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.