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

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 [Me3Si(HCB11H5Br6)], and the nucleophilic phosphine-coordinated palladium(0) complex (o-Tol3P)2Pd, leads to the formation of the cationic silylpalladium adduct [(o-Tol3P)2Pd-SiMe3]+, 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.