C-C reductive elimination in palladium complexes, and the role of coupling additives. A DFT study supported by experiment
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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- [PdR2 (PMe 3)2] themselves, possible intermediates cis-[PdR 2 (PMe3)L] formed in solution or upon addition of coupling promoters (L = acetonitrile, ethylene, maleic anhydride (ma)), and tricoordinated intermediates cis-[PdR2 (PMe3)] (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 〈 PMe3 ≈ 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-[PdMe2 (PPh3)2] in the presence of additives (PPh3, 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-[PdMe2 (PMe3)2]is direct, but PPh 3 retards the coupling for cis-[PdMe2 (PPh 3)2], and DFT calculations support a switch of the coupling mechanism to dissociative for PPh3. (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- [PdMe2 (PR3)(olefin)] intermediates with much lower activation energies than the starting complex or a tricoordinated intermediate. Practical consequences are discussed. © 2009 American Chemical Society.