N([O:3][C:4](C)(C)C)=O.[Cu]C#N.[CH3:11][N:12]1[C:16](N)=[C:15]([CH3:18])[C:14]([C:19]([F:25])([F:24])[C:20]([F:23])([F:22])[F:21])=[N:13]1.[OH-:26].[K+].Cl>C(#N)C.O>[CH3:11][N:12]1[C:16]([C:4]([OH:3])=[O:26])=[C:15]([CH3:18])[C:14]([C:19]([F:25])([F:24])[C:20]([F:23])([F:22])[F:21])=[N:13]18.66 g (85.1 mmol) of tert-butyl nitrite and 6.0 g (67.4 mmol) of copper(I) cyanide are suspended in 360 ml of acetonitrile and heated to 65° C. A solution of 13.0 g (57.2 mmol) of 1,4-dimethyl-3-(pentafluoroethyl)-1H-pyrazole-5-amine in 20 ml of acetonitrile is then slowly added. The reaction mixture is further stirred for 24 hours at 65° C. The reaction mixture is then filtered over Celite. The filtrate is concentrated by evaporation under reduced pressure on a rotary evaporator. The residue is taken up in water and acidified with conc. hydrochloric acid. The aqueous phase is extracted several times with dichloromethane. The combined organic phases are concentrated by evaporation under reduced pressure on a rotary evaporator and the residue is purified by column chromatography on silica gel (hexane:ethyl acetate=5:1). The resulting product (2.3 g, 9.7 mmol) is heated under reflux with 3.44 g (63 mmol) of potassium hydroxide in 20 ml of water. After 3 h the reaction solution is cooled to 0° C. and adjusted to pH 6 with dilute hydrochloric acid. The resulting solid is filtered and dried. This gives 0.3 g of 1,4-dimethyl-3-(pentafluoroethyl)-1H-pyrazole-5-carboxylic acid (2%) as a white solid.
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