Reductive Activation of Aryl Chlorides by Tuning the Radical Cation Properties of N ‐Phenylphenothiazines as Organophotoredox Catalysts

Aryl chlorides as substrates for arylations present a particular challenge for photoredox catalytic activation due to their strong C(sp$^2$)−Cl bond and their strong reduction potential. Electron-rich N-phenylphenothiazines, as organophotoredox catalysts, are capable of cleaving aryl chlorides simply by photoinduced electron transfer without the need for an additional electrochemical activation setup or any other advanced photocatalysis technique. Due to the extremely strong reduction potential in the excited state of the N-phenylphenothiazines the substrate scope is high and includes aryl chlorides both with electron-withdrawing and electron-donating substituents. We evidence this reactivity for photocatalytic borylations and phosphonylations. Advanced time-resolved transient absorption spectroscopy in combination with electrochemistry was the key to elucidating and comparing the unusual photophysical properties not only of the N-phenylphenothiazines, but also of their cation radicals as the central intermediates in the photocatalytic cycle. The revealed photophysics allowed the excited-state and radical-cation properties to be fine-tuned by the molecular design of the N-phenylphenothiazines; this improved the photocatalytic activity.