O-Directed free radical hydrostannation of the β-cyclopropyl propargyl alcohol 57 with stannanes and cat. Et3B in THF/H2O or PhMe/MeOH failed to deliver any of the expected products of α-stannylvinyl cation capture. Instead only α-stannyl-β-cyclopropylvinyl radical intermediates were detected, which underwent fast H-atom abstraction and/or cyclopropane ring-opening as a result of eliminative β-scission. A second alkynol probe 23 was also studied in this O-directed free radical hydrostannation process. It gave rise to an α:β stannyl radical addition regiochemistry that changed markedly in favor of the α-adduct (26) when the reaction was conducted at high stannane concentrations. This outcome confirmed that O–Sn coordinative control must be responsible for the strong α-regiochemical preference of hydrostannations run at higher stannane concentrations, since a non-coordinative, electronically-controlled, stannyl radical addition would always give rise to a fixed and identical ratio of α:β-regioisomers. The formation of 27 further confirmed the exclusively free radical nature of these reactions.
Watson, H. A., Manaviazar, S., Steeds, H. G., & Hale, K. (2020). The stannylvinyl cation that never was! New concentration- and temperature-dependent probe studies confirm an entirely free radical mechanism and O–Sn coordinative control of the hydrostannation of propargylically-oxygenated dialkyl acetylenes with stannanes and cat. Et3B. Tetrahedron. https://doi.org/10.1016/j.tet.2020.131061