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

Abstract

Oxidative homocoupling reaction of various terminal alkynes in supercritical carbon dioxide (scCO 2 ) has been reported. The final optimized reaction conditions for the reaction in scCO 2 were determined to be cupric chloride (2 mmol), sodium acetate (2 mmol), methanol (1 mL) and CO 2 (14 MPa) at 40 °C. Sodium acetate was superior to pyridine in the present reaction system. Methanol, as a co-solvent of scCO 2 , was necessary to help dissolve the inorganic salt and facilitated the reaction. Higher solubility of cupric chloride in the scCO 2 -methanol medium than other cupric salts, e.g. CuSO 4 , Cu(NO 3 ) 2 , Cu(OAc) 2 and cupric bromide, guaranteed its high catalytic ability. Variations of the pressure of seCO 2 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 scCO 2 is an effective and environmental-friendly alternative to the previous methods of homocoupling of terminal alkynes involving volatile organic compounds.