1,1′-Ferrocenoyl–oligoprolines. A synthetic, structural and electrochemical study

The preparations of the eight ferrocenoyl oligopeptides, 1,1′-(Pro–OBzl)2–Fc (1), 1,1′-(Pro–OMe)2–Fc (1a), 1,1′-(Pro2–OBzl)2–Fc (2), 1,1′-(Pro3–OBzl)2–Fc (3), 1,1′-(Pro4–OBzl)2–Fc (4), 1-(Pro2–OBzl)–1′-(OBt)–Fc (5), 1-(Pro3–OBzl)-1′–(OBt)–Fc (6), 1-(Pro4–OBzl)–1′-(OBt)–Fc (7) are described. Crystallographic studies were carried out for 1a, showing a 1,3′-configuration of the two substitutents. The growing oligoproline chain adopts a helical polyproline-II conformation in solution. Compounds 1–4 exhibit reversible one-electron oxidations of the ferrocene moiety, which is influenced by the length of the oligoproline chain. With growing peptide length, the molecule becomes easier to oxidize. For the mixed OBt–ester–oligoprolines, 5, 6 and 7, quasi-reversible oxidations are observed at slow scan rates, which vary with the oligoproline chain length.