Mediating Photoredox Cycloaddition Reactions of Enones Using Surface-Modified Metal Halide Perovskite Nanocrystal Catalyst

Cycloaddition reactions are employed to construct essential chemical motifs in multiple types of bioactive compounds. We utilize the excellent optical property of perovskite nanocrystals to efficiently sensitize these important reactions and explore surface chemistry approaches to control the chemo- and diastereoselectivity of reaction products. By modifying the surface ligand shell of cesium lead bromide perovskite nanocrystals (CsPbBr₃ PNCs), we obtained a stable photocatalyst that can effectively activate different cycloaddition reactions of enones. Time-resolved optical spectroscopic investigation reveals the reactions undergo a photoredox mechanism. The PNC sensitizes [2+2] cycloaddition resulting in a cyclobutyl motif by generating the triplet excited state of enones through photoinduced electron transfer followed by a charge recombination process. The kinetic information collected by transient absorption indicates that it is possible to preserve the anion radicals of enones by introducing appropriate sacrificial electron donors to compete with charge recombination from the PNC. Unlike the triplet excited state, these anion radicals serve as the intermediates for [1+4] cycloaddition and lead to products with cyclopentyl rings. In this specific scenario of photoredox reactions, the stability of our surface-modified PNCs surpasses conventional metal chalcogenide nanocrystals, such as CdSe, due to the fast hole removal ability. Our work demonstrates that metal halide PNCs are versatile photocatalysts for synthetic chemistry, and mediating photoredox processes can realize the selection of reaction pathways to yield different products.