Chemoselective halogenation of 2-hydroperfluoroalkyl aldehydes

2-Hydroaldehydes, RfCH(R)CHO, where Rf = CF3, C2F5, n-C3F7 and R = CF3, C2F5, n-C3F7, Ph, H, were prepared via acid hydrolysis of the corresponding vinyl ethers, RfC(R) = CHOCH3, which can be readily prepared by reaction of [ Ph 3 P + C ¯ HOCH 3 ] with the corresponding ketone. The 2-hydroaldehydes can be chemoselectively converted to the acyl halide, RfCH(R)C(O)X (X = Cl, Br), via free-radical halogenation. The perfluoroalkyl group deactivates the 2-position toward radical abstraction of the 2-hydrogen, and halogenation occurs exclusively at the formyl hydrogen. However, halogenations of the 2-hydroaldehydes in glacial acetic acid chemoselectively gives the 2-haloaldehydes, RfCX(R)CHO, X = Cl, Br. Hydrolysis of the 2-hydroperfluoroacyl halides provides a useful route to 2-hydroperfluoroalkyl branched carboxylic acids, useful ketene precursors. This route avoids the use of toxic fluoroolefins, such as perfluoroisobutylene.