Photochemical activity of a key donor–acceptor complex can drive stereoselective catalytic α-alkylation of aldehydes

Asymmetric catalytic variants of sunlight-driven photochemical processes hold extraordinary potential for the sustainable preparation of chiral molecules. However, the involvement of short-lived electronically excited states inherent to any photochemical reaction makes it challenging for a chiral catalyst to dictate the stereochemistry of the products. Here, we report that readily available chiral organic catalysts, with well-known utility in thermal asymmetric processes, can also confer a high level of stereocontrol in synthetically relevant intermolecular carbon–carbon bond-forming reactions driven by visible light. A unique mechanism of catalysis is proposed, wherein the catalyst is involved actively in both the photochemical activation of the substrates (by inducing the transient formation of chiral electron donor–acceptor complexes) and the stereoselectivity-defining event. We use this approach to enable transformations that are extremely difficult under thermal conditions, such as the asymmetric α-alkylation of aldehydes with alkyl halides, the formation of all-carbon quaternary stereocentres and the control of remote stereochemistry. The combination of organocatalytic and photoredox cycles has attracted much attention for its ability to solve long-standing problems in asymmetric catalysis. Here, it is shown that easily available chiral organic catalysts can guide both the stereoselectivity-defining events and, through the transient formation of photon-absorbing chiral electron donor–acceptor complexes, the photoactivation of the substrates.