Bifunctional Silylene–Aminoborane Enables Cooperative Activation of Unsaturated Bonds and Access to Heteroatom‐Enriched Polycycle

We report the synthesis and cooperative reactivity of a bifunctional silylene–aminoborane compound, 1‐Ph‐2‐ t Bu‐1 H ‐2,1‐benzazaborolyl‐(amidinato)silylene ( 1 ), designed to preserve the ambiphilicity of both functional units while preventing intramolecular deactivation. 1 engages in unprecedented multicomponent activations of unsaturated bonds, including alkynes and phosphaalkynes, to construct novel main‐group element embedded polycyclic frameworks. Reaction with alkynes led to rare main‐group elements enriched semibullvalenes via highly regioselective [2 + 2 + 1] cycloaddition. Notably, reaction with a phosphaalkyne induced a distinct S N Ar‐like B─C bond cleavage and formation of a B─N─Si─P‐embedded barrelene analogue. Structural and computational analyses revealed the synergistic multi‐site activation modes and reaction pathways.