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General procedure: 2-chloro-5-fluoropyridine (20 g, 152 mmol) was dissolved in trifluoroacetic acid (150 mL) under nitrogen protection. Hydrogen peroxide (78 mL, 760 mmol) was slowly added to the solution at 70°C. The reaction mixture was stirred at 70°C for 16 hours. Upon completion of the reaction, the reaction solution was concentrated. Water and dichloromethane (DCM) were added to the residue. The pH was adjusted to 7 with 28% ammonium hydroxide solution and then extracted with DCM. The organic phase was dried with anhydrous magnesium sulfate (MgSO4) and concentrated to give 2-chloro-5-fluoropyridine 1-oxide (20.1 g, 136 mmol, 90% yield), and the product could be used in the next reaction without further purification.LC-MS analysis showed m/z 147.6 (M+H) with retention time of 0.78 min.

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