Theoretical Studies on Coupling Reactions of Carbon Dioxide with Alkynes Mediated by Nickel(0) Complexes

A computational study with the Becke3LYP DFT functional theory was carried out on Ni(0)-mediated coupling reactions of both terminal and internal alkynes with CO₂. We studied the mechanism for the formation of the five-membered metallacyclic intermediates in order to understand the regioselectivity. The steric and electronic factors that determine the regioselectivity have been discussed. The calculations indicate that electronic factors nicely explain the trend observed in the barriers calculated for the coupling reactions of CO₂ with the three terminal alkyne substrates having substituents with different electronic properties, but steric factors are dominant in the regioselectivity for the reaction of a given terminal alkyne substrate. For silyl-substituted internal alkynes, both electronic and steric effects favor the formation of compounds in which CO₂ couples with the silyl-substituted carbon.