Synthesis and Properties of Optically Active Helical Metallocene Oligomers

Syntheses are described for optically active oligomers 9 and 27. The steps required have three essential features. (1) A bromine directs a photocyclization, blocking both the position it occupies and the one adjacent. (2) An acid scavenger (propylene oxide) prevents benzylic ethers from being eliminated during photocyclization. (3) The stereogenicity of one carbon controls the direction in which the helix winds. The oligomers are the first to be prepared in which metallocenes are joined by conjugated systems of double bonds. Their optical activity is very high. The cyclic voltammogram of 9 shows two cathodic waves whose closeness implies that the interaction between adjacent units is weak. When 9 is reduced electrochemically on Pt, a film of neutral oligomers is produced. This film is conductive, and it catalyzes charge transfer. Its weight, measured using a quartz crystal microbalance after the amount of reduction was analyzed coulometrically, measured the molecular weight of 9.