Less Radical Stannylation Mechanism Revealed

For organic chemists, the free-radical hydrostannylation of alkynes to produce vinylstannanes is a key step in important cross-coupling and other reactions. Every published mechanism has hinged solely on radical intermediates. But that isn’t the whole story, according to Michael G. Organ of York University, in Toronto. Chemists have assumed the reaction goes by a purely radical process because radical initiators such as AIBN or triethylborane are used to trigger the reaction. Organ’s group, in collaboration with Robert D. J. Froese of Dow Chemical, discovered that the reaction is oxygen-dependent. When oxygen is rigorously excluded—a difficult task—the reaction fails completely, Organ said. He suggested that the initially formed vinyl radical is instead oxidized by oxygen to form a cationic vinylstannane intermediate and superoxide. The intermediate undergoes hydride addition from a second stannane molecule, is reduced by the superoxide to generate a stannyl radical, and then reacts with another alkyne molecule to continue the catalytic cycle.