Mechanistic significance of the Si-O-Pd bond in the palladium-catalyzed cross-coupling reactions of arylsilanolates

Steven A. Tymonko, Russell C. Smith, Andrea Ambrosi, Michael H. Ober, Hao Wang, Scott E. Denmark

Research output: Contribution to journalArticlepeer-review

Abstract

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.

Original languageEnglish (US)
Pages (from-to)6200-6218
Number of pages19
JournalJournal of the American Chemical Society
Volume137
Issue number19
DOIs
StatePublished - May 20 2015

ASJC Scopus subject areas

  • Catalysis
  • Chemistry(all)
  • Biochemistry
  • Colloid and Surface Chemistry

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