1,2-Dibromoethane → 5-Bromo-7-chloro-2,3-dihydro-benzo[1,4]dioxine

To a solution of 1,2-dibromoethane (1.44 ml, 16 mmol) and tetrabutylammonium bromide (96 mg, 2.5 mol %) in water (8 ml) at reflux under nitrogen was added a mixture of 3-bromo-5-chloro-benzene-1,2-diol (Preparation 27, 2.68 g, 12 mmol) and NaOH (1.06 g, 26.2 mmol) in water (10 ml) over 4 hours, and the resultant mixture stirred overnight. The reaction mixture was cooled to room temperature and diluted with water (100 ml). The mixture was extracted with diethyl ether (3×100 ml), and the combined organic extracts were concentrated in vacuo. Purification by flash chromatography, eluting with 90:10 pentane:dichloromethane, afforded the title compound as a yellow oil which crystallised upon standing to a yellow solid (1.78 g, 60% yield).