An N‐Heterocyclic‐Carbene–Tetracyanoethylene Zwitterion: Experimental and Theoretical Study on Its Formation and Reactivity

The direct addition of tetracyanoethylene (TCNE) to the carbene center of 1,3‐bis(2,6‐diisopropylphenyl)imidazol‐2‐ylidene (IPr) was successfully demonstrated. The IPr–TCNE adduct, that is, zwitterion 1 , clearly differs from the typical [2+1]‐cycloaddition products obtained from the reaction of carbenes with olefins. The results obtained from experimental and theoretical studies suggest that adduct 1 disassembles into IPr and TCNE, which is followed by the addition of IPr to the nitrile carbon atom of TCNE and subsequent isomerization into a thermodynamically more stable singlet nitrene intermediate. This intermediate is then captured by an additional molecule of TCNE to afford another heterocycle. This study reveals a unique example of the reactivity between N‐heterocyclic carbenes and olefins.