Mechanistic Investigation
of the Generation of a Palladium(0) Catalyst from a Palladium(II)
Allyl Complex: A Combined Experimental and DFT Study
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Abstract
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 K<sub>2</sub>CO<sub>3</sub> and PhB(OH)<sub>2</sub> 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 K<sub>2</sub>CO<sub>3</sub> 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 (κ<sup>1</sup>)
of the carbonate ligand. This structural information derived from
spectroscopy is critical because the relative energy of the two κ<sup>1</sup>- and κ<sup>2</sup>-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)<sub>2</sub> to this carbonate
complex was investigated. Both <sup>31</sup>P and <sup>1</sup>H NMR
data provide compelling evidence for the formation of the desired
14-electron Pd(0) complex
