9-(4-Halo-2,6-xylyl)-10-methylacridinium Ion as an Effective Photoredox Catalyst for Oxygenation and Trifluoromethylation of Toluene Derivatives

9-(2,6-Dimethylphenyl)-10-methylacridinium perchlorate ([Acr⁺-Xyl]ClO₄^-), 9-(4-chloro-2,6-dimethylphenyl)-10-methylacridinium perchlorate ([Acr⁺-XylCl]ClO₄^-), and 9-(4-fluoro-2,6-dimethylphenyl)-10-methylacridinium perchlorate ([Acr⁺-XylF]ClO₄^-) were synthesized by the Grignard reaction. The one-electron reduction potentials (E_red vs SCE) of Acr⁺-Xyl, Acr⁺-XylCl, and Acr⁺-XylF in deaerated acetonitrile (MeCN) were determined by cyclic voltammetry to be nearly the same as −0.55, −0.53, and −0.53 V, respectively. On the other hand, the one-electron oxidation potentials (E_ox vs SCE) of Acr⁺-Xyl, Acr⁺-XylF, and Acr⁺-XylCl were determined by the second-harmonic alternative current voltammetric method to be +2.15, +2.20, and +2.21 V, respectively. The E_ox value of Acr⁺-XylCl is higher than the E_ox value of toluene (+2.20 V). Thus, photocatalytic oxygenation of toluene with oxygen occurs efficiently via electron transfer from toluene to the XylCl^•+ moiety of the triplet electron-transfer state of Acr⁺-XylCl under photoirradiation of Acr⁺-XylCl in oxygen-saturated MeCN. By contrast, no oxygenation of toluene occurred with the 9-mesityl-10-methylacridinium ion (Acr⁺-Mes) used as a photocatalyst due to the lack of oxidizing ability to oxidize toluene. The metal-free trifluoromethylation of toluene derivatives has also been made possible by using Acr⁺-XylCl as a photocatalyst and S-(trifluoromethyl)dibenzothiophenium (CF₃DBT⁺) as a trifluoromethyl source. The photocatalytic reaction mechanism was clarified by transient absorption and electrochemical measurements.