Epoxysilanes as substrates in regio- and stereo-specific synthesis of silylated γ-hydroxyphosphane oxides, precursors of stereodefined allylphosphane oxides

Epoxysilanes experienced trans stereospecific α- or β-cleavage by the lithium derivative of methyldiphenylphosphane oxide leading to different compounds. The behaviour of the epoxysilanes towards this nucleophilic reagent depends on the nature of the silyl group, the position of the substituents and the configuration of the epoxysilane. Unsubstituted and cis β-substituted dimethylphenylsilylepoxides underwent α-nucleophilic attack of lithium methyldiphenylphosphane oxide giving stereodefined γ-phosphanyl-β-silylalcohols 2. Nevertheless, in the same conditions the intermediate β-hydroxysilane, formed by α-opening from trans β-substituted dimethylphenylsilylepoxides, experienced Peterson elimination to give stereodefined allylphosphane oxides 3. The steric requirements for the nucleophilic attack determined the regiochemistry of the reaction. α-Substituted phenyldimethylsilyl and tert-butyldiphenylsilylepoxides were exclusively attacked at the β-position affording γ-hydroxy-γ-silylphosphane oxides 4 and γ-phosphanyl silyl ethers 5, resulting from Brook rearrangement.