Ionic Remote α-C–H Allenylation of Silyl Ethers Involving a [1,5]-Hydride Shift Promoted by Silylium-Ion Regeneration

A silylium-ion-promoted α-C-H allenylation of silyl ethers tethered to an internal alkyne is described. The actual intermolecular allenylation event occurs after the trans-selective hydrosilylation of the alkyne, where an in situ-generated β-silicon-stabilized vinyl cation engages in an intramolecular [1,5]-hydride shift. This process transforms the silyl ether into a silylcarboxonium ion, which reacts with propargylsilanes as nucleophiles, formed by the rapid silylium-ion-catalyzed isomerization of allenylsilanes. As part of the allenylation step, the propagating silylium-ion electrophile is regenerated, thereby closing the catalytic cycle. An allylsilane is also applicable in this transformation, producing the corresponding α-C-H allylation product in high yield.