Synthesis and electro-spectroelectrochemistry of ferrocenyl naphthaquinones

A practical approach to ferrocenyl naphthaquinone derivatives involving thermal rearrangement of variously substituted 4-aryl-4-hydroxycyclobutenones was described. The reaction of 3-ferrocenyl-4-isopropoxy-3-cyclobutene-1,2-dione with different aryl lithiums gave the corresponding 4-aryl-4-hydroxycyclobutenones, which were heated in p-xylene at reflux open to the air to yield ferrocenyl naphthaquinones. The redox chemistry of the ferrocenyl naphthaquinones was studied by electrochemical and in situ spectroelectrochemical techniques in CH2Cl2 solution and in CH3CN solution with water, weak and strong acidic additives. Ferrocenyl naphthaquinones displayed reversible two reduction processes involving semiquinone radical anion (Fc–snqradical dot−), dianion (Fc–nq2−) species and a one-electron oxidation process based on the ferrocenium/ferrocene (Fc+–nq/Fc–nq) couple in CH2Cl2. The redox reaction mechanism of the ferrocenyl naphthaquinones in the presence of the additives proceeded via hydrogen bonding or proton-coupled electron transfer. Effects of the substituents on the reduction potentials and intramolecular charge-transfer bands of ferrocenyl naphthaquinones were also discussed.