トリフルオロメチル化合物のエン反応

Abstract

Trifluoromethyl carbonyl compounds were found to react as a good enophile due to the high electronegativity of a trifluoromethyl group. Thus, hexafluoroacetone reacts with terminal olefins without a catalyst, while other trifluoromethyl ketones and trifluoroacetaldehyde are less reactive and react only in the presence of Lewis acid. Interestingly, an inner olefin, 2-octene, hardly reacts with hexafluoroacetone, while it reacts with other trifluoromethyl carbonyl compounds in moderate to good yields. This inversion of the order of reactivity is explained by a large steric effect of the trifluoromethyl group, although it is referred as a “mimic” of a methyl group. It seems to be a little larger than a sec-butyl group. The ene reaction products, trifluoromethylated homoallyl alcohols, are useful intermediates for various types of trifluoromethyl compounds. General procedures for their conversion to trifluoromethylated dienes, tetrahydrofurans, and tetrahydropyrans are introduced. Products from trifluoroacetaldehyde were oxidized to trifluoromethyl ketones, Grignard and Wittig reactions of which are also discussed from the view point of reactivity and steric effect of a trifluoromethly group. Further, application of this reaction for the synthesis of fluorine analogues of natural products, p-and m-menthane derivatives, are presented.