How to Lose a Bond in Two Ways - The Diradical/Zwitterion Dichotomy in Cycloaromatization Reactions

Cycloaromatization reactions decouple two electrons by breaking two bonds to form only one sigma bond and illustrate one of the most common mechanistic dichotomies in chemistry, namely the two ways of breaking a chemical bond: Zwitterionic or diradical. With a suitable choice of reaction conditions, substitution patterns, and catalysts, cycloaromatization processes can be redirected from the usual formation of a diradical towards a variety of zwitterionic pathways. This review illustrates the practical approaches for directing zwitterionic cycloaromatization reactions and lays the mechanistic foundations for the further development of this emerging field.