Copper-catalyzed double borylation of silylacetylenes: highly regio- and stereoselective synthesis of syn-vicinal diboronates.

Phosphite-copper(I) complexes efficiently catalyzed the double borylation of internal silylated alkynes to provide vicinal diboronates with excellent regio- and stereoselectivity. The copper-catalyzed reaction between bis(pinacolato)diboron (B2pin2) and aryl-substituted silylacetylenes in the presence of MeOH resulted in double syn addition of the pinacolboronate moiety (Bpin) and H across the triple bond with complete selectivity. While the double borylation was highly efficient for aryl-substituted alkynylsilanes and silylacetylene, only monoborylation took place with alkyl-substituted alkynylsilanes to yield (Z)-(β-borylvinyl)silanes under the developed catalytic conditions.