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 O-2-saturated acetonitrile (MeCN) solution containing 9,10-dimethylanthracene results in formation of oxygenation product, i.e., dimethylepidioxyanthracene (Me(2)An-O-2). 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 tetra methylethylene), 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 O-2. 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.