NHC-mediated photochemical/electrochemical synthesis of carbonyl compounds

Great progress has been made in the photochemical/electrochemical reactions of carbonyl compounds catalyzed by N-heterocyclic carbene (NHC), allowing for many incredible reactions. The form of active intermediates hugely varies in different reactions. Under light or electricity, different active intermediates can be generated during NHC-catalyzed reactions depending on their interaction with different substrates and the reaction conditions. Intermediates can be converted into α,β-unsaturated acylazolium, Breslow intermediate, homoenolate, and acylazolium and subsequently undergo single- or double-electron conversions. This study reviews the different active intermediates formed by NHC in photochemical/electrochemical catalysis.