Photoredox-Mediated Metal-Free C-H Alkylation and Dual Catalysis

Over the past decade, a resurgence of interest in photo-induced electron transfer has resulted in a new class of organic transformations. The ability to harness over 60 kcal/mol of visible light energy to activate redox-labile substrates—via the intermediacy of a photoredox catalyst—has enabled reactions under extraordinarily mild conditions compared to alternative two-electron modes of activation. Recent research efforts have broadened the scope of trifluoroborate coupling partners, employed 1,4-dihydropyridines (DHPs) in mono- and dual-catalytic manifolds, and accessed new chemical space via C–H functionalization pathways. First, the development of alkyltrifluoroborates as latent radicals for C–H alkylation of heteroarenes under photocatalytic conditions is described. Notably, the catalytic generation of carbon-centered radicals and the BF3 byproduct accomplishes a regioselective and atom-economical approach to the classical Minisci reaction. Subsequent reports disclose DHPs as unique radical precursors that do not require the use of a photocatalyst to effect a single-electron oxidation. Instead, DHPs are oxidized in the presence persulfate, facilitated by their low oxidation potentials. Furthermore, photoredox/Ni dual catalysis protocols have been developed to overcome several inherent limitations of palladium-catalyzed cross-couplings [i.e., forcing reaction conditions, limited scope for C(sp3)–C(sp 2) bond formation] by invoking a single-electron transmetalation pathway. Within the area of photoredox/Ni catalysis, a library of natural and unnatural aryl chromanones are accessed from the corresponding trifluoroboratochromanones and aryl bromides. In an effort to expand the radical toolbox by utilizing feedstock chemicals (e.g., aldehydes) to access radicals inspired the exploration of DHPs as radical partners in the dual catalytic paradigm. Exploiting the one-step procedure to access highly functionalized DHPs, a library of monosaccharide DHPs were synthesized and employed in the dual catalytic cross-coupling procedure with aryl bromides. In summary, the mild, photoredox-mediated C–H alkylation of heteroarenes represents a late-stage functionalization strategy to rapidly access highly functionalized motifs. Additionally, photoredox/Ni dual catalysis has enabled the modular synthesis of functionalized aryl chromanones and monosaccharides. Throughout these reported studies, it is clear the controlled and catalytic nature of photoredox catalysis enables previously challenging transformations and is primed for significant advancements in the near future