Solid-Phase Aromatic Nitration with Mg(NO<sub>3</sub>)<sub>2</sub> on Silica Gel

Abstract

Nitroaromatics are usually prepared using a mixed acid of nitric acid with strong acids. However, the use of strong acids caused dangerous work-up and the disposal of large amounts of acid-waste. Therefore, much effort has been made on the improvement of nitration process without strong acids. We examined solid-phase aromatic nitration with Mg(NO 3 ) 2 on silica gel in order to establish the nitration process without strong acids. The nitration of 1,2-and 1,3-, 1,4-dimethoxybenzenes and 4-methylanisole with Mg(NO 3 ) 2 proceeded by heating on silica gel at 150˚C for 4 -5 h to produce the nitroaromatics. The nitration of 1,3,5-trimethoxybenzene produced the nitrated dimer, 2,4,6,2', 4',6'-hexamethoxy-3-nitrobiphenyl, which was not isolated in other solid-phase nitration. In the cases of naphthalene derivatives, the α-nitrated compounds were obtained. In the cases of p-cresol and 2-naphthol, the esterification occurred at the hydroxyl group to give 4-tolyl nitrate and 2-naphthyl nitrate, respectively. It is synthetic interest to note that nitrate esters were isolated in solid phase. Thus Mg(NO 3 ) 2 -SiO 2 composite was mild reagent for solid-phase nitration. Acidity of Mg(NO 3 ) 2 -SiO 2 composite was determined to be pH 0.96 by the measurement of absorption spectra on a micro spectrophotometer using meso-tetra(p-cyanophenyl)porphyrin as a pH-indicator. Mg(NO 3 ) 2 -SiO 2 composite made acidic conditions. Therefore, it was suggested that Mg(NO 3 ) 2 reacted with proton on silica gel to form the 2 NO + . Thus, electron-rich aromatic hydrocarbons led the efficient nitration through electrophilic attack of 2 NO + . After the nitration, acidic Mg(NO 3 ) 2 -SiO 2 composite could be turned into neutrality by exposing wet conditions and disposed safely since the composite did not involve harmful elements. Thus the solid-phase nitration using Mg(NO 3 ) 2 -SiO 2 composite will provide safety and environmentally conscious chemical process.

Acid-Free Copper-Catalyzed Electrophilic Nitration of Electron-Rich Arenes with Guanidine Nitrate

“…2-Methoxy-1-nitronaphthalene (2u). 22 Reaction temperature, rt; reaction time, 5 h; eluent, 6:1 petroleum ether/ethyl acetate; TLC R f = 0.2 (4:1 PE/EA, UV); yellow solid; 32% yield (32 mg, 0.16 mmol); N-(4-Methoxy-2-nitrophenyl)acetamide (4a). 23 Reaction temperature, rt; reaction time, 12 h; eluent, 10:1 petroleum ether/ethyl acetate; TLC R f = 0.5 (5:1 PE/EA, UV); yellow solid; 95% yield (100 mg, 0.47 mmol); 1 25 Reaction temperature, rt; reaction time, 12 h; eluent, 10:1 petroleum ether/ethyl acetate; TLC R f = 0.1 (10:1 PE/EA, UV); yellow solid; 69% yield (73.2 mg, 0.34 mmol); 1 H NMR (400 MHz, CDCl 3 ) δ 10.66 (s, 1H), 8.02 (s, 1H), 2.34 (s, 3H), 2.28 (s, 3H).…”