Selective Hydrogenation Of C-C/C-N Unsaturated Bonds Catalyzed By Nanoporous Palladium

Nanoporous metals with three-dimensional bicontinuous porous structures,have the controlled feature sizes,low density,high surface area and fine electrical conductivity.Nanoporous metals show wide applications in catalysis,sensors,separation and energy due to their novel structure and properties.Especially the advantages of high specific surface,unsupported structure,and recycling,reusability capacity make the nanoporous metals to be a heterogeneous catalyst with improved performance compared with the traditional nano-catalyst.Therefore,nanoporous metals are regarded as a green and sustainable catalyst.In the first part,the highly chemoselective and stereoselective semihydrogenation of alkynes to Z-internal and terminal alkenes were achieved by using unsupported nanoporous palladium(PdNPore)as a heterogeneous catalyst under mild reaction conditions(room temperature and 1 atm of H2).Under the optimized conditions,the semihydrogenation of various terminal/internal and aromatic/aliphatic alkynes afforded the corresponding alkenes in 70%-95%yields with high selectivities.This protocol shows good functional group tolerance,such as methyl,methoxyl,tert-butyl,trifluoromethyl,formyl,carboxyl and halides(F,Cl and Br).PdNPore further showed high chemoselectivity toward terminal alkynes in the presence of internal alkynes.No Palladium leached from PdNPore during the reaction,and the catalyst was easily recovered and reused without a loss of activity.Mechanistic studies revealed the heterolytic cleavage of the H-H bonds on PdNPore and the in situ-generated Pd-H species play an important role in achieving excellent selectivity with the present heterogeneous catalyst.In the second part,an efficient and highly chemoselective heterogeneous catalyst system for quinoline hydrogenation was developed using unsupported nanoporous palladium(PdNPore).Under the optimized conditions,the PdNPore-catalyzed chemoselective hydrogenation of quinoline proceeded smoothly under mild reaction conditions(low H2 pressure and temperature)to yield 1,2,3,4-tetrahydroquinolines(py-THQs)in 62%-95%yields.This protocol shows good functional group tolerance,such as methyl,methoxyl,hydroxyl,ester,formyl,amide and halides(F,Cl and Br).The chemoselective hydrogenation of quinoline was scaled up to the gram scale,this yield was only slightly decreased,indicating that this method has good practicability.The result of kinetic experiment suggests that not only the CH3CN solvent but also ethylamine and py-THQ,which were produced during the reaction,would act as polar and Lewis-basic additives in this hydrogenation reaction.In the last part,unsupported PdNPore-catalyzed selective hydrogenation of nitriles was studied.This reaction proceeded smoothly without any additives under mild conditions and exhibited good functional group tolerance.Under the optimized conditions,the selective hydrogenation of various aromatic/aliphatic nitriles afforded the corresponding primary amines in 70%-95%yields.The reactivity of nitriles was influenced by the electronic properties of substituents and electron-withdrawing groups were more favorable for this reaction.