Self-Disproportionation of Enantiomers via Sublimation; New and Truly Green Dimension in Optical Purification

Abstract

This Review article presents all available in the literature data on optical purification of enantiomerically enriched organic compounds via simple procedure of sublimation. The material is arranged in a chronological order from early publications to current research. Relatively small amount of the results reported in the literature suggest that this area of research is still in its infancy it terms of commonly accepted experimental procedures, generalizations, interpretation or prediction of the stereochemical outcome of sublimation. On the other hand, the available data clearly show the potential of sublimation as most economical and greener alternative to the conventional optical purification techniques such as chromatography on chiral stationary phase or fractional crystallization. Another important goal of this Review is to bring to the attention of scientific community a warning that special care should always be taken in evaluation of the stereochemical outcome of enantioselective reactions. Thus, due to the always different sublimation rates of racemic and enantiomerically pure crystals, such a routine procedure as drying reaction products in vacuum or even removal of solvents under reduced pressure can significantly alter the true enantiomeric composition (% ee) of a compound under study and lead to erroneous results. Keywords: Sublimation, chiral purification, self-disproportionation of enantiomers, non-linear effects, green methodology, optical purification, chromatography, fractional crystallization, enantioselective resctions, biocatalysis