Imidates: an emerging synthon for N-heterocycles

The unique electronic reactivity of imidates has been recently exploited for the syntheses of diverse classes of N-heterocycles via C-N annulation reactions under acid/base/metal-catalyzed/radical-mediated reaction conditions. As opposed to amides, the imidate functionality provides both electrophilic and nucleophilic centers and eventually enhances its versatility as an organic synthon. In general, imidate motifs act as the soft nucleophiles that coordinate with transition metals to form stable 5-membered metallacycles to activate the proximal C-H bonds followed by annulation reactions to afford the desired N-heterocycles. The imidate precursor also generates in situ nitrogen radicals under suitable conditions to form C-N bonds via 1,5-HAT. This review highlights the recent application of imidates as building blocks for the synthesis of saturated and un-saturated N-heterocycles like oxazolines, oxazines, quinazolines, isoquinolines, imidazoles, and triazoles among others. Different reaction conditions, coupling partners, and imidate substrates reported in the literature have been addressed herein for the nitrogen-containing mono-, bi- and tricyclic ring systems.