Mechanism of the Enantioselective Intramolecular [2 + 2] Photocycloaddition Reaction of Coumarin Catalyzed by a Chiral Lewis Acid: Comparison with Enone Substrates.

The asymmetric catalysis of the intramolecular enone [2 + 2] photocycloaddition reaction relies on a complicated regulation mechanism to control its reactivity and selectivity as well as quantum yield. The multiconfiguration perturbation theory associated with energy-consistent relativistic pseudopotentials offers a mechanistic comparison between representative coumarin and enone substrates. A pair of bright ππ* states govern the unselective background reaction of the free coumarin through the direct cycloaddition in the singlet hypersurface and the elimination of the reaction channel in the triplet manifold due to the existence of anti El Sayed type singlet-triplet crossing. The opening of a reaction channel in the triplet state is repeatedly verified to depend on the presence of relativistic effects, i.e., spin-orbit coupling due to heavy atoms in the chiral Lewis acid catalyst.