A Neutral “Masked” Diborene and Its Reactivity Toward Metal‐Free Pyridine Homocoupling

An unprecedented neutral “masked” non‐external‐donor‐stabilized diborene is generated via intramolecular arene dearomatization of an N‐heterocyclic imine (NHI)‐supported free diborene. The free diborene intermediate is photochemically accessible from the masked form, as supported experimentally through chalcogenation reactions. Computational studies elucidate an alkyne‐like bonding pattern and a triplet ground state with a Δ E S‐T = +1.2 kcal mol −1 for the NHI‐supported free diborene. Remarkably, the “masked” diborene activates pyridines, promoting transition‐metal‐free C−C bond formation with high C2 regioselectivity to afford homocoupling products under mild conditions. Mechanistic investigations reveal that stepwise coordination and B−C bond cleavage produce a bis‐pyridine diborene intermediate, which undergoes reductive coupling via intrinsically strong double single‐π‐electron transfer from the B═B unit to the intramolecular pyridine ligands. Furthermore, oxidation with p ‐benzoquinone allows the release of free 2,2′‐bipyridine derivatives. These findings provide access to insight into a hitherto elusive free diborene species, establish a previously unknown reactivity mode for diborenes, and demonstrate that this “masked” diborene functions as an exceptionally powerful double single‐electron donor with the potential to mimic transition‐metal behavior in the construction of organic molecules.