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

Abstract

AbstractAlkynylferrocenes 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.