Palladium Complexes with Chelating Bis-NHC Ligands in the Mizoroki鈥揌eck Reaction鈥擬echanism and Electronic Effects, a DFT Study

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• 作者：Christoph Allolio ; Thomas Strassner
• 刊名：Journal of Organic Chemistry
• 出版年：2014
• 出版时间：December 19, 2014
• 年：2014
• 卷：79
• 期：24
• 页码：12096-12105
• 全文大小：604K
• ISSN：1520-6904

文摘

Experimental results have shown that palladium complexes with chelating aryl- and alkyl-substituted bis-NHC ligands, including [(H₃C-Im)₂CH₂]PdBr₂, [(C₆H₅-Im)₂CH₂]PdBr₂, and [(H₃CO-C₆H₄-Im)₂CH₂]PdBr₂ are excellent catalysts for the Mizoroki鈥揌eck reaction. To better understand and improve the catalysts, a density functional theory (DFT) study of the Heck reaction has been performed at the B3LYP/6-31G* level of theory, complemented by M06/def2-TZVP single-point calculations. Different mechanistic pathways have been investigated and compared to available experimental results. The most likely mechanism is a cationic catalytic cycle involving the palladium oxidation states 0 and +II. We also looked at other oxidation states, but on the basis of the calculated Gibbs free energy a +II/+IV mechanism can be excluded. Aryl substitution with electron-donating groups at the para position (e.g., the methoxy group in [(H₃CO-C₆H₄鈥揑m)₂CH₂]PdBr₂) was found to reduce the reaction barrier of the rate-determining step. This is in agreement with the experimental findings for the catalysts. The experimentally observed cis selectivity could also be explained by the DFT study.