Copper(II)/Palladium(II) catalysed highly selective cross-coupling of terminal alkynes in supercritical carbon dioxide

An efficient protocol for the synthesis of unsymmetrical 1,3-diynes was developed using supercritical carbon dioxide (ScCO 2 ) as the solvent. The direct coupling of two different terminal alkynes is catalysed by a bimetallic catalyst , CuCl 2 ·2H 2 O/Pd(NH 3 ) 4 Cl 2 ·H 2 O, in the presence of the base tetramethylethylenediamine (TMEDA) and O 2 . In pure ScCO 2 , our bimetallic catalytic system maintains high activity over a wide substrate scope to provide unsymmetrical 1,3-diynes in good to excellent yields. In the proposed reaction mechanism, the synergistic cooperation between copper(II) and palladium(II) centres is responsible for the superior selectivity of the cross-coupling.