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

Abstract

International audienceCross-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

Development of Palladium Precatalysts that Efficiently Generate LPd(0) Active Species

“…Direct hydride transfer was calculated to be a lower energy pathway than β‐hydride elimination followed by reductive elimination of the C−H bond . Under SMC conditions (PhB(OH) 2 , CO 3 2− ), a palladium carbonate complex is formed that undergoes transmetallation with PhB(OH) 2 to give a LPd(η 3 ‐allyl)Ph . This species undergoes reductive elimination to give the LPd(0) fragment and allylbenzene.…”

Designing Pd and Ni Catalysts for Cross-Coupling Reactions by Minimizing Off-Cycle Species

“…base and solvent used). 89,90,91 The activation of Pd(II) Pd(IPr)(Cl)(allyl) and Pd(IPr)(Cl)(indenyl) precatalysts to Pd(0) in the presence of alcoholic solvents was studied in detail by Melvin and co-workers, combining theory with experiments. 92 As shown in Figure 4 for the allyl ligand, the activation mechanism starts with the substitution of the chloride ligand by methoxide, which forms upon deprotonation of the solvent by the t BuOK base.…”

Differences in the Performance of Allyl Based Palladium Precatalysts for Suzuki‐Miyaura Reactions

“…Apart from the prevalence of palladium(I) dimer formation during catalysis, the other key factor in the performance of allyl‐based precatalysts is proposed to be their rate of activation from palladium(II) to palladium(0) [7,9] . To date, three main pathways have been proposed for activation of allyl‐type precatalysts [10] (Figure 3): (A) a process in which a solvent alcohol with a β‐hydrogen coordinates to the metal, is deprotonated by base, and then transfers a hydrogen to the allyl‐type ligand; [11] (B) a process in which a nucleophile such as OH − or O t Bu − directly attacks the allyl‐type ligand; [6b,12] or (C) a process which involves transmetallation of the halide with a boronic acid followed by reductive elimination of the allyl‐type ligand [13] . Preliminary activation studies indicate that the Yale precatalyst activates faster than other allyl‐based systems when the reaction proceeds through a solvent assisted pathway, but information on the relative rates of activation of the different allyl‐based systems is limited [10] .…”