Visible-Light-Mediated Selective Allylic C–H Oxygenation of Cycloalkenes

Abstract A visible-light-mediated selective allylic C–H bond oxygenation of cyclic olefins is presented. Hence, the selective, mild monooxygenation of simple cycloalkenes has been achieved using an acridinium photoredox catalyst in combination with a phosphate base and a disulfide HAT reagent under air atmosphere at room temperature. The combination of both photocatalyst and HAT reagent, which can operate through a single or two different concurrent mechanistic pathways for the formation of the allyl radical, proved highly efficient, while the reaction with exclusively one or the other mediator performs in significantly lower yields. The formed allyl radical further reacts with a molecule of oxygen to build the corresponding peroxyradical that can abstract a hydrogen atom of another cycloalkene substrate, generating the known hydroperoxide intermediate in the formation of the ketone moiety. The advantages of this method rely on the easy use of air as oxygen source, as well as the selective monooxygenation of cycloalkenes without substitution in one of the allylic positions. Besides simple cyclic olefins, the method was also successfully applied in the oxidation of natural products such as the terpene valencene or cholesterol derivatives.