Under protective gas, 10.7 g of 2,3-difluoro-4-propylphenylboronic acid (prepared from 1,2-difluoro-3-propylbenzene by lithiation with n-BuLi in THF at −70° C. and subsequent reaction with trimethyl borate according to J. Chem. Soc., Perkin Trans. II 1989, 2041), 8.9 g of anhydrous potassium fluoride and 0.64 g of tris(dibenzylideneacetone)dipalladium(0) are initially charged, and admixed successively with a solution of 11.5 g of 2-bromo-6-fluoro-4-propylanisole (prepared by etherification of 2-bromo-6-fluoro-4-propylphenol with dimethyl sulfate and potassium carbonate in acetone; 2-bromo-6-fluoro-4-propylphenol was obtained by lithiation of 1-bromo-5-fluoro-3-propylbenzene with LITMP in THF at −70° C. and subsequent reaction with trimethyl borate, acidic workup and oxidation with H2O2 in MTBE according to Tetrahedron Lett. 1996, 37, 6551 and J. Chem. Soc., Perkin Trans. II 1989, 2041; 1-bromo-5-fluoro-3-propylbenzene was obtained from 3-bromo-5-fluorobenzaldehyde [188813-02-7 by Wittig reaction with ethyltriphenylphosphonium bromide and subsequent cat. hydrogenation) in 90 ml of dry 1,4-dioxane and 0.38 g of tri-tert-butylphosphine (dissolved in approx. 5 ml of the same solvent). The mixture is heated to boiling under vigorous stirring for 6 h. After cooling, the reaction mixture is added to water and extracted with MTBE. The org. phase is removed, washed with water and sat. sodium chloride solution, and dried over sodium sulfate, and the solvents are removed under reduced pressure. The crude product is purified by chromatography on silica gel with heptane/dichloromethane (8:2 v/v) as the eluent. The thus obtained 3,2′,3′-trifluoro-2-methoxy-5,4′-dipropylbiphenyl is heated to boiling in a mixture of 75 ml of glacial acetic acid and 65 ml of 48% aqueous hydrobromic acid overnight. After cooling, the reaction mixture is added to ice-water and extracted with ethyl acetate. The combined organic phases are washed with water and 5% sodium hydrogencarbonate solution and dried over sodium sulfate. After the solvent has been removed under reduced pressure, the black-brown residue is purified chromatographically on silica gel using 7:3 heptane/dichloromethane as the eluent. The resulting 3,2′,3′-trifluoro-5,4′-dipropylbiphenyl-2-ol is heated to 90–100° C. with 8.2 g of potassium carbonate in 270 ml of dimethylformamide for 6 h. After cooling, the reaction mixture is added to approx. 1 l of water and extracted twice with dichloromethane, and the organic phases are combined, washed twice with saturated sodium chloride solution and subsequently twice with water and dried over sodium sulfate. The brown residue which is obtained after the solvent has been distilled off is chromatographed using silica gel with toluene as the eluent. After the solvent has been removed under reduced pressure, the product-containing fractions are recrystallized from heptane. 3.2 g of 4,6-difluoro-2,7-dipropyldibenzofuran are obtained.
