Silylium-Ion Regeneration by Protodesilylation Enables Friedel–Crafts Alkylation with Less Isomerization and No Defunctionalization

An improved protocol for the Friedel–Crafts alkylation of benzene as well as its methylated and halogenated derivatives with alkyl and benzyl bromides is reported. The reaction is promoted by a counteranion-stabilized silylium ion in the presence of stoichiometric amounts of a simple phenyl-substituted tetraorganosilane. This additive functions as a proton scavenger, regenerating the catalytically active silylium ion through protonation (protodesilylation) by the Wheland intermediate. It is a productive "proton-into-silylium ion" generator. The higher proton affinity of silylated compared with alkylated arenes results in fast proton transfer from the Brønsted acidic Wheland intermediate to the ipso position of that phenylsilane, thereby preventing otherwise competing defunctionalization and isomerization at the stage of the Wheland intermediate. The additive was also found to be crucial for turnover in the alkylation with primary alkyl bromides and for the suppression of transbenzylation in the benzylation with more reactive benzyl bromides.