Photocatalytic skeletal editing of bicyclic (aza)arenes to (aza)arenosemibullvalenes via cascade energy transfer

Molecular skeletal editing of aromatic systems for selective bond reorganization faces inherent thermodynamic and kinetic challenges. Here, we disclose a photocatalytic skeletal editing strategy that transforms bicyclic (aza)arenes into (aza)arenosemibullvalene frameworks. The reaction is compatible with a wide range of naphthalene and bicyclic azaarenes derivatives, delivering various highly intriguing three-dimensional (3D) strained ring systems. Combined mechanistic investigations—fluorescence quenching studies, cyclic voltammetry, and density functional theory (DFT) calculations—unveiled a cascade energy transfer mechanism, wherein substrate-selective triplet sensitization drives sequential para -[4+2] or ortho -[2+2] cycloaddition followed by Zimmerman di-π-methane rearrangement, as evidenced by transition state analysis. We anticipate this method will find substantial use in facilitating the efficient synthesis of topologically complex 3D scaffolds previously inefficient via conventional pathways.