Boryl Homolytic Aromatic Substitution Enables Radical Borylation via Inverted B-H Polarity of Hydroborane.

Hydroboranes are commercially available, highly atom-economical, and widely used boron sources. Their diverse transformations have traditionally relied on the hydride chemistry of the hydrogen part and the electropositive characteristics of the boron atom, following a two-electron mechanism. However, the single-electron transformation of hydroboranes remains unexplored territory. Here, we present a boryl HAS reaction enabled by the polarity inversion of the B-H bond, which achieves the single-electron transformation of hydroborane for the synthesis of diverse organoboron compounds. The synthetic potential of this hydroborane transformations was demonstrated by a wide substrate scope, gram-scale reactions, and downstream derivatizations. Experimental and computational investigations support a radical mechanism involving radical addition to the boron atom of 1,3,2-benzodiazaborolane (HBaza), followed by single-electron oxidation and an unusual rearomatization-promoted B-H deprotonation of a boronium cation.