Photocatalytic oxygenation of anthracenes and olefins with dioxygen via selective radical coupling using 9-mesityl-10-methylacridinium ion as an effective electron-transfer photocatalyst

Visible light irradiation of the absorption band of 9-mesityl-10- methylacridinium ion (Acr⁺-Mes) in an O₂-saturated acetonitrile (MeCN) solution containing 9,10-dimethylanthracene results in formation of oxygenation product, i.e., dimethylepidioxyanthracene (Me ₂An-O₂). Anthracene and 9-methylanthracene also undergo photocatalytic oxygenation with Acr⁺-Mes to afford the corresponding epidioxyanthracenes under the photoirradiation. In the case of anthracene, the further photoirradiation results in formation of anthraquinone as the final six-electron oxidation product, via 10-hydroxyanthrone, accompanied by generation of H₂O₂. When anthracene is replaced by olefins (tetraphenylethylene and tetramethylethylene), the photocatalytic oxygenation of olefins affords the corresponding dioxetane, in which the O-O bond is cleaved to yield the corresponding ketones. The photocatalytic oxygenation of anthracenes and olefins is initiated by photoexcitation of Acr⁺-Mes, which results in formation of the electron-transfer state: Acr^•- Mes^•+, followed by electron transfer from anthracenes and olefins to the Mes^•+ moiety together with electron transfer from the Acr^• moiety to O₂. The resulting anthracene and olefin radical cations undergo the radical coupling reactions with O ₂^•- to produce the epidioxyanthracene (An-O ₂) and dioxetane, respectively.