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 (eta(3)-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 (eta(3)-allyl)Pd(L)Cl. Specifically, we have prepared and studied a series of well-defined 2-substituted precatalysts of the type (eta(3)-2-R-allyl)Pd(IPr)Cl (R = H, Ph, Me, Bu-t, OMe, CN), as well as their related Pd(I) (mu-2-R-allyl)(mu-Cl)Pd-2(IPr)(2) dimers. The catalytic performance of the Pd(II) monomers and their Pd(I) mu-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) mu-allyl dimers during catalysis. In fact, we find that the catalytic efficiency of (eta(3)-2-R-allyl)Pd(IPr)Cl precatalysts correlates inversely with the thermodynamic stability of the related Pd(I) mu-allyl dimers. Therefore, we have examined the structural and electronic properties of the Pd(I) mu-allyl dimers in detail and probed the mechanism of the (mu-2-R-allyl)(mu-Cl)Pd-2(IPr)(2) dimer/(eta(3)-2-R-allyl)Pd(IPr)Cl monomer interconversion both experimentally and computationally. Overall, this study shows that the formation of Pd(I) mu-allyl dimers can play a crucial role in determining the catalytic efficiency of precatalysts of the type (eta(3)-allyl)Pd(IPr)Cl.