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General procedure for the synthesis of 4-fluoro-1,2-dimethoxybenzene from 3,4-dimethoxyaniline: 1. Continuous diazotization step: material A (50 mL containing 3,4-dimethoxyaniline (100 mmol), fluoboric acid (120 mmol), hydrochloric acid (180 mmol)) and material B (50 mL containing aqueous sodium nitrite solution) were mixed in a reaction tube at 25 °C with a residence time of about 15 seconds. Subsequently, sodium nitrite solution (105 mmol) was pumped into the reaction system through a T-fitting at a flow rate of 4 mL/min. The reaction mixture flowed out through the outlet and was collected in a cooling vessel, maintaining vigorous stirring during the process. The resulting slurry was cooled to -5°C and then pumped. The solids were washed with methanol and subsequently dried under vacuum to give the corresponding diazonium tetrafluoroborate. 2. Continuous fluorination carbonylation process: a slurry of the above prepared diazonium tetrafluoroborate in 300 mL of co-solvent was introduced continuously into the reaction tube at a flow rate of 4 mL/min. The mixture was kept at a set temperature for 1 min and subsequently cooled in the tandem tube. The collected liquid was washed with aqueous NaOH and water to give an almost colorless liquid of 4-fluoro-1,2-dimethoxybenzene.