Catellani-Inspired BN-Aromatic Expansion: A Versatile Tool toward π-Extended 1,2-Azaborines with Tunable Photosensitizing Properties

BN-isosterism, the replacement of carbon-carbon units with boron-nitrogen pairs in organic frameworks, offers a powerful means to create novel compounds, yet methods to access larger BN-containing polyaromatic cores remain scarce. Leveraging our recently developed multigram-scale synthesis of BN-naphthalene, we now combine it with a Catellani-type arene extension (Pd(OAc)₂/P(2-furyl)₃, norbornene) to rapidly access diverse extended BN-embedded polyaromatic cores. This strategy delivers BN-embedded benzo[<i>c</i>]phenanthridines and curved 8- and 7-membered ring-fused derivatives, as well as BN-embedded benzofluorenones in both normal and inverse BN-vector orientations. Importantly, the ability to access both directional BN isomers, in addition to the parent C═C core, provides a rare opportunity to directly interrogate the effect of the presence and sense of the BN moiety. Most notably, light-induced singlet oxygen (¹O₂) generation promoted by the benzofluorenone core shows a more than 10-fold enhancement in the "boron-up" BN isostere, while dropping to negligible levels upon inversion of the BN unit. This work thus offers a blueprint for experimental electronic tuning of optically responsive organic materials through BN-mapping.