GENERAL STEPS: 2-Bromo-1-methyl-4-nitrobenzene (15.0 g, 69.4 mmol) was dissolved in a mixture of peracetic acid (112 mL) and acetic anhydride (105 mL, 1,113 mmol). The reaction mixture was cooled in an ice water bath and concentrated sulfuric acid (15 mL, 281 mmol) was added slowly. Subsequently, chromium (VI) oxide (34.7 g, 347 mmol) was added in batches while the reaction temperature was strictly controlled in the range of 5-10 °C. The reaction mixture was continuously stirred in an ice bath for 1.5 h, during which the temperature needed to be closely monitored, and an ice-acetone bath was used to maintain the temperature at no more than 10 °C if necessary. Upon completion of the reaction, the mixture was poured into a 2 L conical flask containing an appropriate amount of ice, and the total volume was adjusted to 1,500 mL by adding cold water.The precipitate was collected through a Brinell funnel and washed with cold water until nearly colorless. The resulting solid was suspended in cold 2% aqueous Na2CO3 solution (100 mL), stirred thoroughly and the solid was again collected, washed with cold water and partially dried over a Brinell's funnel to give 8.3 g of crude diacetate intermediate. The intermediate (8.3 g) was mixed with ethanol (16 mL), water (16 mL) and concentrated sulfuric acid (2.4 mL) and heated to reflux for 1 hour. After cooling to room temperature, the mixture was transferred to a split funnel containing saturated aqueous K2CO3 solution (50 mL). Fluffy solid formation was observed and identified as non-target product. The aqueous layer was extracted with ethyl acetate (4 x 50 mL), the organic layers were combined and washed with brine (25 mL), dried over MgSO4, filtered and concentrated. Purification by silica gel column chromatography (gradient elution of 5%-15% ethyl acetate in hexane solution) afforded 2-bromo-4-nitrobenzaldehyde (1.41 g, 9% yield) as a colorless fluffy solid.1H NMR (400 MHz, CDCl3) δ 10.41 (s, 1H), 8.51 (d, J = 2.0 Hz, 1H), 8.25 (dd, J = 8.4), 2.1 Hz, 2.1 Hz, 1 Hz. , 2.1 Hz, 1H), 8.06 (d, J = 8.6 Hz, 1H).
