Photo-induced terminal alkyne insertion into arene ring to synthesize boron-doped polycycles.

The classic Büchner reaction involves the insertion of highly reactive carbenoids into benzene rings to form cycloheptatrienes. Achieving a Büchner-type ring expansion using stable terminal alkynes as carbon sources, however, has remained a formidable challenge. Herein, we report that under light irradiation, ethynyl- and BMes2 (Mes = 2,4,6-trimethylphenyl)-substituted heteroarenes undergo an intramolecular Büchner-type ring expansion. This clean reaction generates heteroatom-doped polycyclic compounds through concerted inert aromatic C-C bond cleavage, carbon atom insertion, and hydrogen atom transfer (HAT). To our knowledge, this is the first example of a Büchner-type ring expansion employing a stable terminal alkyne to form a C7 ring, offering a new strategy for constructing boron-containing polycyclic architectures. Theoretical studies reveal that the photochemical transformation is enabled by intramolecular allene- and boron-radical species along with an intramolecular HAT process.