Solvent-Free Synthesis of 2-Pyrones from Alkynes and Carbon Dioxide Catalyzed by Ni(1,5cyclooctadiene)2/Trialkylphosphine Catalysts

Solvent-free, efficient, and mild 2-pyrone synthesis by the cycloaddition of alkynes with CO2 has been achieved under compressed CO2 with a Ni(cod)2/P(C4H9)3 or Ni(cod)2/P(C8H17)3 catalyst; both high yields (up to 98%) and selectivities (up to 99%) are attained. One attractive aspect of these catalysts is their high efficiency at low CO2 pressures (4 MPa). The applicability of this reaction to a broad range of alkyne substrates and the ease of handling the phosphine ligands are additional merits of the present new catalytic systems in comparison to the previously reported Ni(cod)2/P(CH3)3 catalyst in supercritical CO2.