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

In an attempt to generalize the recently introduced concept of 'auto-umpolu ng', nitronyl nitroxides are identified as precursors of a special class of electron-excess carbenes. The latter can be formally derived from nucleoph ilic carbenes of the Wanzlick-, Arduengo type by replacing redox-inactive N -substituents by lone-pair donor functions. A first systematic access to th is class of compounds is achieved by lithiating the nitronyl nitroxide 11a at C(2), the central carbon position. The resulting radical anion equivalen t 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, additi on to an aldehyde as well as silylation at the C(2)-position lead to novel types of functional nitronyl nitroxides. Elemental sulfur and selenium yiel d new types of anionic nitronyl nitroxides. The first characterized C(2)-de rived heavy-metal complex of a nitronyl nitroxide 14g resulted, when Hg(OAc )(2) was used as a trapping agent. More conveniently this complex could als o be obtained directly from 11a and Hg(OAc),. The X-ray structure of 14g is presented and analyzed in terms of cluster-forming secondary Hg-O interact ions. 0 2001 Elsevier Science B.V. Ah rights reserved.