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