Preferential Geminal Bis-silylation of 3,4-Benzothiophane Is Caused by the Dominance of Electron Withdrawal by R3Si over Steric Shielding Effects

Benzylic C–H lithiation of 3,4-benzothiophane and subsequent treatment with triphenyl- or trimethylchlorosilane under a variety of conditions leads to α,α- rather than α,α′-bis-silylation products as a consequence of anion stabilization by R3Si and very fast deprotonation of the intermediate monosilylated product, even with a sterically bulky base such as lithium diisopropylamide.