An n-heterocyclic-carbene-tetracyanoethylene zwitterion: Experimental and theoretical study on its formation and reactivity

Abstract

The direct addition of tetracyanoethylene (TCNto the carbene center of 1,3-bis(2,6-diisopropylphenyl)imidazol-2-ylidene (IPwas 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. © 2017 Wiley-VCH Verlag GmbH & Co. KGaA, Weinheim