Enantioselective stopped-flow multidimensional gas chromatography: Determination of the inversion barrier of 1-chloro-2,2-dimethylaziridine

Enantioselective stopped-flow multidimensional gas chromatography (stopped-flow MDGC) is a fast and simple technique to determine enantiomerization (inversion) barriers in the gas phase in a range of ΔG#gas(T)=70–200 kJmol−1. After complete gas-chromatographic separation of the enantiomers in the first column, gas phase enantiomerization of the heart-cut fraction of one single enantiomer is performed in the second (reactor) column at increased temperature and afterwards this fraction is separated into the enantiomers in the third column. From the observed de novo enantiomeric peak areas aj, the enantiomerization time t and the enantiomerization temperature T, the enantiomerization (inversion) barrier ΔG#gas(T) is determined and from temperature-dependent experiments, the activation enthalpy ΔH#gas and the activation entropy ΔS#gas are obtained. Enantiomerization studies on chiral 1-chloro-2,2-dimethylaziridine by stopped-flow MDGC yielded activation parameters of nitrogen inversion in the gas phase, i.e., ΔG#gas(353 K)=110.5±0.5 kJmol−1, ΔH#gas=71.0±3.8 kJmol−1 and ΔS#gas=−109±11 Jmol−1K−1. By the complementary method of dynamic gas chromatography (GC), the apparent enantiomerization (inversion) barrier of 1-chloro-2,2-dimethylaziridine in the gas–liquid biphase system was found ΔG#app(353 K)=108 kJmol−1. The values obtained by stopped-flow MDGC in the gas phase were used to calculate the activation parameters of nitrogen inversion of 1-chloro-2,2-dimethylaziridine in the liquid phase in the presence of the chiral selector Chirasil–nickel(II), i.e., ΔG#liq(353 K)=106.0±0.4 kJmol−1, ΔH#liq=68.3±1.4 kJmol−1 and ΔS#liq=−106±3.0 Jmol−1K−1.