title: C-C reductive elimination in palladium complexes, and the role of coupling additives. A DFT study supported by experiment.
abstract: A DFT study of R-R reductive elimination (R = Me, Ph, vinyl) in plausible intermediates of Pd-catalyzed processes is reported. These include the square-planar tetracoordinated systems cis-[PdR(2)(PMe(3))(2)] themselves, possible intermediates cis-[PdR(2)(PMe(3))L] formed in solution or upon addition of coupling promoters (L = acetonitrile, ethylene, maleic anhydride (ma)), and tricoordinated intermediates cis-[PdR(2)(PMe(3))] (represented as L = empty). The activation energy ranges from 0.6 to 28.6 kcal/mol in the gas phase, increasing in the order vinyl-vinyl < Ph-Ph < Me-Me, depending on R, and ma < "empty" < ethylene < PMe(3) approximately MeCN, depending on L. The effect of added olefins was studied for a series of olefins, providing the following order of activation energy: p-benzoquinone < ma < trans-1,2-dicyanoethylene < 3,5-dimethylcyclopent-1-ene < 2,5-dihydrofuran < ethylene < trans-2-butene. Comparison of the calculated energies with experimental data for the coupling of cis-[PdMe(2)(PPh(3))(2)] in the presence of additives (PPh(3), p-benzoquinone, ma, trans-1,2-dicyanoethylene, 2,5-dihydrofuran, and 1-hexene) reveals that: (1) There is no universal coupling mechanism. (2) The coupling mechanism calculated for cis-[PdMe(2)(PMe(3))(2)] is direct, but PPh(3) retards the coupling for cis-[PdMe(2)(PPh(3))(2)], and DFT calculations support a switch of the coupling mechanism to dissociative for PPh(3). (3) Additives that would provide intermediates with coupling activation energies higher than a dissociative mechanism (e.g., common olefins) produce no effect on coupling. (4) Olefins with electron-withdrawing substituents facilitate the coupling through cis-[PdMe(2)(PR(3))(olefin)] intermediates with much lower activation energies than the starting complex or a tricoordinated intermediate. Practical consequences are discussed.
authors.affiliation: Departamento de Química Organica, Facultad de Química, Universidade de Vigo, Lagoas-Marcosende s/n, 36310 Vigo, Galicia, Spain.
authors.forename: Martín
authors.initials: M
authors.lastname: Pérez-Rodríguez
authors.forename: Ataualpa A C
authors.initials: AA
authors.lastname: Braga
authors.forename: Max
authors.initials: M
authors.lastname: Garcia-Melchor
authors.forename: Mónica H
authors.initials: MH
authors.lastname: Pérez-Temprano
authors.forename: Juan A
authors.initials: JA
authors.lastname: Casares
authors.forename: Gregori
authors.initials: G
authors.lastname: Ujaque
authors.forename: Angel R
authors.initials: AR
authors.lastname: de Lera
authors.forename: Rosana
authors.initials: R
authors.lastname: Alvarez
authors.forename: Feliu
authors.initials: F
authors.lastname: Maseras
authors.forename: Pablo
authors.initials: P
authors.lastname: Espinet