As part of an ongoing program investigating new reactions of allylsilanes, we observed that epoxysilanes, prepared by epoxidation of the corresponding allylsilane, are cleanly converted to the corresponding fluorohydrin upon treatment with triethylamine trihydrofluoride (HF•Et3N; Table ). Other HF sources such as Olah's reagent (HF•pyridine) resulted in significant decomposition. However, the addition of commercially available HF•Et3N (ca. 37% HF) to a solution of the epoxysilane in dichloromethane (DCM) at room temperature produced the 2-fluoro-3-silylpropan-1-ols in uniformly high yield and with complete regioselectivity. In a typical experiment, the allylsilane was epoxidized using in situ-prepared dimethyldioxirane. This generally gave a sufficiently pure epoxide that could be taken directly into the epoxide opening with HF•Et3N. An exception was allyltrimethylsilane, where the resulting epoxide proved somewhat volatile, making its isolation challenging. Instead, a one-pot epoxidation/epoxide opening was performed by treatment of allyltrimethylsilane with mCPBA and HF•Et3N in DCM. Under these conditions, the fluorohydrin was isolated in 65% yield (Entry 1) containing small amounts (~10%) of 1-hydroxy-3-(trimethylsilyl)propan-2-yl 3chlorobenzoate, resulting from opening of the epoxide by mCPBA-derived 3-chlorobenzoic acid. This one-pot procedure also proved effective for less volatile allylsilanes, and in general gave comparable yields to the two-step procedure (e.g. Entries 4 and 5). The reaction was notably slower with the phenyl-substituted epoxysilanes (Entries 2, 4-7), where greater phenyl substitution resulted in longer reaction times. We rationalize this reduced rate as being due to the electron-withdrawing nature of the phenyl groups, thereby reducing the β-silyl effect 18 in stabilizing the proposed cationic intermediate. There are a few noteworthy aspects of this transformation. First, at the outset we were concerned about competing formation of allyl alcohol and a corresponding fluorosilane, driven by formation of a stable Si-F bond (Scheme 2). By 1 H NMR analysis of the crude product mixtures, however, very little allyl alcohol was produced from any of the silanes tested. Other minor byproducts observed were small amounts of the corresponding diol 1 and aldehyde 2, the latter presumably via a Meinwald-type rearrangement. Second, this fluoride opening of epoxysilanes occurs at room temperature. Other reports of fluorohydrin synthesis by epoxide opening with HF•Et3N generally require heating in order to achieve high conversion. We attribute the rate acceleration, along with control of regioselectivity, to the beta-silyl effect, similar to rate enhancements observed for other reactions involving cationic intermediates with a silicon group at the β-position. Scheme 2. Explanation of regioselectivity observed and structures of byproducts.