Regioselective synthesis of fluorinated pyrazole derivatives from trifluoromethyl-1,3-diketone

1,1,1-Trifluoropentane-2,4-dione (1a) and 1-(thien-2-yl)-4,4,4-trifluorobutane-1,3-dione (1b) reacted readily with per(poly)fluorophenylhydrazines ArfNHNH2 (Arf: C6F5, HC6F4, ClC6F4) to give N-per(poly)fluorophenyl-5-methyl(or thien-2-yl)-3-trifluoromethylpyrazoles 3 and 3-methyl (or thien-2-yl)-5-hydroxy-5-trifluoromethyl-4,5-dihydropyrazoles 4, respectively. Treatment of 4 with P2O5 yielded the dehydrated product N-per(poly)fluorophenyl-3-methyl(or thien-2-yl)-5-trifluoromethyl-pyrazole in good yield.