Stepwise Polymetalation around an sp³ Benzyl Carbon Atom

Herein we describe a sequential metalation process at an sp3 carbon center from μ2- to μ3- to the first benzyl hypercoordinated carbon (HC)-centered μ4-gold­(I) cluster. Starting with a gem-dimetalated precursor, attachment of two gold atoms on the ipso carbon enables the formation of an electron-rich silicon center, and the resulting stable C–Si bond is even resistant to the attack of fluoride. Addition of the third Au­(I) ion results in an HC-centered [Au3–C­(Ar)–Si]+ transition state, which facilitates the formation of unique Au–Si agostic bonding and accordingly promotes the electrophilicity of the silicon center. The subsequent facile C–Si bond cleavage drives further metalation of the trimetalated benzyl carbon to constitute an HC-centered tetrametalated structure. This biased polymetalation effect in every elementary step along with the escalation of the nuclearity has been thoroughly deconvoluted via detailed structural analysis and kinetic and theoretical studies.