Effect of 2‑Substituents on Allyl-Supported Precatalysts for the Suzuki–Miyaura Reaction: Relating Catalytic Efficiency to the Stability of Palladium(I) Bridging Allyl Dimers

One of the most commonly used classes
of precatalysts for cross-coupling are Pd­(II) complexes of the type
(η3-allyl)­Pd­(L)­Cl. Here, we report the first full
investigation of how the steric and electronic properties of the 2-substituent
affect the catalytic properties of precatalysts of the type (η3-allyl)­Pd­(L)­Cl. Specifically, we have prepared and studied
a series of well-defined 2-substituted precatalysts of the type (η3-2-R-allyl)­Pd­(IPr)Cl (R = H, Ph, Me, tBu, OMe,
CN), as well as their related Pd­(I) (μ-2-R-allyl)­(μ-Cl)­Pd2(IPr)2 dimers. The catalytic performance of the
Pd­(II) monomers and their Pd­(I) μ-allyl dimer congeners is compared
for the Suzuki–Miyaura reaction. When Pd­(II) monomers are used
as precatalysts, we observe the formation of the Pd­(I) μ-allyl
dimers during catalysis. In fact, we find that the catalytic efficiency
of (η3-2-R-allyl)­Pd­(IPr)Cl precatalysts correlates
inversely with the thermodynamic stability of the related Pd­(I) μ-allyl
dimers. Therefore, we have examined the structural and electronic
properties of the Pd­(I) μ-allyl dimers in detail and probed
the mechanism of the (μ-2-R-allyl)­(μ-Cl)­Pd2(IPr)2 dimer/(η3-2-R-allyl)­Pd­(IPr)­Cl
monomer interconversion both experimentally and computationally. Overall,
this study shows that the formation of Pd­(I) μ-allyl dimers
can play a crucial role in determining the catalytic efficiency of
precatalysts of the type (η3-allyl)­Pd­(IPr)­Cl.