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 ΔE_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.