Nucleophilic Aromatic Substitution of Hydrogen as a Tool for Heterocyclic Ring Annulation

Publisher Summary SNH methodology finds significant use in organic synthesis. A major advantage of this methodology is that it eliminates the necessity of first introducing a good leaving group into an aromatic substrate. Moreover, in classical SN ipso reactions an acid, such as HHal, H2SO4 or HNO2 is normally liberated that has to be scavenged. However, in SNH processes, a water molecule is typically formed that can contribute to “green chemistry.” This chapter deals with SNH cyclizations. SNH based heterocyclizations allow the synthesis of a great variety of heterocyclic structures differing in ring size, number of heteroatoms and their type, and degree of unsaturation. Various arenes and hetarenes with appropriate electron deficiency can be used as substrates. In context to future prospects, it is observed that a large number of known SNH cyclizations deals with the pyrrole ring closure, therefore search for new substrates, reagents, and cyclization schemes for the annulation of other heterocyclic rings is desirable. It is also important to optimize reaction conditions as many existing transformations do not provide high enough yields.