Ga⁺-catalyzed hydrosilylation? About the surprising system Ga⁺/HSiR₃/olefin, proof of oxidation with subvalent Ga⁺ and silylium catalysis with perfluoroalkoxyaluminate anions.

ABSTRACT: Already 1 mol% of subvalent [Ga(PhF)₂]⁺[pf]^- ([pf]^- = [Al(OR^F)₄]^-, R^F = 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 such as HSiⁱPr₃ are less reactive. Investigating the underlying mechanism by gas chromatography and STEM analysis, we unexpectedly found that H₂ and metallic Ga⁰ formed. Without the addition of olefins, the formation of R₃Si-F-Al(OR^F)₃ (R = alkyl), a typical degradation product of the [pf]^- anion in the presence of a small silylium ion, was observed. Electrochemical analysis revealed a surprisingly high oxidation potential of univalent [Ga(PhF)₂]⁺[pf]^- in weakly coordinating, but polar ortho-difluorobenzene of E _1/2(Ga⁺/Ga⁰; oDFB) = +0.26-0.37 V vs. 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)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 be initiated using only 0.2 mol% of [Ph₃C]⁺[pf]^- as a silylium ion-generating initiator. These results indicate that silylium ion catalysis is possible with the straightforward 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 up new windows for main group catalysis.

SUBMITTER: Barthelemy A 

PROVIDER: S-EPMC8729802 | biostudies-literature | 2022 Jan

REPOSITORIES: biostudies-literature