The Brönsted Basicities of N-Heterocyclic Olefins in DMSO: An Effective Way to Evaluate the Stability of NHO-CO2 Adducts.

A Brönsted basicity scale (∼24 pK units) for 85 commonly seen imidazole-, imidazoline-, triazole-, and thiazole-based N-heterocyclic olefins (NHOs) in DMSO was established using a well-examined computational model. The influence of substituents on the Brönsted basicities of these NHOs was investigated through basicity comparisons and rationalized by geometric analyses. The Gibbs energy (ΔGr) of the reaction between NHO and CO2 was also calculated, which linearly correlates with the basicity of the corresponding NHO, suggesting that the stability of NHO-CO2 adducts can be evaluated by the basicity of NHOs and a stronger basicity leads to a more stable NHO-CO2 adduct.