Copper-Catalyzed Regioselective Formation of Tri- and Tetrasubstituted Vinylboronates in Air

The first “in air” copper-catalyzed method for the selective
synthesis of tri- and tetrasubstituted vinylboronate derivatives is presented.
Three different variants of the borylation of internal alkynes (α-hydroboration,
β-hydroboration, and carboboration) are described using a single
catalyst: [Cu(Cl)(IMes)] (IMes = N,N′-bis-[2,4,6-(trimethyl)phenyl]-
imidazol-2-ylidene) without taking any precaution to avoid the presence of
air. Bis(pinacolato)diboron was used to afford β-hydroborated products in
the presence of methanol. Adding instead another electrophile allowed the
formation of tetrasubstituted vinylboronate species. Finally, the α-products
were obtained using pinacolborane as the boron source. All compounds were
obtained in high yield with excellent regioselectivity at low catalyst loading
(0.04−2 mol %). The protocol constitutes a very convenient route to access these highly valuable molecules.