Theoretical Mechanistic Study of Novel Ni(0)-Catalyzed [6 – 2 + 2] Cycloaddition Reactions of Isatoic Anhydrides with Alkynes: Origin of Facile Decarboxylation

A thorough theoretical analysis was carried out on the novel Ni-catalyzed decarboxylative [6 – 2 + 2] cycloaddition reactions of isatoic anhydrides with alkynes. This is the first theoretical analysis of this new kind of decarboxylative coupling reaction, which has attracted recent attention. The active species is a two-coordinate Ni⁰(phosphine)­(alkyne) complex. The catalytic cycle consists of the four elementary processes: oxidative addition, decarboxylation, alkyne insertion, and reductive elimination. The oxidative addition of the C­(O)–O bond of isatoic anhydride to Ni­(phosphine)­(alkyne) proceeds with a moderate Gibbs activation energy (ΔG°^⧧) of 18.0 kcal/mol. In the transition state, the charge transfer from the Ni 3d_π orbital to the antibonding LUMO of isatoic anhydride plays an important role in the weakening of the anhydride C­(O)–O bond, which determines the regioselectivity of the oxidative addition. Then, the decarboxylation proceeds in a stepwise manner through Ni–N bond formation and CO₂ elimination. In this decarboxylation, the Ni–N bond formation is crucial for a moderate ΔG°^⧧ value of 6.7 kcal/mol. After this step, alkyne and phosphine change their positions around the Ni center, followed by alkyne insertion into the nickel–acyl (Ni–C­(O)­R) bond and reductive elimination. All of these steps occur easily with moderate ΔG°^⧧ values. The facile decarboxylation is the origin of this successful catalytic reaction. This is because the N atom in isatoic anhydride plays an important role by coordinating with the Ni center to accelerate the decarboxylation. The electronic processes of decarboxylation as well as other important elementary steps are discussed in detail.