Highly Enantioselective 6π Photoelectrocyclizations Engineered by Hydrogen-Bonding

Excited-state photoreactions play a pivotal role in organic chemistry. Electrocyclization reactions, in particular, are valued both for their ability to produce structurally complex molecules and their central role in elucidating fundamental mechanistic principles of photochemistry. However, stereochemical control remains challenging to achieve by means of asymmetric catalysis. We present herein the highly enantioselective 6π photoelectrocyclization catalyzed by a chiral Ir(III) photosensitizer. This transformation was successfully realized by engineering a strong hydrogen-bonding interaction between a pyrazole moiety on the catalyst and a basic imidazolyl ketone on the substrate. To shed light on the origin of stereoinduction, we conducted a comprehensive investigation combining experimental and computational mechanistic studies. Results from density functional theory (DFT) calculations underscore the crucial role played by the prochirality and the torquoselectivity in the electrocyclization process, as well as the steric demand in the subsequent [1,4]-H shift step. Our findings not only offer valuable guidance for developing chiral photocatalysts but also serve as a significant reference for achieving high levels of enantioselectivity in the 6π photoelectrocyclization reaction.