Mechanism of the Stoltz–Grubbs (KO <i> ^t </i> Bu/Et ₃ SiH) Silylation: Single‐Electron Transfer is the Missing Link between the Heterolytic and Radical Pathways

A new mechanism for the Stoltz-Grubbs (KO^tBu/Et₃SiH) silylation of heteroarenes is reported that successfully resolves the important unexplained experimental features of the reaction. The mechanism combines hydride transfer, hydrogen atom transfer (HAT), and single electron transfer (SET) steps (with the substrate hydride adduct being identified as a new SET donor) and is supported by extensive theoretical calculations. Previously proposed heterolytic and radical mechanisms do not involve SET. The discovery of a mechanistic role for SET explains why the silylation reaction simultaneously displays both heterolytic and radical character, and why it does not require free silyl radicals. Previously unexplained experimental results from reactions of isotopically labelled reactants and on the inhibitory effects of electron-accepting additives can also be understood based on the SET mechanism.