Revisiting Keteniminium Salts: More than the Nitrogen Analogs of Ketenes

Keteniminium salts are powerful electrophilic heterocumulene reagents well‐known for their selectivity and stereocontrol in [2+2] cycloadditions to olefins and carbonyl derivatives. Furthermore, they are readily accessible from stable and simple precursors under a variety of different conditions. Herein, we present the chemistry of keteniminium salts with the hindsight of time, describe preparation methods, recent [2+2] chemistry, mechanistic studies, and assorted applications that significantly extend the scope of utility of these unique electrophilic heterocumulenes.