Chirality Organization of Ferrocenes Bearing Podand Dipeptide Chains: Synthesis and Structural Characterization

A variety of ferrocenes bearing podand dipeptide chains have been synthesized to form an ordered
structure in both solid and solution states and have been investigated by 1H NMR, FT-IR, CD, and X-ray
crystallographic analyses. Conformational enantiomerization through chirality organization was achieved by
the intramolecular hydrogen bondings between the podand dipeptide chains. The single-crystal X-ray structure
determination of the ferrocene 2 bearing the podand dipeptide chains (-d-Ala-d-Pro-OEt) revealed two C2-symmetric intramolecular hydrogen bondings between CO (Ala) and NH (another Ala) of each podand dipeptide
chain to induce the chirality-organized structure. The molecular structures of the ferrocene 1 composed of the
podand l-dipeptide chains (-l-Ala-l-Pro-OEt) and 2 are in a good mirror image relationship, indicating that
they are conformational enantiomers. An opposite helically ordered molecular arrangement was formed in the
crystal packing of 2 as compared with 1. The ferrocene 2 exhibited induced circular dichroism (CD), which
appeared at the absorbance of the ferrocene moiety. The mirror image of the CD signals between 1 and 2 was
observed, suggesting that the chirality-organized structure via intramolecular hydrogen bondings is present
even in solution. The ferrocene 4 bearing the podand dipeptide chains (-Gly-l-Leu-OEt) also showed an ordered
structure in the crystal based on two intramolecular hydrogen bondings between CO (Gly) and NH (another
Gly) of each podand dipeptide chain, together with intermolecular hydrogen bondings between CO adjacent
to the ferrocene unit and NH (neighboring Leu) to create the highly organized self-assembly. A different
self-assembly was observed in the crystal of the ferrocene 5 composed of the podand dipeptide chains (-Gly-l-Phe-OEt), wherein each molecule is bonded to two neighboring molecules through two pairs of symmetrical
intermolecular hydrogen bonds to form a 14-membered intermolecularly hydrogen-bonded ring. These ordered
structures based on the intramolecular hydrogen bondings in the solution state are also confirmed by 1H NMR
and FT-IR.