Chiral monolithic materials

Obviously, the monolithic material may serve its purpose only if provided with a suitable surface chemistry, which depends on the desired application. For example, hydrophobic moieties are required for reversed phase chromatography, ionizable groups must be present for separation in the ion-exchange mode, and chiral functionalities are the prerequisite for enantioselective separations. Several methods can be used to prepare monolithic columns with a wide variety of surface chemistries. 

Figure 13.24 Separation of racemic DNZ-leucine on polymeric monolithic material. Conditions polymerization mixture, chiral monomer 76 8wt%, 2-hydroxyethyl methacrylate 24wt% ethylene dimethacrylate 8wt%, 1-dodecanol 45 wt%, and cyclohexanol 15wt% UV-initiated polymerization for 16 h at room temperature ...

More immobilized cellulose and amylose phases are currently tmder development at Daicel. Daicel is promoting the concept of a CSP library of more than 100 stationary phases that are not currently commercially available, which can be screened for enantiomer separations. Astec has recently presented an improved linking of antibiotic phases (Chirobiotic). As yet, no chiral monolithic stationary phases are commercially available, although one can assume that various companies are currently endeavoring to change the surface of monolithic material with chiral modifiers. 

The main feature of (meth)acrylate-based support materials is the broad diversity of monomers that is commercially available and that can thus can be used for the fabrication of monoliths. The resulting (meth)acrylate monoliths consequently cover a wide spectrum of surface chemistries and properties. The scope of monomers includes hydrophobic, hydrophilic, ionizable, chiral, as well as reactive (meth)acrylate building blocks [53]—the most popular being mixtures of butyl methacrylate and ethylene dimethacrylate (BMA/EDMA) or glycidyl methacrylate and ethylene dimethacrylate (GMA/EDMA) as cross-linker. 

A final example of chiral additions to aldehydes used monolithic catalyst 48 (Scheme 4.76), using a chiral moiety derived from a waste material from the synthesis... 

One-step method for the preparation of highly enantioselective monolithic columns for CEC has been developed by Frechet et al. The chiral polymer bed of defined pore distribution and chiral ligand concentration has been synthesized within the confines of untreated fused silica capillaries using a mixture of O-[2-(methacryloyloxy)ethylcarbamoyl]-10,ll-dihydroquinidine 76, ethylene dimethacrylate (EDMA), and glycidyl methacrylate or 2-hydroxyethyl methacrylate (HEMA) in the mixture of cyclohexanol and 1-dodecanol as porogenic solvents. Under optimized synthetic and chromatographic conditions, these materials with the desired characteristics were demonstrated to efficiently separate a model racemic DNZ-Leu, Figure 13.24 [146],... 

As in GC, the uses of LC for the separation of chiral species have significantly increased. Column materials now include chiral phases that may, for example, be based on monolithic silica columns with chemically bonded beta-cyclodextrin, teicoplanin, or cellulose tris(3,5-dimethylphenylcarbamate). Hydro-phobic amino compounds have been separated by LC using a crown ether dynamically coated chiral stationary phase. 

Together with a chiral selector, an innert carrier contributes significantly to the performance of a CSP. Although all commercially available chiral columns currently are prepared based on totally porous particulate silica, other organic [15,35—37] and inorganic materials [38], as well monolithic [35,39—41] and core-shell silica [42] materials have been examined as a carrier for a chiral selector [39] and as capillary columns... 

However, not all problems associated with the in situ attachment of a chiral selector to monolithic silica as well as material characterization and optimization are solved currently [37]. It will certainly take some time before competitive chiral columns based on monolithic silica will be commercialized. 

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