Ga-catalyzed Hydrosilylation? About the Surprising System Ga/HSiR/olefin, Proof of Oxidation with Subvalent Ga and Silylium Catalysis with Perfluoroalkoxyaluminate Anions

Already 1 mol% of subvalent [Ga(PhF)][] ([] = [Al(OR)], R = C(CF)) initiates the hydrosilylation of olefinic double bonds under mild conditions. Reactions with HSiMe and HSiEt as substrates efficiently yield -Markovnikov and -addition products, while bulkier substrates such as HSiPr are less reactive. Investigating the underlying mechanism by gas chromatography and STEM analysis, we unexpectedly found that H and metallic Ga formed. Without the addition of olefins, the formation of RSi-F-Al(OR) (R = alkyl), a typical degradation product of the [] anion in the presence of a small silylium ion, was observed. Electrochemical analysis revealed a surprisingly high oxidation potential of univalent [Ga(PhF)][] in weakly coordinating, but polar -difluorobenzene of (Ga/Ga; DFB) = +0.26-0.37 V Fc/Fc (depending on the scan rate). Apparently, subvalent Ga, mainly known as a reductant, initially oxidizes the silane and generates a highly electrophilic, silane-supported, silylium ion representing the actual catalyst. Consequently, the [Ga(PhF)][]/HSiEt system also hydrodefluorinates C(sp)-F bonds in 1-fluoroadamantane, 1-fluorobutane and PhCF at room temperature. In addition, both catalytic reactions may be initiated using only 0.2 mol% of [PhC][] as a silylium ion-generating initiator. These results indicate that silylium ion catalysis is possible with the straightforward accessible weakly coordinating [] anion. Apparently, the kinetics of hydrosilylation and hydrodefluorination are faster than that of anion degradation under ambient conditions. These findings open up new windows for main group catalysis.