Molecular iodine has been used for more than 100 years as a remarkable catalyst for many organic transformations such as cycloadditions, Michael and aldol reactions, or esterifications. Different explanations for the origin of its catalytic effect have been proposed in the last decades including a "hidden" Brønsted acid catalysis by HI, a Lewis-acid (or halogen-bond) activation, or catalysis by an iodonium(I) species. Recently, iodine catalysis again gained more interest due to the latest developments in halogen-bond catalysis. In this Minireview, we first summarize the experimental basis for the proposed modes of activation. Subsequently, we analyze typical iodine-catalyzed reactions to gain more insights into the underlying reaction mechanisms.

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 < ΔG ⧧ < –1.8 kcal mol–1) for reactions involving α,β-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 Brønsted acid catalysis as a competing pathway due to the formation of hydrogen iodide via hypoiodites in aprotic solvents seems less likely for these reactions.

Noncovalent interactions play an important role in many biological and chemical processes. Among these, hydrogen bonding is very well studied and is already routinely used in organocatalysis. This Short Review focuses on three other types of promising noncovalent interactions. Halogen bonding, chalcogen bonding, and anion-π bonding have been introduced into organocatalysis in the last few years and could become important alternate modes of activation to hydrogen bonding in the future.1 Introduction2 Halogen Bonding3 Chalcogen Bonding4 Anion-π Bonding5 Conclusions

Molecular iodine, an easy to handle solid, has been successfully employed as a catalyst in different organic transformations 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 Brønsted 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 in the rate-limiting transition state. Our experimental investigations rule out hidden Brønsted acid catalysis by partial decomposition of I to HI and suggest a halogen bond activation instead. Finally, molecular iodine turned out to be a similar if not superior catalyst for Michael additions compared with typical Lewis acids.

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Assessment of the solution equilibria of [bis(pyridine)iodine(I)]+ complexes by ESI-MS and NMR reveals the preference of iodine(I) to form complexes with a more basic pyridine. Mixtures of symmetric [bis(pyridine)iodine(I)]+ complexes...

[Bis(pyridine)iodine(I)]+ complexes offer controlled access to halonium ions under mild conditions. The reactivity of such stabilized halonium ions is primarily determined by their three‐center, four‐electron [N−I−N]+ halogen bond. We studied the importance of chelation, strain, steric hindrance and electrostatic interaction for the structure and reactivity of halogen bonded halonium ions by acquiring their 15N NMR coordination shifts and measuring their iodenium release rates, and interpreted the data with the support of DFT computations. A bidentate ligand stabilizes the [N−I−N]+ halogen bond, decreasing the halenium transfer rate. Strain weakens the bond and accordingly increases the release rate. Remote modifications in the backbone do not influence the stability as long as the effect is entirely steric. Incorporating an electron‐rich moiety close by the [N−I−N]+ motif increases the iodenium release rate. The analysis of the iodine(I) transfer mechanism highlights the impact of secondary interactions, and may provide a handle on the induction of stereoselectivity in electrophilic halogenations.

Extending earlier studies on iodine catalysis, experimental investigations show that various halogen-bond donors can also be employed to accelerate the Michael addition between trans-crotonophenone and indole. Solvent as well as counteranion effects have been analyzed, and kinetic and computational investigations provide additional insights into the mode of activation.