Mechanistic duality in palladium-catalyzed cross-coupling reactions of aryldimethylsilanolates. Intermediacy of an 8-Si-4 arylpalladium(II) silanolate

(Chemical Equation Presented) The mechanism of palladium-catalyzed cross-coupling reactions of potassium (4-methoxyphenyl)dimethyl silanolate has been investigated. Under catalysis by (t-Bu₃P)₂Pd, the coupling with 1-bromo-4-fluorobenzene displays the following rate equation: rate = k_obs[R₃SiOK]⁰[ArylBr]⁰, with k_obs = k[(t-Bu₃P)₂Pd]^0.98. An independent study of the individual steps of the catalytic cycle has revealed a dual mechanistic pathway. The transmetalation can occur by a thermal process via an 8-Si-4 intermediate without the need for anionic activation. Additionally, arylsilanolates can serve as activators for transmetalation via a hypervalent 10-Si-5 siliconate intermediate.