Merging Photoredox and Palladium Photochemistry: Evidence for a Dual-Photon Approach to Aldehyde Synthesis.

We describe an alternative approach to combine photocatalysts in metal-catalyzed coupling chemistry, where the photoexcitation of an active palladium catalyst is combined with an oxidizing photoredox catalyst to allow the concurrent activation of challenging electrophilic and nucleophilic substrates on both sides of the oxidative addition-reductive elimination cycle. The application of this to carbonylations has opened a new approach to aliphatic aldehyde synthesis that occurs under mild conditions, with an array of alkyl halides and with regiochemistry rigorously defined by the substrate. Analysis shows this transformation proceeds via a synchronous alkyl halide reduction by palladium and photoredox-mediated Hantzsch ester oxidation and operates within a combined redox window well beyond that classically accessible with visible-light photocatalysts.