Silylium-Ion-Promoted Formation of Methylenecyclobutenes by Formal (2 + 2) Cycloaddition of Allenylsilanes and Internal Alkynes.

A (2 + 2) cycloaddition of propargylsilanes and internal alkynes to afford densely substituted methylenecyclobutenes is reported. The propargylsilane is generated in situ by rapid silylium-ion-catalyzed isomerization of the corresponding allenylsilane through the intermediacy of a β,β-bis(silicon)-stabilized vinyl cation. The stepwise cycloaddition is initiated by a silylium ion, and the (2 + 2) pathway hinges on the steric demand of the α-substituent of the allenylsilane (R = 2° alkyl). Smaller groups (R = 1° alkyl) lead to the known formation of the (3 + 2) annulation product.