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

The mechanism of palladium-catalyzed cross-coupling reactions of potassium (4-methoxyphenyl)dimethyl silanolate has been investigated. Under catalysis by (t-Bu(3)P)(2)Pd, the coupling with 1-bromo-4-fluorobenzene displays the following rate equation: rate = k(obs)[R(3)SiOK](0)[ArylBr](0), with k(obs) = k[(t-Bu(3)P)(2)Pd](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.