Emerging applications of the tropylium ion as reagent, catalyst, and chromophore

The cycloheptatrienyl (tropylium) ion is a planar, fully conjugated seven‐membered carbocycle containing six π‐electrons. It features a delocalized positive charge, making it a one‐carbon homolog of benzene. Owing to its unique electronic structure and facile accessibility, the tropylium ion has recently attracted increasing attention as a versatile reagent in organic synthesis. It has been employed as a Lewis acid and an oxidant in various organic transformations. Furthermore, tropylium‐based compounds have emerged as functional materials, including organic dyes and stimuli‐responsive systems. This review summarizes recent developments in tropylium‐mediated organic synthesis and highlights the intriguing properties and potential applications of tropylium ion derivatives.