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)(η(3)-allyl)Cl as precatalysts for cross-coupling, the chemistry of related Pd(I) 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 Pd(I) 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)(η(3)-allyl)Cl type monomers under mildly basic reaction conditions. The catalytic performance of the Pd(II) monomers and their Pd(I) 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)(η(3)-allyl)Cl and monoligated IPr-Pd(0). The microscopic reverse comproportionation reaction of monomers of the type Pd(IPr)(η(3)-allyl)Cl with IPr-Pd(0) to form Pd(I) dimers is also studied. It is demonstrated that this is a facile process, and Pd(I) dimers are directly observed during catalysis in reactions using Pd(II) precatalysts. In these catalytic reactions, Pd(I) μ-allyl dimer formation is a deleterious process which removes the IPr-Pd(0) active species from the reaction mixture. However, increased sterics at the 1-position of the allyl ligand in the Pd(IPr)(η(3)-crotyl)Cl and Pd(IPr)(η(3)-cinnamyl)Cl precatalysts results in a larger kinetic barrier to comproportionation, which allows more of the active IPr-Pd(0) 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)(η(3)-cinnamyl)Cl which are compatible with mild bases.