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-methy1-2-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)(mu-OAc)32 (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)(mu-OAc)12 (8), Pd-O center dot center dot center dot IPh2, which transforms to a transition structure with retention of the O center dot center dot center dot I interaction and formation of a Pd(mu-Ph-eta(1))I bridge in a four-membered ring, Pd center dot center dot center dot Ph center dot center dot center dot I(Ph)center dot center dot center dot O-Pd, followed by elimination of PhI with formation of a binuclear Pd(III) cation containing a Pd Pd bond, [Ph(mpp)Pd(mu-OAc)(2)Pd(mpp)] (14). Cation 14 undergoes mpp center dot center dot center dot Ph coupling at one Pd center to form the binuclear Pd(II) cation [(mppPh-N)Pd(mu-OAc)(2)Pd(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(mu-OAc)(2)Pd(mpp)(+) (I). Computation shows a low-energy pathway for orthopalladation at cation 1 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)(mu-OAc)](2) (8). A Hammett plot obtained from a computational analysis of the reaction of [(p-X-C6H4)(Mes)I]BF4 (X = H, Me, OMe, F, Cl, COMe, CF3) has a reaction constant (rho) of 1.8, which compares well with the experimental result (rho = 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.