Synergistic Silylium‐Ion and Palladium Catalysis Enables Migratory Alkene Silylation With Allylsilanes by Formal C–H/C–Si Metathesis

ABSTRACT A conceptually distinct approach to the Heck‐type silylation of terminal alkenes to yield allylsilanes under synergistic silylium‐ion and palladium catalysis is reported. The combination of an electrophilic silylium carborate, such as [Me 3 Si(HCB 11 H 5 Br 6 )], and the nucleophilic phosphine‐coordinated palladium(0) complex ( o ‐Tol 3 P) 2 Pd, leads to the formation of the cationic silylpalladium adduct [(o‐Tol 3 P) 2 Pd–SiMe 3 ] + , which catalyzes the silylation of allylbenzene derivatives using an allylsilane as the stoichiometric silicon electrophile. Mechanistic studies including DFT calculations support a mechanism involving regioselective stepwise ionic addition of the silylpalladium cation across the double bond followed by β ‐hydride elimination to form a cationic palladium hydride. Exploiting the Brønsted acidity of this intermediate facilitates the regeneration of the active silylpalladium complex by productive proton‐into‐silylium ion interconversion through deallylative protolysis (protodesilylation) of the allylsilane reagent, thereby maintaining the catalytic turnover. Hence, there is no need for an exogeneous base, whereas the conventional silyl‐Heck reaction requires the addition of (over)stoichiometric amounts of a base to trap the formed acid (HX) waste. Moreover, the established silyl‐Heck reaction relies on the oxidative addition of the silicon–halogen bond of a halosilane electrophile as the critical step, while this synergistic process makes use of an allylsilane as the coupling partner, releasing propene as the sole by‐product.