Fluoride-Promoted Hydroxyl-Directed Hydrosilylations of Epoxides and Alkenes

Silicon-oxygen heterocycles are synthetic intermediates that have been used for over 50 years. These compounds are particularly useful when combined with stereospecific desilylation reactions giving access to stereospecific 1,3-diols. To make these heterocycles, precious metal catalysts such as platinum, ruthenium, rhodium, or iridium are typically used. Here we have developed a one-pot reaction using tetrabutylammonium difluorotriphenylsilicate (TBAT) and diphenylsilane to give access to these systems without the use of precious metal catalysts. Subjecting β- or γ-hydroxy alkenes to these conditions gives oxasilolane and oxasilinane systems (5- and 6-memebered silicon oxygen heterocycles), while β-hydroxy epoxides afforded dioxasilinane systems (6-memebered heterocycles with a silicon and two oxygens). These systems are made with syn-selectivity (up to 5.2:1.0) with yields as high as 98% while being highly tolerant of substitutions. This reaction takes place via an anionic intermediate which requires resonance stabilization, but many different conjugated systems are tolerated. Tomao-Flemming oxidations of these systems allow for easy access to syn-1,3-diols. A two-step method was also developed for direct access to 1,3-diols from β-hydroxy epoxides. This reaction first forms a silyl ether using Grubbs 1st generation catalyst and diphenylsilane followed by activation by tetrabutylammonium fluoride (TBAF) or tetrabutylammonium difluorotriphenylsilicate (TBAT) affording 1,3-diols in moderate yields.