Selective photocatalytic aerobic bromination with hydrogen bromide via an electron-transfer state of 9-mesityl-10-methylacridinium ion†

Photocatalytic bromination of aromatic hydrocarbons by molecular oxygen with hydrogen bromide occurs efficiently to produce monobrominated products selectively using 9-mesityl-10- methylacridinium ion (Acr⁺-Mes) as a photocatalyst under visible light irradiation. Both the product yield and selectivity for the bromination of 1,3,5-trimethoxybenzene were 100% with a quantum yield of 4.8%. The photocatalytic turnover number is 900 based on the initial concentration of Acr⁺-Mes. The reactive radical intermediates involved in the photocatalytic cycle have been successfully detected by laser flash photolysis measurements. The photocatalytic bromination is initiated by photoinduced electron transfer from the mesitylene moiety to the singlet excited state of acridinium ion, which results in formation of the electron-transfer state of Acr⁺-Mes (Acrc^.-Mesc^.+), followed by electron transfer from aromatic hydrocarbons to the mesitylene radical cation moiety and electron transfer from the acridinyl radical moiety to O₂. The resulting radical cations of aromatic hydrocarbons react with Br^- to produce the corresponding monobrominated products selectively.