among them, potassium triuoroborate, which was intensively studied by Molander's and other groups, is a common option due to its high shelf stability and easily accessible redox potential. [196][197][198][199][200][201][202][203][204][205][206] By merging photocatalysis and nickel catalysis, the challenging C(sp 2 )-C(sp 3 ) Suzuki-Miyaura coupling with aryl bromides and benzyltriuoroborates was realised by Molander's group in 2014 (Scheme 48A). 207…”

“…Among them, potassium trifluoroborate, which was intensively studied by Molander's and other groups, is a common option due to its high shelf stability and easily accessible redox potential. 196–206…”

Halide Non-Innocence and Direct Photo-Reduction of Ni(II) Enables Coupling of Aryl Chlorides in Dual Catalytic, Carbon-Heteroatom Bond Forming Reactions

“…In contrast, while oxidative quenching cycles (Fig. 1D) have been proposed in the literature occasionally with regards to Ni/PC dual catalytic C─C, C─O, C─N, or C─S bond forming systems, sometimes with computational support, [17][18][19][20][21] experiments capable of distinguishing between oxidative quenching, reductive quenching, and energy transfer (see reviews for relevant spectroscopic and electrochemical methods) [22][23][24] have not been performed in the study of these systems.…”

Theoretical study of NiI–NiIII cycle mediated by heterogeneous zinc in C–N cross-coupling reaction

“…14 For instance, MacMillan et al proposed a dual photoredox/Ni catalysis to realize challenging organic reactions, which made a great breakthrough in the field of organic synthesis. 15,16 Concretely, these photoredox catalysts such as iridium 17 and ruthenium 18,19 bipyridine complexes are stimulated to their excited states by absorbing a photon, which could then regulate the valence of metal catalysts 20,21 or activate the substance through single electron transfer (SET) 22 or energy transfer (ET) 23,24 pathways. However, the traditional homogeneous photocatalytic redox catalysts have the problems of high cost and difficult recovery, which limit the development of low-cost transition-metal catalysts.…”