Novel Trifluoromethylated Spiro-1,3,4-thiadiazoles via (3+2)-Cycloadditions of 2,3-Diphenylcyclopropenethione with Selected in Situ-Generated Nitrile Imines Derived from Trifluoroacetonitrile

Abstract

The in situ-generated N-aryl nitrile imines derived from trifluoroacetonitrile react efficiently with 2,3-diphenylcyclopropenethione to give spirocyclic 1,3,4-thiadiazole derivatives as products of a regio- and chemoselective (3+2)-cycloaddition in good to excellent yields. A stepwise mechanism via initial nucleophilic attack of the S-atom onto the electrophilic C-atom of the electron-deficient 1,3-dipole leading to a zwitterionic intermediate is postulated to explain these formal (3+2)-cycloaddition reactions. The presence of the CF3 group is necessary to activate the nitrile imine for the efficient trapping of the cyclopropenethione. These are the first examples of a successful reaction of this C=S dipolarophile affording 1,3,4-thiadiazoles as formal (3+2)-cycloadducts.