E/Z Switchable Ring-Closing Metathesis in 1,1′-Bis(but-3-enyl)ferrocenes: Synthesis and Characterization of Axially Chiral ansa[6]-Ferrocenes

A series of ansa[6]-ferrocenes were obtained through the ruthenium-catalyzed ring-closing olefin metathesis in 1,1′-bis­(but-3-enyl)­ferrocenes. The stereoselectivity of this reaction depended on the structure of substrates, that is, mixtures of (E) and (Z) isomers were obtained for alkyl-substituted 3a–3d while (Z)-isomers only for aryl-substituted 3e and 3f. The ansa[6]-ferrocenes are axially chiral in the solid state according to single-crystal X-ray analysis. In the case of methyl-substituted derivative 4a, spontaneous resolution of the racemate was observed during crystallization. The solution properties of (E) and (Z) isomers are remarkably different; the latter displays inequivalence of the two formally identical parts of the molecule at low temperature.