Synthesis of 4-Trifluoromethylselenolated Pyrazoles via Tf2O-Promoted Intramolecular Cyclization of α,β-Alkynic Tosylhydrazones with Benzyl Trifluoromethyl Selenoxide.

An expedient electrophilic cyclization strategy has been developed for the construction of 4-trifluoromethylselenolated pyrazoles from α,β-alkynic tosylhydrazones. With benzyl trifluoromethyl selenoxide as an electrophilic trifluoromethylselenolation reagent, Tf2O as the promoter, and under an open air atmosphere at ambient temperature, a wide range of titled compounds were smoothly obtained in good to excellent yields. The method also can be adapted to a one-pot, two-step process directly from BnSeCF3 involving oxidation with mCPBA followed by trifluoromethylseleno-cyclization. The newly developed method featured ready availability of starting materials, mild reaction conditions, and a broad substrate scope.