Transition-Metal-Free Carbonyl Redox: A Ketene in Three Oxidation States

Ketenes are among the most versatile compounds in organic chemistry, participating in a wide range of transformations, including cycloadditions, nucleophilic and electrophilic additions, and polymerizations. However, their redox chemistry, particularly the preparation of stable radical anions and radical cations, remains largely unexplored. Here, we report the isolation and characterization of a cyclic (alkyl)(amino)carbene (CAAC)-derived ketene in three oxidation states, including its radical anion and cation, representing the first example of such species. These ketene radicals exhibit unique electronic properties: the radical anion adopts a bent geometry that places the radical out of conjugation with the CAAC substituent, while the radical cation results from oxidation of an electron-withdrawing carbonyl group. Both radicals were characterized in detail using infrared (IR) and UV-vis electronic absorption spectroscopy, EPR spectroscopy, structural analysis, and computations, revealing parallels with the redox chemistry of transition metal carbonyl complexes. Reactivity studies indicate that the radical anion and radical cation react via nucleophilic and radical pathways, respectively, contrasting with the behavior of neutral ketenes. This work demonstrates a novel redox-based strategy for investigating uncharted transformations of ketenes, expanding their synthetic utility and opening new avenues for redox-organocatalysis of carbenes with small molecules.