Recent Advances in Photoinduced Ketyl and Ketyl-Type Radical Generation from Aldehyde Derivatives

Ketyl radical generation from carbonyls is an important strategy in organic synthesis that enables common electrophilic functional groups to be transformed into nucleophilic radicals. However, the large negative reduction potentials of carbonyls mean that direct conversion to ketyl radicals by single-electron reduction is challenging and requires strongly reducing conditions. As a result, alternative strategies to access these useful radical intermediates have been developed that circumvent traditional reductive pathways. For example, recent reports have demonstrated that aldehydes can be converted into various activated aldehyde derivatives that are readily transformed to ketyl radicals or protected ketyl (ketyl-type) radicals through mechanistically distinct pathways, including photoinduced single-electron oxidation and halogen atom transfer. Herein, we review the synthesis and applications of different aldehyde derivatives that have been developed to allow facile access to ketyl and ketyl-type radicals under mild visible-light photochemical conditions.