Expected and unexpected results in reactions of fluorinated nitrile imines with (cyclo)aliphatic thioketones

A series of (cyclo)aliphatic thioketones have been tested towards trifluoroacetonitrile imines, generated in situ via base-induced dehydrohalogenation of the respective hydrazonoyl bromides. Typically, non-enolisable thioketones yielded exclusively 3-trifluromethyl-2,3-dihydro-1,3,4-thiadiazoles as a result of completely regio-, chemo-, and diastereoselective (3 + 2)-cycloaddition. In the case of 2,2,4,4-tetramethylcyclobutane-1,3-dithione a competitive rearrangement followed by trapping of title 1,3-dipole by the C=S group of the initially formed beta-dithiolactone was observed. Enolizable (1R)-thiocamphor also provided the expected (3 + 2)-cycloadducts along with the insertion products found in a ratio reflecting electronic properties of the nitrile imine. The latter results indicate remarkable electrophilic nature of the electron-deficient fluorinated nitrile imines, and implies a possible step-wise mechanism of the studied reaction.