Spiroindoles as Intermediates/Products in Transition Metal Catalyzed Dearomatization of Indoles

Spirocyclic indole derivatives are fascinating tridimensional molecular scaffolds, from both a synthetic and biological point of view. Among the diversity of strategies developed to access this kind of structures, transition metal catalysis recently led to impressive advances, especially in order to tame the unique reactivity of the dearomatized spirocyclic intermediates. These species can indeed evolve towards both spirocyclic or non-spirocyclic products through rearomatization-driven processes which are at the same time highly challenging to control but also source of a large structural diversity. This review highlights the most prominent methods of the last decade allowing to trigger a spirocyclization on indole derivatives tethered with a transition metal-activable functional group, leading to both spirocyclic and rearomatized products. The discussion is particularly focused on the reactivity and the complex mechanistic features regarding the evolution of the spiroindoleninium intermediate, highly dependent on the catalytic system.