Enantioselective Intermolecular Excited-State Photoreactions Using a Chiral Ir Triplet Sensitizer: Separating Association from Energy Transfer in Asymmetric Photocatalysis

• DOI: 10.1021/jacs.9b06244
• Publication Date: 2019-07-22T20:13:07Z
• Authors: Jian Zheng; Wesley B. Swords; Hoimin Jung; Kazimer L. Skubi; Jesse B. Kidd; Gerald J. Meyer; Mu-Hyun Baik; Tehshik P. Yoon
• Keywords: Maleimide; Alkene; Photoexcitation

Enantioselective catalysis of excited-state photoreactions remains a substantial challenge in synthetic chemistry, and intermolecular have proven especially difficult to conduct stereocontrolled fashion. Herein, we report highly enantioselective [2 + 2] cycloaddition 3-alkoxyquinolones catalyzed by chiral hydrogen-bonding iridium photosensitizer. Enantioselectivities as high 99% ee were measured reactions with range maleimides other electron-deficient alkene reaction partners. An array kinetic, spectroscopic, computational studies supports mechanism which the photocatalyst quinolone form hydrogen-bonded complex control selectivity, yet upon photoexcitation this complex, energy transfer sensitization maleimide is preferred. The sensitized then reacts quinolone–photocatalyst afford enantioenriched cycloadduct. This finding contradicts long-standing tenet photochemistry that held stereoselective require strong preassociation substrate order overcome short lifetimes electronically excited organic molecules. system therefore suggests broader alternate design strategies for asymmetric photocatalysis might be possible.