Green And Highly Efficient Methodology For Glaser Reaction

Here we proposed two novel reaction methods employing polyethyleneglycol (PEG) or ionic liquid (IL) as the reaction media for homocoupling of terminal alkynes, which was also called Glaser reaction. PEG and IL were nonvolatile, nontoxic and environmentally benign "green" solvents. Compared to traditional Glaser reaction in organic solvents, the two methods were highly effective and showed excellent recyclable efficiency. The reactions proceeded smoothly under mild reaction conditions and inorganic bases were applied in the reaction as an alternative base of amines, which were required in traditionally Glaser reaction.PEG-Cu(OAc)₂ system with NaOAc as base could be used as a highly efficient reaction system for the coupling of both arylacetylenes and alkylacetylenes in the absence of other organic solvents. We investigated how the chain length of PEG influenced the reaction and optimized catalyst, base and temperature. Cu(OAc)₂-PEG could be recycled effectively, which was the first example for recycle of Glaser reaction. The mechanism was also investigated and the formation of Cu₂O after reduction from cupric salt was observed.Homocoupling of various terminal alkynes was performed with high yields in water/ionic liquids [1-butyl-3-methylimidazolium tetrafluoroborate ([bmim][BF₄]), 1-butyl-3-methylimidazolium chloride ([bmim][Cl]),1-butyl-3-methylimidazolium hexafluorophosphorate ([bmim][PF₆]), and tetrabutylammonium bromide (TBAB)] in the presence of Cu(OAc)₂ and KOAc acting as an effective base. The effects of different catalyst, base, temperature and the ratio of water and ILs were investigated. [bmim][BF4]/H₂O-Cu(OAc)₂-KOAc was simply recycled 17 times without obvious decrease in efficiency.