We showed that catalytic amounts of 1,3-di(methyl)imidazole-2-ylidene up-convert aldehydes into powerful stoichiometric sources of electrons, allowing light-free reductive transformations of iodoaryls at room temperature and leading to isolable organic oxidized co-products instead of inorganic wastes. This preliminary study shows the efficiency of this NHC in the reduction of iodoaryls to the corresponding arenes, as well as the hydroarylation of 1,1′-diarylethylenes. In the latter case, electrochemical data and isotopic labeling suggest that the outcome of the reaction is determined by the ability of Breslow-type persistent radicals to further reduce alkyl radicals. In particular, this mechanistic proposition accounts for the formation of aryl-acylation products when styrene is the co-reactant, with styrene-derived alkyl radical being then a more difficult-to-reduce benzylic radical.

Prior to our investigations, imidazole-2-ylidenes had been unsuccessfully tested and considered unfit for the promotion of radical transformations of aldehydes. In contrast, this work reveals the unforeseen potential of these NHCs for the generation of strong SED enolates. We are currently reinvestigating the use and scope of this family of organic catalysts for the reductive activation of challenging substrates.

Nadhrata Assani, Ludivine Delfau, Preslav Smits, Sébastien Redon, Youssef Kabri, Eder Tomás-Mendivil, Patrice Vanelle, David Martin and Julie Broggi
Chem. Sci., 2024,15, 14699-14704