Selective Boryl-Anion Migration in a Vinyl sp2−sp3 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)B2Pin2)]− using “soft” BR3 electrophiles (BR3=BPh3 or 9-aryl-BBN). The soft character is key to ensure 1,2-migration proceeds instead of oxygen coordination/B−O activation. The BR3-induced 1,2-boryl-anion migration represents a triple borylation of a vinyl Grignard reagent using only B2Pin2 and BR3 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 B2Pin2, 1,2-boryl-anion migration can be suppressed in favor of intermolecular {BPin}− transfer to BPh3, thus enabling simple access to unsymmetrical sp2−sp3 diboranes.