Strongly Reducing (Diarylamino)anthracene Catalyst for Metal-Free Visible-Light Photocatalytic Fluoroalkylation

Well-defined
9,10-bis­(di­(p-tert-butylphenyl)­amino)­anthracene
serves as a photocatalyst for radical
fluoroalkylation under visible light irradiation. The diarylamine
(Donor)–anthracene (π conjugated system)–diarylamine
(Donor) scaffolds are easily accessed by typical palladium-catalyzed
cross-coupling protocols of the corresponding halogenated anthracenes
with various lithium diarylamides. The anthracene-based photocatalyst
exhibits high reducing power, leading to generation of versatile fluoroalkyl
radicals such as tri- and difluoroethyl and tri- and difluoromethyl
radicals from the corresponding electron-accepting precursors. Catalyst
design strongly influences the absorption capability of visible light
and stability toward redox stimuli. The detailed mechanistic studies
on the metal-free photocatalytic amino-trifluoroethylation of styrene
with diphenyl­(2,2,2-trifluoroethyl)­sulfonium trifluoromethanesulfonate
suggest that the reaction proceeds via catalytic radical processes
rather than radical chain processes. In addition, the static quenching
process is involved in the first single-electron-transfer (SET) process
from the photocatalyst to the fluoroalkylating reagent. Furthermore,
the 1e-oxidized cationic radical species of 9,10-bis­(di­(p-tert-butylphenyl)­amino)­anthracene, a key active
catalytic species with long lifetime and a characteristic IVCT (Intervalence
Charge Transfer) band in the near IR (NIR) region, is detected. From
the viewpoint of elemental strategy initiative and green chemistry,
the present noble metal-free organic photocatalytic system provides
a pivotal technology to replace ruthenium- and iridium-based metal
photocatalysis.