Transition Metal-Catalyzed Carbon-Carbon And Carbon-Nitrogen Cross-Coupling Reactions

The transition-metal-catalyzed cross-coupling reactions are simple,efficient,and versatile routes to the formation of carbon-carbon and carbon-nitrogen bonds,which represents the key step in a wide range of preparative organic processes.In recent years many chemists are searching for better catalysts,more convenient reaction conditions and an insight of the reaction mechanisms, and are using these cross-coupling reactions for the synthesis of interesting or promising compounds.In this thesis,based on the reviews aiming at the developments of Hiyama,Suzuki, Sonogashira cross-eoupling reactions,Glaser coupling and Ullmann-type reactions in recent years,we have investigated these reactions in novel catalytic systems.The details are summarized as follows:1.Mild conditions have been developed to achieve the Pd(OAc)₂-catalyzed fluoride-free cross-coupling between the aryl bromides and arylsiloxanes in good to high yields in aqueous medium.The success of the reactions requires the presence of poly(ethylene glycol) (PEG)and 3 equiv sodium hydroxide.The product was easily separated with ethyl ether extraction and the catalytic system can be reused eight times with high efficiency.A mechanism for this catalytic reaction was proposed.The use of phosphine ligands and fluoride ion(from TBAF or KF)was avoided in this catalytic system.2.A new palladium catalysis system has been developed for the Suzuki-Miyaura cross-coupling reactions of heteroaryl bromides in aqueous media.The method allows the preparation of a variety of heterobiaryls in good to excellent yields without the use of phosphine ligands under mild reaction conditions.Silica gel is found to be crucial to the successful performance of the reactions and the catalytic system can be reused eighteen times with high efficiency.The transmission electronic microscopy(TEM)anlysis visualized that the nice separated palladium nanoparticles were formed and maintained in the catalytic recycles.3.An efficient palladium-catalyzed copper-free Sonogashira reaction in water and acetone has been developed under mild conditions.The results showed that the aryl iodides could carry out the cross-coupling reaction with a variety of terminal alkynes in high yields in water-acetone in the absence of amine,copper(â… )salts,or phosphine ligands at 60℃for one hour,and good yields were obtained for aryl bromides at 60℃for 12-24 hours in the presence of triphenylphosphine and piperidine.The method could be used to synthesize polyethynyl aromatic compounds and 2-phenylbenzofuran in a one-pot reaction.4.Aromatic alkynes and aliphatic alkynes were coupled to afford the corresponding diynes in moderate to good yields in the presence 1.5 equiv I₂,10%HAuCl₄·4H₂O and 2 equiv. K₂CO₃.I₂ was found to be crucial to the successful performance of these reactions.The Sonogashira cross-coupling of 1-iodo-4-methoxybenzene with phenylacetylene was also proceeding successfully in this catalytic system without I₂.A reaction mechanism was proposed.5.2-(pyridin-2-yl)pyridine was found to be an efficient ligand for copper-catalyzed N-arylation reactions of nitrogen-containing heterocycles with aryl- and vinylsiloxane under mild conditions.A variety of functionalized imidazoles,benzimidazoles,pyrazoles,benzotriazole, picolinamides,and arylsiloxanes were transformed in good to excellent yields.Vinylsiloxane were also coupled in moderate yields.The results were better than those in previous reports. A possible reaction mechanism was proposed.