Asymmetric photoredox catalytic formal de Mayo reaction enabled by sensitization-initiated electron transfer

Visible-light-driven photoredox catalysis is known to be a powerful tool for organic synthesis. Its occurrence critically depends on the twice exothermic single-electron transfer processes of photosensitizers, which are governed by the redox properties of the species involved. Hence, the inherently narrow range of redox potentials of photosensitizers inevitably constrains their further availability. Sensitization-initiated electron transfer has recently been found to effectively overcome this substantial challenge. However, feasible and practical strategies for designing such complicated catalytic systems are rather scarce. Herein we report an elaborate dual-catalyst platform, with dicyanopyrazine as a visible light photosensitizer and a pyrenyl-incorporated chiral phosphoric acid as a co-sensitizer, and we demonstrate the applicability of this sensitization-initiated electron transfer strategy in an asymmetric formal de Mayo-type reaction. The catalysis platform enables otherwise thermodynamically unfavourable electron transfer processes to close the redox cycle and allows for precise access to valuable enantioenriched 1,5-diketones with a wide substrate range. The redox properties of visible-light-absorbing photosensitizers are limited by the energy of visible photons, but methods using sensitization-initiated electron transfer have recently been developed to address these challenges. Now a multiphoton dual-catalyst strategy has been used to enable the enantioselective de Mayo reaction for the synthesis of enantioenriched 1,5-diketones.