9-(4-Halo-2,6-xylyl)-10-methylacridinium Ion as an Effective Photoredox Catalyst for Oxygenation and Trifluoromethylation of Toluene Derivatives
Kei Ohkubo, Soichiro Matsumoto, Haruyasu Asahara, Shunichi Fukuzumi
Abstract
9-(2,6-Dimethylphenyl)-10-methylacridinium perchlorate ([Acr + –Xyl]ClO 4 – ), 9-(4-chloro-2,6-dimethylphenyl)-10-methylacridinium perchlorate ([Acr + –XylCl]ClO 4 – ), and 9-(4-fluoro-2,6-dimethylphenyl)-10-methylacridinium perchlorate ([Acr + –XylF]ClO 4 – ) 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 (CF 3 DBT + ) as a trifluoromethyl source. The photocatalytic reaction mechanism was clarified by transient absorption and electrochemical measurements.