A Hierarchy of Ligands Controls Formation of Aryl Radicals in Pd-Catalyzed Ground State Reactions

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-induced homolytic aromatic substitution (BHAS) with benzene to produce 2,6-dimethylbiphenyl 8, and biphenyl 9, in an approx. 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, 10 in varying amounts. However, when any one of a range of diphosphinoferrocenes (e.g dppf or dippf) or BINAP (20 mol% in each case) 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.