Synthesis and [2+2] Cycloaddition with Tetracyanoethylene of Ene–Diyne Scaffolds Bearing Ferrocenes at the Periphery

Abstract

AbstractThe ene–diyne systems 1 and 2, possessing ferrocenyl groups at the periphery, were prepared by a simple one‐pot Sonogashira–Hagihara coupling reaction of ethynylferrocene with 9‐dibromomethylene‐9H‐fluorene (4) and 9,10‐bis(dibromomethylene)‐9,10‐dihydroanthracene (5). Ene–diyne 1 reacted with tetracyanoethylene (TCNE) in a formal [2+2] cycloaddition reaction, followed by ring opening of the initially formed [2+2] cycloadduct, cyclobutene, to afford the corresponding 1,1,4,4‐tetracyanobutadiene (TCBD) derivative 6 in good yield. The redox behavior of the ene–diyne compounds 1 and 2, and the TCBD derivative 6 was examined by cyclic voltammetry (CV) and differential pulse voltammetry (DPV), which revealed their electrochemical oxidation properties with multi‐electron transfer depending on the number of ferrocene units in the molecule, in addition to the two‐electron reduction of the TCBD moiety in the case of TCBD derivative 6. Moreover, a significant color change was observed by visible spectroscopy under electrochemical oxidation conditions and under reduction conditions in the case of TCBD derivative 6.