Decatungstate Photocatalyzed Si-H/C-H Activation in Silyl Hydrides. Hydrosilylation of Electron-Poor Alkenes

Tetrabutylammonium decatungstate has been used for the photocatalytic activation of the Si-H bond in trisubstituted silanes and applied for the hydrosilylation of electron-poor alkenes. The mechanism that occurs depends on the silyl hydride used, as supported by laser flash photolysis and EPR trapping experiments. Homolytic Si-H cleavage through a hydrogen atom transfer from the silane to the excited catalyst operates with dimethylphenylsilane and methyldiphenylsilane, for which the hydrosilylation yields were satisfactory. If we used tertiary silanes that have more labile Si-H bonds, such as triphenylsilane or tris(trimethylsilyl)silane, the reaction worked to some extent even under uncatalyzed conditions because of a radical chain reaction. Competition between C-H and Si-H cleavage was observed, however, if we used trialkylsilanes (e.g., triethylsilane). In favorable cases, the process was also efficient under flow conditions or if promoted by sunlight.