Cyclization of Thiopropargyl Benzimidazoles by Combining Iron(III) Chloride and Diorganyl Diselenides

A practical synthetic approach to the synthesis of 3-(organoselenyl)-imidazothiazines
was developed. The methodology involved the regioselective 6-endo-dig
cyclization of thiopropargyl benzimidazoles promoted by diorganyl
diselenides and iron­(III) chloride. The investigation to determine
the best reaction conditions indicated the use of thiopropargyl benzimidazoles
(0.25 mmol) with diorganyl diselenides (1.0 equiv) and iron­(III) chloride
(2.0 equiv) in dichloromethane at 40 °C for 30 min to be optimal.
Under these conditions, the scope of the substrates was evaluated
varying the structures of thiopropargyl benzimidazoles and diorganyl
diselenides giving 28 3-(organoselenyl)-imidazothiazines in moderate
to good yields. The reaction conditions were also applicable to diorganyl
ditellurides; however, they did not work for diorganyl disulfides.
The mechanism studies were carried out indicating that the cyclization
proceeds via a cooperative action of diorganyl diselenides and iron­(III)
chloride, but a direct electrophilic cyclization, promoted by the
in situ formed electrophilic organoselenium species, cannot be ruled
out.