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.