Organocatalysis represents a synthetic paradigm that has grown exponentially in popularity over the last decade, arguably becoming one of the most desired synthetic methods for the creation of enantiomerically enriched products. Within this field the use of N-heterocyclic carbenes (NHCs) has seen appreciable research activity with the realisation of many novel reaction types. This review provides a comprehensive account of recent advances in the generation and reactivity of azolium enolates with exclusive focus on those processes rendered asymmetric through the use of chiral NHCs. 1 Introduction: NHCs in Organocatalysis 1.1 Azolium Enolates 2 Azolium Enolate Generation via Ketenes 2.1 Asymmetric Formal [2+2] Cycloadditions 2.2 Asymmetric Formal [3+2] Cycloadditions 2.3 Asymmetric Formal [4+2] Cycloadditions 2.4 Asymmetric Protonation and Halogenation 3 Azolium Enolate Generation via α-Functionalised Aldehydes 4 Azolium Enolate Generation via Enals 5 Activated Esters as Azolium Enolate Precursors 6 Conclusions and Outlook

N-Heterocyclic carbenes promote the formal [2+2] cycloaddition of ketenes with N-tosyl imines to give the corresponding β-lactams in good to excellent isolated yields; chiral NHCs give β-lactams in high e.e. after crystallisation.

This tutorial review highlights the organocatalytic Lewis base functionalisation of carboxylic acids, esters and anhydrides via C1-ammonium/azolium enolates. The generation and synthetic utility of these powerful intermediates is highlighted through their application in various methodologies including aldol-lactonisations, Michael-lactonisations/lactamisations and [2,3]-rearrangements.