Oxygenation and chlorination of aromatic hydrocarbons with hydrochloric acid photosensitized by 9-mesityl-10-methylacridinium under visible light irradiation

Efficient photocatalytic oxygenation of toluene occurs under visible light irradiation of 9-mesityl-10-methylacridinium (Acr(+)-Mes) in oxygen-saturated acetonitrile containing toluene and aqueous hydrochloric acid with a xenon lamp for 15 h. The oxygenated products, benzoic acid (70 %) and benzaldehyde (30 %), were formed after the photoirradiation. The photocatalytic reaction is initiated by intramolecular photoinduced electron transfer from the mesitylene moiety to the singlet excited state of the Acr(+) moiety of Acr(+)-Mes, which affords the electron-transfer state, Acr(aEuro cent)-Mes(aEuro cent+). The Mes(aEuro cent+) moiety can oxidize chloride ion (Cl-) by electron transfer to produce chlorine radical (Cl-aEuro cent), whereas the Acr(aEuro cent) moiety can reduce O-2 to O (2) (aEuro cent a') . The Cl-aEuro cent radical produced abstracts a hydrogen from toluene to afford benzyl radical in competition with the bimolecular radical coupling of Cl-aEuro cent. The benzyl radical reacts with O-2 rapidly to afford the peroxyl radical, leading to the oxygenated product, benzaldehyde. Benzaldehyde is readily further photooxygenated to yield benzoic acid with Acr(aEuro cent)-Mes(aEuro cent+). In the case of an aromatic compound with electron-donating substituents, 1,3,5-trimethoxybenzene, photocatalytic chlorination occurred efficiently under the same photoirradiation conditions to yield a monochloro-substituted compound, 2,4,6-trimethoxychlorobenzene.