Asymmetric synthesis involving silicon.
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%)h aveb eenc arriedo ut by 13CN MR methodsi n the presenceo f 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%.