Chirasil metals

Asymmetric Diels-Alder reactions have also been achieved in the presence of poly(ethylene glycol)-supported chiral imidazohdin-4-one [113] and copper-loaded silica-grafted bis(oxazolines) [114]. Polymer-bound, camphor-based polysiloxane-fixed metal 1,3-diketonates (chirasil-metals) (37) have proven to catalyze the hetero Diels-Alder reaction of benzaldehyde and Danishefsky s diene. Best catalysts were obtained when oxovanadium(lV) and europium(III) where employed as coordinating metals. Despite excellent chemical yields the resulting pyran-4-ones were reported to be formed with only moderate stereoselectivity (Scheme 4.22). The polymeric catalysts are soluble in hexane and could be precipitated by addition of methanol. Interestingly, the polymeric oxovanadium(III)-catalysts invoke opposite enantioselectivities compared with their monomeric counterparts [115]. 

A limiting factor of complexation gas chromatography is the low temperature range (25-120°C). Therefore, improved thermostable polymeric stationary phases, e.g., Chirasil-Metal, in which the chiral metal chelates are chemically anchored to a polysiloxane backbone, have been prepared155 156. 

The Chirasil-Metal stationary phase can be thermally immobilized on a glass surface156, thus increasing the temperature stability up to 160 °C. The immobilized Chirasil-Metal stationary phase can also be used in the SFC mode using supercritical carbon dioxide as the mobile phase156. 

A limiting factor of Chirametal stationary phases is the low temperature range (25-120 °C). Improved thermo-stable polymeric stationary phases (Chirasil-Metal) (10) have also been prepared. Figure 3 shows the enantiomer separation of cyclic ethers in less than 20 s. 

The immobilized Chirasil-Metal stationary phase can also be used in supercritical fluid chromatography... 

Figure 10.4. Structures of immobilized poly(siloxane) chiral stationary phases containing (A) a cyclodextrin derivative (Chirasil-Dex) and (B) a metal complex (Chirasil-Nickel).

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