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

摘要

O-Directed free radical hydrostannation of the beta-cyclopropyl propargyl alcohol 57 with stannanes and cat. Et3B in THE/H2O or PhMe/MeOH failed to deliver any of the expected products of alpha-stannylvinyl cation capture. Instead only alpha-stannyl-beta-cyclopropylvinyl radical intermediates were detected, which underwent fast H-atom abstraction and/or cyclopropane ring-opening as a result of eliminative beta-scission. A second alkynol probe 23 was also studied in this O-directed free radical hydrostannation process. It gave rise to an alpha:beta stannyl radical addition regiochemistry that changed markedly in favor of the alpha-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 alpha-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 alpha:beta-regioisomers. The formation of 27 further confirmed the exclusively free radical nature of these reactions. (C) 2020 The Authors. Published by Elsevier Ltd.