Nitroarenes as EnT Photocatalysts for the Cyclopropanation of Cinnamyl Chlorides

Abstract Nitroarenes are versatile building blocks in organic synthesis, and their photochemical reactivity has recently enabled new transformations, including oxidative cleavage reactions. Here, we disclose a sustainable isomerization strategy in which nitroarenes function as energy-transfer (EnT) photocatalysts to convert cinnamyl chlorides into cyclopropanes. In contrast to conventional methods that rely on transition-metal photocatalysts, this approach capitalizes on the intrinsic triplet energy of nitroarenes, offering a metal-free and operationally simple solution. Preliminary studies indicate that the π,π* triplet state of nitroarenes is more efficient in EnT than the n,π* state, revealing a distinct divergence in their excited-state reactivity. Standard silica gel (SiO₂) serves as an effective additive to promote the reverse isomerization of the resulting 1-arylallyl chlorides to cinnamyl chlorides, further enhancing the overall process efficiency.