Mechanistic Investigation of the Generation of a Palladium(0) Catalyst from a Palladium(II) Allyl Complex: A Combined Experimental and DFT Study

Cross-coupling reactions can be efficiently catalyzed using palladium complexes. The formation of low-coordinated, highly reactive Pd(0), which is believed to be the catalytic species, is critical. The mechanism of the reduction of a stable and readily available allyl Pd­(II) complex into Pd(0) by a combination of K₂CO₃ and PhB­(OH)₂ has been studied. We report on the characterization of the associated reactive solution using a combination of density functional theory and experimental methods. First, the stoichiometric reaction of an (allyl)­(phosphine)­palladium­(II) complex with K₂CO₃ was first investigated using trandem mass spectrometry. A palladium–carbonate complex could be characterized in the electrospray mass spectrum of the reactive solution. Gas-phase infrared spectra of mass-selected complexes have been recorded, giving further information on the coordination mode (κ¹) of the carbonate ligand. This structural information derived from spectroscopy is critical because the relative energy of the two κ¹- and κ²-carbonate isomers is difficult to determine theoretically, presumably because of the charge transfers at play between the carbonate and the palladium. Second, the product of the stoichiometric addition of PhB­(OH)₂ to this carbonate complex was investigated. Both ³¹P and ¹H NMR data provide compelling evidence for the formation of the desired 14-electron Pd(0) complex