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.

Automated summary

This study reported the synthesis of acyl azolium salts from various types of carbenes and the generation of their corresponding radicals and enolates, revealing the role of additives in the redox behavior of these compounds and correcting misconceptions about radical transformations of aldehyde derivatives. The reducing ability of enolates derived from certain carbenes was found to be underestimated, and catalytic intermediates were shown to transfer electrons efficiently at room temperature. Enol derivatives of certain carbenes were identified as powerful reducing agents.