Immobilization Of New N-Heterocyclic Carbene Palladium Complex And Its Applications In The Carbon-Carbon Coupling Reactions

Carbon-carbon coupling reactions are very important methods to construct new C-C bonds,which are indispensable means in modern organic synthesis.These reactions have been widely used in the synthesis of chemicals.Palladium complexes such as Pd-PR3,Pd-NHC are major catalysts for the carbon-carbon coupling reactions.But these systems are usually used as homogeneous catalysts, suffering from a number of drawbacks including the difficulty in recovering catalyst, environmental pollution and impurities in the products. These problems have to be overcome in the view of green chemistry and the development of recoverable solid catalysts is highly desirable.NHC-Pd complexes are highly promising catalyst because of their great performances,air-stability and lower toxicity. Heterogeneous NHC-Pd catalysts have been reported, but they generally suffered form poor recyclability. One of the major reasons for the poor recyclability is the aggregation and agglomeration of Pd naonoparticles into less active, large particles during the catalytic reaction.Our previous investigation and other groups'work revealed that support structure and functional groups grafted on the support played an important role in preventing the aggregation of palladium nanoparticles into the less active large particles.In this work, the mesoporous cage-like material SBA-16 was chosen as a support. By grafting imidazolium N-heterocyclic carbene palladium complex and ionic liquid on the internal surface of SBA-16, NHC-Pd /SBA-16-IL catalyst was prepared.N2 sorption,FT-IR.CP-MAS NMR,XPS,TEM and XRD characterizations were employed to characterize the developed catalyst. The characterization results revealed that NHC-Pd/ SBA-16-IL had an ordered mesoporous structure like the parent SBA-16,and NHC-Pd complex along with ionic liquid were successfully grafted on the internal surface of SBA-16.NHC-Pd/SBA-16-IL showed high activity and significantly enhanced recyclability for the Heck and Suzuki reactions. For Heck reaction, at a loading of 0.01 mol%Pd, NHCPd/SBA-16-IL afforded complete conversions of various aryl bromides within 11.5-46 h.For Suzuki reaction, at a Pd loading of 0.01 mol%, this catalyst afforded a fast conversion of the unactivated bromobenzene in 5-10 h under air using 50% aqueous ethanol as a reaction medium. The catalyst NHC-Pd/SBA-16-IL could be recovered by a simple centrifugation and filtration.A good yield was still achieved even after the catalyst was reused ten times.As far as we know, among the grafted catalysts, this was one of the best catalysts for Heck and Suzuki reaction of unactivated aryl bromides in view of the high activity and excellent recyclability.The significantly enhanced recyclability could be attributed to the isolated nanocages of SBA-16 that could efficiently prevent the aggregation and agglomeration of Pd particles formed during the catalytic reaction into the less active, large particles.