Catalytic Behavior of NHC-Silver Complexes in the Carboxylation of Terminal Alkynes with CO2

A number of N-heterocyclic carbene-silver compounds (NHCs)AgX were tested in the direct carboxylation of terminal alkynes using carbon dioxide as the C1 carbon feedstock. The reactions proceed at a pressure of 1 atm of CO2 at room temperature, in the presence of Cs2CO3, and using silver-NHC complexes as catalysts. Thus, phenylacetylene and several alkynes are converted to the corresponding propiolic acids in good to high conversions. The activity of the catalysts is strongly influenced by the substituents on the NHC backbone and the nature of the counterion. Specifically, the most active compound exhibits iodide as the counterion and is stabilized by a benzimidazole derivative. After 24 h of reaction, a quantitative conversion is obtained utilizing DMF as the solvent and phenylacetylene as the substrate.