DFT Study of Iron-Catalyzed [4 + 4] Cycloaddition of 1,3-Dienes: Mechanism and Diastereoselectivity.

The mechanism of diastereoselective [4 + 4] cycloaddition of 1,3-dienes catalyzed by Fe complexes bearing 2-(imino)pyridine (PI) ligands was investigated using density functional theory calculations. The results reveal that the reaction proceeds via diene coordination, oxidative cyclization, allyl isomerization and reductive elimination. Reductive elimination is identified as the rate-determining step. The computational study indicates that the steric repulsion between substituents on 1-substituted dienes and (iPrPI)Fe moiety governs the diastereoselectivity. These computational findings elucidate the detailed reaction pathway and clarify several ambiguities observed experimentally.