Selective Boryl‐Anion Migration in a Vinyl sp(2)−sp(3) Diborane Induced by Soft Borane Lewis Acids

An intramolecular 1,2‐boryl‐anion migration from boron to carbon has been achieved by selective activation of the π system in [(vinyl)B(2)Pin(2))](−) using “soft” BR(3) electrophiles (BR(3)=BPh(3) or 9‐aryl‐BBN). The soft character is key to ensure 1,2‐migration proceeds instead of oxygen coordination/B−O activation. The BR(3)‐induced 1,2‐boryl‐anion migration represents a triple borylation of a vinyl Grignard reagent using only B(2)Pin(2) and BR(3) and forms differentially protected 1,1,2‐triborylated alkanes. Notably, by increasing the steric bulk at the β position of the vinyl Grignard reagent used to activate B(2)Pin(2), 1,2‐boryl‐anion migration can be suppressed in favor of intermolecular {BPin}(−) transfer to BPh(3), thus enabling simple access to unsymmetrical sp(2)−sp(3) diboranes.