Understanding the mechanism of Cu^I-catalyzed N-H carboxylation of heterocyclic rings with CO₂ from a theoretical point of view

The copper catalyzed carboxylation of heterocyclic rings has been studied with density functional theory. We find strong linear correlations between the pK_a of the heterocycles and various steps in the reaction, including the initial copper-bound adduct and the barriers for N-H and CO₂ activations. Using the mechanism proposed by Nolan, Cazin and coworkers, we find that the heterocycles fall into two groups (A and B) and we show that having a nitrogen atom adjacent to N-H in the ring (Group B) is important in lowering the barrier for the two key steps, that is, the N-H and CO₂ activations. However our results for the heterocycles in Group A did not match up to the experimental results. In order to account fully for the observed experimental reactivity we have proposed an alternative mechanism involving promotion by base, i.e. the coordination of OH^- to Cu^I before the N-H and CO₂ activations to occur. This mechanism is the likely one to operate for Group A compounds, and is competitive with the mechanism proposed by Nolan and Cazin for Group B compounds.