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N
O
N
N
N
O
y
NN
Si O
2
O
O
Si
N
H
N
H
N
z
NN
x
x+y+z<7
(OH)
14
SCHEME 11.4
“Click”-immobilization of
-CD on silica support with multiple linkages.
(Adapted from ref. 13.)
which has drawn extensive interests. RAM is usually composed of two layers, the
inner layer is accessible to small molecules and immobilized with chiral functional
groups for chiral separation; the external layer is biocompatible but non-adsorptive
to biomolecules. In this way, the material can be stable to biological samples, adsorb
and separate small molecules, and exclude large biomolecules such as proteins. Due
to its wonderful chiral discrimination capability,
-CD has always been selected to
be immobilized on the inner layer of the RAM.
Using “click” HDC reaction, Wang et al. developed a novel RAM and applied it in
chiral separation [15]. At first, they grafted two layers of poly(glycidyl methacrylate)
(pGMA) on the surface of porous silica gel.
-CDs were then immobilized as the
inner layers by a reaction between the oxirane rings of GMA and the hydroxyl
groups of CDs to give a CD-sil-2 material. As can be expected, the multiple
hydroxyl reaction sites led to the uncertainty of the linker position of
-CD [9].
To address this issue, they turned to “click” chemistry. After immobilization of 3-(2-
bromoisobutyryl)propyltriethoxysilane on silica gel, atom transfer radical polymer-
ization (ATRP) was carried out to introduce the alkyne-functionalized inner surface.
The “click” conjugation with mono-(6-azido-6-deoxy)-
-CD and the subsequent
ATRP gave the CD-click-RAM, which showed better enantioseparation efficiency
than CD-sil-2 (Scheme 11.5). The authors attributed the better separation results to
the more homogeneous linkage of CD with inner support and the stronger hydrogen
bonds of the analytes with triazole.
Guerrouache et al. developed a
-CD “click”-grafted macroporous organic
polymer monolith used as stationary phases in capillary electrochromatography
and nanoliquid chromatography [16]. The photo-initiated copolymerization of
N-acryloxysuccinimide and ethylene dimethacrylate in toluene using azobisisobu-
tyronitrile (AIBN) as initiator produced a macroporous organic polymer mono-
lith, which was alkyne functionalized by nucleophilic substitution of succinimide
moieties with propargylamine. Through the “click” HDC reaction with the mono-(6-
azido-6-deoxy)-
-CD, the polymer monolith was successfully grafted with
-CD,
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