Metal‐Free Protoboration of Dialkyl Ketones: NHC‐Catalyzed Synthesis of α‐Hydroxy Boronate Esters

Protoboration exhibits inverse chemoselectivity relative to classical hydroboration and proceeds via a nucleophilic boron species. Although well established for alkenes and alkynes, its application to carbonyl (C═O) compounds remains unexplored, and all reported methods to date rely on metal catalysis. Herein, we report the first protoboration of carbonyl compounds and the first metal‐free variant of this transformation. An N‐heterocyclic carbene (NHC)‐catalyzed protocol enables the direct protoboration of unactivated dialkyl ketones to afford α‐hydroxyboronate esters in high yields under mild conditions, using readily available bis(pinacolato)diboron (B ₂ pin ₂ ) and isopropyl alcohol (IPA). Mechanistic investigations, including control and sequential addition experiments, variable‐temperature nuclear magnetic resonance, and density functional theory calculations, reveal cooperative activation by the NHC, diboron reagent, and alcohol. A nucleophilic boron species derived from an NHC–borane complex, previously limited to conjugate borylation, is shown here to directly activate relatively unreactive dialkyl ketones, a reactivity previously considered inaccessible without metal catalysis. This work establishes a sustainable, metal‐free strategy for protoboration and significantly expands the scope of nucleophilic boron chemistry to carbonyl functionalization.