Synthesis of 2-Azulenyl-1,1,4,4-tetracyano-3-ferrocenyl-1,3-butadienes by [2+2] Cycloaddition of (Ferrocenylethynyl)azulenes with Tetracyanoethylene

1-, 2-, and 6-(Ferrocenylethynyl)azulene derivatives 1016 have been prepared by palladium-catalyzed alkynylation of ethynylferrocene with the corresponding haloazulenes under SonogashiraHagihara conditions. Compounds 1016 reacted with tetracyanoethylene (TCNE) in a [2+2] cycloadditioncycloreversion reaction to afford the corresponding 2-azulenyl-1,1,4,4,-tetracyano-3-ferrocenyl-1,3-butadiene chromophores 1723 in excellent yields. The redox behavior of the novel azulene chromophores 1723 was examined by using cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their multistep electrochemical reduction properties. Moreover, a significant color change was observed by visible spectroscopy under electrochemical reduction conditions.