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)(eta(3)-allyl)Cl as precatalysts for cross-coupling, the chemistry of related Pd-I dimers of the form (mu-allyl)(mu-Cl)Pd-2(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 (mu-allyl)(mu-Cl)Pd-2(IPr)(2) (allyl = allyl, crotyl, cinnamyl; IPr = 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene) through activation of Pd(IPr)(eta(3)-allyl)Cl type monomers under mildly basic reaction conditions. The catalytic performance of the Pd-II monomers and their Pd-I mu-allyl dimer congeners for the Suzuki-Miyaura reaction is compared. We propose that the (mu-allyl)(mu-Cl)Pd-2(IPr)(2)-type dimers are activated for catalysis through disproportionation to Pd(IPr)(eta(3)-allyl)Cl and monoligated IPr-Pd-0. The microscopic reverse comproportionation reaction of monomers of the type Pd(IPr)(eta(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 mu-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)(eta(3)-crotyl)Cl and Pd(IPr)(eta(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)(eta(3)-cinnamyl)Cl which are compatible with mild bases.