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

The 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.