Synthesis And Catalytic Properties Of NCS Pincer N-heterocyclic Carbene Palladium Complexes

N-Heteocyclic carbene（NHC）is one of the most important ligands for the preparation of transition metal complexes because of its structural diversity,modifiability and coordination ability.Compared with the conventional organophosphine ligands,NHC has strongσ-donating power and weakπ-acceptance capacity.In addition,the electronic and spatial steric effects of NHC are more easily tuned.This excellent property of NHC allows them to form very stable C-M bonds with metal centres and structurally diverse organometallic complexes.In recent years,with the continuous development of green chemistry,various NHC metal complexes have been successfully synthesized and identified,such as monodentate complexes,chelating complexes and pincer complexes.Among them,pincer-shaped NHC metal complexes have superior structural rigidity and stability.Currently,a wide range of NHC metal complexes with different structures and properties have been synthesized and applied in catalysis.In this thesis,a series of pyrimidine and thioether bifunctionalized pincer NHC palladium complexes were designed,synthesized,and their catalytic properties in the C-H activation reaction was investigated.The main studies in this paper are as follows:1)A series of imidazole alcohols were synthesized by the"one-pot reaction"using cheap and easily available L-leucinol or L-phenylglycinol as raw material.The imidazole alcohols were then reacted with sulfoxide chloride,substituted with thiol or thiophenol to form imidazole thioethers,and the thioether-functionalized imidazoles was further reacted with 2-chloropyrimidine to obtain a series of imidazolium salts containing different side chain substituents.A series of pyrimidine and thioether bifunctionalized NHC precursor imidazolium salts with different side chain substituents were obtained.The structures of the products synthesized in each step were characterized by ¹H NMR,¹³C NMR,HR-MS,etc.2)Six new pyrimidine and thioether bifunctionalized pincer NHC-Pd complexes 5a-f were synthesized by direct metallization of the precursor imidazolium salts with palladium chloride.The structures of the final target products were identified and characterized by ¹H NMR,¹³C NMR and HR-MS.3)The catalytic ability of six newly synthesized NHC-Pd complexes was investigated for C-H activated arylation of 2-acetylthiophene and 2-acetylfuran with different aryl bromides.The six prepared novel complexes NHC-Pd 5a-f,were used to catalyze the coupling reaction of 2-acetylthiophene with 4-bromoacetophenone to explore the effect of different kinds of bases and solvents on the catalytic reactions.The bases screened containing Na OAc,KOAc,K₃PO₄,K₂CO₃,^tBu OK and the solvents containing DMF,DMAC,DMSO,CH₃CN,Tol,THF,H₂O（with the addition of TBAB）.The optimized reaction conditions for this coupling reaction were found to be:NHC-Pd 5d,DMAC,KOAc.Under the optimized reaction conditions,the 5d-catalyzed coupling reaction of2-acetylthiophene with 4-bromoacetophenone was found to be almost complete.Replacement of different aryl bromides for the coupling reaction with 2-acetylthiophene or2-acetylfuran,respectively,revealed a relatively wide range of catalytic capabilities for the pyrimidine,thioether bifunctionalised pincer NHC-Pd complexes By analyzing the experimental data of the catalytic reaction,two conclusions can be drawn:firstly,the spatial site hindrance effect has a significant influence on this coupling reaction,with the coupling yields of the aryl bromide isomers substituted at different positions decreasing in the order of para-,meta-,and ortho-;secondly,the electronic effect affects the coupling reaction,with the electron-withdrawing substituents producing better results than that of the electron-donating substituents.4)The activity of six newly synthesized NHC-Pd complexes to catalyze arylation reactions of imidazole derivatives with different aryl bromides was explored.The prepared novel NHC-Pd complexes were used to catalyze the coupling reactions of1,2-dimethylimidazole and 1-methylimidazole with 4-bromoacetophenone to explore the catalytic properties of these complexes and the effect of different types of bases and solvents on this coupling reaction.The bases screened containing Na OAc,KOAc,K₃PO₄,K₂CO₃,^tBu OK and the solvents containing DMF,DMAC,DMSO,CH₃CN,Tol,THF,H₂O（with the addition of TBAB）.The optimized reaction conditions for this coupling reaction were determined by analyzing the results of the experiments data:NHC-Pd 5d,DMAC,KOAc.Under the optimized conditions,it was found that the 5d-catalyzed coupling reaction of1,2-dimethylimidazole with 4-bromoacetophenone was almost complete.The catalytic suitability of the newly synthesized NHC-Pd complexes was explored by changing different aryl bromides under optimized reaction conditions.Experimental data showed that both electron and spatial site resistance effects have a large influence on the coupling reaction,with the coupling yields of the electron-absorbing substituted aryl bromides being greater than those of the electron-donating analogues,and the coupling yields of the ortho-substituted aryl bromides being substantially lower than those of their meta-or para-substituted isomers.