ENANTIOMERIC SEPARATION OF CHIRAL SULFOXIDES - SCREENING OF CELLULOSE-BASED SORBENTS WITH PARTICULAR REFERENCE TO CELLULOSE TRIBENZOATE

The possibility of using cellulose tribenzoate as a chiral selector for the resolution of chiral sulphoxides was investigated on analytical and preparative scales. Ten racemic sulphoxides were separated on eight different cellulose-derivatized chiral stationary phases. As a result from this screening it turned out that Chiralcel-OB using cellulose tribenzoate (CTB) as chiral selector was the most suitable for resolving the enantiomers of sulphoxides. To obtain a better understanding of the separation mechanism, the temperature dependence of enantioselectivity (alpha) was measured for Chiralcel-OB and Chiralcel-OD to determine the Gibbs-Helmholtz parameters Delta((R,S))Delta H-O and Delta ((R,S))Delta S-O In addition, a partial least-squares analysis was performed to find a correlation between molecule-independent parameters and alpha. Both approaches indicated that steric hindrance seems to be the main reason for chiral discrimination. A surprising result was observed for the enantiomeric separation of p-hydroxyphenyl methyl sulphoxide, which significantly improved at higher temperatures. In a further study, the developed column materials and strategies, using CTB as chiral selector, were compared. It turned out that microcrystalline CTB I and CTB beads exhibit enantioselectivity equal to that of chiracle-OB.