Page 192 - Chiral Separation Techniques

170 6 Enantiomer Separations using Designed Imprinted Chiral Phases

rational chirality inherent in the pendant groups of the polymer are to some extent
themselves chiral selectors, and the effect of imprinting is merely to enhance the
selectivity. A good example of this was shown in the imprinting of N-benzyl-L-valine
as a bidentate ligand to a styrene-based chiral cobalt complex (18) [69]. The strong
enantioselectivity of the imprinted polymer should here be viewed with respect to
the enantioselectivity of the control polymer.

Thus, enhanced separations can be obtained using chiral selectors with configu-
rational chirality in combination with molecular imprinting. What about selectors
with conformational chirality ? Can chirality be induced by molecular imprinting ?
This concept was elegantly demonstrated by Welch using a brush-type stationary
phases containing a slowly interconverting (in the order of a day) racemic atropiso-
mer (19) as imprintable selector (Fig. 6-9) [70]. Leaving the selector in contact with
an enantiomerically pure template molecule (20) for more than 2 weeks led to induc-
tion of the most stable selector selectand complex. After washing out the selectand,
the selector could be used to separate the racemate of 20 with similar separation fac-
tors as obtained using the reciprocal phase. However due to interconversion, the CSP
racemized over a period of 2–3 days, a period that possibly can be extended by stor-
ing the CSP at low temperatures. Also mentioned was the interesting possibility of
using a selection of slowly interconverting selectors to achieve a broadly applicable
system for atropisomer-based imprinting.
Alternative approaches to imprint peptides via strong monomer template associa-
tion have recently been reported, although no results of the chromatographic appli-
cation of these phases have been shown. Strong complexation inducing a β-sheet
conformation was possible using a designed functional monomer (21) [71]. Peptides