Gas chromatographic determination of the interconversion energy barrier for dialkyl 2,3-pentadienedioate enantiomers

The enantiomers of dialkyl 2,3-pentadienedioate undergo interconversion during gas chromatographic separation on chiral stationary phases. In this paper the on-column apparent interconversion kinetic and thermodynamic activation data were determined for dimethyl, diethyl, propylbutyl and dibutyl 2,3-pentadienedioate enantiomers by gas chromatographic separation of the racemic mixtures on a capillary column containing a polydimethylsiloxane stationary phase coupled to 2,3-di-O-methyl-6-O-tertbutyldimethylsilyl-beta-cyclodextrin. A deconvolution method was used to determine the individual enantiomer peak areas and retention times that are needed to calculate the interconversion rate constants and the energy barriers. The apparent rate constants and interconversion energy barriers decrease slightly with an increase in the alkyl chain length of the dialkyl 2,3-pentadienedioate esters. The optimum conformation of the dialkyl 2,3-pentadienedioate molecules, their separation selectivity factors and apparent interconversion enthalpy and entropy data changes with the alkyl chain length. The dependence of the apparent interconversion energy barrier (deltaG(app)(a-->b), deltaG(app)(b-->a)) on temperature was used to determine the apparent activation enthalpy (deltaH(app)(a-->b), deltaH(app)(b-->a)) and apparent entropy (deltaS(app)(a-->b), deltaS(app)(a-->b)) (where a denotes the first and b second eluted enantiomer). The comparison of the activation enthalpy and entropy (deltaS(app)(a-->b), deltaS(app)(a-->b)) indicated that the interconversion of dialkyl 2,3-pentadienedioate enantiomers on the HP-5+Chiraldex B-DM column series is an entropy driven process at 160 degrees C. Data obtained for dimethyl 2,3-pentadienedioate enantiomers on the HP-5+Chiraldex B-DM column series at 120 degrees C (deltaG(app)(a-->b) = 123.3 and deltaG(app)(b-->a) = 124.4 kJ mol(-1)) corresponds (at the 95% confidence interval) with the value of deltaG(#) = 128+/-1 kJ mol(-1) found at this temperature by gas chromatography using a two-dimensional stop flow technique on an empty capillary column [V. Schurig, F. Keller, S. Reich, M. Fluck, Tetrahedron: Asymmetry 8 (1997) 3475].