General procedure for the synthesis of 2-bromo-3-nitrobenzaldehyde using 2-bromo-3-nitrotoluene as starting material: first, chromium(VI) oxide (1.86 g, 6.94 mmol, 2.7 eq.) was dissolved in acetic anhydride (30 mL), and the solid was stirred until completely dissolved. Simultaneously, 2-bromo-3-nitrotoluene (1.5 g, 18.7 mmol, 1 eq.) was suspended in acetic anhydride (12 mL). Subsequently, sulfuric acid was slowly added dropwise to the suspension at 0 °C. Next, a previously prepared solution of chromium (VI) oxide was added dropwise over 1 hour and 30 minutes at the same temperature. After completion of the reaction, the mixture was warmed up to room temperature and stirred continuously for 3 hours. The reaction solution was neutralized in a small amount of ice water and the resulting solid product, 2-bromo-3-nitrobenzaldehyde (0.63 g, 39% yield) was collected by filtration, which could be used in subsequent steps without further purification. The product was characterized as follows: melting point 92 °C; IR spectra (KBr) υ 1697, 1539, 1519 cm-1; 1H NMR (DMSO-d6, 300 MHz) δ 14.24 (s, 1H), 8.26 (d, J = 7.8 Hz, 1H), 8.07 (d, J = 7.8 Hz, 1H), 7.79 (t, J = 8.1 Hz, 1H). Calculated elemental analysis (C7H4BrNO3): C, 36.55; H, 1.75; Br, 34.74; N, 6.09. Measured values: C, 36.60; H, 1.74; Br, 34.81; N, 6.04.
