Faculty of Chemistry and Mineralogy, Institute of Inorganic Chemistry, Leipzig University, Johannisallee 29, 04103, Leipzig, Germany.
Department of Chemistry, Cambridge University, Lensfield Road, Cambridge, CB2 1EW, UK.
Chemistry. 2020 May 7;26(26):5765-5769. doi: 10.1002/chem.202000218. Epub 2020 Apr 1.
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.
三级膦通常在中等条件下能够经受反转。在这项工作中，报道了三种手性二茂铁基膦的显著外消旋化过程。作为高度对映富集的化合物，在柱出口收集时，它们在几分钟内经历了极大的外消旋化。最初，在观察到这些配体的超高对映过量（在所有情况下均超过 95%ee）在其相应的钌（II）配合物中几乎损失后，引起了对这种不寻常反转行为的关注。相继排除了可能的外消旋化原因，发现这些手性二茂铁基膦在室温下几分钟内就会发生显著的酸催化外消旋化。在使用常规硅胶或氧化铝的标准柱层析过程中，主要观察到这种过程，但通过添加某些酸也可以故意触发该过程。因此，在对映体纯化为 P-手性膦时，其立体化学的保存本身并不总是能得到保证，因为柱层析是纯化此类化合物最常用的技术。
