Mechanism of Pd-catalyzed Ar-Ar bond formation involving ligand-directed C-H arylation and diaryliodonium oxidants: Computational studies of orthopalladation at binuclear Pd(II) centers, oxidation to form binuclear palladium(III) species, and Ar···Ar reductive coupling

A computational analysis of the Pd-catalyzed coupling of 3-methyl-2-phenylpyridine (mppH) with [Ph₂I]BF₄ to form mppPh is supportive of a prior synthetic and kinetic study implicating binuclear palladium species in a rate-limiting oxidation step. The Pd(OAc)₂ precatalyst forms the "clamshell" orthopalladated complex [Pd(mpp)(μ-OAc)]₂ (8) as the active catalyst, which is oxidized by [Ph₂I]⁺ in a reaction having the highest energy requirement of all steps in the catalytic cycle. In the oxidation reaction, involving formal transfer of Ph⁺, the electrophilic iodine center interacts initially with a bridging acetate oxygen atom of [Pd(mpp)(μ-OAc)] ₂ (8), "Pd-O···IPh₂", which transforms to a transition structure with retention of the O·· ·I interaction and formation of a "Pd(μ-Ph-η¹) I" bridge in a four-membered ring, "Pd··· Ph···I(Ph)···O-Pd", followed by elimination of PhI with formation of a binuclear Pd(III) cation containing a Pd-Pd bond, [Ph(mpp)Pd(μ-OAc)₂Pd(mpp)]⁺ (14). Cation 14 undergoes mpp···Ph coupling at one Pd center to form the binuclear Pd(II) cation [(mppPh-N)Pd(μ-OAc)₂Pd(mpp)]⁺ (Da). Cation Da may fragment to release mppPh and mononuclear palladium species, followed by orthopalladation at a mononuclear center. However, in an environment of very low acetate concentration and high nitrogen-donor concentration, it is considered far more likely that Da undergoes ligand exchange with release of mppPh and formation of [(mppH-N)Pd(μ-OAc) ₂Pd(mpp)]⁺ (I). Computation shows a low-energy pathway for orthopalladation at cation I that involves nitrogen-donor reagents mppH and mppPh acting as bases to remove a proton as [HN-donor]⁺. This orthopalladation would complete the cycle and regenerate the catalyst, [Pd(mpp)(μ-OAc)]₂ (8). A Hammett plot obtained from a computational analysis of the reaction of [(p-X-C₆H ₄)(Mes)I]BF₄ (X = H, Me, OMe, F, Cl, COMe, CF₃) has a reaction constant (ρ) of 1.8, which compares well with the experimental result (ρ = 1.7 ± 0.2). Consistent with this, the analysis reveals the dominant role of the interaction energy for palladium- and iodine-containing fragments in the transition structure.