Transition-metal-free approaches to access N-heterocycles and valuable intermediates from aryldiazonium salts

The discovery of novel and efficient synthetic methods to achieve valuable N-fused heterocycles has been an important research area in organic chemistry for several decades. Owing to the importance of N-fused heterocycles in medicinal chemistry, materials sciences, and organic synthesis, numerous efforts have been devoted to designing robust methods for the construction of N-heterocyclic molecules. Aryldiazonium salts are a privileged and readily accessible class of reagents/synthetic building blocks used in a diverse range of disciplines in chemical sciences. Aryldiazonium salts based on conventional C-C bond construction strategies have been well established. Alternatively, owing to their high electrophilic capacity, aryldiazonium salts have been broadly utilized in the efficient, atom-economic and rapid assembly of structurally unique nitrogen-containing heterocycles via the formation of C-N bonds in recent years. Herein, we review a diverse range of transition-metal-free synthetic methodologies using aryldiazonium salts as key synthetic precursors to afford highly useful N-heterocyclic frameworks over the past nine years (2015 to date). In addition, the application of these salts to prepare synthetically useful intermediates and precursors is also discussed. These N-heterocyclic motifs and intermediates are highly prevalent in a variety of natural products, agrochemicals and material applications.