Visible Light‐Induced [2 + 2]‐Cycloaddition and Photo‐Ene Reactions: Spin‐Dependent Catalysis and Quantum Tunneling Effects

ABSTRACT Electron spin plays a vital role in photochemistry and its precise manipulation can enhance catalytic performance. However, the correlation between spin states and catalytic activity is still elusive, especially in the energy transfer (EnT)‐catalyzed photocycloaddition and photo‐ene reactions. Here, we rationalize the EnT process and clarify the chemoselectivity encompassing Marcus theory and ab initio methods. The initial photophysical process of photocatalyst isopropylthioxanthone ( ITX ) involves a nonadiabatic transition of 1 ππ*→ 3 nπ*→ 3 ππ* upon excitation, populating the long‐lived 3 ππ* state. In the presence of 1,5‐dienes, the intrinsic deactivation pathway of photocatalyst ITX does not suppress the intermolecular spin‐allowed EnT process with substrates, and this process is accompanied by spin multiplicity transfer within the Marcus normal region. From the triplet‐state substrates, both the photocycloaddition and photo‐ene reactions exhibit spin selectivity, whereas the chemoselectivity is governed in the open‐shell singlet state. Furthermore, quantum mechanical tunneling effects play a decisive role in promoting the photo‐ene reaction and reshaping the reaction pathways. Guided by Marcus theory, spin‐dependent catalysis can be further regulated through amino‐group substitution, which not only elevates the EnT efficiency but also enhances the photocycloaddition chemoselectivity.