On the Mechanism of Palladium-Catalyzed Cross-Coupling of Diazonium Salts with Aryltrifluoroborates: A Combined ESI-MS/NMR Study

A combined ESI-MS/NMR mechanistic study on the palladium-catalysed cross-coupling reaction between aryldiazonium salts and aryltrifluoroborates with bis(mu-acetato)bis(4,4'difluoroazobenzene-C-2,N)dipalladium(II) (4) as the precatalyst is reported. The reaction follows a Pd-o/Pd-II cycle after reduction of 4 to a molecular Pd-o species (1), which according to the combined ESI-MS and F-19 NMR studies, bears an arylated azobenzene ligand. Oxidative addition by the diazonium salt generates the arylpalladium(II) intermediate (II), which was also detected in solution. The catalytic cycle completed with a transmetallation between 11 and the organoborate, which is followed by fast reductive elimination of the cross-coupling product to restore the molecular Pd-o species I. A concurrent activation path was also observed which consists of the formation of (4,4'-difluoroazobenzene C-2,N)dipalladium(II) tetrafluoroborate (7) by the reaction of 4 with the diazonium. salt and subsequent reduction by the aryltrifluoroborate to give I.