Anionic Iminoborane as Boryl Anion Synthon.

Classical iminoboranes, extensively investigated since the 1980s, are defined by 1,2-dipolar reactivity in which the boron center remains exclusively electrophilic. Here, we report an anionic iminoborane, [Ar─B≡N─Flu]^-, in which substitution at nitrogen with a fluorenyl anion generates a conjugative B≡N─C: unit that enables cooperative reactivity. This species functions as a nucleophilic boryl anion synthon, engaging N-, P-, Se-, and Au-based electrophiles through a stepwise pathway initiated by carbanion attack followed by 1,3-migration to boron. In parallel, it acts as a BNC-1,3-dipole, undergoing [3 + n] cycloadditions with unsaturated substrates. These results demonstrate a rationally designed anionic iminoborane that provides a versatile platform for building diverse organoboron architectures beyond the classical chemistry of iminoboranes.