Fast Ring-Opening of an Intermediary α-Stannyl-β-Cyclopropylvinyl Radical Does Not Support Formation of an α-Stannylvinyl Cation in the O-Directed Free Radical Hydrostannation of Dialkyl Acetylenes

O-Directed free radical hydrostannation of a β-cyclopropyl propargylic alcohol probe with stannanes and cat. Et₃B in THF/H₂O, or PhMe/MeOH, failed to deliver any detectable products of α-stannylvinyl cation capture. Instead only α-stannyl-β-cyclopropylvinyl radical intermediates could be detected, which underwent fast H-atom abstraction and/or cyclopropane ring-opening as a result of fast eliminative β-scission. These outcomes strongly rule against the claimed intermediacy of α-stannylvinyl cations in the O-directed free radical hydrostannation of disubstituted acetylenes, and provide further good supporting evidence for such reactions proceeding by an exclusively free radical O-coordinatively-controlled mechanism.