Ferrocenyl induced one-pot synthesis of 3,3′-ferrocenylbiindoles

When we used 2-ferrocenethynylaniline (1a) as reactant and NaAuCl₄·2H₂O (5%) as catalyst to prepare 2-ferrocenylindole (2a), we were surprised to find that 3,3′-ferrocenylbiindole (3a) was obtained simultaneously. 3a was characterized by elemental analysis, FT-IR, MS, NMR, and X-ray single crystal diffraction. The reactant substitution reaction, in-situ HNMR, XPS, EPR characterization, and calculations were carried out to interpret why 3a was obtained in one pot. The results showed that the hydroperoxyl radical played a key role in the reaction, which induced synthesis of 3a.