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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 ArAr reductive coupling

Citation

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]

Copyright Statement

Copyright 2013 American Chemical Society

DOI: doi:10.1021/om301013w

Abstract

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-OIPh2", which transforms to a transition structure with retention of the OI interaction and formation of a "Pd(-Ph-η1)I" bridge in a four-membered ring, "PdPhI(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 mppPh 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.

Item Details

Item Type:Refereed Article
Research Division:Chemical Sciences
Research Group:Other Chemical Sciences
Research Field:Organometallic Chemistry
Objective Division:Expanding Knowledge
Objective Group:Expanding Knowledge
Objective Field:Expanding Knowledge in the Chemical Sciences
Author:Canty, AJ (Professor Allan Canty)
Author:Ariafard, A (Associate Professor Alireza Ariafard)
Author:Yates, BF (Professor Brian Yates)
ID Code:88262
Year Published:2013
Web of Science® Times Cited:29
Deposited By:Chemistry
Deposited On:2014-01-23
Last Modified:2014-06-17
Downloads:0

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