Strongly Reducing Helical Phenothiazines as Recyclable Organophotoredox Catalysts

Photocatalysts have recently attracted significant interest due to their potential applications in sustainable energy-conversion systems. In organic chemistry, recyclability of photocatalysts are important characteristics with respect to environmental concerns and economic benefits. Metal-based polypyridyl complexes of ruthenium and iridium have been widely used as recyclable homogeneous photoredox catalysts, whereas organophotoredox catalysts are less explored and usually limited to the use of polymer methods. Herein, we report the design and synthesis of recyclable phenothiazine organophotoredox catalysts (PTHS 1–3). These catalysts exhibit relatively high excited-state oxidation potentials (E1/2ox* = −2.34 to −2.40 V vs. SCE) that can efficiently promote photoredox reactions via the one-electron reduction of 1,3-bis(trifluoromethyl)benzene and 4-trifluoromethyliodobenzene, which have high reduction potentials (Ep/2 = −2.07 to −2.16 V vs. SCE). Notably, when the recycling performance was evaluated in the cross-coupling reaction between an aryl halide and triethylphosphite, PTHS-1 can be recovered at least four times without loss of its catalytic activity. The present recyclable and reusable organophotoredox catalysts thus represent a promising tool for sustainable organic synthesis.