Pd-Catalyzed Hydroborylation of Alkynes: A Ligand Controlled Regioselectivity Switch for the Synthesis of alpha- or beta-Vinylboronates

A ligand controlled selective hydroborylation of alkynes to alpha- or beta-vinylboronates has been developed using a Pd catalyst. The high alpha-selectivity displayed by this reaction can be switched to furnish beta-vinylboronates by altering the ligand from a trialkylphosphine to N-heterocyclic carbene. A variety of terminal alkynes are shown to furnish the corresponding alpha- or beta-vinylboronates in good to excellent selectivity and yield. The mechanistic studies suggest that the solvent is the proton source and bromobenzene functions as an important additive in driving this reaction forward.