One-electron oxidation of N-heterocyclic carbenes (NHCs) to afford NHC radical cations, NHCㆍ+, is highly challenging due to intrinsic instablility of the cations. Previous attempts to react NHCs with one-electron oxidants such as tetracyanoethylene (TCNE) formed dication [NHC-NHC]2+, which was proposed to be formed through dimerization of NHCㆍ+.  To avoid such dimerization, more sterically hindered NHC such as 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPr) was used for one-electron oxidation with TCNE. Surprisingly, one-electron oxidation reaction of IPr with TCNE yields two distinct products, a direct adduct of TCNE to IPr and a heterocyclic compound formed from one IPr and two TCNEs, depending on the reaction conditions. More detailed study will be presented including theoretical calculations to understand such reactivities.
 Ramnial, T.; McKenzie, I.; Gorodetsky, B.; Tsang, E. M. W.; Clyburne, J. A. C. Chem. Comm. 2004, 1054.