Visible‐Light‐Induced Homolysis of Earth‐Abundant Metal‐Substrate Complexes: A Complementary Activation Strategy in Photoredox Catalysis

Abstract

The mainstream applications of visible‐light photoredox catalysis predominately involve outer‐sphere single‐electron transfer (SET) or energy transfer (EnT) processes of precious metal RuII or IrIII complexes or of organic dyes with low photostability. Earth‐abundant metal‐based MnLn‐type (M=metal, Ln=polydentate ligands) complexes are rapidly evolving as alternative photocatalysts as they offer not only economic and ecological advantages but also access to the complementary inner‐sphere mechanistic modes, thereby transcending their inherent limitations of ultrashort excited‐state lifetimes for use as effective photocatalysts. The generic process, termed visible‐light‐induced homolysis (VLIH), entails the formation of suitable light‐absorbing ligated metal–substrate complexes (MnLn‐Z; Z=substrate) that can undergo homolytic cleavage to generate Mn−1Ln and Z. for further transformations.

Visible-light-mediated copper photocatalysis for organic syntheses

“…The radical X • can add to the substrate Y to obtain the stable radical X-Y • . Subsequently, L n−1 Cu I transfers one electron to X-Y • and accepts one ligand to regenerate the intermediate L n Cu II and the final product [39,43] (Scheme 6). Copper photocatalysis is a powerful tool that can be used to construct carbon-heteroatom and carbon-carbon bonds and can be applied to radical chemistry.…”

Recent Advances in Visible-Light-Promoted Copper Catalysis in Organic Reactions

“…Complementary to Cu(I)-catalyzed atom-transfer radical addition (ATRA) reactions, 32 visible-light-induced homolysis of metal-substrate complexes is also an emerging strategy for the difunctionalization of olefins. 33 In 2018, the Reiser group reported a visible-light-accelerated oxo-azidation of vinyl arenes with trimethylsilylazide and molecular oxygen as stoichiometric oxidant (Scheme 16). 34 Compared to classical outer-sphere photoredox such as iridium or ru-thenium, [Cu(dap) 2 ]Cl or [Cu(dap)Cl 2 ] were found to be unique for this transformation, which is attributable to their abilities to interact with the substrates.…”

“…The authors proposed that copper(II) species 80, which is formed by aerobic oxidation and ligand exchange of photoexcited [Cu(dap) 2 ]Cl, combines with TMSN 3 to generate 81 (Figure 8). Complex 81 then undergoes light-mediated homolysis 33 to afford 82 and azide radical, which adds to olefin and reacts with O 2 to provide peroxy radical 84. The radical is captured by 82 to form intermediate 85, which can eliminate the product and close the catalytic cycle.…”

“…Very recently, inspired by the concept of visible-lightinduced homolysis, 33 selective C(sp 3 )-H alkylation and amination of electron-deficient olefins in the presence of cata-lytic CuCl 2 under long-wavelength UV irradiation was reported by the Rovis group (Scheme 21). 41 The catalytic cycle is shown in Figure 10.…”

Ligand-to-Copper Charge Transfer: A General Catalytic Approach to Aromatic Decarboxylative Functionalization

“…Ligand-to-metal charge transfer (LMCT) processes have been employed to generate reactive open-shell intermediates from coordination complexes upon light irradiation. [21][22][23][24][25][26][27] Notably, aliphatic copper(II)-carboxylate complexes can undergo LMCT upon irradiation through innersphere electron transfer from a carboxylate ligand to copper(II), forming a dissociated carboxyl radical and copper(I). We envisioned that this photochemical process could serve as a key design principle in the development of a general light-and copper-enabled decarboxylative functionalization platform for aromatic carboxylic acids.…”