Developing a Reliable Synthetic Route to <i>cis</i>-1,3-Bridge-Disubstituted [3]Ferrocenophane Systems

Treatment of the [3]ferrocenophane cyanide trans-7 with strong aqueous base led to a diasteromeric mixture of carboxylic acids trans-8/cis-8 in a 1:2.8 ratio. Analogous base-induced hydrolysis of the related ortho-bromine-substituted [3]ferrocenophane nitrile trans-12 selectively yielded the cis-carboxylic acid cis-13. DFT calculations showed that in this series the cis-carboxylic acids are thermodynamically more stable than the respective trans-isomers. The dicarboxylic acid cis-15 was prepared in a similar reaction sequence and used for further synthesis of a “TADDOL-analogous” [3]ferrocenophane (cis-20).