Syntheses of substituted benzosiloles and siloles by diisobutylaluminium hydride-promoted cyclization of 1-silyl-2-(2-silylethynyl)benzenes and 1,4-disilylalk-3-en-1-ynes

An efficient method for preparing substituted benzosiloles and unsymmetrically substituted siloles by intramolecular C-Si bond formation has been developed. The reaction of 1-methoxysilyl-2-2-(trimethylsilyl)ethynyl]benzenes with 1.5 equiv of diisobutylaluminium hydride (DIBAL-H) gave benzosiloles in good to high yields. Similarly 4-methoxysilyl-1-silylalk-3-en-1-ynes were cyclized to multisubstituted siloles. Mechanistic study of this transformation uncovered that the methoxysilyl group was initially converted into the corresponding hydrosilyl group by the action of DIBAL-H, and that the resultant hydrosilanes underwent the DIBAL-H-promoted cyclization to benzosiloles. When 1-hydrosilyl-2-2-(trimethylsilyl)ethynyl]benzenes were used as substrates, a substoichiometric amount of DIBAL-H was enough for the cyclization. The DIBAL-H-promoted transformation could be applied to regio-defined synthesis of unsymmetrically substituted siloles from 4-hydrosilyl-1-silylalk-3-en-1-ynes. This pre-installation approach provides a straightforward access to multisubstituted siloles with complete regioselection.