Evidence for a Temperature Dependent Reversal of the Enantioselectivity in Complexation Gas Chromatography on Chiral Phases

Abstract

N-TFA-tert-butyl amides of a-amino acids can intercalate between a diamide solvent in either a parallel (C) or an antiparallel (D) fashion (Scheme 1). The resulting hydrogen bonded structures and the orientation of the asymmetric centers towards each other are not the same. For each of these two modes of intercalation there should be a corresponding In a vs. 1/T plot analogous, respectively, to the curves with a > I and a < I of Figure 2. Superimposi-

Contributions to the theory and practice of the chromatographic separation of enantiomers

“…The existence of an isoenantioselective temperature T isoenant was predicted 39 and proved experimentally for isopropyloxirane and a derivative of 4a (cf. Fig.…”

Salient Features of Enantioselective Gas Chromatography: The Enantiomeric Differentiation of Chiral Inhalation Anesthetics as a Representative Methodological Case in Point

“…Below T iso enantioseparation is governed by the predominant enthalpic contribution to enantiorecognition, whereas above T iso it is governed by the predominant entropic contribution (ΔS D < ΔS L ) to enantiorecognition with the stronger bonded enantiomer D (ÀΔH D > ÀΔH L ) bizzarrely eluted as the first peak. Whereas the sign of the enantioselectivity changes at T iso , the association constants K The existence of an isoenantioselective temperature T iso in enantioselective GC has been observed independently in hydrogen-bonding equilibria [111,112] and in metal complexation equilibria [113,114] whereby the intriguing peak reversal for the enantiomers below and above T iso was clearly evident in the gas-chromatograms. A report of a peak reversal for the enantiomers of methyl lactate on a modified cyclodextrin selector (Lipodex E) [115] could not be ascertained [112] while a temperature-induced reversal of the elution order has been observed for the enantiomers of N-trifluoroacetyl-α-amino acid ethyl esters on the selector Chirasil-Dex [112].…”

High‐performance liquid chromatographic chiral separation of β2‐homoamino acids

“…24,25 It additionally known that there are both achiral and chiral contributions to retention that can vary with a wide variety of experimental parameters. [26][27][28][29] Accordingly, the column temperature has often been optimized in enantioselective HPLC separations. [30][31][32][33][34][35][36] The dependence of the retention of an analyte on the temperature can be expressed by the van't Hoff equation, which may be interpreted in terms of mechanistic aspects of chiral recognition:…”

“…[26][27][28][29][30][31][32][33][34][35][36] After the immobilization of selector molecules on CSPs, they exhibit enantioselective (s) and nonenantioselective (ns) adsorption sites, both of which can contribute to the overall retention factors (k 0 app ) of the enantiomers:…”