Synthesis and Catalysis of Thioethers-Functionalized N-Heterocyclic Carbene Palladium Complexes
Date Issued
2012
Date
2012
Author(s)
Wang, Chia-Ching
Abstract
The hemilability of six- and seven- membered ring metallacycle of thioether-functionalized NHC palladium complexes were observed in solution; therefore, the five-membered ring metallacycle complexes which were constituted by the methyl- tethered thioether-NHC lgands could also exhibit hemilabile behavior in solution.
The methyl- and ethyl-tethered thioether(S)-imidazolium salts have been successfully synthesized and employed to afford the N-heterocyclic carbene (NHC) silver derivatives as carbene precursors of transmetallation. The resulting (S-NHC)PdCl2 complexes constructing with five-membered palladacycles were characterized by NMR techniques and X-ray crystallography. The coordination of thioether group could be displaced by additional PPh3 indicative of the hemilability of S-NHC ligand. Unlike the ethyl-tethered (S-NHC)PdCl2 complex, the cationic methyl-tethered S-NHC palladium complex in the formula of [(S-NHC)Pd(PPh3)Cl]Cl was also accompanied in the reaction resulted from the stronger chelating effect of five-membered palladacycles.
A series of (S-NHC)Pd(allyl-R)X (R = H, Me, Ph and X = Cl, BF4, PF6) complexes has been synthesized and characterized by NMR spectroscopy and X-ray crystallography, which shows the cis configuration of the NHC and unsubstituted allylic groups in their square planar geometries. A rapid exo-endo exchange of the allylic group was observed via η3-η1-η3 isomerization in 2D NMR spectra, which was attributed to the strong trans influence of the carbene donor. The energy barriers of the η3-η1-η3 isomerization and the rapid inversion of sulfur atom were examined by variable-temperature NMR techniques. These S-NHC allylic palladium complexes were evaluated for the reaction of olefins hydrogenation showing comparable activities with literature reports.
Subjects
N-Heterocyclic Carbene
Thioether
Palladium
allyl
Isomerization
Type
thesis
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