1-(4-methylphenoxy)-2-nitrobenzene (prepared by an Ullmann etherification)
Iron powder (Acros Organics, -325 mesh, as received)
Ferric chloride (EM Science, anhydrous, as received)
Ethanol (Pharmco-Aaper, anhydrous, as received)
Glacial acetic acid (J.T. Baker, Baker analyzed, as received)
Sodium nitrate (J.T. Baker, Baker analyzed, as received)
Hydrochloric acid (J.T. Baker, 36.5-38.0%, as received)
Water (deionized)
Diethyl ether (J.T. Baker, sparged with argon and passed through a Seca solvent purification system)
Sodium tetrafluoroborate (Acros Organics, 98%, as received)
Acetone (Brand-Nu Laboratories Inc., TECH grade, as received)
Magnesium sulfate (J.T. Baker, reagent grade, as received)

NOTE - isolated diazonium salts are potentially explosive and should only be prepared and handled by a technically qualified person. Into a 500 mL RBF equipped with magnetic stirring, reflux condenser and nitrogen inlet was added 1-(4-methylphenoxy)-2-nitrobenzene (6.20 g, 27.0 mmol), EtOH (50 mL) and AcOH (5 mL). The head space was purged with nitrogen for 10 min and the solution brought to reflux with an oil bath. To the mixture was rapidly added iron powder (10.0 g, 179 mmol) and ferric chloride (ca. 800 mg, 5 mmol) sequentially down the condenser (Caution! Vigorous effervescence) and the residue was rinsed into the RBF with minimal amounts of EtOH. The heterogeneous mixture was refluxed for 2 h, cooled to RT, filtered by suction through a large Buchner funnel and the filter cake rinsed with small portions of EtOH and Et₂O. The filtrate was transferred to a 1L separatory funnel, diluted with 200 mL of water, extracted with ether (200 mL, 2 x 50 mL) and the pooled organic fractions sequentially washed with equal volumes of saturated bicarbonate and brine. The organic fraction was dried over magnesium sulfate, filtered, the solvent removed under vacuum and the resulting yellow colored oil suspended into 40 mL of water. The oily suspension was chilled in an ice/water bath and 10 mL of conc. HCl was rapidly added affording a finely divided white ppt. To the mixture was added sodium nitrate (2.05 g, 29.7 mmol) dissolved in 20 mL of water dropwise and the mixture stirred magnetically for 20 min. The mixture was filtered by suction and to the filtrate was added sodium tetrafluoroborate (5.20 g, 47.4 mmol) dissolved in 20 mL of water. The resulting yellow ppt. was isolated by suction filtration, washed with minimal amounts of ether and then dissolved into 50 mL of warm acetone. To the solution was added 50 mL of ether dropwise affording white, fibrous needles of the tetrafluorborate salt. NOTE - POTENTIAL HAZARD - the crystals were isolated by suction and then dried to constant weight (6.16 g, 76.4%) under vacuum (P = 0.1 Torr). Mp = 157-159^o C (decomp.).

The intermediate amine can be isolated and chromatographed on silica with 5% EtOAc/petroleum ether (Rf = 0.39), but we found that this material rapidly darkened on standing. In our hands the iron-mediated reduction of the nitroarene proceeds more quickly than traditional heterogeneous hydrogenations (i.e. dihydrogen, Pd/C) and does not require the use of an elaborate apparatus or high pressures of hydrogen gas. TLC analysis revealed incomplete conversion to the amine after 36 h with ca. 1 atm of hydrogen and 10% Pd/C. Likewise, the mild nature of the iron-assisted reaction lead to clean, spot-to-spot conversion by TLC after 1.5 h. As pointed out in Vogel, care needs to be exercised when handling dry diazonium salts. However, tetrafluoroborate diazonium salts are somewhat more stable, but will decompose on heating to the corresponding aromatic fluoride in the so-called Balz-Schiemann reaction. The recrystallized material remained colorless when stored in the dark, but solutions in acetone rapidly turned orange/red when allowed to stand at RT.

¹H-NMR. Acetone-d₆. 8.73 (1H, d, J = 8.4 Hz, arene H), 8.24 (1H, t, J = 7.6 Hz, arene H), 7.61 (1H, t, J = 8.0 Hz, arene H), 7.42 (2H, d, J = 8.2 Hz, arene H’s), 7.32 (2H, d, J = 8.4 Hz, arene H’s), 7.22 (1H, d, J = 8.8Hz, arene H), 2.42 (3H, s, Me H’s). ¹³C-NMR. Delta 162.3, 150.7, 144.3, 137.6, 133.0, 131.4, 124.6, 121.0, 116.9, 103.5, 20.2. IR (KBr pellet) 2277 cm-1. Mp = 157 -159⁰C (dec., lit. 152-153⁰C).

Andrew McCarroll. Iron-mediated reduction of aromatic nitro-groups to amines. Page 261

Frederick W. Wassmundt, Ronald P. Pedemonte. An Improved Synthesis of Dibenzofurans by a Free-Radical Cyclization. Journal of Organic Chemistry, 1995, 60, 4991-4994. B.S. Furniss, A.J. Hannaford, P.W.G. Smith, A.R. Tatchell. Vogel’s Textbook of Practical Organic Chemistry, 5th edition, page 925.