Recent Advances in the Synthesis of Quaternary Ammonium Salts via Transition-Metal-Catalyzed CH Bond Activation

Highly substituted cationic heterocyclic nitrogen salts are versatile building blocks for a number of naturally occurring products and functional materials. In this paper, we review the very recently reported Co(III)-, Rh(III)- and Ir(III)-catalyzed inter- and intra-molecular sp2 CH activation/annulations with alkynes or alkenes for the synthesis of various quaternary ammonium salts containing isoquinolinium, cinnolinium, quinolizinium, mesoionic isoquinolone, 12-azapyrene, protoberberine, indolizinium, and isoindolium salts. In addition, the reaction scope, proposed catalytic mechanisms, and applications are also included. This review will help readers to easily understand the recent advances in this field.