Alkylating Agents Stronger than Alkyl Triflates

A new class of potent electrophilic “R+ ” alkylating agents has been developed using weakly
nucleophilic carborane anions as leaving groups. These reagents, R(CHB11Me5X6) (R = Me, Et, and i-Pr;
X = Cl, Br), are prepared via metathesis reactions with conventional alkylating agents such as alkyl triflates,
using the high oxophilicity of silylium ion-like species, Et3Si(carborane), as the driving force to obtain
increased alkyl electrophilicity. The crystal structure of the isopropyl reagent, i-Pr(CHB11Me5Br6), has been
determined, revealing covalence in the alkyl−carborane bonding. This contrasts with the free i-Pr+
carbocation observed when the anion is less coordinating (e.g. Sb2F11-) or with tertiary alkyl centers, as in
[tert-butyl][carborane] salts. In solution, the reagents exist as equilibrating isomers with the alkyl group at
the 7−11 or 12 halide positions of the CB11 icosahedral carborane anion. These alkylating agents are so
electrophilic that they (a) react with alkanes at or below room temperature via hydride extraction to produce
carbenium ions, (b) alkylate benzene without a Friedel−Crafts catalyst to give arenium ions, and (c) alkylate
electron-deficient phosphorus compounds that are otherwise inert to conventional alkylating agents such
as methyl triflate.