期刊
JOURNAL OF THE AMERICAN CHEMICAL SOCIETY
卷 131, 期 10, 页码 3650-3657出版社
AMER CHEMICAL SOC
DOI: 10.1021/ja808036j
关键词
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资金
- Spanish Ministerio de Educacion y Ciencia [CSD2006-0003, CSD2007-00006]
- FEDER [SAF07-63880]
- Juan de la Cierva [CTQ2005-09000-CO2-01, CTQ2005-09000-CO2-02, CTQ2007-67411/BQU, CTQ2008-06647-C02-01/BQU]
- Xunta de Galicia
- Catalan DURSI [2005SGR00715]
- Junta de Castilla y Leon [VA044A07, GR169]
- UAB
- ICIQ foundation
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(PMe3)(2)] themselves, possible intermediates cis-[PdR2(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 approximate to 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 PPh3 retards the coupling for cis-[PdMe2(PPh3)(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.
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