Asymmetric synthesis involving silicon

Xu, Youli (1996). Asymmetric synthesis involving silicon. PhD thesis The Open University.



Several different types of optically active, synthetically useful, silylated diols (17 pairs) have been prepared by asymmetric dihydroxylation of the corresponding allyl and vinylsilanes using Sharpless catalysts. Chiral analysis of these silyl diols was carried out by 1H NMR methods in the presence of Eu(hfc)3. Enantiomeric purity of some of these silyl diols is greater than 90% e.e.

Synthetically more useful, optically active trimethylsilylepoxides, trimethylsilyl amino alcohols and aziridines have been isolated by a multi-stage chirality transfer from their precursor diol involving no racemization. The main routes for these chirality transfers were via silylated cyclic sulphite or sulphate intermediates and via silylated cyclic ortho esters and halohydrin derivatives. The reactions of these silylated species can be very regioselective, such as the ring opening of epoxysilanes by azide ion, leading exclusively to a single regioisomer. Similarly, the deoxygenation of vicinal silyl diols has been observed without loss of the silyl group.

Chiral analysis of trimethylsilyl amino alcohols and aziridines (with enantiomeric excesses of up to 95%) have been carried out by 13C NMR methods in the presence of Eu(hfc)3.

Asymmetric epoxidation of allyl and vinylsilanes without polar groups have been investigated using manganese (III) salen complexes as a catalyst. A number of axial ligands of the salen complexes has been studied and some of these axial ligands were very effective to influence the reactivity of the catalyst, cis / trans ratio of the silyl epoxides and enantioselectivity of the epoxidation. Several different types of allyl and vinylsilanes have been epoxidized enantiomerically using this catalytic method. Enantiomeric excesses of epoxides were determined by Chiradex G-PN column. Some of these silyl epoxides had e. e. of greater than 95%.

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