N-heterocyclic carbenes: organocatalysts displaying diverse modes of action.

Within the context of Lewis base catalysis N-heterocyclic carbenes represent an extremely versatile class of organocatalyst that allows for a great variety of different transformations. Starting from the early investigations on benzoin, and later Stetter reactions, the mechanistic diversity of N-heterocyclic carbenes, depending on their properties, has led to the development of several unprecedented catalytic reactions. This article will provide an overview of the versatile reactivity of N-heterocyclic carbenes.