Oxidative N‐Heterocyclic Carbene Catalysis

Abstract

N-Heterocyclic carbene (NHC) catalysis is a by now consolidated organocatalytic platform for a number of synthetic (asymmetric) transformations via diverse reaction modes/intermediates. In addition to the typical umpolung processes involving acyl anion/homoenolate equivalent species, implementation of protocols under oxidative conditions greatly expands the possibilities of this methodology. Oxidative NHC-catalysis allows for oxidative and oxygenative transformations through specific manipulations of Breslow-type species depending upon the oxidant used (external oxidant or O 2 /air), the derived NHC-bound intermediates paving the way to non-umpolung processes through activation of carbon atoms and heteroatoms. This review is intended to update the state of the art in oxidative NHCcatalyzed reactions that appeared in the literature from 2014 to present, with a strong focus to crucial intermediates and their mechanistic implications.

Switchable Organocatalysis from N‐heterocyclic Carbene‐Carbodiimide Adducts with Tunable Release Temperature**

“…N ‐Heterocyclic carbenes (NHCs) are highly active and versatile organocatalysts [20–25] that are particularly amenable to this reversible‐coordination strategy to mitigate storage and handling challenges caused by their air‐sensitivity [26–28] . Masked NHC adducts avoid the traditional methods of using (super)stoichiometric base to deprotonate an NHC salt precursor in situ, which often requires demanding purification and can lead to unwanted side reactions [26,29] .…”

NHC-catalyzed formal [3 + 3] annulations of δ-acetoxy allenoates for the synthesis of 4H-pyran derivatives

“…2 NHC catalysts can react with aldehydes, esters, anhydrides, acyl halides, and carboxylic acids to form varieties of reactive intermediates, for example, Breslow intermediates, enolates and acylazoliums. 3 In addition, NHC can catalyze the umpolung of Michael acceptors 4 and the aza-Morita–Baylis–Hillman reaction. 5 Despite these elegant achievements, the development of a new catalytic mode of NHC catalysis that is distinct from the traditional reactive intermediate pathways is still highly desirable.…”

Aerobic Oxidative N‐Heterocyclic Carbene Catalysis

“…In oxidative NHC catalysis, the NHC is combined with an oxidant generally resulting in a reversal of the umpolung reactivity. The oxidative reactivity of NHC catalysis has received widespread attention and several unique reaction types have been developed under this reaction manifold [11–14] . However, the oxidants used are normally stoichiometric high molecular weight quinone‐based oxidants [15] that negatively impacts the atom economy (AE), purification protocols, and operational simplicity.…”