Evolution of enantioselectivity at the liquid gas interface

Unexpectedly, scalemic mixtures (>0% ee) show a significant discrimination during distillation. The most pronounced effects were observed for (S>R)-N-TFA-amino acid esters (valine, leucine, alanine), with differences of up to 20% ee in the fractions, as determined by GC on L- and D-Chirasil-Val. We define the kinetic enantioselectivity αk of the phase transition on the basis of the relative rates: αk = KR/KS where (KR = -Δ[R]/[R]/Δt) ≥ (kS = -Δ[S]/[S]/Δt). In contrast, a merely thermodynamic interpretation was ruled out by vapor pressure measurements: The boiling points of RS- and S-N-TFA-Val-OMe differ by no more than ΔT < 0.25 deg. C, whereas thermodynamic calculations would demand for ΔT > 7.5 deg. C. In the course of the distillation, a gradient of ee is built up along the column. The picture of two-dimensional homochiral domains at the liquid gas interface is corroborated by physicochemical studies in the neat liquid, including 1H- and 13C-NMR spectra, surface tensions and excess volumes of mixing of the enantiomers. Like in living systems, non-equilibrium effects driven by a flux of energy manifest themselves in transient local ordering of the system. (author). 1 ref., 3 figs. Abstract only