Novel reaction of silyl carbonyl complexes with hydride-transfer reagents: Reduction of a carbonyl ligand and coupling with a silyl group

Treatment of CpFe(CO)₂SiR₃ (Cp = ηC₅H₅; R₃ = ^pTol₂Cl, ^pTol₂H, ^pTol₂Me, MePh(1-Nap), and (N-C₅H₁₁)₂H; 1-NaP = 1-naphthyl) with LiAlH₄ in ether or THF at room temperature gave CH₃SiR₃ as the major product in moderate to high yield together with HSiR₃. Reaction of CpFe(CO)₂SiMes₂H with LiAlH₄ afforded only H₂SiMeS₂. Labeling experiments using LiAlD₄ give the deuterated methylsilane. These results proved that the hydrogen source for the methyl group is LiAlH₄. Furthermore, the reduction of CpFe(*CO)₂Si^pTol₂R′ (*C = ¹³C enriched; R′ = H, Me) produced *CH₃Si^pTol₂R′ where the methyl group introduced on Si was ¹³C-enriched. These experimental results suggested that the carbonyl ligand was reduced by LiAlH₄ and coupled with the silyl group to give the methylsilane. Cp*Fe(CO)₂Si^pTol₂H (Cp* = η⁵-C₅Me₅), having a Cp* ligand which is bulkier and more electron-donating than a Cp ligand did not react with LiAlH₄ at room temperature, but it did so at 50°C to give CH₃Si^pTol₂H and HSi^pTol₂H, probably because nucleophilic attack of LiAlH₄ at a carbonyl ligand was retarded both sterically and electronically by the Cp* ligand. When the reactions were performed in sealed NMR tubes and monitored by NMR tubes and monitored by NMR spectroscopy, the signals due to the anionic complexes Li-[CpFe(CO)(CH₃)Si^pTol₂R′]^- were observed in addition to those of silane products. These anionic complexes are considered not to be intermediates but to be byproducts because the anionic complexes did not change to methylsilanes on heating. The anionic complexes reacted with MeOH and MeI to give hydrosilanes HSi^pTol₂R′ and methylsilanes CH₃Si^pTol₂R′, respectively.