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

Abstract

Aryl chlorides as substrates for arylations present a particular challenge for photoredox catalytic activation due to their strong C(sp2)‑Cl bond and their strong reduction potential. Electron‐rich N‐phenyl phenothiazines, as organophotoredox catalysts, are capable to cleave 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 elucidate and compare not only the unusual photophysical properties of the N‐phenylphenothiazines, but also of their cation radicals as the central intermediates in the photocatalytic cycle. The revealed photophysics allowed the fine‐tuning of the excited state and radical cation properties by the molecular design of the N‐phenyl phenothiazines which improved the photocatalytic activity.