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(μ‐acetato)bis(4,4′‐difluoroazobenzene‐C2,N)dipalladium(II) (4) as the precatalyst is reported. The reaction follows a Pd0/PdII cycle after reduction of 4 to a molecular Pd0 species (I), which according to the combined ESI‐MS and 19F 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 is completed with a transmetallation between II and theorganoborate, which is followed by fast reductive elimination of the cross‐coupling product to restore the molecular Pd0 species I. A concurrent activation path was also observed which consists of the formation of (4,4′‐difluoroazobenzene‐C2,N)dipalladium(II) tetrafluoroborate (7) by the reaction of 4 with the diazonium salt and subsequent reduction by the aryltrifluoroborate to give I. (© Wiley‐VCH Verlag GmbH & Co. KGaA, 69451 Weinheim, Germany, 2007)