Regioselectivity and stereoselectivity of intramolecular [2 + 2] photocycloaddition catalyzed by chiral thioxanthone: a quantum chemical study.

Chiral photosensitizer-catalyzed stereoselective olefin cyclization has shown its significance in organic synthesis. In this work, we investigated the reaction mechanism, regioselectivity and stereoselectivity of photochemical intramolecular [2 + 2] cycloaddition reaction catalyzed by a chiral thioxanthone molecule using quantum chemical calculations. The reaction proceeded via an energy transfer from the triplet thioxanthone to the substrate, involving stepwise and sequential C–C bond formation. The first C–C bond formation was calculated to be the rate-limiting and selectivity-controlling step. The origin of stereoselectivity was found to be interaction-controlled by distortion/interaction analysis. In addition, the catalyst substituent effects (O vs. S vs. Se) on the stereoselectivity of the photocycloadditions were explored, which provides helpful mechanistic information for the design of related photoinduced reactions.