Suzuki-miyaura Couplings Catalyzed By Asymmetric Pincer Palladium(Ⅱ)complexes Comprising N-heterocyclic Carbene

In 2010,Richard Heck,Ei-ichi Negishi and Akira Suzuki joined the prestigious circle of Nobel Laureate chemists for their roles in discovering and developing highly practical methodologies for cross-coupling reactions.Palladium-catalyzed cross-coupling reactions have attracted people’s attention and research with their high selectivity,activity and yield.To date,few cross-coupling reactions is used in pharmaceutical synthesis and industrial production.Most of them are still in the research and development stage.Studying the cross-coupling reaction to promote its development and application is of great significance for the healthy survival of human beings.In this dissertation,1-(2-methoxyphenyl)-1H-imidazole was synthesized by condensation of o-Anisidine with glyoxal and formaldehyde.1-(2-methoxyphenyl)-1H-imidazoles were alkylated to synthesis three novel asymmetric NHC pincer ligands(L1,2a,and 3a).2a and 3a are then demethylated with boron tribromide to produce L2 and L3 ligands.Asymmetric NHC pincer ligands not only have the advantages of strong electron-donating ability and large bond energy,but also have the characteristics of the stability and structure of the pincer ligand tridentate chelation mode.The use of such ligands for cross-coupling reactions enables modulation of the activity and selectivity of metal palladium catalysts.Based on the previous work,using different methods,we synthesized four NHC-Pd pincer complexes with different structures,L1-Pd,L2-Pd,L3-Pd and L4-Pd.The four metal palladium complexes were characterized by NMR,high resolution mass spectrometry and single crystal,and the determined single crystal structure diagram was obtained.Through the analysis of chemical shifts in the nucleus and the analysis of bond lengths in the single crystal structure,the reason why the NHC-Pd pincer complex is stable in air and water is obtained.This is also favorable for the use of mild catalysts without the need for anhydrous and anoxic conditions to catalyze cross-coupling reaction.Finally,the Suzuki-Miyaura cross-coupling reactions catalyzed by L1-Pd,L2-Pd,L3-Pd and L4-Pd four NHC-Pd pincer catalysts were investigated.The reaction conditions were optimized using phenylboronic acid and 4-bromobenzaldehyde as the template substrates and isopropanol as the solvent.At 40 °C,potassium carbonate(1.2 equiv)as a base,isopropanol as solvents.catalyst L2-Pd(0.3 mol%),The raction yield of phenylboronic acid(1.1 equiv)and 4-bromobenzaldehyde(1 equiv)reached 90% after 12 h.After optimizing the conditions,we studied the tolerance of the substrate and obtained products with different functional groups.The L2-Pd catalyst also showed high activity and product selectivity in catalyzing different functional group substrates.