To a three-necked flask fitted with a stirrer and a constant-pressure dropping funnel, p-fluorotoluene was added, operated anhydrous and anaerobically, protected by nitrogen, and tert-butyllithium was slowly added dropwise over 1 hr in an ice bath. The reaction was carried out for 1 h. The progress of the reaction was detected by thin layer chromatography. DMF was added slowly dropwise within 15 minutes, and the reaction was carried out for 2 hours in an ice bath, and the progress of the reaction was detected by thin-layer chromatography. Add saturated ammonium chloride solution quenching, dichloromethane extraction, water to wash the organic layer, anhydrous sodium sulfate drying, concentrated as colorless to light yellow liquid 5-fluoro-2-methylbenzaldehyde.
