Fast Halogenation of Some N‐Heterocycles by Means of N,N′‐Dihalo‐5,5‐dimethylhydantoin

Abstract An instantaneous, selective and high‐yielding halogenation process is reported. The method operates with imidazoles, pyrazoles, and indoles under benign reaction conditions. The developed process involves the use of N,N′ ‐dihalo‐5,5‐dimethylhydantoins (halo=chlorine, bromine, iodine) as halogenation reagents that are activated by catalytic quantities of a strong Brønsted acid. Moreover, the halogenation process is switchable to produce either the mono‐ or di‐halogenated products. Issues related to the reaction mechanism are investigated and a proposal for a reaction mechanism is disclosed.