Electrophilic C(2)-functionalization of nitronyl nitroxides: a reference to N-heterocyclic carbenes

In an attempt to generalize the recently introduced concept of ‘auto-umpolung’, nitronyl nitroxides are identified as precursors of a special class of electron-excess carbenes. The latter can be formally derived from nucleophilic carbenes of the Wanzlick–Arduengo type by replacing redox-inactive N-substituents by lone-pair donor functions. A first systematic access to this class of compounds is achieved by lithiating the nitronyl nitroxide 11a at C(2), the central carbon position. The resulting radical anion equivalent 13 can be trapped in solution by various types of electrophiles to give C(2)-substituted nitronyl nitroxides 14, much in the same way as are trapped nucleophilic carbenes of the Wanzlick–Arduengo type. Carboxylation, addition to an aldehyde as well as silylation at the C(2)-position lead to novel types of functional nitronyl nitroxides. Elemental sulfur and selenium yield new types of anionic nitronyl nitroxides. The first characterized C(2)-derived heavy-metal complex of a nitronyl nitroxide 14g resulted, when Hg(OAc)2 was used as a trapping agent. More conveniently this complex could also be obtained directly from 11a and Hg(OAc)2. The X-ray structure of 14g is presented and analyzed in terms of cluster-forming secondary HgO interactions.