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 O2/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 NHC‐catalyzed reactions that appeared in the literature from 2014 to present, with a strong focus to crucial intermediates and their mechanistic implications.

Oxidative N‐Heterocyclic Carbene Catalysis

Carbene-Catalyzed Enantioselective Synthesis of γ-Keto-β-silyl Esters and Amides.

Singlet oxygen (1O2)-mediated oxidation represents an attractive strategy for incorporation of oxygen atoms from air under mild and environmentally benign conditions. However, the 1O2 reaction with enamine suffers from fragmentation, leading to very unsuccessful transformation. Here, Lewis acid is introduced to intercept [2 + 2] or "ene" reaction intermediates of the 1O2 reaction and enables oxidative dimerization of enamines to produce pyrrolin-4-ones in good to excellent yields. Mechanistic studies reveal the formation of the imino ketone intermediate from the interaction of 1O2 and enamine, which is able to interact with Lewis acid, relaying the 1O2 reaction in enamine chemistry. For the first time, selective cross-dimerization of two different enamines is achieved. Due to the advantages of mild conditions, high chemoselectivity, and up to 99% yield, a promising strategy has been developed for synthesizing aza-heterocycles under ambient conditions, which can be further applied for the synthesis of imidazolone, quinoxaline, and highly functionalized imine. 

Lewis Acid-Relayed Singlet Oxygen Reaction with Enamines: Selective Dimerization of Enamines to Pyrrolin-4-ones

The use of functionalized aldimines has been demonstrated as newly structural 1,4-dipole precursors under carbene catalysis. More importantly, enantiodivergent organocatalysis has been successfully developed using carbene catalysts with the same absolute configuration, leading to both ( R )- and ( S )- enantiomers of six-membered heterocycles with quaternary carbon centers. This strategy features a broad substrate scope, mild reaction conditions, and good enantiomeric ratio. DFT calculation results indicated that hydrogen bond C-H…F interactions between the catalyst and substrate are the key factors for controlling and even switching the enantioselectivity. These new 1,4-dipoles can also react with isatin and its imines under carbene catalysis, allowing for access to the spiro oxindoles with excellent enantiomeric ratios.

Asymmetric Carbene-Catalyzed Oxidation of Functionalized Aldimines as 1,4-Dipoles.

Use of Vitamin B12 as a Nontoxic and Natural Catalyst for the Synthesis of Benzoxazoles via Catechols and Primary Amines in Water under Aerobic Oxidation

Sodium pyruvate as a peroxide scavenger in aerobic oxidation under carbene catalysis

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Sodium pyruvate as a peroxide scavenger in aerobic oxidation under carbene catalysis