Synthesis of substituted (trifluoromethyl)alkenes: (Trifluoromethyl)alkylidenation of thioketones via CF3-thiiranes

The synthesis of (trifluoromethyl)alkenes, including fully substituted variants, was achieved from thioketones with in situ-generated (trifluoromethyl)diazoalkanes [CF3C(R)=N2] (the Barton–Kellogg reaction). In the presence of sodium methoxide and a catalytic amount of tetrabutylammonium chloride, trifluoromethylated N-tosylhydrazones [CF3C(R)=NNHTs] derived from trifluoroacetaldehyde hemiacetal (R = H) or trifluoromethyl ketones (R = aryl or alkyl) were employed in the reactions with thioketones. The resulting (trifluoromethyl)diazoalkanes reacted with thioketones, forming (trifluoromethyl)thiirane intermediates. Treatment of these intermediates with trimethyl phosphite readily afforded the substituted (trifluoromethyl)alkenes. Theoretical calculations (DFT, B3LYP/6-31G*) showed that (trifluoromethyl)thiiranes, with less distortion than 2,2-difluorothiiranes, exhibited lower reactivity. Consequently, the involvement of a reducing agent was deemed necessary for the desulfurization step.