Understanding the Unusual Reduction Mechanism of Pd(II) to Pd(I): Uncovering Hidden Species and Implications in Catalytic Cross-Coupling Reactions

The reduction of
Pd­(II) intermediates to Pd(0) is a key elementary
step in a vast number of Pd-catalyzed processes, ranging from cross-coupling,
C–H activation, to Wacker chemistry. For one of the most powerful
new generation phosphine ligands, PtBu3, oxidation state Pd­(I), and not Pd(0), is generated upon reduction
from Pd­(II). The mechanism of the reduction of Pd­(II) to Pd­(I) has
been investigated by means of experimental and computational studies
for the formation of the highly active precatalyst {Pd­(μ-Br)­(PtBu3)}2. The formation of dinuclear
Pd­(I), as opposed to the Pd(0) complex, (tBu3P)2Pd was shown to depend on the stoichiometry
of Pd to phosphine ligand, the order of addition of the reagents,
and, most importantly, the nature of the palladium precursor and the
choice of the phosphine ligand utilized. In addition, through experiments
on gram scale in palladium, mechanistically important additional Pd-
and phosphine-containing species were detected. An ionic Pd­(II)­Br3 dimer side product was isolated, characterized, and identified
as the crucial driving force in the mechanism of formation of the
Pd­(I) bromide dimer. The potential impact of the presence of these
side species for in situ formed Pd complexes in catalysis
was investigated in Buchwald–Hartwig, α-arylation, and
Suzuki–Miyaura reactions. The use of preformed and isolated
Pd­(I) bromide dimer as a precatalyst provided superior results, in
terms of catalytic activity, in comparison to catalysts generated in situ.