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

Abstract

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.