Silylium-Ion-Promoted Cycloisomerization of Silicon-Tethered Enynes

A series of four silylium-ion-promoted enyne cycloisomerization reactions is reported. Various silicon-tethered 1,5-, 1,6-, and 1,7-enynes engage in chemoselective π-activation of either unsaturated branch by the formation of β,β- or β,β’-bis(silicon)-stabilized vinyl cations (from the alkyne branch) or a β-silicon-stabilized carbenium ion (from the alkene branch). Subsequent ring closures lead to a six-membered ring (silinane) in three cases, while one 1,5-enyne cyclizes to give a five-membered ring (silole). The silole system stands out as the exocyclic silyl group is lost by protolysis, followed by proton-mediated, thermodynamically controlled migration of an aryl substituent. The reaction mechanisms can be rationalized by silicon-controlled carbocation formation with termination by either allylation of a carbenium-ion intermediate with an allylsilane or a Friedel–Crafts reaction with an arene. The proton from the Wheland intermediate is captured by proton-into-silylium ion interconversion, again employing an allylsilane reagent.