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

ABSTRACT: 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.

SUBMITTER: Denmark SE

PROVIDER: S-EPMC2812642 | biostudies-literature | 2010 Feb

REPOSITORIES: biostudies-literature

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

Denmark Scott E SE Smith Russell C RC

Journal of the American Chemical Society 20100201 4

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-S ...[more]

PMID: 20058920

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Project description: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.

Dataset Information

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

Publications

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

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