Insight into the Efficiency of Cinnamyl-Supported Precatalysts for the Suzuki–Miyaura Reaction: Observation of Pd(I) Dimers with Bridging Allyl Ligands During Catalysis

Despite widespread
use of complexes of the type Pd­(L)­(η3-allyl)Cl as
precatalysts for cross-coupling, the chemistry
of related PdI dimers of the form (μ-allyl)­(μ-Cl)­Pd2(L)2 has been underexplored. Here, the relationship
between the monomeric and the dimeric compounds is investigated using
both experiment and theory. We report an efficient synthesis of the
PdI dimers (μ-allyl)­(μ-Cl)­Pd2(IPr)2 (allyl = allyl, crotyl, cinnamyl; IPr = 1,3-bis­(2,6-diisopropylphenyl)­imidazol-2-ylidene)
through activation of Pd­(IPr)­(η3-allyl)Cl type monomers
under mildly basic reaction conditions. The catalytic performance
of the PdII monomers and their PdI μ-allyl
dimer congeners for the Suzuki–Miyaura reaction is compared.
We propose that the (μ-allyl)­(μ-Cl)­Pd2(IPr)2-type dimers are activated for catalysis through disproportionation
to Pd­(IPr)­(η3-allyl)Cl and monoligated IPr–Pd0. The microscopic reverse comproportionation reaction of monomers
of the type Pd­(IPr)­(η3-allyl)Cl with IPr–Pd0 to form PdI dimers is also studied. It is demonstrated
that this is a facile process, and PdI dimers are directly
observed during catalysis in reactions using PdII precatalysts.
In these catalytic reactions, PdI μ-allyl dimer formation
is a deleterious process which removes the IPr–Pd0 active species from the reaction mixture. However, increased sterics
at the 1-position of the allyl ligand in the Pd­(IPr)­(η3-crotyl)Cl and Pd­(IPr)­(η3-cinnamyl)Cl precatalysts
results in a larger kinetic barrier to comproportionation, which allows
more of the active IPr–Pd0 catalyst to enter the
catalytic cycle when these substituted precatalysts are used. Furthermore,
we have developed reaction conditions for the Suzuki-Miyaura reaction
using Pd­(IPr)­(η3-cinnamyl)Cl which are compatible
with mild bases.