Pyrimidine Functionalized Phosphine Ligands and Their Application in Catalysis
- The scientific aim of this work was to synthesize and characterize new bidentate and tridentate phosphine ligands , their corresponding palladium complexes and to examine their application as homogenous catalysts. Later on, a part of the obtained palladium catalysts was immobilized and used as heterogonous catalyst.
Pyrimidinyl functionalized diphenyl phosphine ligands were synthesized by ring closure of [2-(3-dimethylamino-1-oxoprop-2-en-yl)phenyl]diphenylphosphine with an excess of substituted guanidinium salts. Furthermore to increase the electron density at phosphorous centre the two aryl substituents on the phosphanyl group were exchanged against two alkyl substituents. Electron rich pyrimidinyl functionalized dialkyl phosphine ligands were synthesized from pyrimidinyl functionalized bromobenzene in a process involving lithiation followed by reaction with a chlorodialkylphosphine.
Starting from the new synthesized diaryl phosphine ligands, their corresponding palladium complexes were synthesized. I was able to show that slight changes at the amino group of [(2-aminopyrimidin-4-yl)aryl]phosphines lead to pronounced differences in the stability and catalytic activity of the corresponding palladium(II) complexes. Having a P,C coordination mode, the palladium complex can catalyze rapidly the Suzuki coupling reaction of phenylbronic acid with arylbromides even at room temperature with a low loading.
Using the NH2 group of the aminopyrimidine as a potential site for the introduction of an other substituent, bidentate and tridentate ligands containing phosphorous atoms connected to the aminopyrimidine group and their corresponding palladium complexes were synthesized and characterized.
Two ligands [2- and 4-(4-(2-amino)pyrimidinyl)phenyl]diphenylphosphine (containing NH2 group) functionalized with a ethoxysilane group were synthesized. The palladium complexes based on these ligands were prepared and immobilized on commercial silica and MCM-41. Using elemental analysis, FT-IR, solid state 31P, 13C and 29Si CP–MAS NMR spectroscopy, XRD and N2 adsorption the success of the immobilization was confirmed and the structure of the heterogenized catalyst was investigated.
The resulting heterogeneous catalysts were applied for the Suzuki reaction and exhibited excellent activity, selectivity and reusability.