Reversible and Irreversible Regioselective Alkyne Insertion into a Silyl-Substituted Stannylene

A range of aryl- and alkyl- substituted alkynes has been shown to insert regio-selectively into the Sn─Si bond of the electron-rich aryl(silyl)stannylene, Ar^MesSnSi(SiMe₃)₃ (Ar^Mes = 2,6-Mes₂C₆H₃, Mes = 2,4,6-Me₃C₆H₂) to generate a series of vinyl-stannylene products. In all cases, the product features a syn arrangement of Sn and Si-containing groups about the resulting carbon-carbon double bond; in the case of unsymmetrical alkynes, the more sterically bulky group is exclusively incorporated in the 1-position (i.e., proximal to Sn). Remarkably, insertion is shown to occur reversibly in the cases of 3-hexyne and trimethylsilylacetylene. The thermodynamic parameters associated with these processes have been determined by variable temperature NMR spectroscopy, and the activation barriers associated with the key mechanistic steps elucidated by quantum mechanical methods.