Reaction Mechanism of Iodine-Catalyzed Michael Additions

Molecular iodine, an easy to handle solid, has been successfully employed as a catalyst in different organic transformation's for more than 100 years. Despite being active even in very small amounts, the origin of this remarkable catalytic effect is still unknown. Both a halogen bond mechanism as well as hidden Bronsted acid catalysis are frequently discussed as possible explanations. Our kinetic analyses reveal a reaction order of 1 in iodine, indicating that higher iodine species are not involved the rate-limiting transition state. Our experimental investigations rifle out hidden Bronsted acid catalysis by partial deconiposition of I-2 to HI and suggest a halogeri bond activation instead. Finally, molecular iodine turned out to be a similar if not superior,catalyst for Michael additions compared with typical Lewis acids.