Aromatization-Driven Borylene Generation from Borepin Precursors Enables Fused Borirane Synthesis and Selective Alkene Functionalization.

Borylenes are among the most intriguing low-valent species in main group chemistry, yet their broader use in synthetic organic chemistry has remained limited by the lack of mild and modular methods for their generation. Here, we report a new class of borepin precursors that undergo aromatization-driven, redox-neutral borylene formation under synthetically practical conditions. This platform enables efficient interception of the resulting borylene intermediates by tethered alkenes through formal intramolecular [1 + 2] cycloaddition, providing direct access to previously unreported fused borirane frameworks. These strained borirane intermediates serve as valuable synthetic linchpins, enabling divergent olefin functionalization through C-B, C-O, and C-C bond-forming processes with notable regioselectivity. The synthetic utility of this platform is further highlighted by the selective functionalization of substrates containing multiple alkene units, guided by a native alcohol directing group, as well as by its application to the formal synthesis of natural products. To the best of our knowledge, this work represents the first application of borylene chemistry for the preparation of valuable building blocks relevant to natural product synthesis, establishing low-valent boron intermediates as practical tools for complex-molecule construction. Mechanistic studies combining DFT calculations, NMR analysis, and X-ray crystallography support a sequence involving Lewis-base coordination, borepin rearrangement, and concerted borylene extrusion driven by naphthalene formation. Overall, this work marks an important milestone in borylene chemistry by demonstrating that these low-valent boron species can be generated and harnessed as practical intermediates in synthetic organic chemistry, thereby opening the door to new reactivity patterns and broader applications in molecular construction.