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

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posted on 20.05.2015 by Steven A. Tymonko, Russell C. Smith, Andrea Ambrosi, Scott E. Denmark
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.