General procedure for the synthesis of 4-bromobenzocyclobutene from 4-bromobenzocyclobutene and N,N-dimethylformamide (DMF): 50 mL of dry tetrahydrofuran (THF), magnesium shavings (2.88 g, 120 mmol), and 1,2-dibromoethane (4 drops) were added to a 500 mL three-necked flask. The reaction mixture was heated and refluxed for 15 min and then a 25 mL THF solution of 4-bromobenzocyclobutene (20.0 g, 109 mmol) was added slowly and dropwise through a constant pressure dropping funnel to prepare the Grignard reagent. After the dropwise addition, the dropping funnel was rinsed with 25 mL of anhydrous THF and the reaction mixture was continued to reflux for 45 min to obtain a greenish brown solution. Subsequently, the reaction mixture was cooled to 0 °C and DMF (15 mL, 210 mmol) was added slowly dropwise, refluxing for 15 min after the drop was completed. Upon completion of the reaction, the mixture was poured into 150 g of ice water, acidified with dilute hydrochloric acid to pH 2-3, and subsequently neutralized with saturated sodium bicarbonate solution to pH 7-8. The crude product was extracted with ethyl acetate (3 × 50 mL), the organic phases were combined, filtered through diatomaceous earth, and concentrated under reduced pressure to remove the solvent. The crude product was purified by silica gel column chromatography with 10% ether/hexane as eluent, and finally further purified by Kugelrohr distillation (145 °C, 0.5 mmHg) to afford the target product 4-formylbenzocyclobutene (11.7 g, 81.2% yield) as a colorless liquid. The product was characterized by IR, 1H NMR and 13C NMR, and the data were consistent with the literature. The elemental analysis results were in accordance with the theoretical calculated values (C9H8O: C, 81.8; H, 6.10; measured values: C, 81.7; H, 5.94).
