Critical Assessment of the Reducing Ability of Breslow‐type Derivatives and Implications for Carbene‐Catalyzed Radical Reactions

We report the synthesis of acyl azolium salts stemming from thiazolylidenes C(NS) , triazolylidenes C(TN) (,) mesoionic carbenes C(MIC) and the generation of their corresponding radicals and enolates, covering about 60 Breslow‐type derivatives. This study highlights the role of additives in the redox behavior of these compounds and unveils several critical misconceptions about radical transformations of aldehyde derivatives under N‐heterocyclic carbene catalysis. In particular, the reducing ability of enolates has been dramatically underestimated in the case of biomimetic C(NS) . In contrast with previous electrochemical studies, we show that these catalytic intermediates can transfer electrons to iodobenzene within minutes at room temperature. Enols derived from C(MIC) are not the previously claimed super electron donors, although enolate derivatives of C(NS) and C(MIC) are powerful reducing agents.