Reversible 1,1‐Hydroboration: Boryl Insertion into a CN Bond and Competitive Elimination of HBR2 or RH

Abstract Boranes with the general formula of HBR 2 have been found to undergo a facile 1,1‐hydroboration reaction with pyrido[1,2‐a]isoindole ( A ), resulting in insertion of a BR 2 unit into a CN bond and the formation of a variety of BN heterocycles. Investigation on the thermal reactivity of the BN heterocycles revealed that these molecules have two distinct and competitive thermal elimination pathways: HBR 2 elimination (or retro‐hydroboration) versus RH elimination, depending on the R group on the B atom and the chelate backbone. Mechanistic aspects of these highly unusual reactions have been established from both experimental and computational evidence. Adduct formation between HBR 2 and A was found to be the key intermediate in 1,1‐hydroboration of A .