N-Heterocyclic Carbene Organocatalysis: Activation Modes and Typical Reactive Intermediates

N-Heterocyclic carbene (NHC) organocatalysis has been developed as an important approach in modern organic synthesis. Versatile activation modes within NHC organocatalysis have been established with countless transformations being realized in both efficient and selective fashion. We would like to provide an overview on the key progresses achieved within this field in the past two decades. Since numerous excellent reviews have been documented within this area, we will mainly focus on the scientific development of this research field based on the basic reaction modes and typical reaction intermediates.