Palladium complexes with chelating bis-NHC ligands in the Mizoroki-Heck reaction—mechanism and electronic effects, a DFT study.

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-Heck 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-Im)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.