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.
Physikalische Organische Chemie, Technische Universität Dresden , 01062 Dresden, Germany.
Christoph
C
Allolio
Thomas
T
Strassner