Regioselective Fluorohydrin Synthesis from Allylsilanes

Allysilanes can be regioselectively transformed into the corresponding 3-silylfluorohydrin in good yield using a sequence of epoxidation followed by treatment with HF-Et3N with or without isolation of the intermediate epoxide. Various silicon-substitution is tolerated, resulting in a range of 2-fluoro-3-silylpropan-1ol products from this method. Whereas other fluorohydrin syntheses by epoxide opening using HF-Et3N generally require more forcing conditions (e.g. higher reaction temperature), opening of allylsilane-derived epoxides with this reagent occurs at room temperature. We attribute this rate acceleration along with the observed regioselectivity to a beta-silyl effect that stabilizes a proposed cationic intermediate. The use of enantioenriched epoxides produces similarly enantioenriched fluorohydrins suggestive of an SN2-type mechanism.