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(center dot+) 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.