Selective boryl-anion migration in a vinyl sp2-sp3 diborane induced by soft borane Lewis acids

A novel 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 on the beta 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, which represents a simple way to access unsymmetrical sp²-sp³ diboranes.