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Título: | C-C reductive elimination in palladium complexes, and the role of coupling additives. A DFT study supported by experiment |
Autor: | Pérez-Rodríguez, Martín ; Braga, Ataualpa A. C.; Garcia-Melchor, Max; Pérez-Temprano, Mónica H.; Casares, Juan A.; Ujaque, Gregori; de Lera, Angel R.; Alvarez, Rosana; Maseras, Feliu; Espinet, Pablo | Fecha de publicación: | 18-mar-2009 | Editor: | American Chemical Society | Citación: | Journal of the American Chemical Society 131(10): 3650–3657 (2009) | Resumen: | 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. | Descripción: | 8 pags., 4 figs., 4 tabs., 2 schs. | Versión del editor: | https://doi.org/10.1021/ja808036j | URI: | http://hdl.handle.net/10261/347305 | DOI: | 10.1021/ja808036j | ISSN: | 0002-7863 |
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