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
Canty, AJ and Ariafard, A and Sanford, MS and Yates, BF, 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, Organometallics, 32, (2) pp. 544-555. ISSN 0276-7333 (2013) [Refereed Article]
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)(”-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)(”-OAc)]2 (8), "Pd-O···IPh2", which transforms to a transition structure with retention of the O···I interaction and formation of a "Pd(”-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-C6H4)(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.