Fluorohydrin epoxide ring opening

Epoxides are regio- and stereoselectively transformed into fluorohydrins by silicon tetrafluoride m the presence of a Lewis base, such as diisopropyleth-ylamme and, m certain instances, water or tetrabutylammonium fluoride The reactions proceed under very mild conditions (0 to 20 C in 1,2-diohloroethane or diethyl ether) and are highly chemoselective alkenes, ethers, long-chain internal oxiranes, and carbon-silicon bonds remain intact The stereochemical outcome of the epoxide ring opening with silicon tetrafluoride depends on an additive used, without addition of water or a quaternary ammonium fluoride, as fluorohydrins are formed, whereas m the presence of these additives, only anti opening leading to trans isomers is observed [17, 18] (Table 2)... 

Epoxide Ring Opening. Recently, Shimizu and Nakahara reported the epoxide ring opening (eq 1) to generate fluorohydrin (2). Using only 47% HF provided 2 in low chemical yield (34%) however, the addition of ammonium hexafluorosilicate (AHFS) increased the yield to 46%, and, further, the addition of CsF also increased the yield, up to 67%. 

Anhydrous hydrogen fluoride itself is not recommended for ring-opening fluorinations of simple epoxides as it causes polymerization, rearrangement and other side reactions. With simple epoxides, low to moderate yields of fluorohydrins can be obtained when anhydrous hydrogen fluoride is diluted with organic solvents. 

Recently, silicon tetrafluoride [207] and tetrabutylphosphonium fluoride [208] have been used to prepare fluorohydrins from epoxides. Generation of a superacid is required for the ring opening of a perfluorinated oxirane [209] (Figure 3.42). 

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