Development of Novel Catalyses Based on Base-Activated Silicates

点击翻译按钮获取中文摘要

Silicon atom expands its coordination sphere to give relatively stable penta- or hexa-coordinated compound, or "hypervalent silicate" due to its vacant 3d-orbital. Since these extracoordinated species are powerful intermediates with abundant reactivities, remarkable developments have been made on various organic reactions based on this unique mechanism. We investigated the base-promoted asymmetric catalyses wherein bases coordinated with silicon atoms to form hypervalent silicates. We found that chiral lithium binaphtholate catalyzed the aldol reaction of trimethoxysilyl enol ethers, which realized the construction of quaternary carbon center using tetra-substituted silyl enol ethers. With this protocol, the highest synlanti selectivity was obtained in the construction of quaternary carbon center using asymmetric aldol reaction. We also found that chiral phosphine oxides catalyzed the aldol reaction of trichlorosilyl enol ethers, wherein complete stereochemical correlation is observed between the diastereomeric ratio (synlanti) of the products and the geometries (E/Z) of the trichlorosilyl enol ethers. This methodology was extended to the direct aldol reaction, the aldol reaction between two carbonyl compounds, not using silyl enol ether as an aldol donor. Phosphine oxides coordinated with tetrachlorosilane to afford more acids complexes than tetrachlorosilane itself, which were efficiently utilized in Lewis acid-catalyzed asymmetric reactions. With this concept, catalytic amounts of chiral phosphine oxides activated tetrachlorosilane to promote the Abramov-type phosphonylations of aldehyde with trialkyl affording the corresponding hydroxyphophonates, which exhibited the first asymmetric version of this type of reaction.