Halogen bond-induced electrophilic aromatic halogenations.

In recent years, there has been increasing interest in utilising halogen bonds in organic synthesis, especially in aromatic halogenation reactions. N-Halosuccinimides and 1,3-dihalo-5,5-dimethylhydantoins are popular sources of halonium ions due to their ease of handling and low toxicities. Traditionally, these N-haloimides are activated by electrophiles, namely Brønsted and Lewis acids. The recent discovery of possible activation by nucleophilic Lewis base catalysts led to a paradigm shift in aromatic halogenation. Active functional motifs in Lewis base catalysts such as CS, R-S-R1, Ar-S-S-Ar, SO, Ar-NH2, and R2NH+Cl- form halogen bonds with the positively charged σ-hole of the halogen atoms: an essential interaction to produce halonium ions. This review highlights the evolution of the two modes of activation. Evidence of halogen bond formation from mechanistic studies of nucleophilic activation is also discussed herein.