New Stereoselective GC Phases: Immobilized Chiral Polysiloxanes with (S)-(-)-t-Leucine Derivatives as Selectors

New chiral polysiloxanes have been prepared as stationary phases for gas chromatography, with (S)-(–)-t-leucine-t-butylamide, (S)-(–)-t-leucine-(S)-(–)-1-phenylethylamide, (S)-(–)-t-leucine-(S)-(–)-1-(α-naphthyl)ethylamide, (S)-(–)-t-leucine-(R)-( + )-1-phenylethylamide, and (S)-(–)-t-leucine-(R)-( + )-1-(α-naphthyl)ethylamide as selectors. Immobilization is achieved by radical-induced cross-linking with 1,3,5,7-tetravinyl-1,3,5,7-tetramethylcyclotetrasiloxane (V4) and dicumyl peroxide (DCUP) as cross-linking reagents and cured at 170°C. Under these conditions, racemization of (S)-(–)-t-leucine is less than 4.5% (R) for 1 h curing, while for polysiloxanes with the conventional (S)-(–)-valine selectors about 20% of R-enantiomers are formed by racemization. In the presence of 5% (w/w) V4 and 6% of DCUP with regard to the phases, 70–80% immobilization is achieved; without V4, the degree of immobilization is about 50% for both the (S)-(–)-t-leucine and (S)-(–)-valine selectors. As the size of the amide moieties of the selectors increases from t-butyl to 1-(α-naphthyl)ethyl, the degree of immobilization decreases. If the curing time is prolonged to 2 h, the extent of racemization increases. The selectivity factors achieved for amino acid enantiomers and similar pharmaceuticals are generally higher than those obtained with the corresponding non-immobilized Chirasil-Val phases.