Critical Assessment of the Reducing Ability of Breslow-type Derivatives and Implications for Carbene-Catalyzed...

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.