Phosphines as Silylium Ion Carriers for Controlled C–O Deoxygenation: Catalyst Speciation and Turnover Mechanisms

We report studies delineating the speciation, kinetics, and deoxygenation catalysis of phosphine-modified mixtures of B­(C6F5)3 (BCF) and R3SiH. Combinations of BCF, a tertiary silane, and PAr3 generate the [H–B­(C6F5)3–]­[R3Si–PAr3+] ion pair with conversions that depend on the silane and the phosphine. Smaller silanes enhance the ionization of the Si–H, as judged by heteronuclear NMR spectroscopy. Kinetic studies indicate that from BCF·PPh2(p-tol), formation of the borohydride/silyl phosphonium ion pair is α [Et3SiH]1[PPh2(p-tol)]0. DFT calculations confirmed the intermediacy of the weakly coordinated BCF···H–SiEt3 adduct en route to the silyl phosphonium. For the catalytic deoxygenation of anisole with Et3SiH, phosphine additives slow the reaction relative to phosphine-free conditions. In situ monitoring confirmed the presence of [H–B­(C6F5)3–]­[Et3Si–PAr3+] at early times, but this slowly converts to [H–B­(C6F5)3–]­[H3C–PAr3+], which is catalytically inactive. These data are reconciled by invoking a competitive demethylation of a key PhOMe­(SiEt3)+ oxonium ion intermediate by H–B­(C6F5)3– (productive) or phosphine (nonproductive).