Enhancement of optical activity by fractional sublimation. An alternative to fractional crystallization and a warning

Self-disproportionation of enantiomers of isopropyl 3,3,3-(trifluoro)lactate via sublimation: Sublimation rates vs. enantiomeric composition

“…In summary, we believe that the data presented here and the recent literature results [20] clearly suggest a glaring gap in our knowledge of SDE via sublimation, which is clearly not as simple as one may assume [9][10][11][12][14][15][16][17][18][19]. For instance, besides the basic thermodynamic factors as energy/stability of crystals, kinetic features, such a minute nuance as shape of crystals [13], and therefore the sublimation surface, should be expected to have a dramatic effect on the difference in the sublimation rates of racemic and enantiomerically pure forms.…”

“…Thus, based on some theoretic considerations and a single set of experimental data available for sublimation of Mandelic acid [11,12,14], the authors suggest three sublimation scenarios with a general prediction that ''initial sublimate always possesses the (enantiomeric) composition of the vapor eutectic'' [15,16]. As it becomes more and more evident, the physico-chemical meaning of ''enantiomeric composition of the vapor eutectic'' has not been clearly defined and awaits an appropriate discussion and study.…”

Attempts to explain the self-disproportionation observed in the partial sublimation of enantiomerically enriched carboxylic acids

“…-In the inverse case when racemic compound whose enantiomeric purity is greater than that of the eutectic, that which first sublimes is less pure than the initial mixture, while the enantiomeric purity of the residue rises and tends toward the pure enantiomer'' It was added ''the initial sublimate possesses the composition of the vapor eutectic''. As before for Garin et al [3], Jacques et al [4] considered that the usual ''sublimation and condensation'' follows the same rules that those deduced from the analysis of the gas phase at the equilibrium.…”

“…Seven years later, Kwart and Hoster studied the sublimation of a-ethylbenzylphenyl sulfide and observed similar enantiomeric enrichments of partial sublimates [2]. In 1977, Garin et al performed a quite complete study on three carboxylic acids, the mandelic, camphoric and bicyclo-[3.1.0]-hex-2-en-6-endo-carboxylic acids [3]. For the first time, increases and decreases of the ee of the sublimate in fractional sublimations were observed depending of the starting material.…”

“…As recalled in a footnote of one of these articles [9], various explanations have been reported in the literature to explain the ee of sublimates: ''the eutectic composition is predicted to preferentially sublime regardless of the initial composition'' [4], ''sublimation process for both the racemate and the enantiomer was found to be enthalpy driven.'' [11] ''the modification with the lowest melting point should exhibit the greater volatility and should sublime preferentially'' [3,4]. That evidences the difficulties to explain many results.…”