Oxidative benzylic C(sp3)-H functionalisation of electron-poor substrates with photoexcited DDQ

Abstract

2,3-Dichloro-5,6-dicyano-1,4-benzoquinone (DDQ) is a common oxidant used for myriad oxidation reactions, including the functionalisation of electron-rich benzylic C(sp3)-H bonds. Under photochemical excitation, DDQ becomes a far more powerful oxidant, though it has scarcely been utilised for C(sp3)-H functionalisation nor is the mechanism by which it operates well understood. Here, we report that photochemical excitation of DDQ enables oxidative functionalisation of previously inaccessible electron-poor benzylic C(sp3)-H bonds to form benzylic quinol ethers. The methodology is amenable to both batch and flow setups and is demonstrated across a series of primary and secondary benzylic substrates. Transient absorption spectroscopy revealed mechanistic insight into the electron transfer processes involved, indicating a single electron transfer pathway operates under these conditions, contrasting a hydride mechanism previously reported to occur when employing ground-state DDQ.