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 [Ph 2 I]BF 4 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 [Ph 2 I] + 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···IPh 2 ”, which transforms to a transition structure with retention of the O···I interaction and formation of a “Pd(μ-Ph-η 1 )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) 2 Pd(mpp)] + ( 14 ). Cation 14 undergoes mpp···Ph coupling at one Pd center to form the binuclear Pd(II) cation [(mppPh- N )Pd(μ-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(μ-OAc) 2 Pd(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-C 6 H 4 )(Mes)I]BF 4 (X = H, Me, OMe, F, Cl, COMe, CF 3 ) 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.