Carbon and Heteroatom Activation via N-Heterocyclic Carbene-Bound Dearomative Intermediates for Stereoselective Annulation Reactions

N-Heterocyclic carbene (NHC) organocatalysis has long been recognized as a powerful and versatile method for the implementation of (asymmetric) transformations enabling the formation of C─C and C-heteroatom bonds. A number of NHC-bound active ionic species have been classically established for activation of carbon atoms as either nucleophiles or electrophiles at diverse positions, such as (aza)Breslow intermediates, homoenolate equivalents, azolium (di)enolates, simple acyl (imidoyl) azoliums, and α,β-unsaturated/alkynyl acyl azoliums. Within this realm, the past decade has witnessed a blossoming interest in the deployment of NHC-tethered dearomative intermediates for the activation of both carbon atoms and heteroatoms in aromatic compounds. Accordingly, functionalization of benzylic carbon atoms was made possible through catalyst-bound o-quinodimethanes (o-QDMs), while remote activation of oxygen and nitrogen atoms was effectively achieved through NHC-bound o-quinone methides (o-QMs) and aza-analogs (aza-o-QMs), along with aza-fulvene type intermediates and triaza-diene species. Aim of this review is to provide an overview of the most relevant literature on the development of NHC-bound dearomative intermediates for carbon and heteroatom activation since 2013, with emphasis on their applications for stereoselective annulation reactions.