Diazo retention transformations of aryldiazonium salts towards nitrogen rich heterocycles

Owing to the valuable contribution of N-fused heterocycles to organic synthesis, medicinal chemistry, and materials research, tremendous efforts have been devoted towards the construction of N-heterocyclic scaffolds. Aryldiazonium salts are a ubiquitous class of building blocks/reagents widely used in organic synthesis owing to their distinct electrophilic nature to undergo substitution reactions with concurrent loss of molecular nitrogen (C-C, C-N bond formations). Conversely, a distinctive category of reactions known as "nitrogen retention reactions" preserve the diazonium-derived "N₂" atoms in the end products. These classes of reactions are of great importance due to their uniqueness in constructing nitrogen-rich heterocycles and azo compounds without sacrificing the -NN- functionality. In this review we discuss 77 synthetic methodologies reported between 2015 and 2025 in which aryldiazonium salts are used as a key precursor and the characteristic nitrogen moiety is retained in the N-heterocyclic frameworks. This report groups nine classes of N-heterocycles, which are synthesized using five catalytic systems (Cu, Au, Ru, Ag and organo-photocatalysts) via three distinct activation modes (thermal, electrochemical, and photochemical). These innovative strategies incorporate several advantages, such as gram-scale experiments, one-pot conversions, solvent/transition metal-free conditions, mechanochemical/electrochemical/continuous-flow setups, and photochemical platforms. Overall, the methods discussed herein highlight the innovative trends and key breakthroughs in aryldiazonium salt based diazo retention chemistry over the years. The scope, limitations, mechanisms, and post-synthetic transformations are specifically emphasized.