Unusual Racemization of Tertiary P ‐Chiral Ferrocenyl Phosphines

Tertiary phosphines are generally known to withstand inversion under moderate conditions. In this work, a remarkable racemization process of three P‐chiral ferrocenyl phosphines is reported. Subjected to conventional column chromatography as highly enantioenriched compounds, they greatly experienced racemization when collected at the column outlet within minutes. Initially, attention was drawn to this unusual inversion behavior after observing that the superb enantiomeric excess of these ligands (>95 % ee in all cases) was almost lost in their corresponding ruthenium(II) complexes. Successively excluding possible racemization causes, these P‐chiral ferrocenyl phosphines were found to undergo a significant, acid‐catalyzed racemization process at room temperature within a few minutes. This process is mainly observed during standard column chromatography by using conventional silica or alumina, but can also be triggered deliberately by addition of certain acids. Therefore, the stereochemical preservation of P‐chiral phosphines during their purification may per se not always be guaranteed, since column chromatography is the most frequently used technique for purifying such types of compounds.
Enantiomeric excess vanished! Subjected to conventional column chromatography as highly enantioenriched compounds, P‐chiral ferrocenyl phosphines suffered dramatically from racemization when collected at the column outlet even within minutes. What has happened? The authors have thoroughly investigated this most frequently used technique for the purification of such compounds and found a remarkable process as the explanation.