Ferrocenes with Simple Chiral Substituents: an In-Depth Theoretical and Experimental VCD and ECD Study

Circular dichroism spectra in the IR range (VCD = vibrational circular dichroism) and in the UV range(ECD = electronic circular dichroism) have been recorded for both enantiomers of simple monosubstitutedferrocenes containing chiral pendants: 1-acetoxyethylferrocene, 1, 1-methoxyethylferrocene,2, and 1-hydroxyethylferrocene, 3; the related disubstituted 1,1'-bis(1-hydroxyethyl)ferrocene, 4, wasalso considered. These two types of spectra, with the support of DFT calculations, concur tounequivocally confirm the absolute configuration for 1-4. In particular, our computational results pointout the clear advantage of using an anharmonic oscillator model for the interpretation of VCD spectraof chiral ferrocenes. Interesting conformational properties are either confirmed or established by thetechnique, like the eclipsed conformation of the two cyclopentadienyl rings and an intra-molecularinteraction involving the OH for 3. For 4, NMR, VCD and IR spectra are compatible with dimer formationand in this case a distorted conformation is predicted. Of utmost importance for the absoluteconfiguration assignment in mono-substituted ferrocenes, we were able to identify a diagnostic VCDband at 950 cm-1 and a (low intensity) ECD band that clearly indicate the absolute configuration of thewhole series.