Ga+-Catalyzed Hydrosilylation? About the Surprising System Ga+/HSiR3/Olefin, Proof of Oxidation with Subvalent Ga+ and Silylium Catalysis with the Perfluoroalkoxyaluminate Anion

Already 1 mol-% of subvalent [Ga(PhF)2]+[pf]– ([pf]– = [Al(ORF)4]– , RF = C(CF3)3) initiates the hydrosilylation of olefinic double bonds under mild conditions. Reactions with HSiMe3 and HSiEt3 as substrates efficiently yield anti-Markovnikov and anti-addition products, while bulkier substrates like HSiiPr3 are less reactive. Investigating the underlying mechanism by gas chromatography and STEM analysis, we unexpectedly found that H2 and metallic Ga0 formed. Without addition of the olefin, the formation of R3Si–F–Al(ORF)3 (R = alkyl) was observed, a typical degradation product of the [pf]– anion in the presence of a small silylium ion. Electrochemical analysis revealed a surprisingly high oxidation potential of univalent [Ga(PhF)2]+[pf]– in the weakly coordinating, but polar ortho-difluorobenzene of E1/2(Ga+/Ga0; oDFB) = +0.26–0.37 V vs. Fc+ /Fc (depending on the scan rate). Apparently, the subvalent Ga+, mainly known as reductant, initially oxidizes the silane, and generates a highly electrophilic, silane-supported, silylium ion representing the actual catalyst. Consequently, the [Ga(PhF)2]+[pf]– /HSiEt3 system also hydrodefluorinates C(sp3)–F bonds in 1-fluoroadamantane, 1-fluorobutane and PhCF3 at room temperature. In addition, both catalytic reactions may also be initiated by using only 0.2 mol-% of [Ph3C]+[pf]– as silylium ion generating initiator. These results indicate that silylium ion-catalysis is possible with the straight forward accessible weakly coordinating [pf]– anion. Apparently, the kinetics of hydrosilylation and hydrodefluorination are faster than that of anion degradation under ambient conditions. These findings open new windows for main group catalysis.