A simple N-heterocyclic carbene for the catalytic up-conversion of aldehydes into stoichiometric super electron donors.

Catalytic amounts of 1,3-di(methyl)imidazole-2-ylidene, one of the simplest and most prototypical N-heterocyclic carbenes, can up-convert aldehydes into powerful stoichiometric sources of electrons (Super Electron Donors) for reductive transformations of iodoaryls (E red < -2 V). In particular, the hydroarylation of 1,1'-diarylethylenes, which may require high temperatures and inherently generate stoichiometric amounts of oxidized waste, was performed at room temperature, with the concomitant formation of esters as oxidized co-products.

On the Redox Properties of the Dimers of Thiazol-2-ylidenes That Are Relevant for Radical Catalysis.

We report the isolation and study of dimers stemming from popular thiazol-2-ylidene organocatalysts. The model featuring 2,6-di(isopropyl)phenyl (Dipp) N-substituents was found to be a stronger reducing agent (Eox = -0.8 V vs SCE) than bis(thiazol-2-ylidenes) previously studied in the literature. In addition, a remarkable potential gap between the first and second oxidation of the dimer also allows for the isolation of the corresponding air-persistent radical cation. The latter is an unexpected efficient promoter of the radical transformation of α-bromoamides into oxindoles.

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 CNS , triazolylidenes CTN, mesoionic carbenes CMIC 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 CNS . 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 CMIC are not the previously claimed super electron donors, although enolate derivatives of CNS and CMIC are powerful reducing agents.