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

摘要

Despite widespread use of complexes of the type Pd(L)(η~3-aUyl)Cl as precatalysts for cross-coupling, the chemistry of related Pd~Ⅰ dimers of the form (μ-allyl) (μ- 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 (μ-allyl)(μ-Cl)Pd_2(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~Ⅰμ-allyl dimer congeners for the Suzuki-Miyaura reaction is compared. We propose that the (μ-allyl)(μ-Cl)Pd_2(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.