Optimization of Enantiomer Separations in HPLC

Optimization of Enantiomer Separations in HPLC 441 2.6.4.1 Cellulose and Amylose Derivatives... 

Optimization of Enantiomer Separations in HPLC 445 Table 6. Systematic method development protocol for chiral tartrate phases. 

Another important issue that must be considered in the development of CSPs for preparative separations is the solubility of enantiomers in the mobile phase. For example, the mixtures of hexane and polar solvents such as tetrahydrofuran, ethyl acetate, and 2-propanol typically used for normal-phase HPLC may not dissolve enough compound to overload the column. Since the selectivity of chiral recognition is strongly mobile phase-dependent, the development and optimization of the selector must be carried out in such a solvent that is well suited for the analytes. In contrast to analytical separations, separations on process scale do not require selectivity for a broad variety of racemates, since the unit often separates only a unique mixture of enantiomers. Therefore, a very high key-and-lock type selectivity, well known in the recognition of biosystems, would be most advantageous for the separation of a specific pair of enantiomers in large-scale production. 

Almost all enantiomer separations show a lower separation factor at higher temperatures when the temperature is increased further, enantiomer separation can be completely suppressed. This means that the entropy term in the above equation approaches the value of the enthalpy term with rising temperature, and that at a certain temperature entropy and enthalpy cancel one another out (In a = 0). For this reason, temperature increase and gradients can be neglected for the optimization of enantioselective HPLC. 

Figure 8.5 depicts the different chiral azaferrocene derivatives synthesized and used in catalysis. The first member in the series bears a TES protected hydroxymethyl group and was prepared in optically pure form in three steps, including a preparative HPLC enantiomer separation (Scheme 8.2) [11]. Optimization studies later showed the greater stereoinduction brought about by bulkier sUyl groups, with the TBS-substituted derivative achieving 77% ee for the addition reaction of methanol to methyl phenyl ketene, whereas the TES derivative shows only 28%...

---