Organocatalytic Halogenation of Indazoles and Pyrazoles with N-Halosuccinimides Enhanced by Gallocyanine as Halogen-Transfer Agent

A screening of organic dyes has led to the discovery of gallocyanine as an organocatalyst for the halogenation of a variety of functionalized pyrazoles, indazoles, and aromatics. This work provides an example of a mild organocatalyst that does not require light, oxidizing agents, transition-metal activation, or high temperatures. Thirty-nine halogenated pyrazoles and indazoles, including pharmaceuticals such as celecoxib, deracoxib, and antipyrine, have been isolated in good to excellent yields using N-halosuccinimides as the stoichiometric halogen source with gallocyanine as the catalyst. An experimental and computational investigation has implicated a mechanistic role in which gallocyanine acts as an acid-dependent halogen transfer agent in the halogenation of (hetero)arene substrates.