Mechanistic Insights into Enantioselective CH Photooxygenation of Aldehydes via Enamine Catalysis

Organocatalytic photooxygenation of aldehydes at the α‐position proceeds via enamine catalysis, though enamines should be easily oxidized by singlet oxygen respectively to amides and carbonyl compounds. Moreover, the formation of a zwitterionic enamine peroxide intermediate was postulated based on experimental and theoretical data. The reaction affords desired diols (after in situ reduction) in a decent yield and ( S )‐ or ( R )‐enantioselectivity depending on a catalyst used. The ( S ) ‐ enantiomer predominated in imidazolidinone‐catalyzed reactions, while prolineamides assured the formation of the ( R )‐stereoisomer. DFT calculation suggests that the enamine–oxygen complex with the lowest energy has the E,   s ‐ cis conformation for the prolineamide derivative and E,   s ‐ trans for the imidazolidinone catalyst, explaining the opposite stereoselectivity in the photooxygenation reaction.