N -Heterocyclic Carbene Catalysis via the α,β-Unsaturated Acyl Azolium

First reported less than a decade ago, the α,β-unsaturated acyl azolium has emerged as a central reactive intermediate for reaction discovery using N -heterocyclic carbene catalysis. In this Perspective, an introduction to the four main reactivity patterns accessible from this intermediate is provided. The Perspective is handled in a largely chronological fashion, with an emphasis on alternate approaches to the key intermediate and first-in-class reaction cascades. Finally, a brief discussion of emerging trends in this field of catalysis is presented. Although not exhaustive, the Perspective provides an overview of this active area of research and serves as a guide for future investigations.