The design and synthesis of imidazolium-containing diselenides 4a–c is described. The introduction of the N-methylimidazolium group gives freely soluble compounds in water, unlike the majority of common organic diselenides. Catalytic amounts of 4a–c effectively promote bromination of organic substrates using a safe and inexpensive NaBr/H2O2 system in water. Kinetics experiments revealed that the bromination of 4-pentenoic acid has a first ...