Methylhydrazine → 5-(4-Fluorophenyl)-1-methyl-1,2-dihydro-3H-pyrazol-3-one

A suspension of (4-fluorophenyl)propynoic acid methyl ester (1.6 g, 9.0 mmol) in MeOH (10 mL) and H2O (10 mL) was heated at 60° C. and MeNHNH2 (0.5 mL, 9.4 mmol) was added. The reaction mixture was stirred at 60° C. for 20 min. then filtered. The residue was washed with H2O (10 mL) and MeOH (5 mL) and dried to give 5-(4-fluorophenyl)-1-methyl-1,2-dihydro-pyrazol-3-one (0.63 g, 36%). The filtrate was then concentrated under reduced pressure and the residue dissolved in EtOAc (20 ml) and washed with H2O (3×15 mL). The aqueous layer was acidified to pH 5.5 with 2 M citric acid solution (3×15 mL) and extracted with EtOAc (3×20 mL). The extracts were dried (MgSO4) and concentrated under reduced pressure to give 5-(4-fluoro-phenyl)-2-methyl-1,2-dihydro-pyrazol-3-one (0.2 g, 12%).