The self-disproportionation of the enantiomers (SDE) of methyl n-pentyl sulfoxide via achiral, gravity-driven column chromatography: a case study

Abstract

This work explores the self-disproportionation of enantiomers (SDE) of chiral sulfoxides via achiral, gravity-driven column chromatography using methyl n-pentyl sulfoxide as a case study. A major finding of this work is the remarkable persistence and high magnitude of the SDE for the analyte. Thus, it is the first case where SDE is observed even in the presence of MeOH in the mobile phase. The study demonstrated the practical preparation, in line with theory, of enantiomerically pure (>99.9% ee) samples of methyl n-pentyl sulfoxide starting from a sample of only modest ee (<35%). Remarkably, it was found that the order of elution was inverted, i.e. enantiomerically depleted fractions preceded later eluting enantiomerically enriched ones, when the stationary phase was changed from silica gel to aluminum oxide. To the best of our knowledge, this is the first occurrence of inverted SDE behavior due solely to a change in the stationary phase. Aberrant SDE behavior was observed in that the ee did not always fall continuously during the progression of the chromatography, and this was attributed to the complexity of the system at hand which cannot be described in simple terms such as the formation only of homo- and heterochiral dimers based on a single interaction. The results nevertheless suggest that all compounds with a chiral sulfoxide moiety in their structure are likely to exhibit the SDE phenomenon and thus this work constitutes the first example of SDE predictability. Moreover, it could well be that optical purification based on the SDE phenomenon is a simple, convenient, and inexpensive method for the optical purification of this class of compounds with a high degree of proficiency.