Anionic Iminoborane as Boryl Anion Synthon

Abstract 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.