Sustainable strategies of C–N bond formation via Ullmann coupling employing earth abundant copper catalyst

Abstract N-arylation reaction has become one of the key strategies in organic synthesis to access several molecules of biological, agrochemical, and medicinal importance. The century-old Ullmann reaction, for the synthesis of C–N has been realized as one of the foremost transition metal catalyzed coupling reaction. However, the limitation associated with the methodology has led to the evolution of several other synthetic strategies for the construction of C–N bonds. However, with the maturity of the copper chemistry, and the realization to look beyond palladium catalysis has led to the revival of the interest in the so-called Ullmann reaction. The need for a cheaper and sustainable catalytic system which makes use of the earth-abundant copper to catalyze the reaction has invigorated from its dormancy. Several research groups for past few years are incessantly working to develop more efficient and sustainable methodologies to address the pertinent issues of poor functional group tolerance, harsh reaction conditions prolonged reaction times associated with the Ullmann reaction and developed several sustainable strategies for the same. Therefore, this review focuses on the summary of the evolution of the Ullmann coupling reaction using earth abundant copper catalyst for the construction of C–N bond under sustainable conditions and provides an outline of the reaction strategies and mechanisms. Despite all these breakthroughs, there is still a demand for more efficient catalytic systems, more benign reaction conditions, as well as overcoming the challenges associated with the sterically and electronically demanding and substrates. This review will therefore inspire the future researchers to establish sustainable methodologies which can have applications on a large industrial scale.