Electrochemical Syntheses of Organofluorine Compounds
Difluoromethylene compounds are one of the current synthetic targets because of their unique biological activity. Among many synthetic methods of difluoromethylene compounds, selective defluorination of trifluoromethyl compounds is promising due to easy availability of trifluoromethylated compounds. Question is how to create the difluoromethylene unit from trifluoromethyl moiety. Deprotonation from difluoromethyl group and dehalogenation from halodifluoromethyl group have been well established. However, there is very few successful demonofluorination from trifluoromethyl group. Bond breaking of C-F bond is not easy due to the large bond energy, however, the bond breaking does easily occur when CF3 group is attached to π-electron system as shown 1, where the added electron would push out a fluoride ion. One of the problems in the conversion of 1 to 2, however, is a further reduction of the initial product 2 and contamination of monofluoro-compounds because 2 mostly has a similar reduction potential to 1. Whereas, demonofluorination would exclusively predominate in the conversion of 1 to 3 due to the large difference of reduction potentials between 1 and 3. In fact, LUMO of difluoromethyl phenyl ketone is close to that of trifluoromethyl phenyl ketone which is much lower than that of silylenolether of difluoromethyl phenyl ketone 1 (Scheme 1). Therefore, silylenolethers are desirable products which would not be reduced under the electrolysis conditions and are synthetic synthons of the enolate of difluoroketones, and should be employed as useful building blocks for difluoro methylene compounds. Here, we describe an electrochemical transformation of trifluoromethyl ketones and imines to the β,β-difluoroenolsilyl ethers and enamines, respectively.
KeywordsBond Breaking Trifluoromethyl Group Active Methylene Compound Aliphatic Ketone Phenyl Ketone
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