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Over the last decades, catalysis promoted by N-heterocyclic carbenes (NHCs) has established itself as one of the most important strategies of current organocatalysis. The striking aspect of the activation promoted by NHCs resides in the polarity inversion of aldehydes (Umpolung), through the formation of the Breslow intermediate, allowing the direct coupling between aldehydes (benzoin condensation), the direct acylation of Michael acceptors (Stetter reaction) and the incorporation of electrophilic species to the b-carbon of a,b-unsaturated aldehydes (polarity inversion via homoenolates). The versatility of NHCs is completed by the possibility of access to acyl-azole intermediates, which can be prepared by several strategies, one of which is the oxidation, via 2-electron transfer, of the Breslow intermediate. These species are electrophilic and, therefore, allow access to a diversity of carbonyl derivatives when combined with nucleophiles in diverse ways.
On the other hand, the removal of a single electron from Breslow intermediates has proven to be challenging when using oxidizing agents, since the generated ketyl radical is still susceptible to a new oxidation step to the acyl azolium salts. However, recent advances have overcome this limitation through the use of photocatalysis and redox-active species (e.g., N-acyloxy-phthalimides, pyridinium salts, and alkyl halides), which are capable of promoting the controlled oxidation of Breslow intermediates to the respective ketyl radicals both thermally and photochemically. The ketyl radical generated from the oxidation of the Breslow intermediate is highly persistent and is therefore capable of reacting via radical-radical coupling with transient species generated in the reaction medium. On the other hand, photocatalysis also enables the SET reduction of acyl azolium salts to their respective ketyl radicals. Both approaches represent a game-changer in this field and unlock connections that were unfeasible through traditional methodologies.
What we want to do:
Our goal is unveil new reactivities using the photoinduced NHC redox chemistry. Both protocols, photoredox, and photoexcitation of electron-donor acceptor complexes, will be explored to convert Breslow intermediates and acyl azolium sats into their corresponding ketil radicals, enabling the coupling with other radical species generated in the reaction medium.
References:
Organocatalytic Reactions Enabled by N-Heterocyclic Carbenes. Chem. Rev. 2015, 115, 9307–9387
Light-Promoted Organocatalysis with N-HeterocyclicCarbenes. ChemPhotoChem 2020, 4, 5147–5153
Recent Advances in Organic Synthesis Using Light-Mediated N-Heterocyclic Carbene Catalysis. Eur. J. Org. Chem. 2021, 2021, 4603– 4610.
Radical NHC Catalysis. ACS Catal. 2022, 12, 11984–11999.