Recent green synthetic approaches toward Ullmann reaction: a review

The traditional Ullmann reaction which represents the copper-catalyzed coupling between two aryl halides to build a C–C bond was first reported in 1901. Over the time, this reaction was widely applied in organic synthesis for the construction of a variety of carbon–heteroatom bonds using different types of catalysts such as palladium nanoparticles, palladium–iron nanoparticles, gold nanoparticles, and gold–copper nanoparticles. The present review describes the recent green methodologies of Ullmann reaction for C–C, C–N, C–O, and C–S bonds formation. Microwave/ultrasound irradiations, easily recoverable heterogeneous catalysts, solvent-free, and ligand-free parameters overcome the problems faced due to classically reported harsh reaction conditions of Ullmann coupling. All such developments with possible mechanistic insights reported since 2010 have been discussed in this review which gives a brief idea to the chemists for the scope, generality, and limitations of the Ullmann reaction. Moreover, considering these methodologies in view, many other exciting pathways can also be developed for the synthesis of different types of heterocycles, drug-like molecules, and natural products. Although a lot of work has been done on the Ullmann reaction, there is still tremendous scope in this field to look for recyclable catalysts, mild conditions, and substrate scope. Moreover, research regarding mechanistic aspects of the reaction can be improved by discovering new sustainable strategies as green procedures will continuously give a substantial impact on Ullmann chemistry.