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

Through
the combination of reaction kinetics (both stoichiometric
and catalytic), solution- and solid-state characterization of aryl­palladium­(II)
aryl­silanol­ates, and computational analysis, the intermediacy
of covalent adducts containing Si–O–Pd linkages in the
cross-coupling reactions of aryl­silanol­ates has been unambiguously
established. Two mechanistically distinct pathways have been demonstrated:
(1) transmetalation via a neutral 8-Si-4 intermediate that dominates
in the absence of free silanolate (i.e., stoichiometric reactions
of aryl­palladium­(II) aryl­silanol­ate complexes),
and (2) transmetalation via an anionic 10-Si-5 intermediate that dominates
in the cross-coupling under catalytic conditions (i.e., in the presence
of free silanolate). Aryl­palladium­(II) aryl­silanol­ate
complexes bearing various phosphine ligands 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 suggested, but did not
prove, that intermediates containing the Si–O–Pd linkage
were involved in the cross-coupling process. The isolation of a coordinatively
unsaturated, T-shaped aryl­palladium­(II) aryl­silanol­ate
complex ligated with t-Bu3P allowed the
unambiguous demonstration of the operation of both pathways involving
8-Si-4 and 10-Si-5 intermediates. Three kinetic regimes were identified:
(1) with 0.5–1.0 equiv of added silanolate (with respect to
aryl­palladium bromide), thermal transmetalation via a neutral
8-Si-4 intermediate; (2) with 1.0–5.0 equiv of added silanolate,
activated transmetalation via an anionic 10-Si-5 intermediate; and
(3) with >5.0 equiv of added silanolate, concentration-independent
(saturation) activated transmetalation via an anionic 10-Si-5 intermediate.
Transition states for the intramolecular transmetalation of neutral
(8-Si-4) and anionic (10-Si-5) intermediates have been located computationally,
and the anionic pathway is favored by 1.8 kcal/mol. The energies of
all intermediates and transition states are highly dependent on the
configuration around the palladium atom.