Nano-Pd/Al?OH?₃ Catalyzed Homocoupling Reactions

As the very important elementary reaction in organic synthesis,the carbon-carbon coupling reaction provides effective synthetic methods for the synthesis of many drugs and natural products,which is also one of the hottest topics in the study of organic chemistry.Carbon-carbon coupling reactions which directly use transition metals as catalysts often need ligands.The disadvantages of these catalysts are expensive and hard to recycle,leading to environmental pollution.Nowadays,environmental problems have attracted much attention,so it is high time to develop green,reusable and highly efficient supported catalyst,which is one of the important ways to solve the problems above.In this paper,we used a supported nano-palladium catalyst to catalyze carbon-carbon coupling reactions.The catalyst is easily prepared,stable in air and water,readily separated and recyclable.To further explore the effect of nano-palladium catalyst in the homocoupling reactions,we chosen and done the following experiments.Firstly,the homocoupling of terminal alkynes was investigated.It was found that the reaction did not require any ligand and the factors of this reaction were optimized including solvent type and dosage,type and amount of base,the reaction temperature and other parameters.Ultimately,the optimal conditions of homocoupling reaction of phenylacetylene came as the follows: the standard substrate phenylacetylene in an amount of 0.2 mmol,nano-palladium catalyst in an amount of 0.1 mol%,1.0 equivalent NaOAc as base,30 mol% Ag2SO4 as oxidant,in 1.0 mL of DMSO at 90 oC for 16 h,and 1,4-diphenyl-1,3-butadiyne was obtained in 96% yield.Under the optimized condition,various alkynes were further surveyed and the results showed that electronic effect and steric effect of substituents were not obvious in this reaction.Secondly,we studied the applications of the nano-palladium catalyst on the homocouplings of aryl boronic acids and potassium aryltrifluoroborates,respectively.Through the investigation of solvents,oxidants and other factors,the optimal conditions of the two catalytic reactions were obtained: One,substrate p-methyl phenylboronic acid 0.2 mmol,nano-palladium catalyst 0.1 mol%,base KOAc 1.0 equivalent,Ag2 O 50 mol%,in 0.8 mL of CH3 OH at 40 oC for 15 h,and 99% yield was obtained.The other,substrate potassium phenyltrifluoroborate 0.2 mmol,nano palladium catalyst 0.1 mol%,base KOAc 1.0 equivalent,Ag2 O 50 mol%,in 0.8 mL H2 O at 50 oC for 16 h,and the biphenyl was provided in 98% yield.At the respective optimal conditions,the scope of substrates was examined and the results showed that moderate to good yields could be obtained for most substrates.And the yields were influenced obviously by electronic effect and steric effect of the substituents on the phenyl ring.The substrates with electron-donating substituents offered better yields than that with electron-withdrawing substituents.The advantages that nano-Pd/Al?OH?₃ catalyzed homocoupling of terminal alkynes,arylboronic acids and potassium aryltrifluoroborates were highly catalytic efficiency,mild reaction conditions,air and water stable.