Isobutylamine (25 mL, 121 mmol) was added dropwise to a toluene solution (40 mL) of 2,4-difluoronitrobenzene (8.0 g, 50.3 mmol) under ice cooling, and the mixture was stirred at 35-40° C. for 2 hours. Water was added to the reaction solution, and toluene extraction was performed, followed by washing with water. The resulting organic layer was concentrated under reduced pressure until reduced to about half (the concentrated solution including a crude product of 5-fluoro-N-isobutyl-2-nitroaniline). Benzyl alcohol (10.4 mL, 100 mmol), tetrabutyl ammonium hydrogen sulfate (1.71 g, 5.0 mmol), potassium carbonate (10.42 g, 75.4 mmol), and water (0.8 mL) were added to the resulting solution, and the mixture was heated under reflux for 3 hours. After the reaction solution was cooled, water (84 mL) was added thereto, followed by toluene extraction. After the resulting organic layer was washed with water, the solvent was distilled off, and cyclopentyl ether was added to the resulting residue. The mixture was stirred for 1 hour. Precipitated crystals were collected by filtration, and the resulting crystals were dried at 50° C., thereby obtaining 12.2 g of a yellow, powdery target compound (yield: 81%).
