Reactions of Alkynyl‐ and 1,1′‐Dialkynylferrocenes with Tetracyanoethylene – Unanticipated Addition at the Less Electron‐Rich of Two Triple Bonds

Alkynylferrocenes react with tetracyanoethylene (TCNE) to form 2-ferrocenyl-1,1,4,4-tetracyanobutadienes. These reactions occur under mild reaction conditions in almost quantitative yields if the triple bond is electron-rich. Systems with more than one triple bond, namely, 1,1′-dialkynylferrocenes or diynylferrocenes, react once at the more electron-rich triple bond, and the second reaction cannot be achieved, even under more-forcing reaction conditions. For a diyne with the reactivity switched by oxidation of the ferrocenyl moiety to the corresponding ferrocenium salt, the addition of TCNE occurred at the less electron-rich triple bond. This unprecedented reactivity is explained by the resonance stabilization of a diradical intermediate. A push–pull 1,1′-dialkynylferrocene with a [4-(dimethylamino)phenyl]ethynyl and a (4-nitrophenyl)ethynyl substituent reacted exclusively at the [4-(dimethylamino)phenyl]ethynyl triple bond.