Iron(II) sulfate heptahydrate, 98% (Aldrich, as received)
water (deionized)
2-(4-Methylphenoxy)benzenediazonium tetrafluoroborate Page 311
diethyl ether (J.T. Baker, sparged with argon and dried by passage through a Seca Solvent Purification System)
magnesium sulfate (J.T. Baker, reagent grade, as received)
silica (Silicycle, 230-400 mesh, as received)
hexanes (J.T. Baker, 95% n-hexane, Ultra Resi-Analyzed, as received)

Into a 1L three-way RBF equipped with magnetic stirring, a reflux condenser, addition funnel and nitrogen inlet was added iron(II) sulfate heptahydrate (2.62 g, 9.41 mmol) and 300 mL of water. The head space was purged with nitrogen for 15 min and the solution brought to a boil with a sand bath. A solution of 2-(4-methylphenoxy)benzenediazonium tetrafluoroborate Page 311(2.81 g, 9.41 mmol) in 300 mL of water was added dropwise to the RBF over the course of 30 min. The resulting oily, orange colored mixture was cooled to RT, extracted with diethyl ether (200 mL, 2 x 50 mL) and the pooled organic fractions washed with an equal volume of brine and dried over magnesium sulfate. The mixture was filtered and evaporated to dryness. The residue was purified by flash column chromatography on 50 g of silica gel with hexanes (Rf = 0.36) affording 1.49 g (87.2%) of a white, crystalline solid after solvent removal.

This procedure was slightly modified from the lead reference as column chromatography was used in lieu of steam distillation to purify the desired product. It should be noted that sonication is necessary to fully dissolve the diazonium salt in water, otherwise suspensions of the salt will clog the addition funnel. The crude residue isolated prior to chromatography was readily miscible in minimal amounts of hexanes, precluding the need for preadsorbing the residue on silica or using a more polar solvent to load the sample on the column. TLC plates were developed with hexanes and visualized both under short wave UV-light (254 nm) and exposure to iodine vapor. Other promoters of this reaction are reported (i.e. hydroquinone, tin(II) chloride, sodium iodide and cupric sulfate), but we chose to work with iron(II) sulfate heptahydrate because it is cheap, readily available in large quantities and easy to handle under ambient conditions.

¹H NMR. CDCl₃. 7.96 (1H, dd, J = 7.7, 0.6 Hz, arene H), 7.78 (1H, m, arene H), 7.60 (1H, d, J = 8.2 Hz, arene H), 7.49 (2H, m, arene H's), 7.37 (1H, td, J = 8.0, 0.9 Hz, arene H), 7.30 (1H, dd, J = 8.0, 1.3 Hz, arene H), 2.56 (3H, s, methyl H's). 13C NMR. CDCl₃. delta 156.7, 154.8, 132.4, 128.4, 127.2, 124.5, 124.4, 122.7, 120.9, 120.8, 111.9, 111.4, 21.6.

Frederick W. Wassmundt, Ronald P. Pedemonte. An Improved Synthesis of Dibenzofurans by a Free-Radical Cyclization. Journal of Organic Chemistry. 1995, 60, 4991-4994.