Cu(II)-Promoted Oxidative Homocoupling Reaction of Terminal Alkynes in Supercritical Carbon Dioxide

Oxidative homocoupling reaction of various terminal alkynes in supercritical carbon dioxide (scCO2) has been reported. The final optimized reaction conditions for the reaction in scCO2 were determined to be cupric chloride (2 mmol), sodium acetate (2 mmol), methanol (1 mL) and CO2 (14 MPa) at 40 °C. Sodium acetate was superior to pyridine in the present reaction system. Methanol, as a co-solvent of scCO2, was necessary to help dissolve the inorganic salt and facilitated the reaction. Higher solubility of cupric chloride in the scCO2-methanol medium than other cupric salts, e.g. CuSO4, Cu(NO3)2, Cu(OAc)2 and cupric bromide, guaranteed its high catalytic ability. Variations of the pressure of scCO2 and temperature in a certain range have almost little impact on the reaction efficiency. A mechanism involving copper(II)-acetylide complex was supposed in which AcO- or Cl- instead of N-containing compounds acted as ligands. The reaction system in scCO2 is an effective and environmental-friendly alternative to the previous methods of homocoupling of terminal alkynes involving volatile organic compounds.