Aldehyde–Olefin Couplings by Photoinduced Reduction of Electron‐Deficient Olefins with Hantzsch Ester Anions

Reductive couplings between aldehydes and olefins, both of which are low-cost chemical feedstocks, provide facile access to valuable alcohol products. Recent advances in photocatalytic and electrochemical methods have provided efficient strategies to achieve reductive aldehyde-olefin couplings through nucleophilic ketyl or olefin radical anion intermediates. However, the strongly reducing conditions required for radical formation result in limitations in substrate generality, especially for reactions of unactivated aliphatic aldehydes with electron-deficient olefins. In this scenario, olefin dimerization or hydrogenation outcompetes aldehyde coupling due to the preferential formation and diminished nucleophilicity of olefin radical anion intermediates. Herein, we report a simple, photocatalyst-free protocol that overcomes this limitation by using visible light-activated Hantzsch ester as a photoreductant under mildly basic conditions. Key to the success of the transformation was the use of water as a protic additive, which enabled nucleophilic addition of the olefin radical anions to aldehydes. Mechanistic experiments support olefin radical anions as the key intermediates and offer insight into the important role of water in the transformation.