Mechanistic Significance of the Si–O–Pd Bond in the Palladium-Catalyzed Cross-Coupling Reactions of Alkenylsilanolates

Through
the combination of reaction kinetics (both catalytic and
stoichiometric) and solid-state characterization of arylpalladium­(II)
alkenyl­silanolate complexes, the intermediacy of covalent adducts
containing Si–O–Pd linkages in the cross-coupling reactions
of organo­silanolates has been unambiguously established. Two
mechanistically distinct pathways have been demonstrated: (1) transmetalation
via a neutral 8-Si-4 intermediate that dominates in the cross-coupling
of potassium alkenyl­silanolates, and (2) transmetalation via
an anionic 10-Si-5 intermediate that dominates in the cross-coupling
of cesium alkenyl­silanolates. Arylpalladium­(II) alkenyl­silanolate
complexes bearing various phosphine ligands (both bidentate and monodentate)
have been isolated, fully characterized, and evaluated for their kinetic
competence under thermal (stoichiometric) and anionic (catalytic)
conditions. Comparison of the rates for thermal and anionic activation
demonstrates that intermediates containing the Si–O–Pd
linkage are involved in the cross-coupling process.