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.