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

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Clark, Kenneth F. and Tyerman, Seb and Evans, Laura and Robertson, Craig M. and Nelson, David J. and Kennedy, Alan R. and Murphy, John A. (2023) A hierarchy of ligands controls formation and reaction of aryl radicals in Pd-catalyzed ground state base-promoted coupling reactions. Journal of the American Chemical Society, 145 (38). pp. 20849-20858. ISSN 1520-5126 (https://doi.org/10.1021/jacs.3c05470)

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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.

ORCID iDs

Clark, Kenneth F., Tyerman, Seb, Evans, Laura, Robertson, Craig M. ORCID logoORCID: https://orcid.org/0000-0002-7780-8986, Nelson, David J. ORCID logoORCID: https://orcid.org/0000-0002-9461-5182, Kennedy, Alan R. ORCID logoORCID: https://orcid.org/0000-0003-3652-6015 and Murphy, John A.;
  • Item type:Article
    ID code:86754
    Dates:
    Date
    Event
    27 September 2023
    Published
    15 September 2023
    Published Online
    1 September 2023
    Accepted
    Subjects:Science > Chemistry
    Department:Faculty of Science > Pure and Applied Chemistry
    Depositing user: Pure Administrator
    Date deposited:22 Sep 2023 11:10
    Last modified:12 Dec 2024 14:59
    URI:https://strathprints.strath.ac.uk/id/eprint/86754
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