A hierarchy of ligands controls formation and reaction of aryl radicals in Pd-catalyzed ground state base-promoted coupling reactions

Kenneth F. Clark, Seb Tyerman, Laura Evans, Craig M. Robertson, David J. Nelson, Alan R. Kennedy, John A. Murphy

Research output: Contribution to journalArticlepeer-review

2 Citations (Scopus)
19 Downloads (Pure)

Abstract

Palladium salts and complexes were tested separately and in the presence of added ligands as potential sources of aryl radicals in ground-state coupling reactions of aryl halide with arenes under basic conditions (KOtBu). Our recently developed assay for aryl radicals was employed to test for aryl radicals. In this assay, aryl radicals derived from the test substrate, 1-iodo-2,6-dimethylbenzene 7, undergo base-promoted homolytic aromatic substitution (BHAS) with benzene to produce 2,6-dimethylbiphenyl 8 and biphenyl 9 in an approximately 1:4 ratio as well as m-xylene 10. The biphenyl arises from a diagnostic radical transfer reaction with the solvent benzene. Using substrate 7 with a range of Pd sources as potential initiators led to formation of 8, 9, and 10 in varying amounts. However, when any one of a range of diphosphinoferrocenes (e.g., dppf or dippf) or BINAP or the monophosphine, diphenylphosphinoferrocene, was added as a ligand to Pd­(OAc)2, the ratio of [2,6-dimethylbiphenyl 8: biphenyl 9] moved decisively to that expected from the BHAS (radical) pathway. Further studies were conducted with dppf. When dppf was added to each of the other Pd sources, the ratio of coupled products was also diverted to that expected for radical BHAS chemistry. Deuterium isotope studies and radical trap experiments provide strong additional support for the involvement of aryl radicals. Accordingly, under these ground-state conditions, palladium sources, in the presence of defined ligands, convert aryl iodides to aryl radicals. A rationale is proposed for these observations.
Original languageEnglish
Pages (from-to)20849-20858
Number of pages10
JournalJournal of the American Chemical Society
Volume145
Issue number38
Early online date15 Sept 2023
DOIs
Publication statusPublished - 27 Sept 2023

Keywords

  • aryl radicals
  • ground-state coupling reactions
  • diphosphinoferrocenes
  • ligands

Fingerprint

Dive into the research topics of 'A hierarchy of ligands controls formation and reaction of aryl radicals in Pd-catalyzed ground state base-promoted coupling reactions'. Together they form a unique fingerprint.

Cite this