Recent Developments on Diverse (Hetero)Arene Functionalization via Nucleophile‐triggered Umpolung of (Hetero)Aryl Phosphonium Salts

ABSTRACT Aryl and heteroaryl phosphonium salts have recently grabbed the attention of synthetic organic chemists as excellent substrates to obtain functionalized (hetero)arenes. The phosphonium moiety renders the adjoining carbon site electrophilic; therefore, they are good candidates for nucleophilic substitutions. The weakness of the C─P + bond paves the way for their activation via single electron reduction and thereby furnishing (hetero)aryl radicals for further functionalization. However, a rather intriguing but underexplored mode of activating the phosphonium salts has been via nucleophilic attack on the P + ‐center, resulting in the formation of the penta‐coordinated phosphorane intermediates. These transient intermediates readily undergo fragmentation and give rise to (hetero)aryl anions suitable to be functionalized by reacting with appropriate electrophiles. Here we document the history and the recent advances of this umpolung strategy to project its potential for future applications.