Origin of the Catalytic Effects of Molecular Iodine: A Computational Analysis

Molecular iodine is an excellent catalyst for many organic transformations, but the origin of its catalytic activity is still unknown. To answer this question, we have analyzed four iodine-catalyzed reactions by density functional theory. Our calculations reveal that molecular iodine significantly reduces the activation free energies (-7.6 < Delta G(double dagger) < -1.8 kcal mol(-1)) for reactions involving alpha,beta-unsaturated carbonyls or nitrostyrenes. Closer analysis of the nature of the interaction between iodine and the corresponding Michael acceptors suggests that halogen bonding is the origin of the catalytic activity. The computational and experimental studies show that hidden Bronsted acid catalysis as a competing pathway due to the formation of hydrogen iodide via hypoiodites in aprotic solvents seems less likely for these reactions.