[Br:1][C:2]1[CH:6]=[C:5](Br)[S:4][C:3]=1[CH2:8][CH3:9].C([Li])CCC.CCCCCC.B(OC)(OC)OC.C1(P(C2C=CC=CC=2)C2C=CC=CC=2)C=CC=CC=1.[Cl:47][C:48]1[CH:53]=[CH:52][C:51](I)=[CH:50][CH:49]=1.C(=O)([O-])[O-].[Na+].[Na+]>C(OCC)C.C([O-])(=O)C.[Pd+2].C([O-])(=O)C.C1COCC1>[Br:1][C:2]1[CH:6]=[C:5]([C:51]2[CH:52]=[CH:53][C:48]([Cl:47])=[CH:49][CH:50]=2)[S:4][C:3]=1[CH2:8][CH3:9]
To a solution of 3,5-dibromo-2-ethyl-thiophene (3.28 g, 12.15 mmol) in diethyl ether (50 ml) at −78° C. under N2 is added, slowly, 2.5M butyl lithium in hexane solution (4.86 ml, 12.15 mmol) over 10 minutes and the reaction stirred at −78° C. for a further 30 minutes. The reaction is then cooled to −78° C. before the dropwise addition of trimethyl borate (1.64 ml, 14.6 mmol) over 5 minutes. The reaction is stirred at −78° C. for 30 minutes, then allowed to warm to room temperature and stirred for a further 60 minutes. Palladium acetate (68 mg, 0.3 mmol), triphenylphosphine (314 mg, 1.2 mmol) and 4-chloro-iodobenzene (4.9 g, 12.15 mmol) is then added to the reaction, followed by THF (50 ml) and 1N sodium carbonate solution (20 ml) and the reaction is heated to reflux for 3 hours. The cooled reaction mixture is partitioned between ether (250 ml) and water (300 ml). The organic layer is separated, dried over magnesium sulphate, filtered and evaporated under reduced pressure. The residue is therefore purified by flash chromatography on silica gel to give 3-bromo-5-(4-chlorophenyl)-2-ethyl-thiophene as a white solid (2.75 g).
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