B(C₆F₅)₃-Catalyzed (5 + 1)-Annulation of Enynamides with Trihydrosilanes en Route to the Synthesis of 1,4-Azasilines

Silaazacycles are emerging as promising heavier bioisosteres of N-heterocycles in drug discovery. Among them, 1,4-azasiline, a versatile scaffold that enables access to diverse silaazacycles, has remained underexplored, largely due to the lack of efficient synthetic methods. Here we report a B(C₆F₅)₃-catalyzed (5 + 1)-annulation of enynamides and trihydrosilanes, providing direct access to 1,4-azasilines and benzo-fused 1,4-azasilines. Mechanistic studies suggest that the reaction begins with a chemoselective and regioselective β-hydrosilylation of the enamide moiety, which exhibits higher reactivity than the ynamide unit within the enynamide substrate.